I own a car without a mass air flow sensor (smart fortwo), and it took me a while to figure out that the MPG readings my Scangauge II was returning weren't accurate. It wasn't made clear in any marketing material I could find, and I did come to some incorrect conclusions about what was good for my gas mileage and what wasn't based on the inaccurate readings.
The Scangauge showed that my engine was most efficient at full throttle and 4000-5000 RPM. Better evidence has since shown that it's really most efficient at about 3/4ths load and 3500-4000 RPM (it isn't exactly a normal engine).
I figured out what was going on when I saw that the reported MPG value didn't change if I just held full throttle in one gear. In my test, I held 4th gear at full throttle from 1500-5000 RPM and the Scangauge showed 22.4 MPG the entire time, only with one quick dip to 22.3. The only way that could happen is if the Scangauge doesn't account for volumetric efficiency changes, but simply calculates fuel consumption as k/(RPM*MAP). It isn't hard to find a good MPG figure on a car with a MAF sensor, but you have to put a little more work into it on a car without one.
I'm still very happy with my Scangauge and have plenty of uses for it, but your marketing material led me to believe it could do something that it could not. I frequent an online forum dedicated to smarts, and the Scangauge is very popular with other smart owners. Despite my explanations about how it's inaccurate in this manner, most of them appear to take its MPG number as absolute truth. While most people haven't gone into the detailed testing I did where it becomes a major issue, I have seen some other people arrive at seriously off-base conclusions due to this problem. The biggest one is that people think they use a whole lot (50-75%?) more gas while idling than they actually do. People also think that it's extremely important to keep cruise speed down to get good gas mileage, while it actually has remarkably little effect for this car.
I understand that you can't derive an exact MPG number through the OBD-II port, but this is a very large problem, resulting in numbers potentially off by more than a factor of two.
I understand that this is a very hard problem to work around from a technical perspective, but I wouldn't have a problem if you simply amended the marketing material to be more realistic. All it would take is adding an entry to the partial compatibility list on your vehicle compatibility page.
For a technical fix, I like how the Ultragauge handles the problem - it has a setup parameter for the vehicle's peak torque RPM, and it extrapolates a volumetric efficiency vs. RPM curve from that (presumably using the same one all the time for VE vs. load). It's far from perfect, but there's a world of difference between that and no correction at all.
Hopefully they fix the marketing. It'd be great if they added the correction, but I'm not getting my hopes up.
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Regarding the amount of fuel used while idling....
A few years ago there was an interesting discussion about the amount of fuel used by the average engine while idling. It's actually a lot less than most people realize. I decided to do a few calculations to determine how much fuel the 4.0L V6 engine in my 4Runner uses while idling. Here's what I came up with:
Using a scan tool, I found that the MAF rate at idle (and full operating temp) was 0.504 lbs/minute. With an air/fuel ratio of 14.7:1, I calculated the fuel consumption at approximately 2.05714 lbs of gas per hour. Applying the density of gasoline, that comes out to about 0.339 gallons of fuel per hour consumed while idling. So my V6 engine will burn less than a 1/2 gallon of gas if I let it sit idling for an hour. And since I have a 23-gallon fuel tank in my 4Runner, I could actually let it idle for more than 2 full days (if the tank was full), which is good to know if I'm ever trapped in the middle of nowhere during a severe winter blizzard. Actually, in that case I'd probably only run the engine occasionally for heat when it was really needed....but still, the point is that very little fuel is used while an engine idles. I know that my method isn't perfect, but I think it provides a pretty decent estimate.
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so what does this have to do with the latent airspeed of an african barn swallow??
Absolutely nothing as far as I can tell. However, it does relate to this statement, which is directly out of DBQ's letter:
"The biggest one is that people think they use a whole lot (50-75%?) more gas while idling than they actually do."
I would imagine that if my 6-cylinder 4Runner uses ~ 1/3 of a gallon of gas idling in an hour, a 3-cylinder smart would probably use roughly half that amount. So....thanks for asking and I'm glad I could provide the insight.
Absolutely nothing as far as I can tell. However, it does relate to this statement, which is directly out of DBQ's letter:
"The biggest one is that people think they use a whole lot (50-75%?) more gas while idling than they actually do."
I would imagine that if my 6-cylinder 4Runner uses ~ 1/3 of a gallon of gas idling in an hour, a 3-cylinder smart would probably use roughly half that amount. So....thanks for asking and I'm glad I could provide the insight.
It was a joke. Quite a funny one, I thought. Monty Python references are very easy to overuse, but if you don't see them coming they're great.
I'd go more by displacement, showing the smart using a quarter of the fuel to idle of the 4Runner.
Completely irrelevant as smart uses a different setup than a 6 cylinder motor, and none of this has any relation to the original posting now does it??
I don't spend much time idling my cars, I get in them, warm them a bit if needed, then go. I have yet to be stuck in a snowstorm where I have been marooned for days in a car, that would be poor planning. Also If the snow was not removed from around the front and back of the car, you would either overheat the motor or load the inside of the car with carbon monoxide and you would expire for the lack of oxygen.
And what about that African Barn Swallow?? If he had a ScangaugeII then there wouldn't be any question about his air speed now would there??
Glad you enjoyed the humor. only a true Monty Python geek would see it..
Completely irrelevant as smart uses a different setup than a 6 cylinder motor, and none of this has any relation to the original posting now does it??
Actually, I disagree. It is relevant....whether any particular car uses mass air flow or speed density doesn't matter because the amount of fuel used to idle wouldn't be significantly different. So if you tested the amount of fuel your car used while idling and then switched it over to a MAF system, it would still use just about the same amount of fuel to idle.
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I don't spend much time idling my cars, I get in them, warm them a bit if needed, then go.
Me too. I think that's how most people do it too...
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I have yet to be stuck in a snowstorm where I have been marooned for days in a car, that would be poor planning. Also If the snow was not removed from around the front and back of the car, you would either overheat the motor or load the inside of the car with carbon monoxide and you would expire for the lack of oxygen.
I haven't been stuck that long in a snowstorm either...but never say never, right? LOL
And thanks for the tip about clearing snow from the tailpipe. I'm well aware of those dangers. A cop around here died of carbon monoxide poisoning after he backed his cruiser into a pile of snow and sat in it idling. Very sad way to go....
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And what about that African Barn Swallow?? If he had a ScangaugeII then there wouldn't be any question about his air speed now would there??
It was a joke. Quite a funny one, I thought. Monty Python references are very easy to overuse, but if you don't see them coming they're great.
I'd go more by displacement, showing the smart using a quarter of the fuel to idle of the 4Runner.
Yeah, I got the joke....and of course I had my own way of responding to it
As for displacement....you're probably correct. Just the number of cylinders alone isn't the only thing that matters. I also tested a V8 vehicle and the numbers were not exactly what I had expected based upon the extra 2 cylinders.
Completely irrelevant as smart uses a different setup than a 6 cylinder motor, and none of this has any relation to the original posting now does it??
I don't spend much time idling my cars, I get in them, warm them a bit if needed, then go. I have yet to be stuck in a snowstorm where I have been marooned for days in a car, that would be poor planning. Also If the snow was not removed from around the front and back of the car, you would either overheat the motor or load the inside of the car with carbon monoxide and you would expire for the lack of oxygen.
There are lots of differences between the different engines that make this comparison fail to work anything close to precisely, but you can still draw some general conclusions. In the same way that you can conclude that the 4Runner's engine probably makes a bit less than 4x the power of the smart's engine, you can conclude that its idle fuel consumption is up there in the same range.
I don't spend much time idling either, but at least one person on here has mentioned that idle time was the biggest thing dragging down their MPG figure, which is highly doubtful, but the Scangauge could definitely give you that idea.
I don't spend much time idling either, but at least one person on here has mentioned that idle time was the biggest thing dragging down their MPG figure, which is highly doubtful, but the Scangauge could definitely give you that idea.
Exactly. And that's the reason for my original thoughts. I'd be surprised if the engine in the smart uses 1/10 of a gallon of gas to idle for an hour. If we added up all of the short periods of idling (for the average smart owner) for the period equal to a full tank of gas usage, I doubt it would be significant in terms of negatively affecting the calculated fuel economy.
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Quite a funny one, I thought. Monty Python references are very easy to overuse, but if you don't see them coming they're great.
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