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.
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