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.

Direct-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.

Advantages	Disadvantages
Minimized surface area raises thermal efficiency and reduces heat loss, resulting in good fuel economy. Simple cylinder head design is durable and reliable, partly because it is largely unaffected by heat or pressure distortion. Engine starts easily, and preheating with a glow plug is not necessary.	Current designs produce more NOx emissions than indirect-injection systems. Not ideally suited to high-revolution vehicles (passenger cars) due to difficulties in creating an ideal swirl.

Indirect-Injection
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.

Advantages	Disadvantages
Suitable for fast engine speeds with high rpm. Less vibration and noise.	Additional chamber adds to design cost. Greater surface area leads to heat loss and reduced fuel economy. Higher temperature operation wears out parts faster.

Intercooler-Equipped Turbocharger

Turbocharger
Intercooler-Equipped Turbocharger
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.

Intercooler
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.

source : http://www.isuzu.co.jp