How Does Engine Speed Affect Diesel Injection Timing?

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Engine speed significantly influences the start of injection (SOI) timing in diesel engines, with higher RPMs requiring an earlier SOI in degrees before top dead center (BTC). As engine speed increases, the combustion process necessitates advancing the injection timing to accommodate the fixed time it takes for the flame to propagate through the combustion chamber. In spark ignition engines, the slower flame front velocity demands substantial timing advancement at high RPMs. Conversely, in compression ignition engines, the delay is more related to the ignition timing after injection starts and the fuel's travel time from the pump to the injector. Understanding these dynamics is crucial for optimizing diesel engine performance.
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How does start of injection effected by speed of the engine? According to data, it seems for higher engine speed, the SOI (in terms of degree BTC) is higher.
 
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Tekneek said:
How does start of injection effected by speed of the engine? According to data, it seems for higher engine speed, the SOI (in terms of degree BTC) is higher.
For an IC engine, the flame will tend to take a fixed time to propagate through the charge in the combustion chamber, so ignition/injection has to be advanced as the RPM increases.
 
There is a distinct flame front in a spark ignition engine. The velocity of the flame front is quite slow so the spark must be significantly advanced at high RPM.

With a compression ignition engine, the fuel burns as it is injected. The delay is not so much in the flame front but in the time to ignite after the start of injection and the speed of sound in the lines from the injection pump to the injector.
 

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