Re: Switch Your Engine Off When In A Queue

On Thu, 18 Sep 2008 08:38:38 +0100, "M.I.5¾"
<no.one@xxxxxxxxxxxxxxxxxxxxxx> wrote:


"Alex Heney" <me8@xxxxxxxxxxx> wrote in message
news:r1q2d49lt6fh37er9jimmb70sgvl288p7s@xxxxxxxxxx
On Wed, 17 Sep 2008 14:35:11 +0100, "M.I.5¾"
<no.one@xxxxxxxxxxxxxxxxxxxxxx> wrote:


"Adrian" <toomany2cvs@xxxxxxxxx> wrote in message
news:6jccumF2dmq3U5@xxxxxxxxxxxxxxxxxxxxx
Maria <oldwoman@xxxxxxxxxxx> gurgled happily, sounding much like they
were
saying:

How much extra fuel is used to re-start the engine?

Negligible, if any at all.

Although probably correct, you have ignored the extra fuel that requires
to
be burned [1] to produce the extra power required to turn the alternator
so
that it can replace the charge lost from the battery when it started the
engine. This is significantly larger for diesel engines than for petrol.
A
rough and ready calculation (making quite a few assumptions [2])

In other words, a complete guess.

I'm surprised at you, since you usually work this sort of thing out
very carefully.


Indeed, but I had to make a lot of assumptions. Either that or spend an
inordinate amount of time researching the factors.

Assumptions included.

Current drawn by starter motor (this varies between car models). I assumed
an average of 150 Amps (diesel engine) , but the inrush current is way
higher than that.

Efficiency of the battery. For a lead acid this is in incredibly low for a
high current load. I was able to look up the discharge efficiency for a
specific battery and found for a 70 Ah battery it was around 20%. But the
charge efficiency also has to be taken into account. This is 85% for a lead
acid, but drops for rapid charging. To where it drops for the charge
currents found in a car I know not. I stuck with 85%.

Efficiency of the Alternator. Can't find a figure for this, but the design
isn't a particularly efficient one as it relies on magnetising the shaft and
diverting the field from the shaft to the salient poles. No other salient
pole alternator is designed this way. I assumed 60%, but I would bet it
could be less than that. It will be a lot less for many French cars fitted
as they are with single phase alternators.

Power loss in the regulator. No idea at all. A total guess at 50%. Could
be more, could be less. But it isn't going to be a lot more.

I did omit the power required to power the glow plugs. (less current but
for longer).

As I said the calculation was rough and ready, and I wouldn't claim any kind
of accuracy. But it is likely somewhere near the right ball park.


I don't believe it is.

The glow plugs are nil, since we are talking of a warm engine.

And even from cold, they are normally only on for a second or two for
a modern diesel.

And again, starting from warm, you are talking no more than a second
or two of cranking the engine, at most. I really do not believe that
can use up more energy than would be used by idling for a minute.

Particularly since the alternator is running all the time anyhow, so
the current drawn for charging is insignificant in terms of additional
power usage.

There are certainly some strong arguments against turning the engine
off when stopped briefly in a queue, but I don't think efficiency is
one of them.

With my car, it would be much more worrying that whenever I turn off
the engine, it automatically turns off the lights, which would not be
a good thing in bad visibility or after dark (mine also turns them on
automatically when the light level drops or the wipers are on, but I
know of other cars where you turn the lights on manually but turning
off the engine also turns off the lights - such as the Fiat Bravo I
rented on holiday last week).
--
Alex Heney, Global Villager
Justice is incidental to law and order.
To reply by email, my address is alexATheneyDOTplusDOTcom
.

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