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Re: driving questions

To: Peter Landy <plandy@idt.net>
Subject: Re: driving questions
From: mmcewen@gpu.srv.ualberta.ca (Mere)
Date: Sat, 29 Nov 1997 21:54:43 -0500
Hello Peter and List:

I must take exception to this "diesel engine thinking" regarding wear on
engines.  Yes, starting places offers wear opportunities on engines but an
engine with good oil and decent oil pressure will not wear unduly at
startup.  However, the idea that engine life will be enhanced by leaving it
running is questionable.

I believe, as others have stated, that there are only so many revolutions
in the life of an engine.  Leaving it idling for lengthy periods does not
allow efficient running as idle mixtures are the least efficient in
carbureted engines which will add greater products of combustion to the
oil, and contribute to poor temperature differential within the engine.  In
cold weather, an idling engine will actually cool off if the car heater is
left on.  As manufacturers have recognized, one of the most important ways
of avoiding wear is to drive the car as soon as possible after cold starts
so that the warmup is not prolonged.  The sooner it is warmed the less wear
can occur.

A cold engine has larger clearances than a warm engine and cold oil has
higher viscosity than warm oil.  Thus the cold oil in the cold engine will
stick better to vertical surfaces and will fill larger engine clearances
better than hot oil.  As the oil and engine warm the clearances decrease as
viscosity declines.  The real culprit in startup is cylinder wear but
proper oil and pressure reduces this problem almost instantaneously.

What determines cylinder wear is the simply the number of times the piston
travels up and down in the bore.  Every one of those cycles causes wear.
Why does a large displacement V8 engine seriously outlive a small
displacement four cylinder engine?  It does less work at much fewer RPM.

It is not at all unusual for a small-block Chev or Ford taxicab to
accumulate over a million kilometers in service.  This is possible because
of two things.  It is started less often and it does less work because of
its large displacement and lower revs.

Diesel engines however thrive on steady running and don't like to be shut
off because of problems inherent with compression ignition and proper
mixture control.  Diesel trucks go for millions of miles, not so much
because they are diesels but because they run at very low RPM and have very
large displacements.  If diesels ran at the speeds of our LBCs they
wouldn't last much longer relative to the work they do.

John McEwen



>DANMAS@aol.com wrote:
>>
>> If it's true that 90% of engine wear occurs at start-up, as has been
>> reported, then you will wear out the engine from starting it long before you
>> wear it out from driving it!
>>
>
>I agree that most of the wear occurs during engine startups.  During
>shopping or other stop'n'go errands I leave the car running - providing
>one of my members is watching.  Some might see it as a waste of gas but
>in the long run it pays back more in parts and engine life.  Which
>reminds me about one of the DieHard battery commercials.  A bunch of
>guys decide to play a game of hoops after dark.  In the scene the guys
>jump out, looking so smart, while the truck's headlight is focused on
>the ball field with engine off.  After the game they get back in to
>start with no problems - of course. In reality such practice would kill
>the best battery, starter, alternator, as well as the engine.  But I
>wonder  how many watching that commercial actually grasped the truth
>beyond the obvious: That is don't shut the engine unless you really
>parked for the day/work or out of sight.
>
>Peter Landy
>'74 Bgt



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