Diesel engine

The diesel engine is a type of internal combustion engine. It is a compression ignition engine, in which the fuel ignites as it is injected into the engine. By contrast, in the gasoline engine the fuel is mixed first and then ignited by a spark plug. Also, diesels generally have high compression ratios, to enable compression ignition, whereas in gasoline-burning engines, compression ignition is undesirable.

The engine operates using the diesel cycle.

The engine is named after German engineer Rudolf Diesel, who invented it in 1892 based on the hot bulb engine and received the patent on February 23, 1893. Diesel intended the engine to use a variety of fuels including coal dust and peanut oil. He demonstrated it at the 1900 Exposition Universelle (World's Fair) using peanut oil (see biodiesel).

Early history timeline

1862: Nicolaus Otto develops his coal gas engine, similar to a modern gasoline engine.

1891: Herbert Akroyd Stuart, of Bletchley perfects his oil engine, and leases rights to Hornsby of England to build engines. They build the first cold start, compression ignition engines.

1892: Hornsby engine No. 101 is built and installed in a waterworks. It is now in the MAN truck museum in Northern England.

1892: Rudolf Diesel develops his Carnot heat engine type motor which burnt powdered coal dust. He is employed by refrigeration genius Carl von Linde, then Munich iron manufacturer MAN AG, and later by the Sulzer engine company of Switzerland. He borrows ideas from them and leaves a legacy with all firms.

1892: John Froelich builds his first oil engine powered farm tractor.

1894: Witte, Reid, and Fairbanks start building oil engines with a variety of ignition systems.

1896: Hornsby builds diesel tractors and railway engines.

1897: Winton produces and drives the first US built gas automobile; he later builds diesel plants.

1897: Mirrlees, Watson & Yaryan build the first British diesel engine under license from Rudolf Diesel. This is now displayed in the Science Museum at South Kensington, London.

1898: Busch installs a Rudolf Diesel type engine in his brewery in St. Louis. It is the first in the United States. Rudolf Diesel perfects his compression start engine, patents, and licences it. This engine, pictured above, is in a German museum.

1899: Diesel licences his engine to builders Burmeister & Wain, Krupp, and Sulzer, who become famous builders.

1902: F. Rundlof invents the two-stroke crankcase, scavenged hot bulb engine.

1902: A company named Forest City [1] start manufacturing diesel generators.

1903: Ship Gjoa transits the ice-filled Northwest Passage, aided with a Dan kerosene engine.

1904: French build the first diesel submarine, the Z.

1908: Bolinder-Munktell starts building two stroke hot-bulb engines.

1912: First diesel ship MS Selandia is built. SS Fram, polar explorer Amundsen’s flagship, is converted to a AB Atlas diesel.

1913: Fairbanks Morse starts building its Y model semi-diesel engine. US Navy submarines use NELSECO units.

1914: German U-Boats are powered by MAN diesels. War service proves engine's reliability.

1920s: Fishing fleets convert to oil engines. Atlas-Imperial of Oakland, Union, and Lister diesels appear.

1924: First diesel trucks appear.

1928: Canadian National Railways employ a diesel shunter in their yards.

1930s: Clessie Cummins starts with Dutch diesel engines, and then builds his own into trucks and a Duesenberg luxury car at the Daytona speedway.

1930s: Caterpillar starts building diesels for their tractors.

1934: General Motors starts a GM diesel research facility. It builds diesel railroad engines—The Pioneer Zephyr—and goes on to found the General Motors Electro-Motive Division, which becomes important building engines for landing craft and tanks in the Second World War. GM then applies this knowledge to market control with its famous Green Leakers for buses and railroad engines.

1936: Mercedes-Benz builds the 260D diesel car. A.T.S.F inaugurates the diesel train Super Chief.

1936: Airship Hindenburg is powered by diesel engines.

How diesel engines work

When a gas is compressed, its temperature rises (see the combined gas law); a diesel engine uses this property to ignite the fuel. Air is drawn into the cylinder of a diesel engine and is compressed by the moving piston at a compression ratio as high as 25:1, much higher than needed for a spark-ignition engine. At the end of the piston stroke, diesel fuel is injected into the combustion chamber at high pressure through an atomising nozzle. The fuel ignites directly from contact with the air, the temperature of which reaches 700–900 °C (1300–1650 °F). The combustion causes the gas in the chamber to heat up rapidly, which increases its pressure, which in turn forces the piston outward. The connecting rod transmits this motion to the crankshaft, which delivers rotary power at its output end. Scavenging (pushing the exhausted gas-charge out of the cylinder and drawing in a fresh draught of air) of the engine is done either by ports or valves. To fully realize the capabilities of a diesel engine, use of a turbocharger to compress the intake air is necessary. Use of an aftercooler/intercooler to cool the intake air after compression by the turbocharger further increases efficiency.

In cold weather, diesel engines can be difficult to start because the cold metal of the cylinder block and head draw out the heat created in the cylinder during the compression stroke, thus preventing ignition. Engine generally use small electric heaters called glow plugs inside the cylinder to warm the cylinders prior to starting. A small number use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. Engine block heaters (electric resistive heaters in the engine block) plugged into the utility grid are often used when an engine is shut down for extended periods (more than an hour) in cold weather to reduce startup time and engine wear. Diesel fuel is also prone to 'waxing' in cold weather. This is when the fuel begins to solidify into a crystaline state. The crystals build up in the fuel system (especially the fuel filters), eventually starving the engine of fuel. Low-output electric heaters in fuel tanks and around fuel lines are used to solve this problem. Also, most engines have a 'spill return' system, by which any excess fuel from the injector pump and injectors is returned to the fuel tank. Once the engine is warmed up, this warm fuel will prevent waxing in the tank. Fuel technology has improved in recent years, with special additives preventing waxing in all but the coldest climates.

A vital component of older diesel engine systems is the governor, which limits the speed of the engine by controlling the rate of fuel delivery. Unlike in petrol (gasoline) engines, incoming air is not throttled and an engine without a governor can overspeed. Older injection systems were driven by a gear system from the engine and thus supplied fuel in proportion with engine speed. Modern, electronically controlled engines apply controls similar to those of petrol engines and limit the maximum RPM through an electronic control module (ECM) or electronic control unit (ECU)—the engine-mounted computer. The ECM/ECU receives an engine speed signal from a sensor and controls the amount of fuel and (start of injection) timing through electric or hydraulic actuators.

Controlling the timing of the start of injection of fuel into the cylinder is a key to minimizing emissions, and maximizing fuel economy (efficiency), of the engine. The timing is usually measured in units of crank angle of the piston before Top Dead Center (TDC). For example, if the ECM/ECU initiates fuel injection when the piston is 10 degrees before TDC, the start of injection, or timing, is said to be 10 deg BTDC. Optimal timing will depend on the engine design as well as its speed and load.

Advancing the start of injection (injecting before the piston reaches TDC) results in higher in-cylinder pressure and temperature, and higher efficiency, but also results in higher emissions of oxides of nitrogen (NOx) because of the higher temperatures. At the other extreme, delayed start of injection causes incomplete combustion. This results in higher particulate matter (PM) and unburned hydrocarbon (HC) emissions and more smoke.

Fuel injection in diesel engines

Early fuel injection systems

The modern diesel engine is a combination of two inventors' creations. In all major aspects, it holds true to Diesel's original design, that of the fuel being ignited by compression at an extremely high pressure within the cylinder. However, nearly all present-day diesel engines use the so-called solid injection system invented by Herbert Akroyd Stuart, for his hot bulb engine (a compression-ignition engine that precedes the diesel engine and operates slightly differently). Solid injection is where the fuel is raised to extreme pressures by mechanical pumps and delivered to the combustion chamber by pressure-activated injectors in an almost solid-state jet. Diesel's original engine injected fuel with the assistance of compressed air, which atomised the fuel and forced it into the engine through a nozzle. This is called an air-blast injection. The size of the compressor needed to power such a system made early diesel engines very heavy and large for their power outputs, and the need to drive a compressor lowered power output even more. Early marine diesels often had smaller auxiliary engines whose sole purpose was to drive the compressors to supply air to the main engine's injector system. Such a system was too bulky and inefficient to be used for road-going automotive vehicles.

Solid injection systems are lighter, simpler, and allow for much higher RPMs, and so are universally used for automotive diesel engines. Air-blast systems provide very efficient combustion under low-speed, high-load conditions, especially when running on poor-quality fuels, so some large cathedral marine engines use this injection method. Air-blast injection also raises the fuel temperature during the injection process, so is sometimes known as hot-fuel injection. In contrast, solid injection is sometimes called cold-fuel injection.

Because the vast majority of diesel engines in service today use solid injection, the information below relates to that system.

Mechanical and electronic injection

Older engines make use of a mechanical fuel pump and valve assembly which is driven by the engine crankshaft, usually from the timing belt or chain. These engines use simple injectors which are basically very precise spring-loaded valves which will open and close at a specific fuel pressure. The pump assembly consists of a pump which pressurizes the fuel and a disc-shaped valve which rotates at half crankshaft speed. The valve has a single aperture to the pressurized fuel on one side, and one aperture for each injector on the other. As the engine turns, the valve discs will line up and deliver a burst of pressurized fuel to the injector at the cylinder about to enter its power stroke. The injector valve is forced open by the fuel pressure, and the diesel is injected until the valve rotates out of alignment and the fuel pressure to that injector is cut off. Engine speed is controlled by a third disc, which rotates only a few degrees and is controlled by the throttle lever. This disc alters the width of the aperture through which the fuel passes, and therefore how long the injectors are held open before the fuel supply is cut, which controls the amount of fuel injected.

Older diesel engines with mechanical injection pumps could be inadvertently run in reverse, albeit very inefficiently as witnessed by massive amounts of soot being ejected from the air intake. This was often a consequence of bump starting a vehicle using the wrong gear.

This contrasts with the more modern method of having a separate fuel pump (or set of pumps) which supplies fuel constantly at high pressure to each injector. Each injector then has a solenoid which is operated by an electronic control unit, which enables more accurate control of injector opening times that depend on other control conditions, such as engine speed and loading, resulting in better engine performance and fuel economy. This design is also mechanically simpler than the combined pump and valve design, making it generally more reliable, and less noisy, than its mechanical counterpart.

Both mechanical and electronic injection systems can be used in either direct or indirect injection configurations.

Indirect injection

An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a prechamber, where combustion begins and then spreads into the main combustion chamber, assisted by turbulence created in the chamber. This system allows smoother, quieter running, and because combustion is assisted by turbulence, injector pressures can be lower, which in the days of mechanical injection systems allowed high-speed running suitable for road vehicles (typically up to speed of around 4,000 rpm). The prechamber had the disadvantage of increasing heat loss to the engine's cooling system and restricting the combustion burn, which reduced the efficiency by between 5-10% in comparison to a direct injection engine, and nearly all require some form of cold-start device such as glow plugs. Indirect injection engines were used widely in small-capacity high-speed diesel engines in automotive, marine and construction uses from the 1950s, until direct-injection technology advanced in the 1980s. Indirect injection engines are cheaper to build and it is easier to produce smooth, quiet running vehicles with a simple mechanical system, so such engines are still often used in applications which carry less stringent emissions controls than road-going vehicles, such as small marine engines, generators, tractors, pumps. With electronic injection systems, indirect injection engines are still used in some road-going vehicles, but most prefer the greater efficiency of direct injection.

During the development of the high-speed diesel engine in the 1930s, various engine manufacturers developed their own type of pre-combustion chamber. Some, such as Mercedes, had complex internal designs. Others, such as the Lanova pre-combustion chamber, used a mechanical system to adjust the shape of the chamber for starting and running conditions. However, the most commonly-used design turned out to be the 'Comet' series of swirl chambers developed by Harry Ricardo, using a two-piece spherical chamber with a narrow 'throat' to induce turbulence. Most European manufacturers of high-speed diesel engines used Comet-type chambers or developed their own versions (Mercedes stayed with their own design for many years), and this trend continues with current indirect-injection engines.

Direct injection

Modern diesel engines make use of one of the following direct injection methods:

Distributor pump direct injection

The first incarnations of direct injection diesels used a rotary pump much like indirect injection diesels; however the injectors were mounted in the top of the combustion chamber rather than in a separate pre-combustion chamber. Examples are vehicles such as the Ford Transit and the Austin Rover Maestro and Montego with their Perkins Prima engine. The problem with these vehicles was the harsh noise that they made and particulate (smoke) emissions. This is the reason that in the main this type of engine was limited to commercial vehicles— the notable exceptions being the Maestro, Montego and Fiat Croma passenger cars. Fuel consumption was about fifteen to twenty percent lower than indirect injection diesels, which for some buyers was enough to compensate for the extra noise.

One of the first small-capacity, mass-produced direct-injection engines that could be called refined was developed by the Rover Group.^{[citation needed]} The '200Tdi' 2.5-litre 4-cylinder turbo diesel (of 111 horsepower) was used by Land Rover in their vehicles from 1989, and the engine used an aluminum cylinder head, Bosch two-stage injection and multi-phase glow plugs to produce a smooth-running and economical engine while still using mechanical fuel injection.

This type of engine was transformed by electronic control of the injection pump, pioneered by Volkswagen Audi group with the Audi 100 TDI introduced in 1989. The injection pressure was still only around 300 bar, but the injection timing, fuel quantity, exhaust gas recirculation and turbo boost were all electronically controlled. This gave much more precise control of these parameters which made refinement much more acceptable and emissions acceptably low. Fairly quickly the technology trickled down to more mass market vehicles such as the Mark 3 Golf TDI where it proved to be very popular. These cars were both more economical and more powerful than indirect injection competitors of their day.

Common rail direct injection

In older diesel engines, a distributor-type injection pump, regulated by the engine, supplies bursts of fuel to injectors which are simply nozzles through which the diesel is sprayed into the engine's combustion chamber.

In common rail systems, the distributor injection pump is eliminated. Instead an extremely high pressure pump stores a reservoir of fuel at high pressure - up to 1,800 bar (180 MPa, 26,000 psi) - in a "common rail", basically a tube which in turn branches off to computer-controlled injector valves, each of which contains a precision-machined nozzle and a plunger driven by a solenoid, or even by piezo-electric actuators, which are found on experimental diesel race car engines.

Most European automakers have common rail diesels in their model lineups, even for commercial vehicles. Some Japanese manufacturers, such as Toyota, Nissan and recently Honda, have also developed common rail diesel engines.

Different car makers refer to their common rail engines by different names, e.g. DaimlerChrysler's CDI, Ford Motor Company's TDCi (most of these engines are manufactured by PSA), Fiat Group's (Fiat, Alfa Romeo and Lancia) JTD, Renault's DCi, GM/Opel's CDTi (most of these engines are manufactured by Fiat, other by Isuzu), Hyundai's CRDi, Mitsubishi's DI-D, PSA Peugeot Citroën's HDi (Engines for commercial diesel vehicles are made by Ford Motor Company), Toyota's D-4D, and so on.

Unit direct injection

This also injects fuel directly into the cylinder of the engine. However, in this system the injector and the pump are combined into one unit positioned over each cylinder. Each cylinder thus has its own pump, feeding its own injector, which prevents pressure fluctuations and allows more consistent injection to be achieved. This type of injection system, also developed by Bosch, is used by Volkswagen AG in cars (where it is called a Pumpe-Düse-System - literally "pump-nozzle system") and by Mercedes Benz (PLD) and most major diesel engine manufacturers in large commercial engines (CAT, Cummins, Detroit Diesel). With recent advancements, the pump pressure has been raised to 2,050 bar (205 MPa), allowing injection parameters similar to common rail systems.

Hypodermic injection injury hazard

Because many diesel engine fuel injection systems operate at extremely high pressure, there is a risk of injury by hypodermic injection of fuel, if the fuel injector is removed from its seat and operated in open air.

Types of diesel engines

There are two classes of diesel (and gasoline) engines: two-stroke and four-stroke. Most diesels generally use the four-stroke cycle, with some larger diesels operating on the two-stroke cycle, mainly the huge engines in ships (see also Nissan UD3, UD4 and UD6 engine series). Most modern locomotives use a two-stroke diesel mated to a generator, which produces current to drive electric motors, eliminating the need for a transmission. To achieve operational pressure in the cylinders, two-stroke diesels must utilize forced aspiration from either a turbocharger or supercharger. Diesel two-strokes are ideal for such applications because of their high power density--with twice as many power strokes per crankshaft revolution compared to a four-stroke, they are capable of producing much more power per displacement.

Normally, banks of cylinders are used in multiples of two, although any number of cylinders can be used as long as the load on the crankshaft is counterbalanced to prevent excessive vibration. The inline-6 is the most prolific in medium- to heavy-duty engines, though the V8 and straight-4 are also common. Small-capacity engines (generally considered to be those below 5 litres in capacity) are generally 4- or 6-cylinder types, with the 4-cylinder being the most common type found in automotive uses. 5-cylinder diesel engines have also been produced, being a compromise between the smooth running of the 6-cylinder and the space-efficient dimensions of the 4-cylinder. Diesel engines for smaller plant machinery, boats, tractors, generators and pumps may be 4-, 3-, 2-cylinder types, with the single cylinder diesel engine remaining for light stationary work.

The desire to improve the diesel engine's power-to-weight ratio produced several novel cylinder arrangements to extract more power from a given capacity. The Napier Deltic engine, with three cylinders arranged in a triangular formation, each containing two opposed-action pistons, the whole engine having three crankshafts, is one of the better known. The Commer van company of the United Kingdom developed a similar design for road vehicles. The Commer engine had 3 horizontal in-line cylinders, each with two opposed action pistons and the engine had two crankshafts. Whilst both these designs succeeded in producing greater power for a given capacity, they were complex and expensive to produce and operate, and when turbocharger technology improved in the 1960s this was found to be a much more reliable and simple way of extracting more power.

In the early decades of the 20th century, when large diesel engines were first being fitted to ships, the engines took a form similar to the compound steam engines common at the time, with the piston being connected to the connecting rod via a crosshead bearing. Following steam engine practice, double-acting 4-stroke diesel engines were constructed to increase power output, with combustion taking place on both sides of the piston, with two sets of valve gear and fuel injection. This system also meant that the engine's direction of rotation could be reversed by altering the injector timing. This meant the engine could be coupled directly to the propeller without the need for a gearbox. Whilst producing large amounts of power and being very efficient, the double-acting diesel engine's main problem was producing a good seal where the piston rod passed through the bottom of the lower combustion chamber to the crosshead bearing. Again, it was found easier and more reliable to fit turbochargers to the engines, although crosshead bearings are still used to reduce the stress on the crankshaft bearings in large long-stroke cathedral engines.

As a footnote, prior to 1949, Sulzer started experimenting with two-stroke engines with boost pressures as high as 6 atmospheres, in which all of the output power was taken from an exhaust turbine. The two-stroke pistons directly drove air compressor pistons to make a positive displacement gas generator. Opposed pistons were connected by linkages instead of crankshafts. Several of these units could be connected together to provide power gas to one large output turbine. The overall thermal efficiency was roughly twice that of a simple gas turbine. (Source Modern High-Speed Oil Engines Volume II by C. W. Chapman published by The Caxton Publishing Co. Ltd. reprinted in July 1949)

Carbureted compression ignition model engines

Simple compression ignition engines are made for model propulsion. This is quite similar to the typical glow-plug engine that runs on a mixture of methanol (methyl alcohol) and lubricant (typically castor oil) (and occasionally nitro-methane to improve performance) with a hot wire filament to provide ignition. Rather than containing a glow plug the head has an adjustable contra piston above the piston, forming the upper surface of the combustion chamber. This contra piston is restrained by an adjusting screw controlled by an external lever (or sometimes by a removable hex key). The fuel used contains ether, which is highly volatile and has an extremely low flash point, combined with kerosene and a lubricant plus a very small proportion (typically 2%) of ignition improver such as Amyl nitrate or preferably Isopropyl nitrate nowadays. The engine is started by reducing the compression and setting the spray bar mixture rich with the adjustable needle valve, gradually increasing the compression while cranking the engine. The compression is increased until the engine starts running. The mixture can then be leaned out and the compression increased. Compared to glow plug engines, model diesel engines exhibit much higher fuel economy, thus increasing endurance for the amount of fuel carried. They also exhibit higher torque, enabling the turning of a larger or higher pitched propeller at slower speed. Since the combustion occurs well before the exhaust port is uncovered, these engines are also considerably quieter (when unmuffled) than glow-plug engines of similar displacement. Compared to glow plug engines, model diesels are more difficult to throttle over a wide range of powers, making them less suitable for radio control models than either two or four stroke glow-plug engines although this difference is claimed to be less noticeable with the use of modern schneurle-ported engines.

Advantages and disadvantages versus spark-ignition engines

Power and fuel economy

Diesel engines are more efficient than gasoline (petrol) engines of the same power, resulting in lower fuel consumption. A common margin is 40% more miles per gallon for an efficient turbo diesel. For example, the current model Skoda Octavia, using Volkswagen engines, has a combined Euro rating of 38 mpg for the 102 bhp petrol engine and 53 mpg for the 105 bhp diesel engine. However, such a comparison doesn't take into account that diesel fuel is denser and contains about 15% more energy. Adjusting the numbers for the Octavia, one finds the overall energy efficiency is still about 20% greater for the diesel version, despite the weight penalty of the diesel engine. When comparing engines of relatively low power for the vehicle's weight (such as the 75 hp engines for the VW Golf), the diesel's overall energy efficiency advantage is reduced further still, to between 10 and 15 percent.

While higher compression ratio is helpful in raising efficiency, diesel engines are much more economical than gasoline (petrol) engines when at low power and at engine idle. Unlike the petrol engine, diesels lack a butterfly valve (choke) in the inlet system, which closes at idle. This creates parasitic drag on the incoming air, reducing the efficiency of petrol/gasoline engines at idle. Due to their lower heat losses, diesel engines have a lower risk of gradually overheating if left idling for a long periods of time. For example, in many applications, such as marine, agriculture and railways, diesels are left idling unattended for many hours or sometimes days. These advantages are especially attractive in locomotives (see dieselization for more information).

Naturally aspirated diesel engines are heavier than gasoline engines of the same power for two reasons. The first is that it takes a larger displacement diesel engine to produce the same power as a gasoline engine. This is essentially because the diesel must operate at lower engine speeds [2]. Diesel fuel is injected just before ignition, leaving the fuel little time to find all the oxygen in the cylinder. In the gasoline engine, air and fuel are mixed for the entire compression stroke, ensuring complete mixing even at higher engine speeds. The second reason for the greater weight of a diesel engine is it must be stronger to withstand the higher combustion pressures needed for ignition, and the shock loading from the detonation of the ignition mixture. As a result, the reciprocating mass (the piston and connecting rod), and the resultant forces to accelerate and to decelerate these masses, are substantially higher the heavier, the bigger and the stronger the part, and the laws of diminishing returns of component strength, mass of component and inertia - all come into play to create a balance of offsets, of optimal mean power output, weight and durability.

Yet it is this same build quality that has allowed some enthusiasts to acquire significant power increases with turbocharged engines through fairly simple and inexpensive modifications. A gasoline engine of similar size cannot put out a comparable power increase without extensive alterations because the stock components would not be able to withstand the higher stresses placed upon them. Since a diesel engine is already built to withstand higher levels of stress, it makes an ideal candidate for performance tuning with little expense. However it should be said that any modification that raises the amount of fuel and air put through a diesel engine will increase its operating temperature which will reduce its life and increase its service interval requirements. These are issues with newer, lighter, high performance diesel engines which aren't "overbuilt" to the degree of older engines and are being pushed to provide greater power in smaller engines.

The addition of a turbocharger or supercharger to the engine (see turbodiesel) greatly assists in increasing fuel economy and power output, mitigating the fuel-air intake speed limit mentioned above for a given engine displacement. Boost pressures can be higher on diesels than gasoline engines, and the higher compression ratio allows a diesel engine to be more efficient than a comparable spark ignition engine. Although the calorific value of the fuel is slightly lower at 45.3 MJ/kg (megajoules per kilogram) to gasoline at 45.8 MJ/kg, diesel fuel is much denser and fuel is sold by volume, so diesel contains more energy per litre or gallon. The increased fuel economy of the diesel over the gasoline engine means that the diesel produces less carbon dioxide (CO₂) per unit distance. Recently, advances in production and changes in the political climate have increased the availability and awareness of biodiesel, an alternative to petroleum-derived diesel fuel with a much lower net-sum emission of CO₂, due to the absorption of CO₂ by plants used to produce the fuel.

The two main factors that held diesel engine back in private vehicles until quite recently were their low power outputs and high noise levels (characterised by knock or clatter, especially at low speeds and when cold). This noise was caused by the sudden ignition of the diesel fuel when injected into the combustion chamber. This noise was a product of the sudden temperature change, hence it was more pronounced at low engine temperatures. A combination of improved mechanical technology (such as two-stage injectors which fire a short 'pilot charge' of fuel into the cylinder to warm the combustion chamber before delivering the main fuel charge) and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures) have almost totally solved these problems in the latest generation of common-rail designs. Poor power and narrow torque bands have been solved by the use of turbochargers and intercoolers.

Emissions

Diesel engines produce very little carbon monoxide as they burn the fuel in excess air even at full load, at which point the quantity of fuel injected per cycle is still about 50% lean of stoichiometric. However, they can produce black soot (or more specifically diesel particulate matter) from their exhaust, which consists of unburned carbon compounds. This is often caused by worn injectors, which do not atomize the fuel sufficiently, or a faulty engine management system which allows more fuel to be injected than can be burned completely in the available time.

The full load limit of a diesel engine in normal service is defined by the "black smoke limit", beyond which point the fuel cannot be completely combusted; as the "black smoke limit" is still considerably lean of stoichiometric it is possible to obtain more power by exceeding it, but the resultant inefficient combustion means that the extra power comes at the price of reduced combustion efficiency, high fuel consumption and dense clouds of smoke, so this is only done in specialised applications (such as tractor pulling) where these disadvantages are of little concern.

Likewise, when starting from cold, the engine's combustion efficiency is reduced because the cold engine block draws heat out of the cylinder in the compression stroke. The result is that fuel is not combusted fully, resulting in blue/white smoke and lower power outputs until the engine has warmed through. This is especially the case with in-direct injection engines which are less thermally efficient. With electronic injection, the timing and length of the injection sequence can be altered to compensate for this. Older engines with mechanical injection can have manual control to alter the timing, or multi-phase electronically-controlled glow plugs, that stay on for a period after start-up to ensure clean combustion- the plugs are automatically switched to a lower power to prevent them burning out.

Particles of the size normally called PM10 (particles of 10 micrometres or smaller) have been implicated in health problems, especially in cities. Some modern diesel engines feature diesel particulate filters, which catch the black soot and when saturated are automatically regenerated by burning the particles. Other problems associated with the exhaust gases (nitrogen oxides, sulfur oxides) can be mitigated with further investment and equipment; some diesel cars now have catalytic converters in the exhaust.

Power and torque

For commercial uses requiring towing, load carrying and other tractive tasks, diesel engines tend to have more desirable torque characteristics. Diesel engines tend to have their torque peak quite low in their speed range (usually between 1600–2000 rpm for a small-capacity unit, lower for a larger engine used in a truck). This provides smoother control over heavy loads when starting from rest, and crucially allows the diesel engine to be given higher loads at low speeds than a petrol/gasoline engine, which makes them much more economical for these applications. This characteristic is not so desirable in private cars, so most modern diesels used in such vehicles use electronic control, variable geometry turbochargers and shorter piston strokes to achieve a wider spread of torque over the engine's speed range, typically peaking at around 2500–3000 rpm.

Reliability

The lack of an electrical ignition system greatly improves the reliability. The high durability of a diesel engine is also due to its overbuilt nature (see above) as well as the diesel's combustion cycle, which creates less-violent changes in pressure when compared to a spark-ignition engine, a benefit that is magnified by the lower rotating speeds in diesels. Diesel fuel is a better lubricant than gasoline so is less harmful to the oil film on piston rings and cylinder bores; it is routine for diesel engines to cover 250,000 miles or more without a rebuild.

Unfortunately, due to the greater compression force required and the increased weight of the stronger components, starting a diesel engine is a harder task. More torque is required to push the engine through compression.

Either an electrical starter or an air start system is used to start the engine turning. On large engines, pre-lubrication and slow turning of an engine, as well as heating, are required to minimize the amount of engine damage during initial start-up and running. Some smaller military diesels can be started with an explosive cartridge, called a Kaufman starter, which provides the extra power required to get the machine turning. In the past, Caterpillar and John Deere used a small gasoline pony motor in their tractors to start the primary diesel motor. The pony motor heated the diesel to aid in ignition and utilized a small clutch and transmission to actually spin up the diesel engine. Even more unusual was an International Harvester design in which the diesel motor had its own carburetor and ignition system, and started on gasoline. Once warmed up, the operator moved two levers to switch the motor to diesel operation, and work could begin. These engines had very complex cylinder heads (with their own gasoline combustion chambers) and in general were vulnerable to expensive damage if special care was not taken (especially in letting the engine cool before turning it off).

As mentioned above, diesel engines tend to have more torque at lower engine speeds than gasoline engines. However, diesel engines tend to have a narrower power band than gasoline engines. Naturally-aspirated diesels tend to lack power and torque at the top of their speed range. This narrow band is a reason why a vehicle such as a truck may have a gearbox with as many as 16 or more gears, to allow the engine's power to be used effectively at all speeds. Turbochargers tend to improve power at high engine speeds, intercoolers do the same at lower speeds, and variable geometry turbochargers improve the engine's performance equally (or make the torque curve flatter).

Quality and variety of fuels

Petrol/gasoline engines are limited in the variety and quality of the fuels they can burn. Older petrol engines fitted with a carburetor required a volatile fuel that would vaporise easily to create the necessary fuel/air mix for combustion. Because both air and fuel is admitted to the cylinder, if the compression ratio of the engine is too high or the fuel too volatile (with too low an octane rating), the fuel will ignite under compression, in a similar fashion to in a diesel engine, before the piston reaches the top of its stroke. This pre-ignition causes a power loss and over time major damage to the piston and cylinder. The need for a fuel that is volatile enough to vaporise but not too volatile (too avoid pre-ignition) means that petrol engines will only run on a narrow range of petroleum-based fuels.

In diesel engines, a mechanical injector system vaporises the fuel (instead of a Venturi jet in a carburetor as in a petrol engine). This forced vaporisation means that less volatile fuels can be used. More crucially, because only air is inducted into the cylinder in a diesel engine, the compression ratio can be much higher as there is no risk of pre-ignition provided the injection process is accurately timed. This means that cylinder temperatures are much higher in a diesel engine than a petrol engine allowing less combustible fuels to be used.

Diesel fuel is a form of light fuel oil, very similar to kerosene, but diesel engines, especially older or simple designs that lack precision electronic injection systems, can run on a wide variety of other fuels. One of the most common alternatives is vegetable oil, which some engines can be run on without modification, and most others require fairly basic alterations. Bio-diesel is a pure diesel fuel refined from vegetable oil and can be used in nearly all diesel engines. The only limits on the fuels used in diesel engines are the ability of the fuel to flow along the fuel lines and the ability of the fuel to lubricate the injector pump and injectors adequately.

Most large marine diesels (often called cathedral engines due to their size) run on heavy fuel oil, which is a thick, viscous and almost un-flammable fuel which is cheap to buy in bulk and safe to store. The fuel must be heated to thin it out and is often passed through multiple injection stages to vaporise it.

Dieseling in spark-ignition engines

A gasoline (spark ignition) engine can sometimes act as a compression ignition engine under abnormal circumstances, a phenomenon typically described as pinging or pinking (during normal running) or dieseling(when the engine continues to run after the electrical ignition system is shut off). This is usually caused by hot carbon deposits within the combustion chamber that act as would a glow plug within a diesel or model aircraft engine. Excessive heat can also be caused by improper ignition timing and/or fuel/air ratio which in turn overheats the exposed portions of the spark plug within the combustion chamber. Finally, high-compression engines that require high octane fuel may knock when a lower-octane fuel is used.

Fuel and fluid characteristics

Diesel engines can operate on a variety of different fuels, depending on configuration, though the eponymous diesel fuel derived from crude oil is most common. Good-quality diesel fuel can be synthesised from vegetable oil and alcohol. Biodiesel is growing in popularity since it can frequently be used in unmodified engines, though production remains limited. Petroleum-derived diesel is often called petrodiesel if there is need to distinguish the source of the fuel.

The engines can work with the full spectrum of crude oil distillates, from compressed natural gas, alcohols, gasolene, to the fuel oils from diesel oil to residual fuels. The type of fuel used is a combination of service requirements, and fuel costs.

Residual fuels are the "dregs" of the distillation process and are a thicker, heavier oil, or oil with higher viscosity, which are so thick that they are not readily pumpable unless heated. Residual fuel oils are cheaper than clean, refined diesel oil, although they are dirtier. Their main considerations are for use in ships and very large generation sets, due to the cost of the large volume of fuel consumed, frequently amounting to many tonnes per hour. The poorly refined biofuels straight vegetable oil (SVO) and waste vegetable oil (WVO) can fall into this category. Moving beyond that, use of low-grade fuels can lead to serious maintenance problems. Most diesel engines that power ships like supertankers are built so that the engine can safely use low grade fuels.

Normal diesel fuel is more difficult to ignite than gasoline because of its higher flash point, but once burning, a diesel fire can be fierce.

Diesel applications

The vast majority of modern heavy road vehicles (trucks), ships, large-scale portable power generators, most farm and mining vehicles, and many long-distance locomotives have diesel engines. (See dieselisation.)

Besides their use in merchant ships and boats, there is also a naval advantage in the relative safety of diesel fuel, additional to improved range over a gasoline engine. The German "pocket battleships" were the largest diesel warships, but the German torpedo-boats known as E-boats (Schnellboot) of the Second World War was also diesel craft. Conventional submarines have used them since before the First World War. It was an advantage of American diesel-electric submarines that they operated a two-stroke cycle as opposed to the four-stroke cycle that other navies used.

Mercedes-Benz, cooperating with Robert Bosch GmbH, has had a successful run of diesel-powered passenger cars since 1936, sold in many parts of the World, with other manufacturers joining in the 1970s and 1980s. Other car manufacturers followed, Borgward in 1952, Fiat in 1953 and Peugeot in 1958.

In the United States, diesel is not as popular in passenger vehicles as in Europe. Such cars have been traditionally perceived as heavier, noisier, having performance characteristics which make them slower to accelerate, sootier, smellier, and of being more expensive than equivalent gasoline vehicles. General Motors' Oldsmobile division produced a low-powered and unreliable diesel version of its gasoline-powered V8 engine which is one reason for this reputation. In addition, before the introduction of 15 parts per million ultra-low sulfur diesel, which started at 15 October 2006 in the U.S. (1 June 2006 in Canada), diesel fuel used in North America still has higher sulphur content than the fuel used in Europe, effectively limiting diesel use to industrial vehicles, which had further contributed to the negative image. Ultra-low sulfur diesel is not mandatory until 2010 in the US. This image does not reflect recent designs, especially where the very high low-rev torque of modern diesels is concerned — which have characteristics similar to the big V8 gasoline engines popular in the US. Light and heavy trucks, in the U.S., have been diesel-optioned for years. After the introduction of ultra-low sulfur diesel, Mercedes-Benz has marketed passenger vehicles under the Bluetec banner.

In Canada, Smart Fortwo was first introduced in 2004 with a diesel engine, up until 2008.[3][4]

European governments tend to favor diesel engines in taxation policy because of diesel's superior fuel efficiency.

In Europe, where tax rates in many countries make diesel fuel much cheaper than gasoline, diesel vehicles are very popular and newer designs have significantly narrowed differences between petrol and diesel vehicles in the areas mentioned. Often, among comparably designated models, the turbodiesels outperform their naturally aspirated petrol-powered sister cars. One anecdote tells of Formula One driver Jenson Button, who was arrested while driving a diesel-powered BMW 330cd Coupé at 230 km/h (about 140 mph) in France, where he was too young to have a gasoline-engined car hired to him. Button dryly observed in subsequent interviews that he had actually done BMW a public relations service, as nobody had believed a diesel could be driven that fast. Yet, BMW had already won the 24 Hours Nürburgring overall in 1998 with a 3-series diesel. The BMW diesel lab in Steyr, Austria is led by Ferenc Anisits and develops innovative diesel engines.

Mercedes-Benz, offering diesel-powered passenger cars since 1936, has put the emphasis on high performance diesel cars in its newer ranges, as does Volkswagen with its brands. Citroën sells more cars with diesel engines than gasoline engines, as the French brands (also Peugeot) pioneered smoke-less HDI designs with filters. Even the Italian marque Alfa Romeo, known for design and successful history in racing, focuses on diesels that are also raced.

A few motorcycles have been built using diesel engines, but the weight and cost disadvantages generally outweigh the efficiency gains in this application.

High-speed: High-speed (approximately 1200 rpm and greater) engines are used to power trucks (lorries), buses, tractors, cars, yachts, compressors, pumps and small electrical generators.

Medium-speed: Large electrical generators are driven by medium speed engines, (approximately 300 to 1200 rpm) optimised to run at a set speed and provide a rapid response to load changes.

Low-speed: The largest diesel engines are used to power ships. These monstrous engines have power outputs over 80MW, turn at about 60 to 100 rpm, and are up to 15 m tall. They often run on cheap low-grade fuel, which require extra heat treatment in the ship for tanking and before injection due to their low volatility. Companies such as Burmeister & Wain and Wärtsilä (e.g., Sulzer diesels) design such large low speed engines. They are unusually narrow and tall due to the addition of a crosshead bearing. Today (2006), the 12 cylinder MAN B&W Diesel K98MC turbocharged two-stroke diesel engine built by MAN B&W Diesel licencee Hyundai Heavy Industries in Korea is the most powerful diesel engine put into service, with a cylinder bore of 980 mm delivering 74.8 MW (101,640 bhp). When put into service in 2007, the 14 cylinder Wärtsilä-Sulzer RTA96-C will become the most powerful diesel engine with 80 MW (110,000 hp).

Unusual applications

Aircraft

Main article: aircraft diesel engine

The zeppelins Graf Zeppelin II and Hindenburg were propelled by reversible diesel engines. The direction of operation was changed by shifting gears on the camshaft. From full power forward, the engines could be brought to a stop, changed over, and brought to full power in reverse in less than 60 seconds.

Diesel engines were first tried in aircraft in the 1930s. A number of manufacturers built engines, the best known probably being the Junkers Jumo 205, which was moderately successful, but proved unsuitable for combat use in WWII. Postwar, another interesting proposal was the complex Napier Nomad. In general, though, the lower power-to-weight ratio of diesels, particularly compared to kerosene-powered turboprop engines, has precluded their use in this application.

The very high cost of avgas in Europe, and the advances in automotive diesel technology have seen renewed interest in the concept. New, certified diesel-powered light planes are already available, and a number of other companies are also developing new engine and aircraft designs for the purpose. Many of these run on the readily-available jet fuel, or can run on either jet fuel or conventional automotive diesel. To gain the high power:weight ratio needed for an aero engine, these new 'aero-diesels' are usually two-strokes and some, like the British 'Dair' engine, use opposed-action pistons to gain further power.

Automobile racing

Although the weight and lower output of a diesel engine tend to keep them away from automotive racing applications, there are many diesels being raced in classes that call for them, mainly in truck racing and tractor pulling, as well in types of racing where these drawbacks are less severe, such as land speed record racing or endurance racing. Even diesel engined dragsters exist, despite the diesel's drawbacks of weight and low peak rpm, specifications central to performance in this sport.

1931 - Clessie Cummins installed his diesel in a race car, hitting 162 km/h at Daytona and 138 km/h at the Indianapolis 500 race, where Dave Evans drove it to 13th place by finishing the entire race without a pit stop, relying on torque and fuel efficiency to overcome weight and low peak power. [5],[6]

In 1933, A 1925 Bentley with a Gardner 4LW engine was the first diesel-engine car to take part in the Monte Carlo Rally when it was driven by Lord Howard de Clifford. It was the leading British car and finished fifth overall. [7]

In 1952, Fred Agabashian in a Cummins diesel won the pole at the Indianapolis 500 race with a turbocharged 6.6 liter diesel car, setting a record for pole position lap speed, 222.108 km/h or 138.010 mph[8]. Don Cummins and his chief engineer Neve Reiners recognized that the low center of gravity of the flat engine configuration (designed to lie beneath the floor of a bus) plus the power advantage gained by the novel use of Elliott turbocharging would be a winning combination[9]; Agabashian led most of the race until the badly situated air intake of the car swallowed enough debris from the track to disable the turbocharger at lap 71; he finished 27th[10].

With turbocharged diesel cars getting stronger in the 1990s, they were also entered in touring car racing, and BMW even won the 24 Hours Nürburgring in 1998 with a 320d, against other factory-entered diesel competition of Volkswagen and about 200 normally powered cars. Alfa Romeo even organized a racing series with their Alfa Romeo 147 1.9 JTD models.

The VW Dakar Rally entrants for 2005 and 2006 are powered by their own line of TDI engines in order to challenge for the first overall diesel win there. Meanwhile, the five time 24 Hours of Le Mans winner Audi R8 race car was replaced by the Audi R10 in 2006, which is powered by a 650 hp (485 kW) and 1100 N·m (810 lbf·ft) V12 TDI common rail diesel engine, mated to a 5-speed gearbox, instead of the 6 used in the R8, to handle the extra torque produced. The gearbox is considered the main problem, as earlier attempts by others failed due to the lack of suitable transmissions that could stand the torque long enough.

After winning the 12 Hours of Sebring in 2006 with their diesel-powered R10, Audi obtained the overall win at the 2006 24 Hours of Le Mans, too. This is the first time a sports car can compete for overall victories with diesel fuel against cars powered with regular fuel or methanol and bio-ethanol. However, the significance of this is slightly lessened by the fact that the ACO/ALMS race rules encourage the use of alternative fuels such as diesel.

In 2006, the JCB Dieselmax broke the diesel land speed record posting an average speed of over 328 mph. The vehicle used "two diesel engines that have a combined total of 1,500 horsepower. Each is a 4-cylinder, 4.4-liter engine used commercially as a backhoe loader." See MSNBC news See BBC news

Motorcycles

With a traditionally poor power-to-weight ratio, diesel engines are generally unsuited to use in a motorcycle, which requires high power, light weight and a fast-revving engine. However, in the 1980s NATO forces in Europe standardised all their vehicles to diesel power. Some had fleets of motorcycles, and so trials were conducted with diesel engines for these. Air-cooled single-cylinder engines built by Lombardini of Italy were used and had some success, achieving similar performance to petrol bikes and fuel usage of nearly 200 miles per gallon. This led to some countries re-fitting their bikes with diesel power.

Development by Cranfield University and California-based Hayes Diversified Technologies led to the production of a diesel powered off road motorbike based on the running gear of a Kawasaki KLR650 petrol-engine trail bike for military use. The engine of the diesel motorcycle is a liquid cooled, single cylinder four- stroke which displaces 584 cm³ and produces 21 kW (28 bhp) with a top speed of 85mph (136kph). Hayes Diversified Technologies mooted, but has subsequently delayed, the delivery of a civilian version for approx USD$19,000. Expensive compared to comparable models.

In 2005 the United States Marine Corps adopted the M1030M1, a dirtbike based on the Kawasaki KLR650 and modified with an engine designed to run on diesel or JP8 jet fuel. Since other U.S. tactical vehicles like the Humvee utility vehicle and M1 Abrams tank use JP8, adopting a scout motorcycle which runs on the same fuels made sense from a logistical standpoint.

In India, motorcycles built by Royal Enfield can be bought with 650cc single-cylinder diesel engines based on the similar petrol engines used, due to the fact that diesel is much cheaper than petrol and of more reliable quality. These engines are noisy and unrefined, but very popular due to their reliability and economy.

Current and future developments

Already, many common rail and unit injection systems employ new injectors using stacked piezoelectric crystals in lieu of a solenoid, which gives finer control of the injection event.

Variable geometry turbochargers have flexible vanes, which move and let more air into the engine depending on load. This technology increases both performance and fuel economy. Boost lag is reduced as turbo impeller inertia is compensated for.

Accelerometer pilot control (APC) uses an accelerometer to provide feedback on the engine's level of noise and vibration and thus instruct the ECU to inject the minimum amount of fuel that will produce quiet combustion and still provide the required power (especially while idling.)

The next generation of common rail diesels is expected to use variable injection geometry, which allows the amount of fuel injected to be varied over a wider range, and variable valve timing similar to that on gasoline engines.

Particularly in the United States, coming tougher emissions regulations present a considerable challenge to diesel engine manufacturers. Other methods to achieve even more efficient combustion, such as HCCI (homogeneous charge compression ignition) are being studied.

Modern diesel facts

(Source: Robert Bosch GmbH)

Fuel passes through the injector jets at speeds of nearly 1500 miles per hour (2400 km/h)

Fuel is injected into the combustion chamber in less than 1.5 ms – about as long as a camera flashes.

The smallest quantity of fuel injected is one cubic millimetre – about the same volume as the head of a pin. The largest injection quantity at the moment for automobile diesel engines is around 70 cubic millimetres.

If the camshaft of a six-cylinder engine is turning at 4500 rpm, the injection system has to control and deliver 225 injection cycles per second.

On a demonstration drive, a Volkswagen 1-litre diesel-powered car used only 0.89 litres of fuel in covering 100 kilometres (264 mpg {US}, 317 mpg {Imperial/English}) – making it probably the most fuel-efficient car in the world. Bosch’s high-pressure fuel injection system was one of the main factors behind the prototype’s extremely low fuel consumption. Production record-breakers in fuel economy include the Volkswagen Lupo 3 L TDI and the Audi A2 3 L 1.2 TDI with standard consumption figures of 3 litres of fuel per 100 kilometres (78 mpg {US}, 94 mpg {Imperial}). Their high-pressure diesel injection systems are also supplied by Bosch.

In 2001, nearly 36% of newly registered cars in Western Europe had diesel engines. By way of comparison: in 1996, diesel-powered cars made up only 15% of the new car registrations in Germany. Austria leads the league table of registrations of diesel-powered cars with 66%, followed by Belgium with 63% and Luxembourg with 58%. Germany, with 34.6% in 2001, was in the middle of the league table. Sweden is lagging behind, in 2004 only 8% of the new cars had a diesel engine.

Diesel car history

The first production diesel cars were the Mercedes-Benz 260D and the Hanomag Rekord, both introduced in 1936. The Citroën Rosalie was also produced between 1935 and 1937 with an extremely rare diesel engine option (the 1766 cc 11UD engine) only in the Familiale (estate or station wagon) version. [11]

Following the 1970s oil crisis, turbo diesels were tested (e. g. by the Mercedes-Benz C111 experimental and record-setting vehicles). The first production turbo diesel car was, in 1978, the 3.0 5-cyl 115 HP Mercedes 300 SD, available only in North America. In Europe, the Peugeot 604 with a 2.3 litre turbo diesel was introduced in 1979, and then the Mercedes 300 TD turbo.

Many Audi enthusiasts claim that the Audi 100 TDI was the first turbo charged direct injection diesel sold in 1989, but actually it isn't true, as the Fiat Croma TD-i.d. was sold with turbo direct injection in 1986 and 2 years later Austin Rover Montego.

What was pioneering about the Audi 100 however was the use of electronic control of the engine, as the Fiat and Austin had purely mechanically controlled injection. The electronic control of direct injection really made a difference in terms of emissions, refinement and power.

It's interesting to see that the big players in the diesel car market are the same ones who pioneered various developments (Mercedes, BMW, Peugeot/Citroën, Fiat, Alfa Romeo, VW/Audi/SEAT/Skoda), with the exception of Austin Rover- although Austin Rover's ancestor, The Rover Motor Company had been building small-capacity diesel engines since 1956, when it introduced a 2051cc 4-cylinder diesel engine for its Land Rover 4x4.

In 1998, for the very first time in the history of racing, in the legendary 24 Hours Nürburgring race, a diesel-powered car was the overall winner: the BMW works team 320d, a BMW E36 fitted with modern high-pressure diesel injection technology from Robert Bosch GmbH. The low fuel consumption and long range, allowing 4 hours of racing at once, made it a winner, as comparable petrol-powered cars spent more time refueling.

In 2006, the new Audi R10 TDI LMP1 entered by Joest Racing became the first diesel-engined car to win the 24 Hours of Le Mans. The winning car also bettered the post-1990 course configuration lap record by 1, at 380. However, this fell short of the all-time distance record set in 1971 by over 200 km.

References

D J Blair., "Sales & Marketing Executive" - Forest City [12]

External links