The hum of an engine

[Peregrine1983]

Hi all,

Since the people on this forum always astonish me with their smarts, I'm wondering if someone can explain something to me about engine noise.

I would think that the bang of fuel burning in each cylinder in a motor would lead to a "clapping/clattering" sound - which in some cases, it does - like when you hear an engine run straight off the exhaust manifold like a Harley or a dragster.

But in boats, reciprocating engine planes, and normal cars, the sound normally turns into a hum rather than a clatter.

I understand that a clattering sound played fast enough will yield a hum, but there's often even a hum from single cylinder engines at very low rpm.

Is this hum generated by resonance in the exhaust and the vibrations resonating through the object holding the engine?

[danstanford]

From a very high level perspective, engines are generally designed to have balancing motions of their mass to reduce wear and vibration. I would guess that this results in more and softer noises which could easily blend into a less clattery noise.

Look at a crankshaft design which has weights opposite the piston assembly so that when the mass of the piston, rod, and rings goes up the counterweight goes opposite.

[smith8273]

I would think what we hear is a combination of combustion and other noises. Valves opening and closing, water pumps, the whine of the belts and alternator and everything else.

[rslifkin]

The quieting effects of the intake and exhaust system design also tend to smooth out the pulses as well. But if you listen to an older mechanically injected diesel idle without much sound isolation between you and the engine itself, you can certainly hear each individual firing event (the noise radiates through the metal of the engine).

[Peregrine1983]

These are good points. I blanked on the fact that engines have many other moving parts inside that also make noises.

[Rapanui]

What you are listening to is the result of many billions of £, $ and many other currencies investment into the internal combustion engine for transportation over 100 years. Each succeding design making small improvements. Improved tolerances, vibration dampers, mounting systems, insulation, better combustion etc etc.

Even though makers of boat engines cannot afford that level of investment, the spin-off from the car and truck world has made boat engines much smoother, but even the best boat engines sound far more agricultural than my car turbo diesel. Part of the problem is that typically, boat structures are dynamically less stiff which limits the effectiveness of the mounting system, the engine mounts are pretty crude and rigid connections to a propeller shaft do not help. Your car design will go through a complex and expensive dynamic modelling design to reduce vibration and noise justified by amortising the cost over millions of cars sold, boat builders cannot afford that. Only the most sophisticated naval warships can justify the cost and time involved.

[hjohnson]

Originally Posted by rslifkin

The quieting effects of the intake and exhaust system design also tend to smooth out the pulses as well. But if you listen to an older mechanically injected diesel idle without much sound isolation between you and the engine itself, you can certainly hear each individual firing event (the noise radiates through the metal of the engine).

The clatter is more the injectors doing their thing. Your high pressure fuel pump pulses the pressure and when it hits the threshold, it overcomes the spring tension in the injector which opens momentarily. This is where most of the clatter comes from.

[rslifkin]

Originally Posted by hjohnson

The clatter is more the injectors doing their thing. Your high pressure fuel pump pulses the pressure and when it hits the threshold, it overcomes the spring tension in the injector which opens momentarily. This is where most of the clatter comes from.

Injectors make noise as well for sure. But on plenty of diesels you can hear the combustion clatter as well (and the resulting change in the sound when you increase engine load at the same RPM). In many cases it's hard to pinpoint exactly what portion of the mechanical noise is combustion, injectors, valvetrain, etc. beyond the somewhat more obvious intake and exhaust noises.

[donradcliffe]

Its all about the sound insulation. The better the insulation, the more it stops the high frequency noise.

The boat I just delivered was a recent Jeanneau with about 3-4 inches of insulation in the engine compartment. I slept in the quarterberth without earplugs.

This weeks delivery is a Santa Cruz 40 with only 1/4 of marine plywood between me an the motor. Earplugs are essential.

[doog]

Ive thought that from a physics and analysis viewpoint that a sound recording of an engine could concievably diagnose most any issue, if properly slowed down and each sound identified, like the tick of a watch can tell you if its fast or slow per day within seconds. Waiting for the App to come out!

[AKA-None]

Originally Posted by doog

Ive thought that from a physics and analysis viewpoint that a sound recording of an engine could concievably diagnose most any issue, if properly slowed down and each sound identified, like the tick of a watch can tell you if its fast or slow per day within seconds. Waiting for the App to come out!

Actually available for machinery for preventative maintenance

You can make one yourself for cheap with a smallish accelerometer and a wad to capture and analyze the output

Challenge is where do you get the vibrations for a “normal “ operation?