There are two types of heavy duty engines, two stroke and four stroke. Both designs have been available for many years, and both are proven and well accepted in the diesel industry. Until now two stroke diesels are mainly used in the heavy shipping industry, Wärtsiliä-Sulzer is manufacturing one of the largest two stroke marine engines in the world: the RTA96-C. To understand their difference with four stroke diesels, one must first be clear on the basic design characteristics of each type.
Every engine has a combustion cycle of intake, compression, power and exhaust for each power impulse. Some engines, identified as two stroke or two cycle, use two piston strokes and one crankshaft revolution to accomplish these functions.
A four stroke engine performs intake, compression, power and exhaust in four piston strokes and two crankshaft revolutions, half as often as a two stroke engine. And this big difference gives the two stroke diesel its remarkable advantages.
This type uses intake and exhaust valves to regulate the gas flow. At the intake stroke the piston moves downward and the intake valve is open. When the piston start to move up again (after reaching bottom dead center, BDC) the compression stroke has started and both intake and exhaust valves must be closed. Temperature and pressure rises quickly in the cylinder.
When the piston is almost at top dead center (TDC) fuel is injected and after a slight delay the finely atomized fuel will start to burn. Temperature and pressure rises even more and will push the piston down in what is called the power stroke. The heat of combustion will also heat the cylinder.
In the last stroke the piston moves up and the exhaust valve is open thereby forcing the burnt gases out the cylinder.
During the above process the valves do not open and close when the piston is precisely at the top or bottom of the cylinder. That would not be a very efficient way to discharge the gas. Valve opening and closing occurs with some overlap so that the energy of the moving gas is used to intake fresh air and remove the burnt gas from the cylinder in a continuous motion so that optimum cylinder breathing is accomplished without too much energy losses.
Both two and four stroke diesel can use either a prechamber (indirect injection or IDI) or direct injection (DI). Prechamber design use a small space where fuel in injected and connected through a small canal to the main combustion area. Direct injection engines have the fuel injected into the cylinder just above the piston.
The advantage of a direct injection is a reduced fuel consumption up to 15% compared with indirect injected engines. Disadvantage of direct injection systems are higher combustion noise and restricted maximum RPM and this system requires higher injection pressures and a more complex fuel system.
Indirect injection use lower pressure fuel systems (to 300 bar) and have a softer combustion than direct injection. These engines also have lower toxic exhaust emissions particular with respect to HC and NOX. With an optimized pre-chamber the particulate emission is about 40 % less, mainly because of better air fuel mixing in the prechamber.
For that you will need to increase fuel flow, but there is a limit. Optimum fuel/air ratio is about 1:14.7, thus you will need more air together while adding fuel. One way to do that is with a super- or turbocharger. But as compressing air raises its temperature and density, the air from a super- or turbocharger must be cooled with an air radiator (intercooler).
Power is then raised and can be held constant up until a certain altitude where the turbo can not deliver any more air pressure and reaches its maximum RPM. For example: the Rotax 914 reaches this situation at 16000 feet.
As the diesel intakes air during its intake stroke, the way to regulate RPM or power is by varying the amount of fuel injected. Be it a two or four stroke engine. This system must regulate the fuel quantity requirements at all speeds and loads. The engine requires the correct amount of fuel at the correct time, pressure, sequence and point in the combustion process.
Modern four stroke diesels have a full authority digital engine controller (FADEC) that regulates the fuel injection system and takes into account the multitude of variables of the engine with respect to smoke, combustion pressure, EGT, torque and speed limits.
Nowadays, the FADEC is extended to control the propeller too. In this configuration the pilot sets the power between idle and 100% and the system regulates engine settings and propeller RPM. This results in a much reduced workload and only one (1) control to operate. No more fiddling with manifold pressure, RPM and mixture (in case of a gasoline engine), just set the required power level and that's it!Written by EAI.