Wild MPG instant reading - Page 4 - Ram 1500 Diesel Forum
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post #31 of 40 Old 12-01-2014, 12:33 PM
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Originally Posted by ED_Lover View Post
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)

Bob
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post #32 of 40 Old 12-01-2014, 01:04 PM
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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.
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post #33 of 40 Old 12-01-2014, 01:31 PM
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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
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.
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post #34 of 40 Old 12-01-2014, 01:34 PM
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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

Bob
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post #35 of 40 Old 12-01-2014, 01:41 PM
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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?).

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.
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post #36 of 40 Old 12-01-2014, 01:56 PM
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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).

Bob
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post #37 of 40 Old 12-01-2014, 02:10 PM
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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.

Bob
Current; '04.5, 2500HO,Auto,QCLB,4x4,CAI,4" TB,Smarty Jr,AD II 100,Flex-A-Lite fans, homebrew headache rack, "wedge" cap, rear bumper'03, 3500,QCLB,4x4,Dually,RIP
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post #38 of 40 Old 12-01-2014, 07:34 PM
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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).
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.
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post #39 of 40 Old 12-01-2014, 08:34 PM
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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.

Bob
Current; '04.5, 2500HO,Auto,QCLB,4x4,CAI,4" TB,Smarty Jr,AD II 100,Flex-A-Lite fans, homebrew headache rack, "wedge" cap, rear bumper'03, 3500,QCLB,4x4,Dually,RIP
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post #40 of 40 Old 12-01-2014, 11:09 PM
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The Ram eco diesel uses a Bosch EDC17 ecm and there is an internal barometer sensor in it. It is used to determine altitude and corresponding corrections in boost setpoint, injection timing, egr rates, etc. Bosch also has some diagnostic routines that compare the barometer reading and MAP reading at idle in certain conditions as a check to see if the MAP sensor is drifting.

There are injector maps in the software that give the fuel flow based on rpm/load. These are calibrated on a test bench. They are not extremely accurate due to build variation, but typically within 3%. There are also corrections in there for fuel temp. Generally speaking, if you want to know you real fuel economy, use actual miles driven divided by fuel fill at the pump. Most manufacturers bias the calculated fuel economy a few % to give you that 'feel good' feeling.
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