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Indirect injection and Direct injection diesels

Posted on 3/7/2016 2:41:51 PM

Indirect injection, or IDI diesels, were much more common in the 80's and early 90's, but have altogether replaced by more efficient direct injection, or DI engines. The names can be misleading, as the difference between the two engines is not in how fuel is injected, but rather where it is in injected. IDI and DI engines have different combustion chamber designs, the later having a more traditional design while the IDI has a "pre-chamber" in which fuel is injected into before traveling into the actual combustion chamber. All versions of the Power Stroke, Cummins, and Duramax diesel models are direct injection diesels, where as International's earlier 6.9L/7.3L and General Motor's 6.2L/6.5L motors had pre-chambers and therefore fall under the category of IDI's.

IDI diesel engines utilize a pre-combustion chamber, generally referred to as a pre-chamber. Fuel is injected into the pre-chamber, where it rapidly mixes with oxygen and ignition occurs. As the flame front expands in the pre-chamber, it forces fuel to enter the combustion chamber rapidly, effectively mixing the fuel with air in the cylinder and atomization is achieved. The glowplug is also located in the prechamber, and the shape of the pistons in an IDI resemble those of a gasoline engine.
DI engines inject fuel directly into the combustion chamber, right into the top of the piston. The pistons on a DI engine have a bowl or cup machined into them that the fuel is directed into. DI engines operate at higher injection pressures and therefore more complete atomization occurs, meaning these engines do not require a pre-chamber to ensure proper diffusion of the fuel into the air.
• IDI engines are known for their simplicity and ease of maintenance since the injection systems are mechanical and the technology more primitive (no computer controls or sensors to malfunction).
• IDI and DI engines have distinct differences in sound to a well trained ear. 
• IDI Engines, in general, are quieter than DI engines, though many newer DI engines utilize intake baffles and advanced injection controls to suppress sound, making them extremely quiet. 
• Direct injection is much more efficient than indirect injection due to the level of atomization that is achieved. 
• Naturally aspirated IDI engines have much higher compression ratios than DI turbodiesels. 
• IDI engines can be naturally aspirated or turbocharged, where as DI engines are always turbocharged.
• IDI engines operate at much lower fuel injection pressures than DI engines. 
• Injectors for IDI engines are much cheaper than DI engines. Lower injection pressures also mean IDI injectors will outlast DI injectors.
Direct injection diesels are favored due to their efficiency, performance potential, and cleaner exhaust emissions. IDI diesels, however, have built a reputation of reliability and their ease of service/repair is alluring nonetheless.

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