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
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.
Tom Lawrence wrote:
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.
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
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.
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.
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+
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 :)
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