Re: Power = f(t) ?

[John Greenstreet <jgreenst@motown.ge.com>]

> From quattro-owner@swiss.ans.net Fri Oct 28 11:46:56 1994
> From: "Arun Rao" <rao%frisbee@frisbee.pixar.com>
> 
> 
> 	I drive my 5000ST every day to work and back, and I notice that
> 	the car is noticeably more powerful on some days than on others.  There
> 	is almost certainly a correlation with temperature: afternoons are
> 	always worse than mornings and hot days are always worse than
> 	cooler ones.
> 
> 	My ME friend tells me that high temperature and humidity are
> 	bad for turbos in general.  He's also an amateur pilot, and
> 	he mentioned that climb rates were drastically affected by
> 	temperature and humidity in turbocharged (piston-engined, of course!)
> 	aircraft.  I was theorizing that it had to do with intercooler
> 	efficiency: higher ambient temperatures would reduce the heat
> 	transfer rate.
> 
> 	Any thoughts, O Knowledgeable Ones?  I would be interested in knowing
> 	if other people experience similar behavior in their cars.

What you have observed also affects normally-aspirated engines as well,
but the impact of temperature differences will vary with engine design,
generally depending on how close to pre-detonation the engine is run.
For example, many early-80s cars had their static ignition timing set
to top-dead-center rather than advanced.  Since the full energy
potential of the fuel was not being exploited, intake air temperature
made little difference.

In turbo and many of the new normally-aspirated designs, the engine is
run close to pre-detonation and knock sensors are used to maintain that
point of operation, thereby extracting the most energy from every cc of
fuel-air mixture.  All other things being equal, as intake air
temperature increases, pre-detonation will increase, and the ignition
point will have to be retarded, producing less power.  This effect will
also be evident in cars without knock sensors if they have spark
advance curves which vary with temperature.

At a dragstrip the difference is quite apparent and can result in up to
a two-second increase in 1/4-mile times from early morning to
mid-afternoon when using the same, normally-aspirated car.  Temperature
one big reason why the acceleration times for a specific make and model
can vary widely from test to test.  I think R&T has the right idea when
they publish the temperature and humidity at the time each car is
tested.

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John Greenstreet, Senior Engineer           (jgreenst@motown.ge.com)
Martin Marietta Government Electronic Systems    Moorestown NJ 08057
WPI Class of '75, Temple Class of '94

My new car history:
  1975    1978    1982       1986        1989      1992      1995
   VW ->  Audi -> Audi  -> Mercedes -> Mercedes -> Audi -> Mercedes
Scirocco Fox GTI  4000S    190E 2.3    190E 2.6    100CS     S320

POSSLQ's* new car history:
         1978       1981       1985      1988        1990     1993
       Triumph ->  Toyota ->  Toyota  ->  VW    ->   Audi  -> Audi
       Spitfire    Tercel     Corolla   Jetta GL      80      90S

*POSSLQ = Person of Opposite Sex Sharing Living Quarters
Note: All Audis and Mercedes above were sold to friends or family.
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