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Calculating Injector size course equation

EFI Tuning Fundamentals

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Very nice explanation starting from simple and then developing the topics. The course is clear.

I have a question about the injector sizing equation which is given in the course. I will expose my understanding and please correct me when I make a mistake. I am sorry if that question was addressed before.

Injector_size[g/h] = (Target_Power[kW] x BSFC [g/kWh]) / Number_injectors [-] is supposed to give 100% of fuel to be injected for that BSFC in order to get the target power.

If we want injector to opens only at 85% of duty cycle in order to reach that fuel mass, it would mean that we have to devide 100% injector flow by 0.85.

Multiplying by 0.85, as mentioned in the course, will decrease the injector size and thus make it 17.6% shorter than requested instead of making it 15% larger.

Am I right or wrong saying that ?

Thank you very much !



Is my question already addressed elsewhere ?

Thank you

You want headroom on injector flow. Put it this way: if you need to flow 850cc at peak power, size the injectors to 1000C.

The real trick is knowing the BSFC at peak power to know how much fuel you are flowing. And that's tricky because it depends on airflow AND air fuel ratio required to make the power.

If you have to run rich with retarded spark and more of boost/airflow to make power safely, bsfc gets worse and more injector flow is needed.

It also depends on the lower heating value (energy per mass) of the fuel, in megajoules per kilogram. More ethanol means lower energy content, meaning more grams of fuel needed.

Thank you !

I am familiar with BSFC and what is behind it. If we want to be more accurate we should use High Pressure ISFC as we are doing when matching a turbocharger in order to account for LP IMEP induced by the turbine as well as for FMEP and so on. However it is another topic.


Yes there is an error in the equation which is why it isn't giving you the expected answer. We are aware of this and expect to rectify it early next year. Apologies for the confusion.

Thank you Andre !

We usually reply within 12hrs (often sooner)

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