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Car Forum / Dodge / Dodge Trucks / April 2005Measured Fuel Use / Computers
How does the computer that determines what MPG for the new vehicles, work? Is there a fuel flow meter or what?  SignatureBILL P. Just Dog & ME > How does the computer that determines what MPG for the new vehicles, > work? Is there a fuel flow meter or what? It calculates "fuel injected" from injector pulse width, fuel rail pressure and possibly also intake manifold vacuum readings. Nothing too fancy ;) Peter The EPA has their own testing system, you can see how it works at http://www.fueleconomy.gov/feg/how_tested.shtml I always take their ratings and knock off about 10% though, they usually seem to be a little optimistic. But they have been getting better lately it seems. I now use their site as research when buying any vehicle, or making any recommendations. > The EPA has their own testing system, you can see how it works at > http://www.fueleconomy.gov/feg/how_tested.shtml [quoted text clipped - 3 lines] > better lately it seems. I now use their site as research when buying > any vehicle, or making any recommendations. I was already aware about the un realistic .gov checking. What I wanted to know is how does the computers installed in the late model vehicles work. That is for both the gas as well as diesel vehicles. SignatureBILL P. Just Dog & ME > I was already aware about the un realistic .gov checking. What I wanted to > know is how does the computers installed in the late model vehicles work. > That is for both the gas as well as diesel vehicles. Yep - the computer knows how much fuel should be used on each injection event (let's see... I opened the injector for 3 milliseconds, with fuel rail pressure at 15,000psi - that used <X> amount of fuel... subtract that... carry the one... okay, next event...), so it keeps track of that, and sees how much fuel was used to travel a particular distance. From that, it makes an MPG calculation. That's why when, with a diesel, you add a pressure box to increase rail pressure, the fuel economy calculations get all out of whack. The computer thinks it injected <X> amount of fuel, becuase it did a 3ms injection event at 15,000psi rail pressure. But the rail pressure was really at 17,500psi, so it really injected about 1.2<X> fuel... the computer thinks it used 17% less fuel than it actually did. So, instead of your actual 17MPG that you're getting, the computer's reporting 19.8MPG. (All numbers are used as examples only, and may not have any real meaning - so put down the calculators :) Now I'm a bit puzzled, Tom. The Cummins is continually measuring the fuel pressure in the rail. Doesn't the trip computer use that data? Or does it merely use a fixed number for the fuel pressure? The latter approach would, I admit, help explain why the computer is mostly a bit off in its calculations. As an aside, for me the most valuable calculation that trip computer does is DTE. It often helps me to decide to refill now or later. John >> (snip) > [quoted text clipped - 12 lines] > - > so put down the calculators :) > Now I'm a bit puzzled, Tom. The Cummins is continually measuring the fuel > pressure in the rail. Doesn't the trip computer use that data? Or does it > merely use a fixed number for the fuel pressure? The latter approach > would, > I admit, help explain why the computer is mostly a bit off in its > calculations. It wouldn't matter with respect to a pressure box, because the way the pressure boxes work is to report false pressure readings to the rail pressure sensor, which causes the pressure to rise higher than it otherwise would. The ECM isn't aware that it's being lied to. In thinking about it some more, I may be giving the trip computer too much credit. It's probably just using pre-determined fuel flow rates from a table of engine RPM, throttle position, and calculated engine load. All this data is available right on the CCD bus, so they can use the same design for all gas and diesel engines - just with different data sets for the particular engine in use. >> Now I'm a bit puzzled, Tom. The Cummins is continually measuring the >> fuel pressure in the rail. Doesn't the trip computer use that data? [quoted text clipped - 14 lines] > use the same design for all gas and diesel engines - just with > different data sets for the particular engine in use. Does petrol engines use fuel rail pressure sensor at all? My 99 Durango does not have one, instead fuel pressure regulator is supposed to maintain 49 +- 5 psi at all times. That alone could account for error margin of 10% or so... Peter > Does petrol engines use fuel rail pressure sensor at all? None that I'm aware of (at least in the scope of this group, anyway) > 5 psi at all times. That alone could account for error margin of 10% or > so... Yes, although when operating in closed-loop mode, the varience in fuel pressure would be overcome by tuning the fuel delivery to keep the O2 sensors happy. > As an aside, for me the most valuable calculation that trip computer > does is DTE. It often helps me to decide to refill now or later. I've never been able to figure that DTE stuff out... I'm owner manual-less ;) Peter DTE is 'distance to empty.' An example of use would be if you are leaving a town and your DTE shows about 50 miles but the highway sign indicates the next town is 100 miles, it seems likely that you couldn't make it. So, fill 'er up now. >> As an aside, for me the most valuable calculation that trip computer >> does is DTE. It often helps me to decide to refill now or later. [quoted text clipped - 3 lines] > > Peter > Yep - the computer knows how much fuel should be used on each injection > event (let's see... I opened the injector for 3 milliseconds, with fuel [quoted text clipped - 13 lines] > (All numbers are used as examples only, and may not have any real meaning - > so put down the calculators :) Where does all this common rail pressure come from, surely not from the punie little lift pump. I would have thought that the actuation of the injector would create the much higher pressure. I have come to this conclusion because of the location of the lift pump being between the tank and filter. That would make the filter an extremely high pressure can, with a low pressure gasket on it. SignatureBILL P. Just Dog & ME The lift pump simply supplies fuel to the main injection pump which supplies the high pressure side. SignatureIf at first you don't succeed, you're not cut out for skydiving > > Yep - the computer knows how much fuel should be used on each injection > > event (let's see... I opened the injector for 3 milliseconds, with fuel [quoted text clipped - 20 lines] > between the tank and filter. That would make the filter an extremely > high pressure can, with a low pressure gasket on it. > Where does all this common rail pressure come from, surely not from the > punie little lift pump. Nope - the electric lift pump only supplies about 12psi or so of pressure. The gear-driven injection pump (for '03 and up trucks, it's a Bosch CP3 pump) pressurizes the fuel up to about 23,500psi (again, for the '03+ engines). > I would have thought that the actuation of the injector would create the > much higher pressure. In the older trucks ('02 and earlier), it was the pressure in the injector lines leading to the injector that would cause them to "pop off"... somewhere around 3500psi. The pressure had to be created first to get them to open, and then the higher the pressure, the more fuel that would flow in a given time, and the more atomized the fuel would become. On the '03+ trucks, the injectors are electrically actuated via a solenoid. The rail pressure is present in the injectors at all times. This gives the computer direct control over all phases of the injection event (or events, as the '03-'04 trucks have two injection events, the '04.5+ engines have three injection events per power stroke). > to this conclusion because of the location of the lift pump being between > the tank and filter. That would make the filter an extremely high pressure > can, with a low pressure gasket on it. Yeah - a pleated paper filter isn't going to hold up to 1600 atmospheres :) |
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