Date: Tue, 19 Feb 2002 21:45:22 EST
Reply-To: FrankGRUN@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Frank Grunthaner <FrankGRUN@AOL.COM>
Subject: Re: Engine Wear
Content-Type: text/plain; charset="US-ASCII"
In a message dated 2/19/02 10:47:45 AM, al_knoll@PACBELL.NET writes:
<< Donning my Nomex...
I seem to recall that engine wear is proportional to piston feet per mile.
How far a piston travels in moving the car one mile.
Given same lubricants, operating temperatures, the motor with more p-ft/mi
will wear out sooner.
Of course there are many other elements in the wear equation too but P-F/mi
seemed important enough to "Rod and Truck" to include as a parameter on
their road tests. >>
Al,
True as far as it goes. More details in a long soliloquy I put in the
archives about a year ago on piston speeds. To summarize, the current
materials set (post 1980 for VW/Audi) allows the choice of wall finishes,
materials, ring surfaces and compositions such that continual operation at
5000 rpm with high loads (1 HP per cubic inch) will give a 75% wear point in
300,000 km. This is in contrast with a 75% wear point on a 1985 Ford V8 at
80,000 miles. The point is that the engineer designs for a load-wear-life p
rofile. Now it is true that you can reduce the piston ring wear for the same
engine by 20% by operating it at 4000 rpm vs. 5000. But not the whole story.
The thermal transmission from friction and conduction is also a strong
function rpm, load, etc., etc. But this curve is the locus of a multi
dimensional plot in which there are real optimal values. Here the engineers
(plural) set some finely tuned use parameters because this effects the
thermal efficiency of the heat engine. These and other changes like cam
timing, valve lift, chamber turbulence, intake runner length, exhaust gas
pressure phasing, etc. all combine for optimal operating points.
So a slugmobile (Ford Expedition) is tuned for maximum fuel efficiency
(oxymoron, sorry) at 1700 rpm and probably gets 0.1 mpg more here than at
2000. The optimal operating point for the waterboxers and the inline 4's is
given in the brake specific power per unit fuel flow curves in the various VW
design papers I have posted on Alistair Bell's web site. For any car, max
fuel economy and maximum life will coincide (per unit work done). This
requires operation at that rpm, full throttle and gearing for maximum load.
For any given engine rpm and extraction load, there is a consumption graph
(Alistair doesn't have it posted yet) that shows how much excess fuel you
need to use to move a load at the non optimal values. Varies by 50% over the
whole operating space.
Oh well, if the engine is designed for high rpm run at high rpm. The loss in
engine life compared to lower rpm running will be unmeasurable. Car goes for
200000 miles, or 250000 I still will have my money out of the conveyance.
BTW, as near as I can tell no similar data charts have ever been published by
Fuji Heavy Industries. I can only conclude that they are shamed by the
comparison with other competitive technologies!
Frank Grunthaner
