Discussion and questions related to the course Understanding AFR
Is there any cons in running a fixed and constant 5bar pressure for the fuel?
in other words, the fuel pressure doesn’t increase with boost, just fixed at 5 bars across all MAP regions.
Yes, the compensations based on the linear range of the injectors are based on a constant differential pressure, if your differential pressure is getting reduced in boost, then the compensations and other fueling calculations will get off. Now you can "bake in" the errors into your fuel map. But the correct answer is DON'T DO IT -- ALWAYS USE A MANIFOLD PRESSURE REFERENCED FUEL PRESSURE REGULATOR. Here's some other input on this topic:
As David said, bad idea as it compromises fueling at both ends - in short -
At low load and rpm, when the fuel demand is low, the higher pressure means more fuel is injected for the same opening period and, especially with larger injectors, it is difficult to get the metering correct. The delta (difference) between the fuel rail at +5 bar and manifold runner at -0.5 bar is 5.5 bar.
At high rpm and boost pressure, when the fuel demand is high, there is a much smaller pressure difference forcing fuel through the injector, and this greatly reduces the fuel flow. Fuel rail pressure +5bar and manifold pressure +2bar, difference is 3bar.
By referencing the pressure regulator to the boost pressure, and using a base fuel line pressure of 3bar we now have...
Idle fuel rail pressure of - +3 plus -0.5bar - which is +2.5bar and so less than half the fuel is injected for the same injection opening period, this means much better control of the metering.
At full boost fuel rail pressure we have - the base +3bar plus +2bar for the reference correction - which is what you'd've had in the first place, but without the aforementioned benefits. If you were running 3bar of boost, though, with referenced rail pressure you would have 6bar at the rail, a significant improvement as you'd still have 3bar more at the rail than in the manifold but with fixed it would only be 2 bar - so you'd have 50% more fuelling available.
More importantly in your sample -- the differential pressure (Fuel Rail Pressue - Manifold Manifold Pressure) is exactly 3 bar in all cases. So a 10% increase in pulse width results in a 10% increase in fuel delivered -- so compensations like for environmental conditions (air temp, barometric pressure) will produce the correct result under all operating conditions.
Yup, I've modified the last paragraph ;-)
thanks guys for the input. good inputs v informative. It is in fact much better to have a 1:1 increase in fuel pressure.