Station Information - Diesel engine

The diesel engine is an internal combustion engine, also called a reciprocating compression ignition engine. It was invented and patented by Rudolf Diesel in 1892. Diesel intended the engine to use a variety of fuels including coal dust. He demonstrated it in the 1900 World's Fair using peanut oil.

How diesel engines work

A diesel engine works by igniting the air-gas mixture without a spark. The heat that ignites the mixture comes from a temperature increased, caused by raising the pressure inside the combustion chamber of the engine. 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. 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. (See direct injection vs indirect injection for a discussion of the types of fuel injection.)

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 is a major manufacturer of diesel engine governors.

In very cold weather, the compression itself is insufficient to heat the air enough to ignite the fuel. Glow plugs are electrical heaters inside the cylinder to help warm the air. Once the engine is started and warmed up, the glow plugs are no longer needed and may be turned off.

Types of diesel engines

There are two classes of diesel engines: two-stroke and four-stroke. Many larger diesels operate on the two-stroke cycle. Smaller engines generally use the four-stroke cycle.

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 to prevent excessive vibration.

Advantages and disadvantages versus spark-ignition engines

Diesel engines are more massive than gasoline/petrol engine of the same power because of the higher combustion pressures needed for ignition.

The increased fuel economy of the diesel over the petrol engine means that mile-for-mile the diesel produces less carbon dioxide (CO₂). The possible development of biofuel alternatives to fossil fuels could lead to an effective zero emission of CO₂, as it is re-absorbed into plants that are then used to produce the fuel.

Diesel engines can produce black soot from their exhaust. This consists of unburnt carbon compounds. Other problems associated with the exhaust gases (high particulates, nitrogen oxide, sulfurous fumes) can be mitigated with further investment and equipment.

The addition of a turbocharger or supercharger to the engine greatly assists in increasing fuel economy and power output. 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 megajoules/kilogram to gasoline at 45.8 megajoules/kilogram.

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 extra power required to get the machine turning.

Fuel and fluid characteristics

Diesel fuel is a product of crude oil, although other oils can be burned inside an adapted engine. Good quality diesel fuel can be synthesised from vegetable fat and alcohol.

Diesel engines can work on thicker, heavier oil, or oil with higher viscosity, as long as it is heated to ease pumping and injection. These fuels are cheaper than clean, refined diesel oil, although they are dirtier.

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

The use of low-grade fuels can lead to serious maintenance problems.

See also: biofuel, biodiesel, petrodiesel

Diesel applications

The vast majority of modern heavy road vehicles, ships, long-distance locomotives, large-scale portable power generators, and most farm and mining vehicles have diesel engines. 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 gasoline and diesel vehicles in the areas mentioned. One anecdote tells of Formula One driver Jenson Button, who was arrested driving a diesel-powered BMW coupe at 230 km/h (about 140 mph).

High-speed (approximately 1200 rpm and greater) engines are used to power lorries (trucks), buses, tractors, carss, yachts, compressors, pumps and small generatorss. The largest diesel engines are used to power ships along the sealanes. These monstrous engines have power outputs up to 90,000 kW, turn at about 60 to 70 rpm and are 15 m tall. They often run on cheap low grade oil, which needs extra heat treatment in the ship for tanking and before injection. Companies such as Burmeister & Wain and Wartsila NSD (e.g. 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.