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Home> Maintaining your Continental  and Lycoming engine

One Limitation to Power Output on Piston Aircraft Engines

Increasing engine power eventually reaches a design imitation and something breaks. Some recent breaking includes:

For each problem the emphasis has been on finding the flaw in the product and fixing the flaw. Examples include: Continental and Lycoming changes to crankshaft metallurgy in an effort to prevent crankshaft cracking; crank gear attachment changes on Lycoming engines; changing to a snap-ring style impulse coupling; and changes to the Lycoming oil pump gears are all efforts to beef-up the weak point. 

When something breaks it breaks at a weak point. This weak point will always be a flaw because it is where the part broke - such is the circular logic of the metallurgist. Metallurgist make a living finding these weak points and we love them for it. However, they overstep their competence by blaming the weak point on the break. The weak point will always be there in any part,  broken or not.

Impulse coupling with failed rivet. High torsional vibrations (the kind you can't feel) vibrate the impulse coupling causing wear to the rivet.

A part does not break until energy is applied to the weak point. Energy in the form of stress and the nature of the weak point are partners in any failure. The solution to the Energy Problems lie not only in improving the metallurgy but also making sure that the energy input is within bounds. Since the emphasis has been solely on the metallurgical holy grail of finding the material flaw we have shifted responsibility for the failure to the foundry and ignored energy side of the problem.

All of the Energy Problems listed above have a common part that controls the energy level -- the engine counterweight system.

 Gas pressures from combustion causes crankshaft torsional flexing. The counterweight system is used to these absorb torsional vibrations in the crankshaft. If the counterweight system is not working, Stress increases dramatically. For this reason the counterweight system should not be left out of the failure analysis.

Close-up of Lycoming counterweight showing plates (the part with 3 holes in it) that hold the pins in place. Pin diameter determines the pendulum length and thus the frequency.

The factories use the term "detuning" to describe a counterweight system that has stopped working. This is an engineering term that means nothing to the pilot so I will attempt to explain the meaning. A counterweight is a pendulum similar to a child's swing. When operating properly the pendulum swings in an arc. If you push the child on the swing too high the swing falls and no longer follows a smooth arc - it has "detuned". Once the swing starts bouncing you must stop the swing and re-establish the smooth arc. Same thing happens to the counterweight system. Too much energy and it detunes. Once it detunes you must re-establish the smooth arc by drastically reducing the power,  just like the child's swing. Such as system has what engineers call "hysteresis". 

Energy input into the counterweight system isn't the same as engine power. Increasing the throttle or flow porting may increase engine power but the level of energy the counterweight must absorb is not directly related. Each counterweight is tuned to absorb a specific crankshaft vibration frequency by adjusting the length of the pendulum arm. For example, assume that a Continental IO-520 crankshaft has a 6th order resonant frequency at 2200 engine rpm.  Less than 2200 rpm or more than 2200 rpm and the 6th order counterweight(s) have little to do. But At 2200 rpm the crankshaft natural frequency is the same as the firing impulses of the engine and the crankshaft torsionally vibrates. It is the job of the 6th order counterweights to absorb this energy. If the pilot increased power by increasing engine rpm past 2200 rpm the energy to the engine is increased while the energy to the 6th order counterweight is reduced. This may explain some of the reason why only a very small percentage of engines have a failure as listed above - a very specific set of engine operating, wear, and repair conditions  must exist before the counterweights "detune"

(This is an illustration only and not an operational recommendation. it gets more complicated than this in real life and the exact numbers here are not correct)

Counterweights absorb torsional energy up till a point and then they stop. The point where they stop is called the "jump point" on the energy curve. Besides design, repair, and balance of the counterweight system, the two inputs that are critical are:

1. Energy input

2. Counterweight bushing wear. As the counterweight bushing wears the amount of energy it is able to absorb is reduced.

What this means to the airplane owner:

1. High time engines may be less able to absorb torsional energy (and more susceptible to the problems listed above) due to counterweight bushing wear. 

2. Engine modifications to increase engine power may exceed the counterweight's ability to absorb energy and lead to problems listed above. The counterweight pendulum returns to mid-position by virtue of the centrifugal field acting on the mass. There is a point where one cannot Increasing torsional vibration without increasing the counterweight mass or centrifugal force.

3. Combining 1 and 2 together i.e. high time engine that has had power increase modifications increases the Energy Problems listed above.

Whenever an Energy Problem is encountered the counterweight system should be inspected. Since, to my knowledge their hasn't been anything published on how to inspect the counterweights to determine if they have "jumped",  I suggest the following:

Counterweight hangar or Bifilar suspension

When the counterweights stop swinging they pound against the crankshaft counterweight hangars causing polishing and impingement marks. They also may damage the bushings, something the factories term "brinelling". In my experience impingement marks and hangar polishing are much more common than bushing damage. Indeed, I have seen very bad polishing and bruises the counterweights and hangars without any bushing damage. It is very rare for a pilot to feel a counterweight vibration. If you suspect counterweight detuning, inspect the main bearings. Increased torsional crankshaft twisting should increase main bearing loading. Anytime you are fixing a failure one must fix the damage and correct the problem.

Estimating Power Gain So your friend just installed a new tuned exhaust system and claims a 25% increase in engine power and you think this is a little much.  Given that the indicated horsepower of an engine is proportional to the weight of mixture induced into the engine per unit time for a constant fuel/air mixture, there should be a corresponding increase in fuel consumption at full throttle. For those with fuel flow transducers you should be able to accurately measure the increased horsepower. Cylinder head temperature method Power requires heat and more power produces more heat. Some of this excess heat finds its way into the cylinder head. To maintain a constant average cylinder head temperature, the mass flow of air must be increased approximately in direct proportion to the indicated horsepower. During climb at full throttle using the same airspeed, outside air temperature, air density, and approximate mixture ratio, cylinder head temperatures should be proportionally hotter with the increase in indicated power.  Because of the heat capacity of the cylinder materials, the CHT doesn't react instantly to changes in engine or cooling conditions. For comparison purposes the cylinder temperatures need to reach equilibrium conditions.

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