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.
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.
Manifold Absolute Pressure.....MAP
Which is a fancy way of say air pressure or lack thereof.
Wouldn't an even more accurate measurement be a flow meter on the fuel line?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
Why would you need 6? Two would be sufficient I would think (a supply and a return).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.
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
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.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)
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.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).