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Wild MPG instant reading

15980 Views 39 Replies 13 Participants Last post by  GDE
Was in heavy traffic for about 1 hour on I 5 goin south through Tacoma WA today for about 10 to15 miles or so and the reading would go from 0 standing still to 99 As I crawled along one or two mph, then down too 25, up down and even when I got out of the slow traffic it was still bounceing up and down. When I got too my destination I cleared all the readings and it would still do it as I moved the truck to park it. It run good other wise and drove fine, it started right up three or four times after I had it parked. This is a 360 mile trip over and the same going back so hope it runs fine when we head back Saturday. Do you think I should see if I have it checked Friday before I head back to Eastern Washington :(
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1st no manifold vacuum on a diesel.
2nd, MPG is calculated using a number of factors (Probably forgot a few) including speed, engine load, throttle position, engine temp, etc. Don't reset for awhile and it should average settle down. On the big Rams 2500/3500 diesels we refer to these as lie-o-meters for a reason. The only mpg numbers we believe are hand calculated. Anything from the display is BS. They might get close but that's about it. :)
I'm guessing the ECM counts the number and duration of injections and calculates overall fuel flow from that as well as estimated rail pressure. Once flow is known, MPG = Speed(miles/sec) / Flow(gallons/sec).
rpm, barometric pressure, ambient temperature, engine temp., vehicle speed, estimated fuel flow @ ?? RP (RP varies with with rpm/load, ECM can't calculate exact fuel flow it doesn't know what's going back to the tank/return line), boost pressure, transmission gear, TC locked/unlocked, throttle position. (missing a few I'm sure) as you can see with all these parameters the mpg display "might" vary just a touch. That is why the ONLY real MPG numbers are hand calced. distance traveled / gallons used. The less the display is reset the more consistent/stable "realistic" the display will become to calced mpg. When "tuners" come out the numbers that you will see reported will be all over the board. Tuners, depending on how they interact with the factory ECM and what tune level they are operated on, will give really wild extremes on the display.
How does barometric pressure have any bearing on MPG? Once fuel flow is known, it doesn't matter what temperature anything is, it's all about speed and fuel consumption.
Barometric pressure is the measure of air density. ECM adjusts for that. for example: given the same weather conditions (NO tornadoes/hurricanes in the area) barometric pressure is lower in Denver (mile high city ring a bell/thinner air) than it is at sea level. System has to compensate for the amount of air ingested. Even with a turbo engine MAP (Manifold Absolute Pressure) sensor is still employed. More boost more fuel/less boost less fuel.
I really doubt this truck has a barometric pressure sensor on it. The truck does of course adjust for changes in O2 density (lower ambient pressure). But the sensor for adjusting for O2 fluctuations is the O2 sensor, not a barometric sensor.
Manifold Absolute Pressure.....MAP
Which is a fancy way of say air pressure or lack thereof.
Am I missing something? Why can't the ECM know fuel flow by counting the number of times the injectors fire? I would think the flow would be easier to control digitally by injecting the same amount of fuel each time and modulating the firing rate. The distance traveled is known. Divide the two to get mpg. Who cares about temperatures, pressures, etc? MPG is about what has already happened, not predicting the future. What I am reading here sounds like an awfully complicated way to do this.
It is a complicated way to do this. I didn't write the algorithm taking into account all the parameters/variables that the designers did include. I guess if it was easy the MPG display would be dead nuts on like the odometer is when measuring distance (well almost this accurate AND consistent too). The odometer doesn't give wildly variable numbers when we take our foot off the throttle or accelerate does it. Problem is all the variables (as I stated before) in the program. IF there was a way to accurately WEIGH the fuel in the tank then it would be REAL easy get an accurate/consistent MPG number. distance traveled / lbs/hr fuel burned. That would be THE way to go.
MAP is NOT the same as ambient pressure. The air filter, inter-cooler, turbo all contribute to pressure differences between ambient and manifold pressure. But more importantly the MAP sensor is there for boost calculations, not MPG.
Ok, guess I should be a clearer. MAP is the absolute pressure difference between ambient pressure (atmosphere) and manifold (intake) pressure. The MAP sensor is required to determine the pressure differential between the manifold pressure and atmospheric pressure. It can also be used to calculate airflow.
here to save a lot of typing. getting lazy. :)
MAP sensor - Wikipedia, the free encyclopedia
Wouldn't an even more accurate measurement be a flow meter on the fuel line?

I still don't see why the designers would include any temps, boost, etc. into the MPG calculation when the injection duty cycle would be a much closer measurement of fuel consumption.

The variation between the truck MPG and real-world is likely due to variations in the amount of energy contained in the fuel. It could also be that assuming each injector delivers the same amount of fuel every time is inaccurate (rail pressure varies?).
That's why I said fuel weight in the tank. Flow meters would be nice BUT there is also the fuel return line to deal with. Not all the fuel that goes through the pump is injected into the engine. More is returned to the tank than is actually used. With the V6 Motori Common Rail engine there would have to be 6 (or however many cylinders there are on any given Common Rail engine) flow meters (1 for each cylinder) as all fuel leaving the high pressure pump is under high pressure to the fuel "log" then to the individual injector lines. The fuel that is not used (metered) through the injectors is "dumped/returned" from the fuel "log" back to the tank.
Why would you need 6? Two would be sufficient I would think (a supply and a return).
2 would work you are correct. I was just thinking for accurate data acquisition (fuel flow) for each cylinder. Getting a little to in depth. we use individual pyro's for each cylinder when testing. I was going a little overboard I guess. Old habits....as many channels of data as possible. removes more variables :eek:
So do you work for one of the Big 3?
No, let's just say that I do "third party independent evaluations" i.e unbiased quality control. Plus I still do R & D. I'm not "in it" like I used to be but I've been "at it" for about 38 years (+/-). I started right around the time the first emissions hardware was being implemented. (mid-'70's)
So the obvious question then is, have you actually seen the algorithm used for the MPG calculation and it is using the 6 or more parameters you listed earlier? I design medical devices for a living so I'm certainly not an automotive expert but I have a hard time understanding why they go to such lengths to have a result that is so inaccurate.
for this particular application no I haven't seen it. But I am familiar with what is used (sensors) in the so called "modern/computer era" applications. correct it doesn't make a lot of sense. But the thinking is "we have all that information anyway let's use it". If they would do away with the instant MPG end of the display/readout and just do an average over time/fuel burned/used I think there would be a lot less confusion and a lot more stable/realistic numbers reported. IMO the instant display/readout is what throws everything/everyone off. IF they wanted a reset/clear function it should delete/erase the last AVERAGE MPG and start a new average. Forget the instant which in reality means NOTHING. JMO :)
I believe that is not correct. Manifold absolute pressure is the difference between zero pressure and the manifold pressure. Boost is the difference between atmospheric pressure and manifold pressure.

One of my engines, for example, will show ~14 psi MAP when the engine is not running and ~29 psi at full load. So, it is getting ~15 psi of boost above atmospheric pressure.
I believe you are just stating the same thing taking into consideration that atmospheric pressure is 14.7 psi. When I try to explain this I don't even go into the "normal/average" atmospheric pressure is 14.7. gets way too complicated way too quick. You are correct, the MAP sensor measures the differential between manifold and atmospheric pressure. (which changes with elevation and weather conditions).
A flow meter in the fuel line doesn't work because there is a return line and only part of the fuel that is flowing is delivered to the injectors.

Knowing fuel pressure, injection time width and nozzle size, the amount of fuel can be calculated. Energy in the fuel doesn't matter. Only the fuel used and miles covered matter. Overall accuracy is probably dictated by injector pulse width accuracy and rail pressure and perhaps other parameters to a lesser degree.
If two flow meters were used (one in the line from the pump to the rail and another in the return line to the tank) it could be made to work. Probably a lot easier and more accurate that's for sure.
Actually if the MAP sensor is reading 14.7 psi with the engine off that implies it is an absolute sensor not a differential sensor in the traditional sense. What makes a sensor absolute is that it is a differential sensor measuring pressure relative to a vacuum (ambient pressure places 14.7 pounds per sq inch on a completely evacuated chamber). Otherwise the MAP sensor would read 0 psi when the engine is off since ambient and the manifold are at the same pressure.
you are correct! In trying to simplify this I'm getting my defintions backwards. MAP is absolute. Differential would be equated to a boost/vac gauge that starts at-0- when the engine is OFF. MY error. :(hope I didn't confuse anyone other than myself. so much for simplify! I'll just go back to the way I usually have to explain. Heaven help us. 0:)
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