mounting of 3-cylinder engine

[okeng]

I'm working on an engine installation with a small 1.4L 3-cylinder diesel.

Customer's previous experience with another brand of 3-cylinder was pretty bad--lots of vibration problems.

I'm working with the engine "manufacturer" (actually importer) and one of the isolation-mount companies (Barry), but would like to hear from some engineers who have plowed this field already.

Thanks,

Jess

 

[MikeHalloran]

Make the mounts real soft, with a lot of stroke, and let it wobble around however it wants. You'll need at least two flex joints in the exhaust pipe, or if it's in a boat, a 'wet' exhaust and some rubber elbows.





Mike Halloran
Pembroke Pines, FL, USA

 

[okeng]

Mike,
I keep seeing references to "focused" mounts, but no useful information regarding what that is. I believe they mean that the mounts are angled inward (like automotive mounts) rather than pointing vertical (like most industrial engines). I ran across something that said they should aim at the CG of the engine or at the engine's roll axis. Not sure how to find the roll axis, either.

Yes, the exhaust system is going to be tough. It's a mercifully simple system on the existing machine--just a 90 degree elbow with a light muffler pointing down. I figure we will have to at least add a mount to tie the end of the muffler to the engine so it all shakes together. I really hate to use flex joints.

Any guidance on that?

Thanks,

Jess

 

[MikeHalloran]

I think the word 'focused' here might indeed mean mounts with their compression axis pointed at the engine's roll axis. It assumes that the mounts are soft in shear, so that rotary oscillations are not strongly damped. If they were, the mounts' reaction forces would induce structureborne noise, which I guess is what you are trying to suppress.

Even a small muffler may not like being rigidly mounted to a vibration source. At least be sure the mount doesn't restrict the muffler's growth from thermal expansion.







Mike Halloran
Pembroke Pines, FL, USA

 

[okeng]

Good point on the thermal expansion--easy to get carried away making things stiff and rigid.

Any idea how to find the roll axis of the engine? The mfg info has tons of stuff on the cg, and has a nice general picture showing the roll axis to be typically tilted down at the flywheel end. It shows it passing through the CG of the engine and implies that it also passes through the CG of the transmission (or pump stack in my case). Any idea of whether that's correct? I can't quite visualize why that would be true.

Thanks,

Jess

 

[GregLocock]

That roll axis sounds about right. I'm not sure I really agree witht he idea behind focussed mounts in practice.

Can I ask, what is the application?

Is vibration an issue?

do you have a reduction gear in the gearbox? reverse? shock loads?

Mounting three cylinder engines is non trivial due to the rocking couple.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[okeng]

The application is in a small self propelled sweeper.

Vibration is an issue because of the problems with the previous 3-cylinder installation--they have had lots of vibration-related failures of bolted joints, exhaust systems, etc. It's not important to have a luxury-car quality of smoothness--we're talking riding-mower levels of comfort being acceptable.

The engine has a tandem stack of hydraulic pumps bolted onto the back, driven by a Hayes coupler. No reversal, torque loads, shock loads.

We are fighting a perception thing after the problems with the other engine, so I want this installation to be as good as is practical.

I'm not really a fan of 3-cylinder engines unless they are in old british motorcycles. Unfortunately, with industrial and construction equipment, it's harder to pass off vibration, leakage, and parts-shedding as "character".

Jess

 

[GregLocock]

2 strategies come to mind

1), get a screw driver, run the engine through its rev range and try and find the quietest position along the engine. Put a pair of engine mounts with their soft axis at 45 degrees in line with that. That'll take most of the static weight, and react torque.

The other mount has to react the rest of the weight while not transmitting the rocking couple. That is tricky.

2) put the pair of mounts under the cg. now the third mount has only to restrain the beast, not take any static load.


I prefer option 2.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

Because of the direct mount pump stack, the mounts don't have to transmit the engine's reaction torque, just its weight and imbalance loads. Greg's idea of mounting it at a node, is excellent, if you can find one.

Do you have flexible connections in the hydraulics? E.g. helices of metal tubing, or at least a one- turn helix of hydraulic hose.



Mike Halloran
Pembroke Pines, FL, USA

 

[Warpspeed]

Interesting problem.

As the whole thing will be completely isolated from any external drive torque reaction, the engine mounts can be made soft. Just allow the whole thing to rock and wobble around as it wants to.

The usual simple solid "rubber block" engine mounts may not be a good choice because while being soft in shear, they will be stiff in compression.

Instead of using hollow rubber mounts, how about supporting the engine on coil springs? (or even air bags)? These springs could be long and soft to carry the static weight, while having a very low natural bounce frequency, and allow engine movement in all planes.

That may be important if the engine is relatively heavy compared to the mass of the supporting frame. I suspect the only way to reduce vibration transmission will be through very soft compliant mountings.

There would then only need to be some hard rubber bump stops provided to limit any extreme motion, but these would always have some clearance during normal operation.

Either tension or compression springs would do the job, perhaps three or four identical springs arranged equally around the CG. Air bags may be even more effective.

 

[hallb]

Okeng,

We have designed focused engine mounts by pointing our Barry mounts (Lord is another good source for engine mount information) axially toward the center of gravity of the engine. Most general aviation airplanes point the mounts towards the engine's center of gravity as well. Both Continental and Lycoming state that this is the desired configuration in their respective owner's manual.


Good Luck.