Can't help you with the first question too much. Although I'm sure it has a lot to do with monitoring injector pulse width with the engine on a dyno going through all possible scenarios.
Air fuel ratio is not always stoichiometric on a fuel injected car. Newer engines operate almost entirely at stoichiometric air fuel ratios, but not always.
as long as you have LOD, RPM, and vehicle speed, you can predict fuel consumption.
Does this only apply to an engine operating in closed loop mode?
This is one of the drawbacks of the ScanGauge, in my understanding. It's relatively accurate provided you stay in closed loop, where it can make calcs based on stoich A/F ratios. But when you get into open loop fuel enrichment at higher loads, it doesn't necessarily know how much more fuel the computer is injecting.
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Does this only apply to an engine operating in closed loop mode?
This is one of the drawbacks of the ScanGauge, in my understanding. It's relatively accurate provided you stay in closed loop, where it can make calcs based on stoich A/F ratios. But when you get into open loop fuel enrichment at higher loads, it doesn't necessarily know how much more fuel the computer is injecting.
In my experience, this is where the ScanGauge falls flat, but only in reporting Instantaneous MGP. Last summer, I drove my F350 Diesel on a 1600 mile round trip over two long days, pulling a big, heavy, very un-aerodynamic trailer over a lot of long steep hills. In the steepest parts, to keep the speed above 50mph I had to use full throttle. During full throttle, the ScanGauge would indicate the instant-mpg at crazy numbers: 53mpg, 217mpg, 9999mpg, etc. The numbers would jump all over the place. At one point, I was at full throttle (never lifting) for over an hour... But the average MPG was correct, and at the end of the tank, it was only off by 1/10th of a gallon, on the pessimistic side. If they can get the average correct, I don't know why they can't get the instant correct.
If you slap down the throttle at low rpm and LOD goes to ~100, it's based on peak torque. If it increases w/ rpm at WOT, it's based on peak power. If it's based on torque, as long as you have LOD, RPM, and vehicle speed, you can predict fuel consumption.
I tried that last night and it did NOT go to 100. But maybe my RPM wasn't low enough (I was accelerating onto the freeway), so I'll try it again.
Question: What are the units for "Peak Power"? Is there a unit translation/mapping to/from the BSFC chart?
Does this only apply to an engine operating in closed loop mode?
I will make sure to display the Open/Closed loop parameter when I repeat the WOT+low RPM test.
Quote:
This is one of the drawbacks of the ScanGauge, in my understanding. It's relatively accurate provided you stay in closed loop, where it can make calcs based on stoich A/F ratios. But when you get into open loop fuel enrichment at higher loads, it doesn't necessarily know how much more fuel the computer is injecting.
Are you implying that it is using the 02 sensor feedback? From my experience, I do not think it does.
I think it makes calcs based on the assumption of 14.7/1 AFR ratios, but it *derives" it's MPG from all the other data inputs.
CarloSW2
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Nope, not implying that. I don't know exactly how it works. I've only read in other threads that fuel consumption is estimated from air consumption, through other sensors.
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I am curious about how bsfc maps are produced. Could anyone point me to literature on the subject?
Also, is it correct that with fuel injection the mixture (except for warming up/choke) is always stoichiometric (in correct amounts for full combustion)? If so, then is it solely the amount of fuel/air mixture that is in the cylinder that determines the power output, and this amount is lineally dependent upon pressure in the inlet manifold, and this pressure is lineally related to the throttle position?
Thanks in advance.
Brake specific fuel consumption (BSFC) measures how efficiently fuel energy is converted into engine shaft power. Wiki does a fairly good job of defining the BSFC but one common practice is to measure this with the engine on a dynamo, full-throttle, and adjusting the load. This typically results in a graph like this ...
CarloSW2
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roflwaffle -
I tried that last night and it did NOT go to 100. But maybe my RPM wasn't low enough (I was accelerating onto the freeway), so I'll try it again.
As long as it went to what, 80-85 out of 100 if you slap it down at ~1500rpm or greater, it should be measuring torque at some engine speed. If it's significantly below that it's probably measuring power.
As long as it went to what, 80-85 out of 100 if you slap it down at ~1500rpm or greater, it should be measuring torque at some engine speed. If it's significantly below that it's probably measuring power.
At around 1100 RPM in 5th gear, I would see as high as 91 LOD. But it wouldn't stay there. As the RPMs ever-so-slowly increased with the MPH, it would fall off. I will do the test again at 1500 RPM, but now I am leaning toward assuming that that it's measuring torque.
CarloSW2
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Nice find. I thought the Excel chart was particularly interesting.
Quote:
A couple of years ago, I used a Graham scanner to monitor engine performance over a full tank of gas, over 500 miles. I then reduced the huge file to a smaller set to plot my best approximation:
Now Excel is not very good about drawing areas so as a quick hack, I used 'bubbles" of different sizes to stack the relative BSFC. What is interesting is the small 'red dots', the highest efficiency metrics, overlap some of the formal BSFC data but also extends quite is[sic] outside on the low power range.
One hypothesis, speculation on my part, I suspect ordinary vehicle operation involves partial throttle operation that is not evident in the standard BSFC charts. It may be that partial throttle operation shifts the high efficiency, engine operations into lower speed ranges.
A second hypothsis is that "heretical mode," or as Toyota calls it energy recirculation mode, may change the load characteristics. This may also change the peak BSFC regions.
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