Date: Mon, 26 Mar 2012 12:14:07 -0400
Reply-To: pickle vanagon <greenvanagon@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: pickle vanagon <greenvanagon@GMAIL.COM>
Subject: Re: Fuel consumption in different gears - how does the energy /
fuel work?
In-Reply-To: <CAEuQn0ZmkVP-Q6G=o=PUeUP0nNqiguN3Ak-EU-BpYmyA0qdV5w@mail.gmail.com>
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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
> >
>
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