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Power to weight ratio
Engine efficiency is basically judged by how much mechanical
energy is generated per gallon of fuel. In aviation, weight is
also a significant factor. An aviation engine should put out as
much power as possible from the lightest weight in components.
Aside from engine management systems there are two basic ways to
increase the power per cubic inch. Increase the compression
ratio or increase the intake air pressure (also known as
manifold pressure).
Since engine management systems for aircraft engines are
still pretty much non-existent we will concentrate on making do
with the existing fixed magneto ignition and either carbureted
or fuel injection systems that are the norm today. These archaic
systems if anything present a worst case scenario under which
changes in other operating parameters such as combustion chamber
temperatures can be shown to have a significant impact on the
ability to generate more power per cubic inch.
The high combustion chamber temperatures of air-cooled
engines present a major problem in attempting to generate more
power per cubic inch by either increasing the manifold pressure
or raising the compression ratio. Why? Because higher combustion
chamber temperatures substantially increase the chances of
detonation occurring.
Detonation - The explosion inside your
cylinder
Many volumes have been written about detonation, its causes
and what can be done to minimize it. Most people would simply
like to know what it is. In short it is an explosion which is
defined in the dictionary as: A release of chemical energy in a sudden and often violent manner with the generation
of high temperature and usually with the release of gases.
Basically, detonation is the instantaneous burning of the
air-fuel mixture within the cylinder. Such an event releases a
tremendous amount of heat in a very short period of time often
before the piston has reached top dead center. The result is
enormous stresses pushing back against the piston head,
combustion chamber, rod bearings, etc. In a large bore aircraft
engine like the ones we currently fly, the result (at least in
the dyno chamber) sounds like taking a 10 pound sledge hammer
and smacking it against the concrete floor. Aside from the
distinct noise the engine itself rocks violently.
The fundamental requirement for detonation is heat - too much
heat. Normally the burning of the air-fuel mixture takes anywhere
from 10 to 20 milliseconds. This burning is a smooth even event
that results in a flame front that moves through the fuel-air
charge and consumes all of the available oxygen and fuel to produce
an overall heating of the combustion gases. This smooth burn time
is what contributes to the setting of the ignition timing on an
engine. In the vast majority of cases the fuel mixture is ignited
several degrees before the piston reaches top dead center. This
allows time for the combustion charge to burn and to produce the
heat and thermal expansion that will ultimately drive the piston
back down providing mechanical force.
During the compression cycle the fuel-air charge heats up as
a product of just being compressed into a smaller volume. In
addition the fuel-air charge is also soaking up leftover heat
from the cylinder walls, head, valves and spark plugs. The
temperature of the fuel-air mixture can reach a point where self
ignition is possible producing an uncontrolled burning of the
fuel-air charge prior to the the desired point of ignition. This
pre-ignition may be a relatively controlled burn or if the
conditions are right may be an outright explosion. This
variation has often lead to heated debates regarding whether or
not pre-ignition is detonation. The answer is that sometimes if
the temperature of the fuel-air mixture is high enough pre-ignition can lead to the explosive burning the fuel-air
mixture however in many cases the fuel-air mixture starts to
burn just prior to the firing of the spark plug. This usually
produces two or more flame fronts and it is the collision of
these flame fronts that produces a characteristic ping sound.
Pinging while not as destructive as detonation still robs the
engine of power and produces abnormally high combustion
pressures which stresses the piston and rod bearings. The second possibility is that the
fuel-air mixture may reach its critical temperature at about the
time the spark plug is fired and the result is almost always an
explosive detonation. Regardless of how it happens, anytime the
fuel-air mixture burns explosively (as in detonation) or prior
to the timed spark ignition the result is less power and more
mechanical and thermal stress on the engine.
Reducing or eliminating detonation
There are several methods that can be used to reduce or
eliminate detonation and virtually all of them reduce the
overall power output of the engine and are in some way
tied to lowering the compressed temperature of the fuel-air
mixture.
- Super rich fuel mixture
- Lower induction air temperatures
- Lower compression ratio
- Later ignition timing
- Higher octane fuel
- Lower the combustion chamber temperatures
One method that is widely used on our air-cooled engines is
to run a super rich fuel mixture. As we discussed earlier a
fluid has much more thermal capacity than a gas so by running
a super rich mixture we are using fuel to cool the compressed
fuel-air mixture. The excess fuel in liquid form (as in small
droplets in the fuel-air charge) readily absorbs a lot of the
heat from the compressed mixture and also helps to remove waste
heat from the cylinder walls and head. This keeps the temperature
below the point of detonation by effectively throwing fuel out
of the exhaust. In this day and age of $4.50+ per gallon of fuel
this is incredibly wasteful and contributes to significantly
higher operating costs.
Lowering induction temperatures is only a viable option for
turbocharged engines. An intercooler is used to cool off the
compressed air from the turbocharger prior to it entering the
internal engine manifold. Lowering the compression ratio of the
engine is yet another method but it results in the exact
opposite of what is needed to produce more power per cubic inch
and so the power to weight ratio of the engine is reduced. The
same is true in setting a later ignition timing.
Finally there is good old high octane fuel. This is the
almost magic solution that allows the use of higher compression
ratios without sacrificing ignition timing or running super rich
mixtures. The problem is that higher octane fuels can only do
so much and the higher the octane rating the more expensive the
fuel is
to produce. A compound known as tetra-ethyl lead is used to
boost the octane rating of aviation fuel. Unfortunately it is
the lead component that is bad for the environment and in many
ways also bad for our engines. It is very difficult to formulate
100+ octane fuel using non-lead compounds. Such high octane
unleaded fuel does exist. It can usually be found at automotive
race tracks at a price of around $8.00 a gallon!
Aviation fuel is the last fuel being produced that contains
tetra-ethyl lead compounds and the writing is on the wall that
someday all aviation fuels will have to use non-lead containing
compounds in the formulation. Reliance on 100+ octane fuels to
reduce or eliminate detonation is going to be a very
expensive option in the future.
Lowering the combustion chamber temperature
One other method to reduce detonation is to significantly
lower the combustion chamber temperatures. As mentioned
previously - air-cooled engines typically run with cylinder head
temperatures of 350-450F. As the air-fuel mixture is compressed
it is forced against the piston, cylinder walls and head which causes it
to soak up the excess heat in the surrounding metal. This excess
heat exacerbates the heat buildup in the compressed air-fuel
mixture which is already heating itself due to the compression
forces alone.
The only viable method to lower the combustion chamber
temperatures is to convert to water cooling. A water cooled
cylinder head typically runs around 200F - virtually half the
temperature of the air-cooled counterpart. In addition - the
thermal gradient is substantially reduced so that internal temperatures are
lowered to around 350F vs. 700-800F in an air cooled cylinder head. That amount
of reduction in temperature is equivalent to increasing the fuel octane 10
or more points, say from 100 to 110! Or, using the same octane fuel it is theoretically
possible to safely increase the compression ratio by 2 to 3 full points and
still not have any detonation problems. Or similarly the octane requirement
of the engine can be reduced to around 90 using the same compression ratio
Of course water cooling doesn't come without some penalty -
mainly in terms of weight gain. However, the weight gain is
minimal with respect to the additional power that is now
possible. Most air-cooled aircraft engines run compression
ratios that around 8.5:1 or less. In fact many are as low as
7.0:1 and the average tends to hover around 8.0:1. These are very low
compression ratios when compared to today's automotive engines
many of which now routinely run 10.0:1 or higher.
Higher compression ratios are one of the easiest ways to
achieve more power per cubic inch and thus better power to
weight ratio. Water cooling accomplishes two things in this
quest for more power. It keeps the combustion chamber
temperatures low, allowing the use of lower octane fuels and
higher compression ratios to achieve more power per cubic inch and
keeps the exhaust valve and cylinder heads from being subjected
to excessive temperatures thus increasing component life and
overall engine reliability.
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