# Re: Measured Fuel Use / Computers

• posted on April 7, 2005, 1:38 pm
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.
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• posted on April 7, 2005, 3:19 pm
bucky_katt snipped-for-privacy@yahoo.com wrote:

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.
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BILL P.
Just Dog
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• posted on April 8, 2005, 1:34 am

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 :)
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• posted on April 8, 2005, 2:47 am
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
Tom Lawrence wrote:

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• posted on April 8, 2005, 4:12 am

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.
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• posted on April 8, 2005, 6:17 am

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
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• posted on April 8, 2005, 12:48 pm

None that I'm aware of (at least in the scope of this group, anyway)

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.
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• posted on April 8, 2005, 6:23 am

I've never been able to figure that DTE stuff out... I'm owner manual-less ;)
Peter
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• posted on April 8, 2005, 8:39 pm
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.
Peter wrote:

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• posted on April 8, 2005, 1:39 pm
Tom Lawrence wrote:

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.
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BILL P.
Just Dog
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• posted on April 8, 2005, 4:23 pm
The lift pump simply supplies fuel to the main injection pump which supplies the high pressure side.
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If at first you don't succeed, you're not cut out for skydiving

"William Boyd" < snipped-for-privacy@direcway.com> wrote in message
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• posted on April 9, 2005, 1:20 am

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).

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).

Yeah - a pleated paper filter isn't going to hold up to 1600 atmospheres :)