BSFC Mapping

[BHarvey]

How would you go about it and what gauges would you use to map it?

Been thinking about it and not sure which gauges on my UG will be best.
Relative throttle position/absolute position
Load
MAP

[serialk11r]

I think you'd need a dyno that can hold the engine at a constant rpm and vary load.

[some_other_dave]

In addition to a good dyno that will let you impose a specific load on the engine, you will need two fuel flow-measuring devices--very sensitive ones. One measures the fuel coming to the engine from the tank, the other measures the fuel going back to the tank. The ones with the sort of precision you use are quite expensive, last I heard.

BSFC is fuel used per time per unit of power (or per unit of torque or per unit of pressure in the cylinder). So you have to get quantities for all three of those things--fuel used (generally by weight), time, and power or torque developed.

You can approximate BSFC for some specific points in the load/power regime if you get readings of gallons/hour of fuel use (the UG will approximate that from fuel injector duty cycle) and power or torque currently being developed. (Your UG will, if it provides those numbers at all, be approximating them from ... probably from a whole lot of inputs frankly.)

So you get approximations multiplied by approximations divided by approximations. Which means any results you get are going to be a little bit better than total wild guesses, but only a little.

-soD

[BHarvey]

Originally Posted by some_other_dave

In addition to a good dyno that will let you impose a specific load on the engine, you will need two fuel flow-measuring devices--very sensitive ones. One measures the fuel coming to the engine from the tank, the other measures the fuel going back to the tank. The ones with the sort of precision you use are quite expensive, last I heard.

BSFC is fuel used per time per unit of power (or per unit of torque or per unit of pressure in the cylinder). So you have to get quantities for all three of those things--fuel used (generally by weight), time, and power or torque developed.

You can approximate BSFC for some specific points in the load/power regime if you get readings of gallons/hour of fuel use (the UG will approximate that from fuel injector duty cycle) and power or torque currently being developed. (Your UG will, if it provides those numbers at all, be approximating them from ... probably from a whole lot of inputs frankly.)

So you get approximations multiplied by approximations divided by approximations. Which means any results you get are going to be a little bit better than total wild guesses, but only a little.

-soD

So, since the new UG has HP, torque and kilowatts, could you combine those with VE, Load, and whatever other gauge and map it?

[jdchmiel]

If you have a fast sampling rate, you should be able to calculate it from logging the engine sensors through the OBD2 port.

[some_other_dave]

Originally Posted by BHarvey

So, since the new UG has HP, torque and kilowatts, could you combine those with VE, Load, and whatever other gauge and map it?

The HP is an approximation. The torque is also an approximation. Ditto the kilowatts (which is power just like HP, but in a different scale). VE is somewhere between an approximation and a wild guess.

Using those to calculate BSFC means you will have numbers with several sets of approximations all multiplied together. The results will be little if any better than picking numbers out of a hat, sad to say.

Originally Posted by jdchmiel

If you have a fast sampling rate, you should be able to calculate it from logging the engine sensors through the OBD2 port.

That's exactly what the UG does. I think the "fast" sampling rate available is about 1/second, which isn't very fast for a gauge. The consumption figures that it shows you are approximations, every single one of them. No car that I know of (except possibly some very specialized and $$$$ test cars) measures fuel flow into and out of the fuel tank. So you get estimates of fuel used based on the amount of time the injector is open, and (if you're lucky) some notion of the fuel pressure versus the manifold pressure. And maybe a fudge factor for the amount of time it takes to open and close the injector itself. Still an approximation.

And the engine management system doesn't know the exact amount of torque or HP that it is producing at any time. There aren't any sensors that provide that information. It may be able to guess, using the vehicle speed and a time counter, and some guess as to the current weight of the car and current traction available and air density and so on, but that's an approximation of an approximation.

So again, you've got graphs of things approximated by multiplying approximations, plotted against things approximated from guesses.

It would take a decent amount of work to get results, and the results would be about a half-step better than random noise.

IMHO. (Well, not that humble!)

-soD

[jmcc]

Our test benches run expensive (accurate) dynos that are calibrated very accurately, and v expensive fuel measurement systems, along with fuel conditioning to hold temperatures accurately (40degC +/-0.5degC). This last point is critical as variation in temperature changes the fuel density and therefore the measured flow into the system. The air is supplied at controlled conditions (25DegC and 40% relative humidity, from memory).

Even with that, and with our dynos at sea level we then correct for atmospheric conditions.

Personally, I don't think you could get anything meaningful from an OBD measurement - sorry!

[drmiller100]

what you could get is instantaneous mpg.

which is meaningful.

[E4ODnut]

I agree completely with soD and JMCC. The only way I know of determining power with any reasonable degree of accuracy is on a high quality load dyno.
Accurate fuel flow measuring instruments are expensive and as was mentioned, fuel density has a major effect on the outcome. I calculate my fuel flow based on accumulated injector on time over a set time period. The best I seem to be able to get is about +- 3% of actual averaged over a range of speeds and loads.

[jmcc]

Originally Posted by drmiller100

what you could get is instantaneous mpg.

which is meaningful.

For back to back under controlled conditions maybe. Question was about generating a BSFC map and the errors in the fuel calculation in the ECU make this too inaccurate to be useable (in my opinion).

Reason for inaccuracy: fuel flow is calculated as a function of rail pressure and injection duration for a nominal injector. Injector flow particularly will vary, and needle lift vs electrical pulse will also be inconsistent.