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The operation of an internal combustion engine relies on the rapid pressure pulses generated by the combustion of the air-fuel mixture above the piston. These powerful pulses cause the engine to vibrate. While engine designers strive to minimize these vibrations by balancing forces, it’s impossible to eliminate all inherent vibrations. Hence, it’s normal for an internal combustion engine to exhibit a characteristic vibration spectrum signature. Therefore, the analysis of engine vibrations should focus on deviations from this “normal” signature.
The rotational reference speed of an engine is the crankshaft or also called â1X orderâ. This corresponds to the cycles that the crankshaft rotates per minute (RPM) or per second (Hz). This frequency is used to know the guide us on the degree of use of the engine. But this does not necessarily mean that the main vibration is at this rotational frequency. As indicated on the introduction, Vibration is originated by eccentricity generated by the powerful pulses on the pistons.Â
Since the crankshaft is a well-balanced element within the engine, the order 1X should not be very evident. The pulsations of the pistons, on the other hand, should be very visible. Depending on the number of cylinders we will be able to see â2Xâ â3Xâ or â4Xâ. For example, on a 4 cyl engine every rotation of the crankshaft, there are 2 movements of pistons. See the below video.
Each combustion pulse resembles a hammer blow, impacting the engine block with a burst of energy. This results in a vibration spectrum characterized by spectral lines at integer multiples of the firing rate of each piston. In a four-stroke engine, where the piston fires every other revolution, the fundamental spectral line occurs predominantly on the order that follows this pattern:
Since the crankshaft is a well-balanced element within the engine, the order 1X should not be very evident. The pulsations of the pistons, on the other hand, should be very visible. Depending on the number of cylinders we will be able to see â2Xâ â3Xâ or â4Xâ. For example, on a 4 cyl engine every rotation of the crankshaft, there are 2 movements of pistons. See the below video.
2cyl= predominantly by 1,5th order, followed by 3, 4.5, 6 order
3cyl= predominantly by 1,5th order, followed by 3, 4.5, 6 order
4cyl= predominantly by 2nd order, followed by 4, 6, 8 order.
6cyl= predominantly by 3rd order, followed by 6,9,12 order
RPMS OF THE ENGINE= 851 RPM
On this example we can see first order (1x) at 851 RPM (14,19×60 ) but the order 1,5x is the most important one. (14,19×1,5)
RPMS OF THE ENGINE= 856 RPM
On this example we can see 1,5x very visible but not the first order (1x). The way to know the crankshaft speed is by dividing the main peak by 1,5 and multiply it by 60. (21,40/1,)x60=856
RPMS OF THE ENGINE= 777 RPM
On this example we can see 2x very visible but not the first order (1x) neither the 1,5x . The way to know the crankshaft speed is by dividing the main peak by 2 and multiply it by 60. (25,91/2)x60=777
When all pistons produce nearly identical combustion pulses, the 1/2 order vibration remains small, ranging from 2,5-8mm/sec (0.1-.3 In/sec). However, if one cylinder produces less power than the others, the 1/2 order vibration increases from 7mm/sec (0.3 In/sec) upwards to over 25mm/sec ( 1 In/sec) in the case of a misfire. Various issues such as plug misfire, bad plug, plugged injector, broken ring, leaky valve, low compression, bad magneto, worn cam, or collapsed lifter can cause higher than normal 1/2 order vibrations.
1/2 order vibrations present significant challenges as they can be felt by the pilot in the cabin. Low-frequency vibrations are poorly isolated by most engine mounts, causing the entire system to shake. If left unattended, a 1/2 order vibration can loosen rivets, hinges, and pivots throughout the sailboat frame, in addition to causing premature fatigue, posing a serious safety concern.
1P vibrations are often attributed to propeller imbalance, which can be corrected by replacing or cleaning the propeller. They can also result from unequal piston mass, sometimes caused by the inadvertent use of the wrong weight piston during replacement. Â
X/P vibrations are often attributed to propshaft imbalance. If the Reduction factor of the transmission is 2,64, the peak would be observed at the RPMs of the crankshaft divided by 2,64 like the video below.
Misalignment is often the root cause of this vibration order. This can be corrected by rechecking the alignment.
A shaft bent could also be the case. In such case, extracting the shaft and checking it with a lathe would be the most appropriate approach.
Ellebogen has made an app for Android and iOS that allows to measure vibrations using the mobile phone as it can be seen on the below video.
The app that helps you to find the correct engine mount
