Diesel Engine Combustion Systems
The quality of combustion in diesel engines depends on how quickly and how completely the fuel mixes with the air as it is injected into the combustion chamber. Two basic systems have been devised to improve this mixing: direct-injection and indirect-injection.
The direct-injection system introduces the fuel directly into the combustion chamber. Direct-injection promotes good fuel economy, but the air swirling is not strong enough to achieve an ideal mixture with the fuel. This weakness is overcome with specially designed chambers and air-intake ports, and by the use of high-pressure fuel injection. Direct-injection diesel engines are gathering increasing popularity. They are now used in nearly all trucks with payloads of four tons or more and also in a significant proportion of passenger cars in Europe. The most popular form of direct-injection system provides a strong swirl of air in the combustion chamber to aid the air-fuel mixing process, with the fuel being injected under high pressure from four or five nozzle holes.
The indirect-injection system is currently limited to use in passenger cars and light-duty trucks. The most popular design features a spherical swirl chamber in the cylinder head. Air is forced into the chamber by the piston and begins swirling rapidly, which promotes a good mix when the fuel is injected. A preliminary combustion of the mixture takes place and heat rises, forcing the remaining unburned fuel into the chamber at high velocity, where it mixes well with the air and undergoes complete combustion.
A turbocharger is a mechanism that increases the amount of air supplied to an internal combustion engine at higher than normal pressure by means of a turbine powered by the exhaust gases. By allowing more air to enter the cylinder while maintaining the exhaust amount at the same level, a turbocharger can improve combustion efficiency and improve the power output.
An intercooler is a device that cools the supplied air, which is heated to a high temperature upon being compressed in the turbocharger. Then, it will send the cool high-density air to the cylinder.
|Advantages of Turbo-Charged Engines|
The turbocharger can supply large displacement to the cylinder, so that a high level of output can be obtained with a small exhaust volume. Achieving high power with a small exhaust volume means that the engine's weight and size can be made smaller, and this translates into a lighter vehicle weight and improved fuel efficiency. Moreover, a turbo-charged engine can generate 20% to 50% more torque* compared to a non-turbo-charged engine with the same displacement. These advantages make turbo-charged engines ideal for vehicles used for long-distance, high-speed transportation. On the other hand, non-turbo-charged engines feature high levels of torque in the low speed range, which gives them a better startup and acceleration performance and makes them suitable for vehicles used mainly for city driving involving repeated starting and stopping. In recent years, turbo-charged engines are getting more popular for their high fuel economy and remarkable power performance.
Torque is the rotational force generated by the movement of the crankshaft. The unit of torque is the Newton meter (Nm) or the kilogram meter (kgm). In principle, the higher an engine's combustion power, the greater the amount of torque it generates. For example, if a one-meter-long arm is fixed at right angles to a shaft and a 1kg weight is set at the tip, the force exerted on the shaft is 1Nm (1kgm).