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Looking for some clarification regarding injection timing and fuel vapor. The way I understand it is that the injector sprays on the port wall and evaporates at the same rate that the injector fuel puddle forms. So the injector is never actually spraying fuel directly into the engine (besides in the case of DI). Its really the fuel vapor that is being introduced that mixes with the air. And this is why Tip-In compensations are needed. Is this correct?
Yes that is correct , when you're working on closed valve injection. If you're firing the injector early in the cycle when the valve is closed, the fuel hitting the back of the closed intake valve and also the amount going onto the port walls (an amount of fuel will always impact the port wall) the heat on these components will cause the fuel to vaporize, then when the valve opens the vaporized is drawn in. This is more combustible than raw fuel.
However, if you fire the injector later in the cycle, then you will be working on open valve injection.
Example you might find an engine likes closed valve injection down low RPM (say around 380BTDC end of injection) then as RPM is increased you want to advance the injection to around 200BTDC for best power. There are many factors involved which will determine this, injector location, angle, engine designs etc.
Tip in compensation is required in regards to many factors, injection timing and angle is part of that, along with other aspects. The main goal with tip in is compensating for the amount of fuel which is coming from the injector and impacts the fuel wall. There is a great webinar below which explains this I have attached below.
Thank you very much!