Engine vibrations at idle

FlyWright said:

Don't expect much difference at idle, Mark. They are twin 4-cyl Perkins. It's what they are supposed to do at idle.

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FlyWright and I have "brotherly" teasing concerning our boats' relative levels of boat vibration between our two boats. My 4-cyinder JD transmits much less vibration. Is it due to steel versus fiberglass hull? Different engine mounts? Or different engine makes?

JD has balance shafts right?

Greetings,
MR. mp. " Is it due to steel versus fiberglass hull? Different engine mounts? Or different engine makes?" Could be all or none of the above. It could also be sympathetic vibrations specific to one's individual vessel. I would be very surprised if two identical boats (factory built side by side at the same time) "vibrated" exactly the same. Slightly thicker resin in one bulkhead tab, three instead of four fasteners holding a locker together etc. A plethora of causes.

Brooksie said:

JD has balance shafts right?

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Don't all engine makers strive to balance the shaft?

markpierce said:

Don't all engine makers strive to balance the shaft?

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Different animal. Engine design often has a counterbalance installed on the main crank, on the flywheel or possibly on the cam shaft. (Most often on the main crank).

These are NOT all the same counterbalance. Often when rebuilding the 'guy' grabs the wrong one (stb engine, versus port) or other model. I have had 3 Detroit Generators shake themselves to pieces in short order.

Although I have not found the specific RPM on my Perkins 4.236, There does seem to be a certain RPM that shakes the stainless galley stove and the toaster oven. Luckily, when I am up on the flybridge the vibration goes away!

On the other engines I run (EMDs), they definitely are bad vibrating between 700 and 750 rpm. It is a well known phenomenon that the operator must avoid that RPM window if at all possible.

Are you referring to vibration at idle in neutral or clutched in gear? Same in reverse as forward?

The balance shafts are seperate rotating shafts that offset some of the vibration in our 4 cylinder JD engines.

Ted

O C Diver said:

The balance shafts are seperate rotating shafts that offset some of the vibration in our 4 cylinder JD engines.

Ted

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That's what I am talking about, even my old '62 Greymarine has them, so does JD, but not Perkins.

Balance shafts ... news to me ... thanks for the education!


So ... which popular marine engines do or do not have balancing shafts other than JD (does) and Perkins (doesn't)?

markpierce said:

Balance shafts ... news to me ... thanks for the education!


So ... which popular marine engines do or do not have balancing shafts other than JD (does) and Perkins (doesn't)?

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Can't say really, like listing them. Except to say only 4 cyl and under would normally have them as 6's are naturally balanced. My 5 cyl MB diesel seems well balanced too. But in a 4, you have 2 pistons rising and two falling at all times so unless it has opposed cylinders, you have vibration of the first order.

4cyl 4stroke has a second order vibe. Balance shafts run 2x crank speed to cancel. This second order vibe is only evident up in the higher revs. Not the same as idle shake, balance shafts will do nothing there.

Idle shake is from the gap in compression strokes/firing pulses- slower engine turns, bigger each "bump" feels.

Usually only the larger displacement 4's have balance shafts, and only then if they run up there in rpm.

Idle shake can be managed with well selected resilient mounts. And even then, some boats are better than others. A gazzilion variables.

Ski in NC said:

Idle shake can be managed with well selected resilient mounts. And even then, some boats are better than others. A gazzilion variables.

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Exactly.

The amount of vibration is affected not only the quality and condition of the engine mounts, but how well they are adjusted.

Often, prop shaft alignments are done using just the engine mount adjustments, sometimes setting them at the top or bottom end of their range. This greatly reduces their effectiveness.
The weight distribution between mounts must also be kept even, especially side to side, as all mounts are designed to carry a certain range of weight.

With nor without a balance shaft a 4 cyl will never be smooth ay all RPM.

The mfg usually try to create the harmonics at a speed not usually operated at.

So a slower than book idle might be the cause of shaking, because that's where the engine mfg put it.

Talking about " harmonics" Ive seen different harmonic pulleys fitted to the same make of engine and in some cases the harmonics pulley had been machined to except a v belt to run a water pump this can also upset the idle balance

All very good and valid replies. And there may be other sources of vibration like pulse rate from the exhaust system. I'm assuming we are talking about idle in neutral.

Engine manufacturers make engines and buy mounts from various sources. Often the manufacturer doesn't know where the engine will end up. To narrow that down if it's a marine engine is it going in a planing hull or displacement? What's the gearing? Will the mounts take thrust or not? Point being if the engine is installed with factory supplied mounts they are most probably very conservative towards keeping the engine secure and usually at the expense of isolation.

Engine mount selection for optimum isolation involves taking many factors into account. Weights, CG, mount relation to CG, torque, thrust (or not) RPM range, plus a safety factor. Engine manufacturers don't usually do this. Then as mentioned the mounts have to be installed properly, loaded evenly and loaded square. Age also plays a part as creep will degrade isolation performance over time, and for natural rubber mounts 10 years is about the working lifespan for optimum isolation.

It's probably the mounts, but might not be.

Greetings,
Correction to post #3. Rather than sympathetic vibrations, I meant harmonic vibrations....brain fart.