Diesel engine

The Diesel engine is an internal combustion engine. It is also called a reciprocating compression ignition engine. A Diesel engine works by igniting the air-gas mixture without a spark. The heat that ignites the mixture comes from raising the termperature by raising the pressure. Diesel fuel is injected at the top of the stroke under great pressure so that it atomises, into the combustion chamber where it mixes with air at high temperature and pressure. The resulting mixture ignites and burns very rapidly. This contained explosion causes the gas in the chamber to expand, driving the piston down with considerable force and creating power in a vertical direction. The connecting rod transmits this motion to the crankshaft which is forced to turn, delivering rotary power at the output end of the crankshaft. The Diesel engine was invented by Rudolf Diesel in 1892.

The motor builds up a strong pressure on the fuel which self-ignites and pushes down on the piston to turn the crankshaft. Because of the higher pressures than in the normal engine are needed, diesels must be built heavier than gasoline engines of the same size.

The vast majority of modern heavy road vehicles, ships, most long-distance locomotives, large-scale portable power generators, and most farm and mining vehicles have diesel engines, as they are more fuel-efficient than comparably powerful petrol engines and have proven to be extremely reliable and dependable. However, they are not nearly as popular in passenger vehicles as they are heavier, noisier, have performance characteristics which makes them slower to accelerate, and more expensive than petrol vehicles. In Europe, where tax rates in many countries make diesel fuel much cheaper than petrol, diesel vehicles are very popular and newer designs have significantly narrowed differences between petrol and diesel vehicles in the areas mentioned - in one amusing (to some) example Formula One driver Jenson Button was arrested driving a diesel-powered BMW coupe at 230 km/h (about 140 mph).

Normally banks of cylinders are used in multiples of 2, 4, 6 or 8 although any number of cylinders can be used as long as the load on the crankshaft is counterbalanced some-how; otherwise excessive vibration can occur. Each cylinder in a bank is set to push at a different point on the rotation of the crank-shaft, thus smoothing out the delivery of power.

Ignition of the mixture in the combustion chamber above the piston crown is due to the very high temperature of the compressed air inside the chamber. Self-ignition of the explosive fuel-air mixture then occurs.

Direct Injection vs Indirect injection

Many larger diesels operate on the two stroke cycle.
Smaller engines generally use the four stroke cycle .

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.

Highspeed (approximately 1200rpm and greater) engines are used to power lorries(trucks), buses, cars, yachts , compressors, pumps and small generators. They tend to be heavier than a similar

Today the largest Diesel engines are used to power ships along the sealanes. These monstrous engines have power outputs upto 90,000 kW, turn at about 60 to 70 rpm and are as tall as a four storey building. Companies such as Burmeister & Wain and Wartsila NSD(ex Sulzer Diesels), design such large slow speed engines. They are unusually narrow and tall due to the addition of a crosshead bearing.

Large Electrical Generators are driven by Medium speed engines, optimised to run at a set speed and provide a rapid response to load changes.

A vital component of any Diesel engine system is the Governor, which limits the speed of the engine by controlling the rate of fuel delivery. Woodward seems to be the major manufacturer of governors.

The additon of a Turbo-charger or Super-charger to the engine greatly assists in increasing fuel economy / power output. The higher compression ratio and superior concentration of stored energy in diesel fuels allows a diesel engine to be more efficient than a comparable spark ignition engine (Gasoline/Petrol/Butane powered). Diesel is a product of crude oil (Petroleum), although other oils can be burned inside an adapted engine. Good quality diesel fuel can be synthesised from vegetable fat and alcohol, see Bio-Fuels.

Fuel viscosity. Diesel engines can work on thicker, heavier oil (i.e. oil with higher viscosity) as long as it is heated to ease pumping and injection. These fuels are cheaper but dirtier than clean, refined diesel oil.

The lack of an electrical ignition system greatly improves the reliability. 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 are used to start the engine turning. On large engines pre-lubrication and slow turning of an engine as well as heating are required to minimise the possibility of damaging the engine during initial start-up and running( some smaller military diesels are started with an explosive cartridge that provides the oomph required to get the machine turning).

The contribution of the diesel engine to the transport network of the world should not be under stated. The problems associated with the exhaust gases (high particulates, NOX, Sulfurous fumes) can be mitigated to a degree with further investment and equipment. The increased fuel economy of the Diesel over the Petrol engine means that mile-for-mile the diesel produces less C0₂. The possible development of Bio-fuel alternatives to fossil fuels could lead to an effective zero emission of C0₂, as it is re-absorbed into plants that are then used to produce the fuel.
The decreased fire risk, due to the lower flash point of the fuel is a further boon.
(note: although Diesel fuel is harder to ignite, once burning a fire can be extremely fierce).
The use of cheaper, poor grade fuels can lead to serious maintenance problems.

One of the problems with diesel engines is that they can produce black soot from their exhaust. This consists of unburnt carbon compounds.