Indirect Injection vs. Direct Injection Diesel
IDI vs. DI
The differences between IDI and DI diesel engines is a commonly asked question, and there are many misconceptions surrounding the topic. IDI stands for indirect injection, and DI stands for direct injection. All Power Stroke, Duramax, and Cummins diesels are direct injection engines. Older motors, such as the 6.9L/7.3L International engines (pre-Power Stroke) and 6.2L/6.5L GM engines are indirect diesels.
IDI - Indirect Injection Diesel:
IDI diesel engines utilize a pre-combustion chamber, sometimes referred to as a prechamber. Fuel is injected into the prechamber, where it rapidly mixes with oxygen and ignition occurs. As the flame front expands in the prechamber, it forces fuel to enter the combustion chamber rapidly, effectively mixing the fuel with air in the cylinder. The glowplug is also located in the prechamber, and the shape of the pistons in an IDI resemble those of a gasoline engine.
DI - Direct Injection Diesel:
DI engines inject fuel directly into the combustion chamber. 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 a prechamber to ensure proper diffusion of the fuel into the air.
IDI vs. DI
• IDI engines are known for their simplicity and ease of maintenance since the injection systems are mechanical and the technolog 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, and 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.
• Naturally aspirated IDI engines have much high compression ratios than DI turbo diesels.
• 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.
Director 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.