Direct Vs. Indirect Injection Diesel Motors

Written by sachiko schott
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Direct Vs. Indirect Injection Diesel Motors
DI and IDI diesel motors use different methods of fuel ignition. (diesel image by Andrey Kiselev from

A diesel motor can use either a direct or an indirect fuel injection system. These two systems differ with respect to how they ignite, their utilisation factor and their fuel economy.

DI and IDI Systems

In a direct-injection, or DI, system the fuel is sprayed directly into the main combustion chamber and ignites. By contrast, an indirect-injection, or IDI, system sprays the fuel first into a pre-combustion chamber, where ignition occurs before spreading into the main combustion chamber.

Utilisation Factor

The utilisation factor is the ability of a diesel engine to have smoke-free combustion. IDI systems have a utilisation value of about 80 per cent because the fuel has already been ignited before it reaches the main combustion chamber. DI systems tend to have a lower utilisation value, of 50 to 80 per cent, depending on the specific type of system.

Compression Ratio

DI and IDI systems have different combustion chamber shapes. The shape of the combustion chamber in an IDI system gives it more thermal efficiency than the DI design, which in turn gives it slightly better fuel efficiency.

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