More power per cubic inch

About the Benefits
  • Overview
  • Getting rid of the heat
  • Heat - the top end killer
  • More power per cubic inch
  • Better performance and fuel economy
  • Radiator technology - past and present
  • Shock cooling - Problem solved!
  • The secret to achieving longer TBOs
  • Flying faster on the same power - reducing cooling drag
  • Side benefits - Safer cabin heat
  • Dispelling the Myths

  • 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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    Proudly MADE in the U.S.A.