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Re: Combustion theory

To: William Eastman <william.eastman@medtronic.com>
Subject: Re: Combustion theory
From: edward <engconn@pottsville.infi.net>
Date: Sat, 02 Nov 1996 10:43:55 -0500
William Eastman wrote:
> 
> I have read a lot of questions on ignition system performance both here and
> on the MGA BBS.  Since I spent a goodly portion of the  sober moments of my
> college career studying internal combustion theory and since my present job
> does not provide an outlet for this pent up wisdom, allow me to expound.
> 
> Spark ignition is a lot simpler than the experts make it sound.  The job is
> to use electrical energy to bridge an air gap as a spark.  This spark
> ignites the fuel air mixture inside the cylinder and bang, we have
> propulsion.
> 
> The ignited fuel air mixture (FA)  heats up rapidly.  As a gas warms, it
> expands.  The expanded FA exerts pressure on the piston head which turns
> the crankshaft and causes our LBC's to move forward, scare neighborhood
> women and children, etc.
> 
> It would stand to reason, then that the most important thing you can do to
> increase the power of an IC engine is to increase the pressure acting on
> the piston top.  In fact, the standard measure of combusiton activity brake
> mean effective pressure (BMEP).  BMEP is the average piston top pressure
> necessary to develop the torque output as measured at the "brake" of the
> dynomometer.  So BMEP X Area of Piston X Number of Pistons X Stroke / 12 =
> torque (lb-ft).   Increasing any of these terms will increase torque
> output.
> 
> So how can you increase BMEP?  Here is a list of engine attributes and how
> they effect BMEP.
> 
> Compression ratio-  Higher CR = higher BMEP
> Cylinder Filling-  Higher efficeincy = higher BMEP
> Peak cylinder pressures-  Higher is better for BMEP
> Valve Overlap-  All things being equal, less overlap increases BMEP
> Intake air pressure- More is better
> Intake air temperature- Lower is better
> Cylinder Temperature- Higher is better
> Combustion speed- faster givers higher BMEP
> 
> So the answer is to build supercharged, high compression engines with no
> valve overlap.  Not quite.  The FA mixture has some unusual characteristics
> when it is ignited.  It's been fifteen years so I might have some of the
> technical terms wrong, but this is basically what happens after the spark
> plug does its thing.
> 
> There is a short period of time where nothing happens.  This is called the
> ignition delay period and is one of the main reasons why timing must be
> advanced when engine speed is increased.  Soon a flame front is established
> and this thin wall of flame moves outward from the spark plug throughout
> the cylinder.  The flame front tends to get increase in speed the longer
> that it travels.  For any FA mixture, there is a critical flame speed at
> which the process of detonation occurs.  At this speed, something happens
> and all the rest of the FA ignites at the same time. All of thes are
> effected by pressure.  In general, the higher the pressure, the faster
> things happen.  That is why you take timing away at higher power (lower
> vacuum) settings.  Also, as engine speed increases, the mechanical movement
> of the piston can reduce pressure fast enough to stop detonation.  So the
> worst possible situation for detonation is a high compression engine with a
> long flame path operated under high load at low speeds.
> 
> If getting high pressure quickly is good for power, then detonation must be
> really good, right?  Unfortunately, there are limitations to how much
> pressure is a "good thing".   The most obvious is the mechanical strength
> of the engine.  Mechanical device can survive only so much stress.
> However, very few engines die from mechanical overstress caused by
> detonation.  The most common side effect of detonation, a wholed piston, is
> caused by a differant physical property of gasses.  The ability of a gas to
> transfer heat to its container varies with the pressure of the gas.  The
> higher the pressure, the more heat that is rejected to the surrounding
> structure, such as the piston crown.  Gray exhaust is often seen coming
> from an engine expreiencing detonation.  That gray is from aluminum in the
> exhaust gasses that has been eroded from the top of the piston.  This is
> definately not a "good thing."
> 
> So, in theory, more pressure is better.  In practice, all engines are
> detonation or "knock" limited to how much power they can produce.  Things
> that reduce detonation tendancies include the use of higher octane
> (detonation resistant) gasoline.  Also, design of the cumbustion chamber is
> important.  If the longest distance from the spark plug to the the edge of
> the combustion chamber can be reduced, detonation will be controlled.  This
> is one reason why 4-valve heads with their centrally located spark plugs
> run higher compression ratios and make more power.  This is also why many
> racing head include a second spark plug across from the first- the thought
> is that the two flame fronts will meet at the middle.
> 
> A second detonation control technique is to keep the FA charge cool.  Cool
> intake air is important to detonation resistance.  Inside the combustion
> chamber, many engines are designed with "squish area."  This means that the
> combustion chamber farthest from the spark plug is squished down.  This
> higher surface area to volume ration increases heat transfer (but not to
> the dangerous  levels that occur during detonation) and cool the charge,
> reducing detonation tendencies.  The term "wedge head" refers to this
> combustion chamber shape.  Other methods of increasing heat transfer are
> also effective.  For instance, all things being equal, an aluminum head
> engine can run a higher compression ration than an iron head engine.
> 
> So, to close, ignition timing is important to BMEP which effects power.
> Detonation is very bad.  In general, LBC's have lousy combustion chamber
> design and, as such, don't develop a lot of power for their engine size.
> This is more because of cost considerations than poor engineering since
> most of this has been known since WWII.  Now that I think about it, the
> engine on my "A" was probably designed befor WWII, so maybe some ignorance
> was invloved.
> 
> I hope that I didn't bore anyone to death.  And to ease the minds of anyone
> who may feel responsible for triggering this self abuse, I can assure you
> that no textbooks were opened to prepare this oration.
> 
> Regards
> Bill Eastman
> 61 MGA
> 
> Timing of the spark is critical to performance an engine.
That was a truly excellent example of how the internet should properly
be used.  Thank you.

Geoff Love, The English Connection

Geoff Love, The English Connection


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