Message-ID: <39059E39.D9207F41@regandesigns.com>
Date: Tue, 25 Apr 2000 08:31:37 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>, marklisalally@mindspring.com
Subject: Re: Engines
Mime-Version: 1.0

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Mark Lally writes:

<450 lbs of torque (at 2700 RPM) will move a plane of lancair size off the
runway
quite smartly I think.>

Horsepower is the product of torque, in foot pounds, times RPM divided by
5252, or in this case 450 x 2700 /5252 = 231 horsepower. Not nearly enough.

Actually you need about 700 foot pounds of torque at 2700 RPM to match the
TSIO-550.

Torque is proportional to the average cylinder pressure (aka Brake Mean
Effective Pressure or BMEP) and the piston area. As the saying goes "If you
want to make horsepower, the only thing better than cubic inches......is
cubic feet."

What about turbocharging to increase the BMEP? you ask. As it turns out a
single stage turbo is limited to how much it can increase the deck pressure
(the pressure just before the throttle butterfly) over the ambient pressure.
This pressure ratio is limited to about 3:1 for a high performance
turbocharger. If you want to make full horsepower at FL250 where the ambient
pressure is about 11 in.Hg then you need to make full horsepower at 33
inches of manifold pressure. So, all you can hope for is about 10% over sea
level naturally aspirated performance. If you want to fly high and fast then
a turbo is a must BUT you don't get high, fast AND a lot more horsepower
(over sea level performance).

What about turning at higher RPM and using a PSRU? you also ask. PSRUs weigh
a lot and they don't produce any horsepower so as you reduce an engines
weight (by reducing it's displacement) you are also adding it back as PSRU.
It works out that there is a "no man's land" between 2700 RPM and around
4000 RPM where decreasing displacement and adding a PSRU does not decrease
the installed weight of the power plant. You need to be turing the engine
around 5000-6000 RPM before a PSRU makes any real sense weight wise.

There aint no such thing as a free lunch. TANSTAAFL.

Regards
Brent Regan

PS. I know that I have made some broad statements and taken some liberties
with definitions. I do know that BMEP differs from the mean cylinder
pressure by the friction horsepower and other factors, for example. My goal
is to provide the broadest view in the simplest terms. I am not ignorant of
the minutia, I just don't think it is relevant, or interesting, at all
levels of discussion.

BR

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