Diesel cycle

The diesel cycle refers to the combustion process of a type of internal combustion engine in which the burning of the fuel is not triggered by a spark plug as in the otto cycle, but rather by the heat generated in compressing the fuel air mixture.

In the Otto cycle the fuel and air is mixed outside the cylinder, and draw into it through the intake valve. It is then compressed, and ignited at the right time with a spark plug. This is timed so that the charge is ignited just before the piston reaches the top of the compression stroke, and by the time the combustion is complete it has just started its downward motion. This means that the heating takes place at a constant volume, so the pressure and temperature of the charge increases dramatically.

In the Diesel cycle only air is drawn into the engine, and compressed. The fuel is then injected directly into the cylinder with a high-pressure fuel injector. The fuel is thicker and burns more slowly than petrol, so the piston is alreading moving down but the time the combusion completes. Thus the Diesel actually loses some of the potential energy of the charge, energy that the Otto cycle captures.

Given that the Diesel is not as efficient, and is more complex due to the nature of the injectors, it may be surprising that it is used at all. However the key to the Diesel cycle is that the fuel cannot possibly lead to the dreaded condition in the Otto cycle known as knock (or ping). This occurs when the fuel ignites at the wrong time, but the fuel only reaches the engine at the "right time" in the Diesel. Moreover the slower burning fuel means less stress on the engine as it runs. However it is also that slow burning fuel that leads to lower maximum RPM compared to an Otto.

The combination of these factors allows a diesel engine to be run at much higher compression ratios, typically twice as high for any given contemporary Otto cycle engine. Since efficiency is strongly related to compression ratio, in practice the Diesel engine is considerably more efficient than a similar power Otto. In fact the Diesel engine has the lowest specific fuel consumption of any prime mover engine, 0.26 lb/hp/hr for very large marine engines.

In addition it is easier to make a slower-burning fuel than petrol, which means that diesel fuel is less expensive to produce and more common. Diesel fuel also has more energy for a given volumn than petrol, meaning it takes up less room and longer-range is a common benefit.

In North America diesel engines are primarily used in large trucks, where the low-stress cycle leads to much greater engine life. However cars continue to use gasoline, primarily due to consumer desire for "peppy" cars. In Europe the use of diesel engines is far more common in cars as well.

Diesel is particularily well suited to the aviation role, where the combination of low specific fuel consumption and low volumn make for a smaller aircraft for any given range requirement. They became common only in the 1940s however, as the metalugy of the era required large blocks to contain the higher compression ratios. They fell from favour after WWII, and from that point on everyone was thinking turbines only. More recently they appear to be making something of a comback, notably in Europe where avgas is very expensive and hard to find.