The diesel engine was developed by Rudolf Diesel and was patented in 1892. Diesel engines are very similar to petrol or gasoline engines in that both rely on the Otto cycle to convert the chemical energy in fuel into mechanical energy and, in so doing, produce power. The major difference is the way fuel is delivered to the combustion chamber and the way the fuel mixture is ignited. Firstly, in gasoline engines, the fuel is usually fed into the intake manifold or the intake port where it is combined and mixed with the intake air, which is also called the intake charge. In modern diesel engines, the fuel is injected directly into the combustion chamber. This means that only the intake charge is compressed during the compression stroke and the diesel is only introduced once the intake charge has been compressed. Secondly, in gasoline engines, the fuel mixture is ignited by a spark plug, while in diesel engines the fuel is ignited by the heat from the compressed air in the combustion chamber. However, diesel requires a much higher temperature than petrol before ignition (not spontaneous ignition) can take place.
These differences have important consequences for the modification of diesel engines, especially when you consider the differences between diesel fuel and gasoline.
The Differences Between Diesel and Gasoline
For starters, diesel is a heavier fuel than gasoline. In other words, it contains more carbon atoms in longer chains than gasoline (technically, gasoline is typically C9H20, while diesel fuel is typically C14H30). Because it is heavier, diesel is much more stable that gasoline and vaporizes at a much higher temperature than gasoline. It also vaporizes much slower than gasoline and burns much slower. The result is that diesel requires a much higher temperature to ignite. Gasoline, for example can burn at temperatures of -40° F while diesel requires a temperature of at least 143° F!
The main point, however, is that diesel burns slower than petrol. This means that it will produce a steady pressure on the piston for longer. Consequently, diesel can be ignited at a higher temperature, and indeed can be allowed to reach the point at which it will ignite spontaneously. The interesting thing is that diesel needs a temperature of 410° F to ignite spontaneously but will ignite or burn at a much lower temperature of 143° F. Consequently, diesel cannot be introduced into the combustion chamber until the correct temperature is reached, or else it will pre-ignite. Now, to reach the required temperature, air in the combustion chamber must be compressed much more than in a gasoline engine, and because there is not fuel in the combustion chamber, the intake charge can be safely compressed without the danger of pre-ignition. Thus a gasoline engine will typically have the compressions ratio would of somewhere between 1:9 and 1:12 while a diesel engine will typically a compression ratio of around 1:25! And it is this higher compression ratio, as well as its higher vaporization point and slower burning rate, and the fact that diesel has about 17% more energy density than gasoline, that makes diesel much more efficient than gasoline.
Now you're thinking why not use direct injection in a gasoline engine so we can increase the compress without pre-ignition? Indeed some manufacturers to employ direct injection on gasoline engines, but without the higher compression ratio because gasoline will burn too quickly at higher temperatures, hence the need to keep the temperature of the intake charge down in a gasoline engine. Remember, diesel burns at a slower rate than gasoline and therefore can be ignited at higher temperatures.
Diesel Engine Modifications
When it comes to modifying a diesel engine, you can apply the same techniques that you would apply to a gasoline engine, except for ignition system obviously as diesel engine has no spark plug. All the basics apply, i.e., increasing the engine displacement, increasing the engine speed, improving and increasing the air intake, and increasing the volumetric efficiency.
Nonetheless, there are a number of things to consider before attempting to modify a diesel engine.
- Firstly, components in the diesel engine are exposed to far higher pressures and temperatures than the components in gasoline engines. Therefore, diesel engines need to be more robust with thicker cylinder walls and stronger pistons. Should you decide to increase the displacement of your diesel engine by boring out the cylinders you should ensure that you improve your cooling system.
- Secondly, diesel burns at a much slower rate than gasoline; therefore a diesel engine will operate at a much lower RPM. This is natural, and getting the diesel engine to operate at higher speed will mean increasing the temperatures in the combustion chamber, which would require thicker cylinder wall and much a better cool system, and improving the cooling system is easier said than done because of diminishing returns!
- Furthermore, increasing the temperatures in the combustion chamber will increase the heat in the intake manifold, and will result in a reduction of air density. Consequently, we're dealing with even more diminishing returns! Still, maximum power will be reached at relatively low RPMs because of the slow rate at which diesel burns and will drop off dramatically at higher RPMs.
- Thirdly, increasing the amount of air ingested by the engine will require a proportionate increase in the amount of fuel injected into the engine. Thus bigger injectors, a higher fuel pressure will be required, or a remapped engine control unit (ECU) would be required. On some turbo-diesel engines, a remapped ECU has led to impressive improvements in power and should be the starting point in your quest to squeeze more power from a diesel engine.