Cylinder Pressure

[ROD BRANDON]

A "just because I'm curious" question:

Does anyone know the peak pressure (in psi) developed in the cylinder of
a four-cycle engine during the power stroke? I'm sure this varies
somewhat from engine to engine depending on maybe the compression ratio
etc., but just an average pressure for most engines would do.

Thanks
=rod=

[Kevin Mouton]

If you install a compression gauge in a cylinder and run the engine the
normal reading would be about 60 psi, but that is without any combustion
taking place. The actual pressure during combustion can reach pressures in
excess of 2,000 psi for short periods. This depends a great deal on the rpms
and the load on the engine as well as many other factors.

ROD BRANDON wrote:

--
Please direct all E-mail replies to:
kevinm@*REMOVE THIS*eatel.net
http://www.eatel.net/~kevinm/homepage.htm
************************************************
Kevin Mouton - Automotive Technology Instructor
************************************************
"If women don't find you handsome they
should at least find you handy!"
Red Green of Possum Lodge
************************************************

[dbug]

On Wed, 15 Apr 1998 16:47:33 -0700, ROD BRANDON <rbra...@mdausa.org>
wrote:

>A "just because I'm curious" question:
>
>Does anyone know the peak pressure (in psi) developed in the cylinder of
>a four-cycle engine during the power stroke? I'm sure this varies
>somewhat from engine to engine depending on maybe the compression ratio
>etc., but just an average pressure for most engines would do.
>
>Thanks
>=rod=

We used to routinely check firing pressure on locomotive Diesels. At
ful power and speed, they usually checked around 31-3200 psi.. Of
couse, the BMEP was down in the 260 psi. That is the brake mean
effective pressure which is essentially the average pressure over the
stroke. From that, you can surmise that maximum pressures are
developed very early in the power stroke and diminish very quickly.
Most of the fuel is burned within about the first 15 degrees after
ignition. Can't say these numbers are correct for every engine as
every engine has different characteristics. These engines may have
been different if they had been tested under stall conditions.

[ChrisCWa]

Naturally aspirated production gasoline engines rarely exceed 1200 psi peak
pressures. There really is no set value since peak pressure is a strong
function
of spark timing and volumetric efficiency.

I you look at peak pressure versus rpm, peak cylinder pressure (in the absence
of knock of course) reaches a maximum at the speed where volumetric efficiency
is maximized if the engine is running with optimized spark timing. In the
engine
development community optimum spark timing is usually refered to as MBT,
Minimum spark timing for Best Torque.

However, the above rule of thumb can be significanly changed depending upon
the spark timing settings. As spark timing is advanced peak pressure rises
(nearly linearly with spark) and as spark is retarded, peak pressure declines.
( As a side
note here, because of the strong dependency of peak pressure on spark one
should be very careful about over-advancing spark. Bearing loads and piston
side loads increase as peak pressure increases).

Two valve per cylinder engines would be somewhat lower, probably closer
to 1000 psi.