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
|