"In principle it should be possible to count the fuel injector pulses.
(Here's an example of a project to do this kind of thing:
http://ecomodder.com/wiki/index.php/MPGuino).  One problem with coming up
with a real-time output of mpg is that the speedometer in a vanagon is
mechanical.  So you need to grab the fuel injector pulses but also have a
speed input to the calculation somehow."

I like this idea, sounds very workable except as you pointed out that the
default speedo is mechanical.  I wonder if a GPS input would work, perhaps
there is one of the Android apps that would output the measured speed to a
couple of pins on the general purpose connector.

Also, I am not certain, but I think the device would have to measure the
duration of power applied to the injectors, not just count pulses, since I
believe the WBX/ECU use time to vary the amount of fuel delivered.

I wonder how the missing variable might also be available, and that is
load:  I wonder if that could be simulated by measuring how far the
accelerator is pushed?  Or probably better a vacuum sensor on the intake
somewhere.

Roland

On Mon, Mar 26, 2012 at 9:14 AM, pickle vanagon <greenvanagon@gmail.com>wrote:

> I think it's very easy to convince yourself that engine losses are much
> greater at higher rpms.  Try engine-breaking in low versus high gear.
> (push down on the gas pedal with the ignition off while you're doing this
> if you're worried the differences might just be due to throttle
> restriction).  There's a huge difference at high-rpms versus low-rpms.  In
> fact, you should even be able to calculate the resistive torque (or force,
> with respect to a fixed gear) of the engine at different rpms, by observing
> your deceleration rate and comparing it with the rate you decelerate in
> neutral from rolling and aerodynamic resistance alone.
>
> But you're right, this is not enough to decide conclusively that the
> engine is more efficient at lower rpms (and certainly, there are cases
> where this is not the case).  The system is complicated enough that I think
> you can't really reason about it extremely precisely from first
> principles.  But, for practial purposes, I suspect that driving on flat
> ground at 30mph at 2500 rpm is significantly more efficient than driving on
> flat ground at 30mph at 3500rpm, since the differences in engine losses are
> *so* large.
>
> The problem with what your asking for in terms of data is that I don't
> know an accurate way of measuring fuel consumption in a WBX short of
> long-term averaging of fuel-ups.  In principle it should be possible to
> count the fuel injector pulses.  (Here's an example of a project to do this
> kind of thing: http://ecomodder.com/wiki/index.php/MPGuino).  One problem
> with coming up with a real-time output of mpg is that the speedometer in a
> vanagon is mechanical.  So you need to grab the fuel injector pulses but
> also have a speed input to the calculation somehow.
>
>
>
>
>
> On Sun, Mar 25, 2012 at 5:58 PM, Roland <syncronicity1@gmail.com> wrote:
>
>> Thanks all for the discussion.  (this is an '89 2.1 WBX by the way)
>>
>> I don't understand all of the responses, for example I am not sure what
>> torque has to do with my question.  Sure there are some torque differences
>> in 3rd vs 4th gear, I can't understand how that might affect fuel
>> consumption.
>>
>> But what David wrote makes good sense.   In 3rd gear there would be more
>> losses in the engine itself (piston / conn rod weight reversal, valve
>> springs, compression events, oil/water pump).  So that suggests that the
>> MPG would be lower while driving in 3rd gear.
>>
>> But then as a counter argument (if I read it correctly), the engine might
>> run more efficiently at higher RPMs, at 3400 in 3rd vs 2400 in 4th.
>>
>> I think we need to find some graphs of fuel consumption vs engine speed.
>> Ideally they would be for the WBX, but perhaps all engines have similar
>> characteristics.  About 5 different plots, from 1000 RPM to 5000 RPM with
>> each plot being a different load on the engine.  I think this is what I am
>> searching for / asking about.  It is almost like reverse engineering the
>> ECU.
>>
>> Roland
>>
>>
>>
>> On Sun, Mar 25, 2012 at 3:25 AM, David Beierl <dbeierl@attglobal.net>
>> wrote:
>>
>> > At 09:56 PM 3/24/2012, Roland wrote:
>> >
>> >> consumed in 3rd gear vs 4th gear?  Now the immediate answer is none!
>>  It
>> >> takes the same energy to drive at 45 mph in 3rd or 4th.  The wind
>> >> resistance is the same, the drive-line loss is the same, other friction
>> >> like tires are the same.
>> >>
>> >
>> > But it takes more energy to run the engine faster.  The inertial losses
>> > from reversing the pistons' direction of travel, the frictional losses
>> from
>> > scrubbing the piston rings up and down in their bores, the energy lost
>> by
>> > compressing the fuel/air charge and operating the valve train, the
>> > additional output of the water and oil pumps - these parasitic losses
>> all
>> > increase more-or-less linearly with increasing rpm.
>> >
>> > It would be interesting for someone with an OBDII engine to do some
>> > calculations of no-load operation at various rpm, using the rpm and
>> engine
>> > load numbers to determine the relative amount of fuel required to merely
>> > spin the engine at those rpm.  I wish I'd done it while I still had my
>> > Honda.
>> >
>> > Yours,
>> > David
>> >
>>
>
>