Date: Sun, 25 Mar 2012 18:26:54 -0700
Reply-To: Poppie Jagersand <poppie.jagersand@YAHOO.CA>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Poppie Jagersand <poppie.jagersand@YAHOO.CA>
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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You are right that you will need graphs of specific fuel consumption in g/kWh from detailed engine testing. I seldom see these for gas engines, but they can be found for the Diesels.
They are often broadly similar for similar engines. Here are ones for the IDI/prechamber diesels.
A graph summarizing power, torque and specific fuel consumption:
http://www.4crawler.com/Diesel/SAE/figure_35.jpg
However, for a more detailed calculation we need to plot the fuel consumption for different loads. Here the load is indexed by the manifold pressure. As seen peak efficiency for an IDI diesel is at 1/2 max pressure and 1400-2000rpm. At max load the peak efficiency is at 2000-2500rpm.
http://www.4crawler.com/Diesel/SAE/figure_54.jpg
Figures are from this paper.
http://www.4crawler.com/Diesel/SAE/vwtdsae.shtml
A university library should be able to get this in a more legible copy and any other technical papers.
For the TDI peak efficiency is at 1700-1900 rpm and at/near max load.
Martin (and Poppie 1.9TD, IDI)
________________________________
From: Roland <syncronicity1@GMAIL.COM>
To: vanagon@GERRY.VANAGON.COM
Sent: Sunday, March 25, 2012 3:58:13 PM
Subject: Re: Fuel consumption in different gears - how does the energy / fuel work?
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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