Easy and accurate test (based on relativity) ?
 

Ok, so I thought I would post this up here....

Is it possible relatively speaking, to judge whether BSFC is better or worse, by comparing RPM and duty cycle change?

I.e, say at 60km/hr, rpm is 1800, and duty cycle is 14% in 5th gear, and in 4th gear, rpm is 2000 and duty cycle 13%, therefore an increase in rpm of 11%, and a decrease in duty cycle of 8%.

Meaning, on those relative changes, 5th gear, cruising at 60km/hr would be more fuel efficient?

I should point out, that these numbers are just out of thin air... and I'm assuming fuel pressure doesn't rise, with different load?

I'm also assuming the instrument error margin is constant and linear across the range being measured, for rpm and duty cycle.

Any potential holes you guys can see?

This isn't to calculate exactly how much my BSFC is, just whether it is less or more, in a specific gear, cruising along.

:)

Sounds pretty good, I would think that could show a rough picture of relative BSFC.

there is a difference in injector opening time and closing time, so there will be some deviation from consumption vs duty cycle there.

I would guess this would be possible. If you have the ability to get the %dutycycle from your car (OBD2?) and the RPM, you should be able to plug them into a BSFC chart for your car's engine, and see how much better/worse you would do by changing gears.

You cannot use fuel injector duty cycle by itself for figuring out fuel consumption. As someone said above, the opening and closing affects the actual open time. The other part that you are overlooking is the fuel pressure. There are very few cars on the road that use steady state fuel pressure, so figuring out flow will be tough. As for using engine load, that is different, although, still unable to tell you consumption without a BSFC map.

All the fuel injection systems I've worked on have constant fuel pressure across the injector. And there are of course opening and closing times that add error into using the duty cycle as linear, but as long as you are using mid-range duty cycles, the error is manageable for a relative calculation.

Most factory, naturally aspirated cars that i have seen use a static fuel pressure. All the forced induction cars i have experienced and seen use a rising rate fuel pressure regulator, so that it adds 1psi of fuel pressure for every 1psi of boost pressure (so as to maintain the static pressure at any boost).

Turbo vehicle maintain static pressures relative to manifold pressure. There are "rising rate" regulators that add more pressure relative to boost, but those are for adding more fuel to richen the mixture outside of computer control.

http://www.youtube.com/watch?v=UHTV3W5VI5w

http://www.youtube.com/watch?v=dBbuG...ext=1&index=48

I know that there is someone that has worked on more cars than me here, there has to be. With that said, most cars that I have worked on DO NOT USE CONSTANT FUEL PRESSURE. The fuel pressure is regulated by engine vacuum. As engine speed and throttle position change, vacuum changes. That changes pressure and therefore the amount of fuel that enters the engine even if the duty cycle is the same.

Fuel pressure is regulated relative to vacuum to maintain constant pressure across the injectors. If you measure the pressure relative to atmosphere, it appears to vary. but it does not.