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Old 01-31-2014, 12:16 PM   #1 (permalink)
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Calculating BSFC from injector pulse and speed

Sometime, somewhere, on the internet, I came across someone who had developed a program/board/what-have-you that connected to the injectors and vehicle speed sensor and drew a BSFC map. From the vehicle speed sensor it could tell when the vehicle was accelerating (and how much etc.) From the injector pulse it could tell fuel flow and the like.

Constants were the vehicle mass, and some injector characteristics. The key thing was that it outputted a full map, no further calculations were needed for most usage.

Can anyone point me at that thread/project? I can't seem to find it any more. It would be a great thing to attach to any lean burn vehicle, or any vehicle where lean burn has been forced to operate full time (such as any lean burn capable vehicle that has had an interceptor fitted).


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Old 01-31-2014, 03:57 PM   #2 (permalink)
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You need at least three measurements for a BSFC map. Power (or torque), fuel consumption, and RPM.

Fuel consumption can be quite well approximated by the amount of time the injectors are open.
RPM can be derived from how often a given injector opens. Or, assuming you're only in one gear, from vehicle road speed.
Power or torque can be very roughly approximated by the injector duty cycle, but it would be exceedingly rough and would probably match almost 1:1 with fuel consumption.

Even if you can directly measure all of those things, you would need to spend a whole lot of time driving in order to get reasonable data for a good map. You'd have to drive at pretty much every torque level possible in every RPM, and log consumption at each. And ideally, you'd want a large number of samples at each torque/RPM point, to help average out any measurement errors.

Lots of work. The data will be very nice to have if you get it, though!!

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Old 01-31-2014, 07:31 PM   #3 (permalink)
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The problem with using a speed sensor only is that ground slope is ignored. Add that in and you can calculate a vehicle power at the wheels. A(n accurate) G sensor would do it.

Aero - including wind speed - will have an effect but you could just draw the BSFC plot using low road speeds and collect enough data for wind direction to average out.

I would be interested if anyone can find that project though.

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