Date:         Fri, 8 Nov 2002 00:16:43 -0000
Reply-To:     Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject:      Re: BMEP question
Content-Type: text/plain; charset="iso-8859-1"

> Can anyone define it, is it different from engine to engine (I assume it
> is), and can an engine builder change this? What factors affect it?

BMEP is Brake Mean Effective Pressure

It is effectively an integration (summing) of pressure in the cylinder
against crank rotation for a whole engine cycle (2 rotations in the case of
a 4 stroke)

So,  the peak pressure on the piston might be in the 1000's of psi, but at
other times, it might be negative (exhaust slug disappearing down a
tuned exhaust). Using a mechanical 'indicator' diagram (these days
electronic transducers), a plot of the actual variations can be had and then
by various means (planimeter, these days digital curve analysis) the mean
value established.

What does this mean (sic)?
It is a measure of the TORQUE, as pressure on the piston acting against the
leverage of the crank (varying sinusoidally) is exactly this.

However, note that it can be derived by reverse engineering! Knowing the
torque (easily measured), the mean cyclic pressure in the cylinder to
achieve this can be calculated. And there are no approximations of note in
this back calculation - its exact.

Can it be changed?
Yes, by improving cylinder filling, changing timing, exhaust/inlet mods etc
etc - anything that changes torque.

What does it relate to exactly?
Well, torque and by direct inference SFC (specific fuel consumption),
nominally (hey pensioner, I know!, I know!) the inverse of bmep.

OK!

Clive
'88 Syncro Transporter
----- Original Message -----
From: "Chris Mills" <scmills@TNTECH.EDU>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Thursday, November 07, 2002 10:57 PM
Subject: BMEP question


> I think I've got the term correct.
>
> Can anyone define it, is it different from engine to engine (I assume it
> is), and can an engine builder change this? What factors affect it?
>
> What is it's consequences? Differences in power, efficiency, durability,
etc?
>
> Thanks.
> Chris Mills in TN
>