THEORY: Reciprocating Engine Vibration
Piston Engine Vibration
An internal combustion engine produces power using the extremely rapid pressure pulse of burning air fuel mixture above the piston. These powerful pulses of energy cause the engine to vibrate in response. Engine designers do their best to make these forces cancel out to minimize vibrations. But, no matter how well the designer does his job, he cannot eliminate all inherent vibrations in an engine. Therefore we need to remember that it is perfectly normal for an IC (Internal combustion) engine to produce a characteristic vibration spectrum signature. Vibration analysis of IC engines then must focus on "variations" from the "normal" vibration signature.
Normal Vibration Signature
Each combustion pulse acts much like a hammer blow, hitting the engine block with a pulse of energy. The vibration spectrum of such a pulse is a series of vibration spectral lines. These spectral lines will be at integer multiples of the firing rate of each piston. In a four stroke engine the piston fires every other revolution, therefore the fundamental spectral line will be at 1/2 the engine RPM, often called the 1/2 order vibration. The result will be a vibration signature that has spectral lines at the 1/2 order, 1P, 1-1/2P, 2P, 2-1/2P, 3P ... etc. In our experience, we have found that most 4 and 6 cylinder horizontally opposed IC engine produce these spectral lines in varying patterns but the levels are usually in the range of .2-.5 IN/S. For unknown reasons we have found it is not uncommon for an engine to produce a 2P or 2-1/2P vibration in the range of .5-1.0 IN/S. These readings are taken on the ground at typically 2500 RPM.
1/2 Order Vibrations
It turns out that if all of the pistons produce nearly identical combustion pulses, the 1/2 order vibration will be very small, .1-.3 IN/S. When any one cylinder produces less power than the rest, the 1/2 order vibration will increase from .3 IN/S on up to over 1 IN/S for a misfire. Mechanics know very well what can cause one cylinder to be weak. Plug misfire, bad plug, plugged injector, broken ring, leaky valve, low compression, bad magneto, worn cam, collapsed lifter, etc. Any of these things can cause a higher than normal 1/2 order vibration.
1/2 order vibration are especially troublesome as they can be felt in the cabin by the pilot. Low frequency vibrations are not well isolated by most engine mounts, and the vibration will shake the entire aircraft. If not taken care of, a 1/2 order vibration can loosen rivets, hinges, and pivots all over the airframe, as well as causing premature pilot fatigue. This is a serious safety issue.
1P vibrations are usually dominated by propeller imbalance. Happily we can correct this with propeller dynamic balance. 1P vibrations can also be caused by unequal piston mass. Pistons and cylinders are often replaced 1 jug at a time, and once in a great while the wrong weight piston is used due to error. In horizontally opposed engines, this will produce a 1P vibration in the horizontal plane, but little vibration in the vertical plane. An out of balance prop will produce nearly equal vertical and horizontal vibrations. If the rear of the engine has a high 1P vibration that is not corrected by balancing the prop, piston mass imbalance is the most likely cause.
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