NOx production in the combustion chamber is dependant on flame temperature and
combustion pressure.
Increasing the compression ratio will have little effect
on the flame temperature but it will ncrease the combustion pressure which
will increase NOx.
I have no experience with reciprocating engines but in a
gas turbine engine the NOx increases by the square root of the pressure.
Assuming that this is the case in a recip, then going from ,say 9 :1
compression ratio ro 10 :1 will increase the Nox by 5.5%
The EGR valve system
is calibrated to admit sufficient exhaust gas into the combustion chamber to
reduce the NOx to whatever the regulations require. This calibration is done
with the standard compression ratio for the engine. If you increase the
compression ratio the calibration will not be correct.
despite the above I
think that since the California smog test is done at very low power settings
I would be surprised if increasing the compression ratio had any significant
effect on NOx readings.
However when you raise the compression ratio you
will get detonation unless you retard the ignition, because the best fuel you
can get in Ca is 91 Octane, and this will reduce power. So don't think that
you can get the engine back to MkIII power levels.
I suggest if you want more
power sell the Spit and buy a TR !
Stuart A Greenwood
71 MK I Stag 71 MK
IV Spitfire
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To:
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Sent: Monday, August 26, 2013 11:00 AM
Subject:
Spitfires Digest, Vol 7, Issue 48
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Today's Topics:
1. High compression pistons and smog test (Hans)
----------------------------------------------------------------------
Message: 1
Date: Sun, 25 Aug 2013 17:14:21 -0700
From: Hans
<sukaspit@gmail.com>
To: spitfires@autox.team.net
Subject: [Spits] High
compression pistons and smog test
Message-ID:
<356B6E8E-7020-4D5B-A0DF-9188CA7C22E8@gmail.com>
charset=us-ascii
I am rebuilding my California 1976 spitfire's engine and
considering using
Moss's HC flat top pistons instead of the LC (dished) ones.
I have been told
that I risk failing the NOx smog test max requirement. I
understand that this
is due to the combustion chamber's temperature
potentially exceeding 2500
degrees , which generates NOx. Assuming the EGR to
function perfectly and the
fuel mixture to a bit rich, is this still
potentially a high risk problem?
There must be a few a guys out there that
have been tempted to do this to get
the nearly 20 extra hp. That is so badly
needed.
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