3 cylinder diesel resonances, what's the best solution?

[Tony Brooks]

50 minutes ago, cuthound said:

In answer to the question posed in the thread title, surely the answer is to replace  the engine with one with an even number of cylinders... 😅😂

 

or a properly designed three pot with internal balance weighs. I noticed no excess vibrations with the Bukh DV36 at any speed

[IanD]

4 minutes ago, Tony Brooks said:

 

or a properly designed three pot with internal balance weighs. I noticed no excess vibrations with the Bukh DV36 at any speed

Unless it has a counter-rotating balance shaft (does the Bukh?), balance weights on the crank just convert a vertical rocking couple into a horizontal one...

[Tacet]

32 minutes ago, cuthound said:

In answer to the question posed in the thread title, surely the answer is to replace  the engine with one with an even number of cylinders... 😅😂

Maybe but the pedant says check the crankshaft arrangement.  There are three cylinder engines with 180 degree crankshafts that avoid the rocking couple.  And two cylinder engines with one up and one down pistons, that will be more unbalanced than a 120 degree triple.

 

If you want perfect primary balance with less than four cylinders find a V twin.  90 degree works with a common crankpin. Knife and fork conrods avoids the small rocking couple.  Two crankpins can give primary balance at less than 90 degrees.

 

A horizontally opposed twin will give perfect primary and secondary balance but with a narrow rocking couple.  Don't really know why, but the BMW motorcycle I have with this arrangement is not especially smooth.  I don't much like the bike anyway.

 

Good luck in finding a suitably arranged engine driving a suitable generator

 

 

 

 

[Tony Brooks]

8 minutes ago, IanD said:

Unless it has a counter-rotating balance shaft (does the Bukh?), balance weights on the crank just convert a vertical rocking couple into a horizontal one...

 

Yes, and they have to be timed, get that wrong and you are in all sorts of trouble.

[IanD]

6 minutes ago, Tony Brooks said:

 

Yes, and they have to be timed, get that wrong and you are in all sorts of trouble.

The Bukh must be in a fairly small minority if it has a balance shaft -- my point was that counterweights alone don't fix the problem... 😉

[cheesegas]

Update on this - the mount with the most weight on it eventually failed resulting in an awful racket, unsurprising as to get the engine level it was at the end of its deflection.

 

Called R&D and their shear mounts are low enough to fit between engine and bearers, but the deflection is very small, only 3mm allowed! They didn't offer any advice, just referred me to the spec sheets again and again.

 

Ordered some more AV Mounts, this time two 90kg (75kg fitted before) for the heavy side and two softer ones. These have marginally higher deflection at 4mm. 

edit: Another update - got a call back from a chap at R&D who was very very helpful and recommended I order four compression mounts of the same 86kg rating, he said not to mix mounts of different capacities. Going to order them and return the AV Mounts I think!

Edited March 28 by cheesegas

[Bee]

Might find placing a sandbag on top of it helps. Not entirely joking, these things can be really hard to solve.

[Peugeot 106]

On 28/03/2024 at 10:24, cheesegas said:

Update on this - the mount with the most weight on it eventually failed resulting in an awful racket, unsurprising as to get the engine level it was at the end of its deflection.

 

Called R&D and their shear mounts are low enough to fit between engine and bearers, but the deflection is very small, only 3mm allowed! They didn't offer any advice, just referred me to the spec sheets again and again.

 

Ordered some more AV Mounts, this time two 90kg (75kg fitted before) for the heavy side and two softer ones. These have marginally higher deflection at 4mm. 

edit: Another update - got a call back from a chap at R&D who was very very helpful and recommended I order four compression mounts of the same 86kg rating, he said not to mix mounts of different capacities. Going to order them and return the AV Mounts I think!

R&D recommended different ratings front and back for my 2 cylinder LPWS2 with PRM 120 gearbox. ( Ididn’t use them as my problem I discovered was down to the engine bed flexing) . R&D were very prescriptive and did not fill me with confidence that they actually knew what they were talking about. They just sent me curt emails and data sheets. I  got the impression that the £200 or so was not of interest to them and they showed little interest in discussing anything. I won’t be using them again unless I have to. It sounds like your experience is similar

[cheesegas]

52 minutes ago, Peugeot 106 said:

R&D recommended different ratings front and back for my 2 cylinder LPWS2 with PRM 120 gearbox. ( Ididn’t use them as my problem I discovered was down to the engine bed flexing) . R&D were very prescriptive and did not fill me with confidence that they actually knew what they were talking about. They just sent me curt emails and data sheets. I  got the impression that the £200 or so was not of interest to them and they showed little interest in discussing anything. I won’t be using them again unless I have to. It sounds like your experience is similar

With me it was hit and miss. The first guy I spoke to didn’t have the time of day for me and wouldn’t recommend anything, just sent me a link to their (crap) site and various data sheets. 
 

I then tried to call later, left a message and got a call back from a much more helpful man who chatted to me about everything engines and resonances for over half an hour, eventually recommending four of the same compression mounts. He said different to the first guy though and not to mix ratings of mounts which I think other people have also got the same mixed message!

 

The studs are captive though so it makes installation more challenging…I think I can slide the engine over each corner in turn though, the whole thing is only 170kg so with a bit of scaff as a lever it should be fine. 

[Peugeot 106]

Scaffold as a lever makes a lot of sense which I didn’t think of  although it will be easier with two people. When I was thinking of doing mine I looked at using some air wedges/bags that they use for doors etc to slide underneath the engine. There are some pretty heavy duty ones available that I eventually found if you are on your own

Just keep your fingers and toes out of the way. Good Luck

[cheesegas]

Another update...

 

Got the mounts fitted, as the studs are captive and very long I had to chop an inch off to get them in without having to lift the whole engine at once. Did it by lifting the back and then the front with a bit of scaff across the gunnels. Took a while to get it aligned!

 

Started the engine, idle set to 1000rpm warm which means it idles at 900rpm for a while until it warms up. Good points is that the resonance off idle has gone and it's much much quieter as the vibrations aren't being transferred to the hull! In gear or out of gear there's no resonance off idle now.

 

However...as the mounts are softer, the engine does vibrate a lot more throughout the rev range, whereas previously it would only noticeably move around the resonance RPM band. These vibrations are being isolated by the mounts, but the engine visibly moves a lot more. There's a new resonance almost at redline which think I can live with. Movement appears to be parallel with the crank so it's not bouncing up and down and it's not enough to rattle the propshaft around in the stern tube. I'm not sure of the effect this has on the lifespan of the Centaflex coupling and gearbox output shaft bearing though!

 

The vibrations were enough to shear two of the four M8 bolts (again!) holding the air filter assembly to the intake manifold within a few minutes, but this is a recurring problem I was having. It seems that the assembly, which is quite a heavy steel affair, has a resonance at 1000rpm, so at idle whilst the engine barely moves, it's waggling around like a mad thing.

 

The intake hose and filter bracket gets in the way of changing the alternator belt and the filter elements are expensive so I'm going to bin it and replace with a pleated cotton K&N style filter mounted directly on the intake I think.

 

Partial success!

[Bee]

I wonder if its possible to fit a couple of shock absorbers to it? I'm thinking maybe from a scrap washing machine or something?  Might be possible to take one each side from a handy nut on the engine to a bracket on the side of the boat, Of course there will not be any handy brackets or bolts 'cos that's the way it goes but it might just work.

[Alan de Enfield]

Many years ago I was working with Peugeot at Ryton developing new products for them. It was for the '405' so you can tell how long ago it was.

 

As the cars went into production and the 1st ones were sold, it was found that there was a big problem with engine vibration (it was really thrashing about) and a quick fix was needed.

 

The Lead Engineer ( a guy by the name of Fergie Raye) was struggling to come up with a rectification that did not involve re-designing the engine, and then it came to him - what stops an out of balance washing machine drum from making the washing machine 'walk about'.

Answer - a big concrete weight !

 

Long story short, the affected engine model had a concrete weight bolted onto it at the 'right spot', a few cars recalled for a retrofit and all new vehicles going down the line were fitted with the weight. 

Sorted !

[cheesegas]

17 minutes ago, Bee said:

I wonder if its possible to fit a couple of shock absorbers to it? I'm thinking maybe from a scrap washing machine or something?  Might be possible to take one each side from a handy nut on the engine to a bracket on the side of the boat, Of course there will not be any handy brackets or bolts 'cos that's the way it goes but it might just work.

It does seem to be a high frequency, low displacement vibration so this might work, but as you say, there's no suitable strong mounting points nearby without welding something on!

 

3 minutes ago, Alan de Enfield said:

Long story short, the affected engine model had a concrete weight bolted onto it at the 'right spot', a few cars recalled for a retrofit and all new vehicles going down the line were fitted with the weight. 

Sorted !

The chap at R&D actually suggested a weight above the mount that's most lightly loaded - I've tried this with both my own 80kg mass and varying cast iron ballast weights on top of the gearbox but movement is in the wrong axes. Appears to be the reaction force spinning the block in the other direction to the crank each time a cylinder fires, so the weight would need to be high up to have an effect... It's got a fairly hefty cast alu camshaft cover but I don't think its bolts would hold up to a weight on top! 

[Peugeot 106]

5 minutes ago, Alan de Enfield said:

Many years ago I was working with Peugeot at Ryton developing new products for them. It was for the '405' so you can tell how long ago it was.

 

As the cars went into production and the 1st ones were sold, it was found that there was a big problem with engine vibration (it was really thrashing about) and a quick fix was needed.

 

The Lead Engineer ( a guy by the name of Fergie Raye) was struggling to come up with a rectification that did not involve re-designing the engine, and then it came to him - what stops an out of balance washing machine drum from making the washing machine 'walk about'.

Answer - a big concrete weight !

 

Long story short, the affected engine model had a concrete weight bolted onto it at the 'right spot', a few cars recalled for a retrofit and all new vehicles going down the line were fitted with the weight. 

Sorted !

And to think my distant past at about the same time I  worked with Peugeot to manufacture the car boot from carbo fibre to reduce weight. They were really stuck on “every gm weight saves x amount of petrol”.  Maybe it was to make up for the concrete you added! 

[IanD]

On 07/04/2024 at 09:33, cheesegas said:

It does seem to be a high frequency, low displacement vibration so this might work, but as you say, there's no suitable strong mounting points nearby without welding something on!

 

The chap at R&D actually suggested a weight above the mount that's most lightly loaded - I've tried this with both my own 80kg mass and varying cast iron ballast weights on top of the gearbox but movement is in the wrong axes. Appears to be the reaction force spinning the block in the other direction to the crank each time a cylinder fires, so the weight would need to be high up to have an effect... It's got a fairly hefty cast alu camshaft cover but I don't think its bolts would hold up to a weight on top! 

 

If the problem now is just the air filter resonance at 1000rpm, adding some mass to it (bolt on a lump of steel?) could help -- an extra 20% (of the air filter mass) will bring the resonance down to 910rpm which could still be a problem, adding 40% will bring it down to 850rpm which should be fine. Your suggestion of a lighter air filter assembly will move the resonance up the rev range which might be an even bigger problem when you're cruising...

[cheesegas]

Just now, IanD said:

If the problem now is just the air filter resonance at 1000rpm, adding some mass to it (bolt on a lump of steel?) could help -- an extra 20% (of the air filter mass) will bring the resonance down to 910rpm which could still be a problem, adding 40% will bring it down to 850rpm which should be fine. Your suggestion of a lighter air filter assembly will move the resonance up the rev range which might be an even bigger problem when you're cruising...

To be honest the replacement is so small, made completely of rubber and mounts directly onto the intake I can't see it resonating at all. Should be delivered today so I'll fit it and see how it goes.

 

The existing filter assembly has a thick steel bracket that bolts to the manifold into blind threaded holes, then the canister is bolted to that with a short flexible hose to join it to the intake. One of the bolts sheared flush with the manifold and my stud extractor can't get it out so I think it's easier to scrap the whole assembly!

[IanD]

1 minute ago, cheesegas said:

To be honest the replacement is so small, made completely of rubber and mounts directly onto the intake I can't see it resonating at all. Should be delivered today so I'll fit it and see how it goes.

 

The existing filter assembly has a thick steel bracket that bolts to the manifold into blind threaded holes, then the canister is bolted to that with a short flexible hose to join it to the intake. One of the bolts sheared flush with the manifold and my stud extractor can't get it out so I think it's easier to scrap the whole assembly!

Should be fine then, a rubber assembly will have plenty of damping unlike a steel one.

[Tracy D'arth]

There is a question of whether you need a air filter at all. 

Harmonic balancing an engine is an interesting subject.  Have you tried adding weight to the sump rather than the top of the engine? Most engines are very top heavy with reference to the crank center, it is one reason for using aluminium cylinder heads.  

[dmr]

In a modern car one or more mounts are often quite high up to give a "neutral axis" installation where the mounts are on the axis about which the engine wants to rotate. In boats the mounts are low down and not "designed" so more engine movement is likely.  The three cylinder engine is quite difficult as it produces two harmonic series, one at the firing frequency (1.5 times rotation) and one at rotation frequency, so its difficult to steer clear of all the resonances.

[cheesegas]

Just now, Tracy D'arth said:

There is a question of whether you need a air filter at all. 

Harmonic balancing an engine is an interesting subject.  Have you tried adding weight to the sump rather than the top of the engine? Most engines are very top heavy with reference to the crank center, it is one reason for using aluminium cylinder heads.  

I'd rather have one, if only because it's under a cruiser stern and lifting the boards above the engine always results in a bit of grit etc falling into the engine bay. The intake end of the manifold is also perfectly positioned to suck in belt dust from the alternator

 

It's a full cast iron block and head so yeah, probably quite top heavy. I haven't tried adding weight to a point lower than the mounts but I'm not sure how feasible it is to get weights down there because clearance is quite tight.

[Ray T]

Bit late to the party but we had vibration problems with a Lister LPWS3.

This was solved by fitting side rail bearers.
Do Isuzu make them for the 3 cylinder Isuzu 3KC1?

[IanD]

33 minutes ago, dmr said:

In a modern car one or more mounts are often quite high up to give a "neutral axis" installation where the mounts are on the axis about which the engine wants to rotate. In boats the mounts are low down and not "designed" so more engine movement is likely.  The three cylinder engine is quite difficult as it produces two harmonic series, one at the firing frequency (1.5 times rotation) and one at rotation frequency, so its difficult to steer clear of all the resonances.

 

There's also a big low-frequency (rotation frequency) rocking couple which is hard to suppress, it's why modern cars with 3-cyl engines use complex and carefully placed engine mounts, often soft and with added damping, and sometimes added masses, as well as sophisticated computer analysis of all the vibration modes -- it's all this which has made them usable (and popular) in modern cars.

 

In boats which don't bother with any of this (modern technology, uurgh...) the vibration can be a problem with 3-cyls, the rocking couple wants to make the prop shaft wiggle up and down (and side to side) which is not good for flexible couplings and/or stern tube, especially if there's an engine mount resonance within the rev range -- which there probably will be unless the mounting feet are very soft (e.g. Yanmar), which they often aren't... 😞 

 

Technically speaking, the proper solution is either to use an internal balancer shaft (e.g. Bukh) or have a slower speed engine bolted to the bedplates instead of flexible mounted, which is what trad engines do, but at the cost of vibration throughout the boat -- which can be really bad if this then hits a hull resonance, like the Bolinder-powered boat (yes, a single-cylinder...) I saw on the Trent where the entire stern was bouncing up and down by several inches. Most marinisers seem to just use flexible feet and hope there won't be a problem, and they usually get away with this -- but not always, as cheesegas has found... 😞 

 

5 minutes ago, cheesegas said:

I'd rather have one, if only because it's under a cruiser stern and lifting the boards above the engine always results in a bit of grit etc falling into the engine bay. The intake end of the manifold is also perfectly positioned to suck in belt dust from the alternator

 

It's a full cast iron block and head so yeah, probably quite top heavy. I haven't tried adding weight to a point lower than the mounts but I'm not sure how feasible it is to get weights down there because clearance is quite tight.

 

The problem is that to affect resonances like this you need a *lot* of weight, especially with a heavy cast iron engine like yours -- a few tens of kg is unlikely to fix the problem.

Edited April 8 by IanD

[john.k]

The cross river ferries here had JP3 Listers in them........I doubt the motors ever hit 800 revs ,and idled at around 100..........the whole wooden 30ft boat used to shudder and shake all the time........that was part of the charm of them.

[dmr]

17 minutes ago, IanD said:

 

There's also a big low-frequency (rotation frequency) rocking couple which is hard to suppress, it's why modern cars with 3-cyl engines use complex and carefully placed engine mounts, often soft and with added damping, and sometimes added masses, as well as sophisticated computer analysis of all the vibration modes -- it's all this which has made them usable (and popular) in modern cars.

 

In boats which don't bother with any of this (modern technology, uurgh...) the vibration can be a problem with 3-cyls, the rocking couple wants to make the prop shaft wiggle up and down (and side to side) which is not good for flexible couplings and/or stern tube, especially if there's an engine mount resonance within the rev range -- which there probably will be unless the mounting feet are very soft (e.g. Yanmar), which they often aren't... 😞 

 

Technically speaking, the proper solution is either to use an internal balancer shaft (e.g. Bukh) or have a slower speed engine bolted to the bedplates instead of flexible mounted, which is what trad engines do, but at the cost of vibration throughout the boat -- which can be really bad if this then hits a hull resonance, like the Bolinder-powered boat (yes, a single-cylinder...) I saw on the Trent where the entire stern was bouncing up and down by several inches. Most marinisers seem to just use flexible feet and hope there won't be a problem, and they usually get away with this -- but not always, as cheesegas has found... 😞 

 

 

The problem is that to affect resonances like this you need a *lot* of weight, especially with a heavy cast iron engine like yours -- a few tens of kg is unlikely to fix the problem.

 

Yes, the 1E (rotational frequency) component is actually a pitch. Many off the shelf 3 cylinder engines will have an out of balance flywheel and front pulley to turn half of this into a yaw as a best compromise.

A proper (automotive) design would establish the sensitivity of the vehicle structure to the various force inputs and design the mounts and pitch/yaw split correctly. This is also sometimes done on big plastic boats but I don't know if it has been tried on a narrowboat, it would be an interesting project.

Balance shafts use/waste a bit of power and would have limited gain on a three due to the various frequencies, they work a treat on the I4 (standard inline four cylinder engine) as this has one major harmonic (but do have to run at twice engine speed).

 

Traddy boats have a rigidly mounted big heavy engine, I don't know how well a lighter modern regidly mounted engine would work but suspect boaters who choose not to have a "proper" engine aspire to near xero noise and vibration so mounts are the way to go.

Putting the engine right at the back of the boat is probably good for noise but maybe not optimum for vibration.