Theory_ Reciprocating Engine Vibration Analysis

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  • 8/22/2019 Theory_ Reciprocating Engine Vibration Analysis


    THEORY: Reciprocating Engine Vibration

    B yL l o y d Jo h n s o n

    Piston Engine Vibration

    An internal combustion engine produces power in the form of controlled explosions. These explosionsproduce powerful pulses of energy that cause the engine to vibrate in response. Engine designers dotheir best to make these forces cancel out to minimize vibrations. But, no matter how well the designerdoes his job, he cannot eliminate all inherent vibrations in an engine. Therefore we need to rememberthat it is perfectly normal for an IC (Internal combustion) engine to produce a characteristic vibrationspectrum 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 otherrevolution, therefore the fundamental spectral line will be at 1/2 the engine RPM, often called the 1/2order 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 horizontallyopposed IC engine produce these spectral lines in varying patterns but the levels are usually in therange of .2-.5 IN/S. For unknown reasons we have found it is not uncommon for an engine to produce a2P or 2-1/2P vibration in the range of .5-1.0 IN/S. These readings are taken on the ground at typically2500 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 ordervibration will increase from .3 IN/S on up to over 1 IN/S for a misfire. Mechanics know very well whatcan 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 higherthan normal 1/2 order vibration.

    1/2 order vibration are especially troublesome as they can be felt in the cabin by the pilot. Lowfrequency vibrations are not well isolated by most engine mounts, and the vibration will shake the entireaircraft. If not taken care of, a 1/2 order vibration can loosen rivets, hinges, and pivots all over theairframe, as well as causing premature pilot fatigue. This is a serious safety issue.

    1P Vibrations

    1P vibrations are usually dominated by propeller imbalance. Happily we can correct this with propellerdynamic balance. 1P vibrations can also be caused by unequal piston mass. Pistons and cylinders areoften 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 littlevibration in the vertical plane. An out of balance prop will produce nearly equal vertical and horizontalvibrations. 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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    Theory: Reciprocating Engine Vibration Analysis (1 de 2) [21/11/2010 04:26:10 p.m.]
  • 8/22/2019 Theory_ Reciprocating Engine Vibration Analysis


    Theory: Reciprocating Engine Vibration Analysis (2 de 2) [21/11/2010 04:26:10 p.m.]

    Los mejores resultados parasignature vibration in


    Therefore we need to remember that it is perfectly normal for an IC (Internal

    combustion) engine to produce a characteristic vibration spectrum

    signature. Ir al texto


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