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

[cheesegas]

Got an ongoing issue with engine RPMs and resonances which I'm hoping someone can advise on.

 

I've got a 3 cylinder Isuzu 3KC1 diesel in my narrowboat, coupled to a PRM Delta 30 (basically an older 150) hydraulic box with a Centaflex coupling, about 1' of shaft to the conventional stern gland. Engine is mounted on four AV Mounts' marine series, all brand new as of last year. The rear mounts are in line with the flywheel so the gearbox overhangs and the front ones in line with the belts roughly. The left side of the engine is around 30kg heavier so these mounts are 100kg rated, the other two are 75kg.

 

When I got the boat the idle speed was set to 800rpm (Isuzu state 1000rpm in the manual), one of the mounts was sheared and one of the brackets had worked loose so it all bounced around. Ever since replacing the mounts, I haven't been able to get things quite right. 

 

Currently for faultfinding purposes I have the propshaft disconnected at the gearbox flange, engine idle is set to 1000rpm (verified by a handheld laser tach), alternator regulator switched off so there's no load. Between 1100rpm and 1250rpm there's a really bad resonance which is characteristic of 3 pot diesels - the whole engine jumps around, everything bounces and shakes. If I cruise at this RPM, it moves enough to vibrate the propshaft against the stern tube and cause the gland to leak excessively...and it'll shear the bolts that hold the air filter bracket on! Above 1250rpm it's perfectly smooth.

 

Last summer I set the idle to 1300rpm to avoid the resonance band as it's difficult to go from idle straight to 1300rpm when cruising without the engine trying to shake itself out the boat. However - and it may be coincidence - the splines on the drive plate sheared after a couple of months. The gearbox never sounded very happy engaging drive at 1300rpm but the PRM manual doesn't mention an idle speed. https://www.prm-newage.com/media/File/delta man.pdf

 

Is this normal for 3 cylinder diesels? And what's the consensus on the high idle speed causing drive plate failure and/or being bad for the gearbox?

 

This engine was also used in mini diggers, generators and reefers - all applications where it's either idling or at full power RPMs with no variation. I wonder if it was never intended for variable speed use? There's quite a few around in narrowboats though, including its bigger brother the 3KR1.

[magnetman]

Why were the engine mounts changed last year? 

It seems odd to use different sizes. Why not just use 4 x 100kg mounts? 

 

And why is one side 30kg heavier? Is that as designed? 

[cheesegas]

4 minutes ago, magnetman said:

Why were the engine mounts changed last year? 

It seems odd to use different sizes. Why not just use 4 x 100kg mounts? 

 

And why is one side 30kg heavier? Is that as designed? 

Forgot to mention - initially I fitted 75kg mounts all round, but as I didn't account for the increased weight on the left side of the engine those mounts collapsed after a few months as they were bottomed out at full deflection. The 100kg mounts are better and deflect a similar amount to the 75kg ones on the other side of the engine.

 

edit - might it be worth trying 100kg mounts all round in case I'm exciting some resonance with two different compliances of mount? Advice I got from AV Mounts was to use different ratings of mounts depending on how the engine's weight is distributed.

 

edit two, just seen your edit - the left side of the engine is a lot heavier as both manifolds, starter and alternator are on that side. The rear right mount takes almost no downward force at all - removing it entirely doesn't cause the engine to droop at all! In fact, when I had all 75kg mounts fitted, it was taking upward force.

Edited November 7, 2023 by cheesegas

[IanD]

6 minutes ago, cheesegas said:

Got an ongoing issue with engine RPMs and resonances which I'm hoping someone can advise on.

 

I've got a 3 cylinder Isuzu 3KC1 diesel in my narrowboat, coupled to a PRM Delta 30 (basically an older 150) hydraulic box with a Centaflex coupling, about 1' of shaft to the conventional stern gland. Engine is mounted on four AV Mounts' marine series, all brand new as of last year. The rear mounts are in line with the flywheel so the gearbox overhangs and the front ones in line with the belts roughly. The left side of the engine is around 30kg heavier so these mounts are 100kg rated, the other two are 75kg.

 

When I got the boat the idle speed was set to 800rpm (Isuzu state 1000rpm in the manual), one of the mounts was sheared and one of the brackets had worked loose so it all bounced around. Ever since replacing the mounts, I haven't been able to get things quite right. 

 

Currently for faultfinding purposes I have the propshaft disconnected at the gearbox flange, engine idle is set to 1000rpm (verified by a handheld laser tach), alternator regulator switched off so there's no load. Between 1100rpm and 1250rpm there's a really bad resonance which is characteristic of 3 pot diesels - the whole engine jumps around, everything bounces and shakes. If I cruise at this RPM, it moves enough to vibrate the propshaft against the stern tube and cause the gland to leak excessively...and it'll shear the bolts that hold the air filter bracket on! Above 1250rpm it's perfectly smooth.

 

Last summer I set the idle to 1300rpm to avoid the resonance band as it's difficult to go from idle straight to 1300rpm when cruising without the engine trying to shake itself out the boat. However - and it may be coincidence - the splines on the drive plate sheared after a couple of months. The gearbox never sounded very happy engaging drive at 1300rpm but the PRM manual doesn't mention an idle speed. https://www.prm-newage.com/media/File/delta man.pdf

 

Is this normal for 3 cylinder diesels? And what's the consensus on the high idle speed causing drive plate failure and/or being bad for the gearbox?

 

This engine was also used in mini diggers, generators and reefers - all applications where it's either idling or at full power RPMs with no variation. I wonder if it was never intended for variable speed use? There's quite a few around in narrowboats though, including its bigger brother the 3KR1.

3-pots are balanced vertically but have a strong rocking couple end-to-end i.e. crank nose and tail going in opposite directions at the rotation speed, which is about 20Hz in your case. If the mounting feet aren't soft enough and the resonance of the engine on the feet coincides with this, you get a *lot* of movement. It's why 3-cyl suppliers like Yanmar supply extra-soft feet, to move this resonance down below idle speed.

 

You mention the load rating of your engine feet, but not what the static deflection under load is -- soft feet suitable for 3-cyl this is normally something around 4mm. Do you know what the weight of the engine/gearbox is, and what the rated deflection of the feet is?

5 minutes ago, cheesegas said:

Forgot to mention - initially I fitted 75kg mounts all round, but as I didn't account for the increased weight on the left side of the engine those mounts collapsed after a few months as they were bottomed out at full deflection. The 100kg mounts are better and deflect a similar amount to the 75kg ones on the other side of the engine.

 

edit - might it be worth trying 100kg mounts all round in case I'm exciting some resonance with two different compliances of mount? Advice I got from AV Mounts was to use different ratings of mounts depending on how the engine's weight is distributed.

 

edit two, just seen your edit - the left side of the engine is a lot heavier as both manifolds, starter and alternator are on that side. The rear right mount takes almost no downward force at all - removing it entirely doesn't cause the engine to droop at all! In fact, when I had all 75kg mounts fitted, it was taking upward force.

 

The resonant frequency for a rocking mode (likely culprit for 3-cyl) doesn't depend so much on the static load as the inertia -- when the resonance happens can you tell whether end-to-end rocking is the problem?

[cheesegas]

Just now, IanD said:

3-pots are balanced vertically but have a strong rocking couple end-to-end i.e. crank nose and tail going in opposite directions at the rotation speed, which is about 20Hz in your case. If the mounting feet aren't soft enough and the resonance of the engine on the feet coincides with this, you get a *lot* of movement. It's why 3-cyl suppliers like Yanmar supply extra-soft feet, to move this resonance down below idle speed.

 

You mention the load rating of your engine feet, but not what the static deflection under load is -- soft feet suitable for 3-cyl this is normally something around 4mm. Do you know what the weight of the engine/gearbox is, and what the rated deflection of the feet is?

Interesting, thanks for the reply. The rear right mount I originally replaced with a 75kg one, but it was so soft it spent all its time bottomed out and eventually failed. The rear left mount was actually taking a small amount of upward force when I installed it.

 

The datasheet for the mounts is here: https://www.avindustrialproducts.co.uk/files/ww/33-34. Silent Marine Mounts.pdf . Unfortunately these mounts are the only ones that are low enough and fit the holes in the bearers, the R&Ds are 5mm too high.

 

The gearbox is 30kg and the engine is 130kg dry without the alternator, so around 150kg wet, total of 170kg. As mentioned though, it's not evenly distributed and most of it is on the back right mount.

 

Had a think - is it worth going to 50kg mounts for the three which are less heavily loaded, and trying a 100kg on the loaded one?

[IanD]

1 hour ago, cheesegas said:

Interesting, thanks for the reply. The rear right mount I originally replaced with a 75kg one, but it was so soft it spent all its time bottomed out and eventually failed. The rear left mount was actually taking a small amount of upward force when I installed it.

 

The datasheet for the mounts is here: https://www.avindustrialproducts.co.uk/files/ww/33-34. Silent Marine Mounts.pdf . Unfortunately these mounts are the only ones that are low enough and fit the holes in the bearers, the R&Ds are 5mm too high.

 

The gearbox is 30kg and the engine is 130kg dry without the alternator, so around 150kg wet, total of 170kg. As mentioned though, it's not evenly distributed and most of it is on the back right mount.

 

Had a think - is it worth going to 50kg mounts for the three which are less heavily loaded, and trying a 100kg on the loaded one?

Which foot/feet from the range are you using? The small one (SM1600) has models rated at 40/50/75/90kg...

 

Such uneven weight distribution is really not going to make things easy -- you really want to aim for similar deflection on all the feet, hopefully the stiffest one (90kg) will be OK for rear right but you definitely want softer ones elsewhere. Which I think is what AV mounts suggested... 😉

 

But you still might have a problem getting the resonance down below idle speed. 3-cyl engines are worse than 4-cyl for this because the rocking couple is once per revolution, 4-cyl have a vertical vibration but at double the frequency. I know Yanmar 3-cyl engines use *really* soft feet with something like 6mm static deflection to try and do this.

Edited November 7, 2023 by IanD

[cheesegas]

5 minutes ago, IanD said:

Which foot/feet from the range are you using? The small one (SM1600) has models rated at 40/50/75/90kg...

 

Such uneven weight distribution is really not going to make things easy -- you really want to aim for similar deflection on all the feet, hopefully the stiffest one (90kg) will be OK for rear right but you definitely want softer ones elsewhere. Which I think is what AV mounts suggested... 😉

 

But you still might have a problem getting the resonance down below idle speed. 3-cyl engines are worse than 4-cyl for this because the rocking couple is once per revolution, 4-cyl have a vertical vibration but at double the frequency. I know Yanmar 3-cyl engines use *really* soft feet with something like 6mm static deflection to try and do this.

Oops, sorry forgot to mention - I'm using the SM1600 series, 75kg on three and 90kg on the one heavier. 

 

By the sounds of it, I might be better off with three 50kg and one 90kg? There's much more deflection on the 50kg. 

 

Otherwise, I can raise the idle to above the resonance, around 1300rpm but I'm not entirely convinced the gearbox likes this. Does 1300rpm sound high to you compared to other similar 1000cc Kubota based engines for example? The sheared drive plate splines might have been coincidence though as it hadn't been touched since I got the boat around 4 years ago, and the paperwork doesn't mention a new drive plate being fitted prior to that.

[IanD]

7 minutes ago, cheesegas said:

Oops, sorry forgot to mention - I'm using the SM1600 series, 75kg on three and 90kg on the one heavier. 

 

By the sounds of it, I might be better off with three 50kg and one 90kg? There's much more deflection on the 50kg. 

 

Otherwise, I can raise the idle to above the resonance, around 1300rpm but I'm not entirely convinced the gearbox likes this. Does 1300rpm sound high to you compared to other similar 1000cc Kubota based engines for example? The sheared drive plate splines might have been coincidence though as it hadn't been touched since I got the boat around 4 years ago, and the paperwork doesn't mention a new drive plate being fitted prior to that.

If the load on the other three feet is small and roughly equal 50kg (or even 40kg) might work -- do you have any way to measure the loads? Or deflection between no-load and loaded, since you have the plots showing deflection vs. load.

 

1300rpm is very high for idle speed, usually about 900rpm is used -- at least, it is for the Beta engines which are also Kubota based, Beta 20 looks like the closest to yours.

[cheesegas]

16 minutes ago, IanD said:

If the load on the other three feet is small and roughly equal 50kg (or even 40kg) might work -- do you have any way to measure the loads? Or deflection between no-load and loaded, since you have the plots showing deflection vs. load.

 

1300rpm is very high for idle speed, usually about 900rpm is used -- at least, it is for the Beta engines which are also Kubota based, Beta 20 looks like the closest to yours.

Confusingly the 50kg is the squishiest one they make, despite showing 40kg on the datasheet... I can measure deflection yep, from there I can get an estimate of weight. I did this last time which is how I worked out that one mount was taking 30kg more but it's definitely worth doing again with these. I'll get them all at exactly the same height from the bearers with the propshaft disconnected and measure again.

 

The mounts are only £20 each so I'll order three 50kg anyway. I'm out the country for work for a bit so I'll update in Dec, thanks for your help! I was going to go the other way and buy firmer mounts. 

[IanD]

6 minutes ago, cheesegas said:

Confusingly the 50kg is the squishiest one they make, despite showing 40kg on the datasheet... I can measure deflection yep, from there I can get an estimate of weight. I did this last time which is how I worked out that one mount was taking 30kg more but it's definitely worth doing again with these. I'll get them all at exactly the same height from the bearers with the propshaft disconnected and measure again.

 

The mounts are only £20 each so I'll order three 50kg anyway. I'm out the country for work for a bit so I'll update in Dec, thanks for your help! I was going to go the other way and buy firmer mounts. 

 

The Yanmar 3-cyl mounts (and replacements from suppliers like R&D) are squishier still, with deflection up to 8mm or so. These might bring the resonance down low enough that you can set an idle rpm above it.

 

Don't Isuzu have any recommended feet for this engine/gearbox?

[cheesegas]

5 minutes ago, IanD said:

 

The Yanmar 3-cyl mounts (and replacements from suppliers like R&D) are squishier still, with deflection up to 8mm or so. These might bring the resonance down low enough that you can set an idle rpm above it.

 

Don't Isuzu have any recommended feet for this engine/gearbox?

The flattest R&D mounts may just fit if I replace the bottom nut with a half-nut actually - the softest one is rated at 13-40kg (they seem to give a range rather than a value) but the deflection is only 3.4mm which is less than the AV Mounts 50kg model at 4mm. No datasheet showing deflection is available on the R&D site though, see attached for info and link here https://shop.tnorrismarine.co.uk/products/r-d-flexible-engine-mount-shear-type-30-90lbs-12mm-stud

 

There's very little info about this engine on the web, parts are equally scarce as well so I doubt there's any recommended feet to go with it. The originals were similar to the AV Mounts, but given how one had sheared and one bracket had vibrated loose, the issue has been there for a while. 

[IanD]

3 minutes ago, cheesegas said:

The flattest R&D mounts may just fit if I replace the bottom nut with a half-nut actually - the softest one is rated at 13-40kg (they seem to give a range rather than a value) but the deflection is only 3.4mm which is less than the AV Mounts 50kg model at 4mm. No datasheet showing deflection is available on the R&D site though, see attached for info and link here https://shop.tnorrismarine.co.uk/products/r-d-flexible-engine-mount-shear-type-30-90lbs-12mm-stud

 

There's very little info about this engine on the web, parts are equally scarce as well so I doubt there's any recommended feet to go with it. The originals were similar to the AV Mounts, but given how one had sheared and one bracket had vibrated loose, the issue has been there for a while. 

That's not the R&D mounts I meant, it was the "Yanmar replacement" ones -- have a look at ASAP supplies.

 

The problem you might have is that you haven't got enough height to fit mounts in which are flexible enough to fix the problem... 😞

 

What height do you have available from top of engine bed to underside of engine mounting lug?

[cheesegas]

3 minutes ago, IanD said:

That's not the R&D mounts I meant, it was the "Yanmar replacement" ones -- have a look at ASAP supplies.

 

The problem you might have is that you haven't got enough height to fit mounts in which are flexible enough to fix the problem... 😞

 

What height do you have available from top of engine bed to underside of engine mounting lug?

These ones, or a variant thereof? https://www.asap-supplies.com/products/r-d-flexible-engine-mount-1gm10-for-yanmar-engines-40-131lbs-207074

 

And yep, you're right - they're too tall unfortunately, I only have 65mm from bed to bottom of the mount bracket! It looks like the R&Ds might be the only option, I remember looking around a lot when selecting new ones and the height was the limiting factor.

[IanD]

7 minutes ago, cheesegas said:

These ones, or a variant thereof? https://www.asap-supplies.com/products/r-d-flexible-engine-mount-1gm10-for-yanmar-engines-40-131lbs-207074

 

And yep, you're right - they're too tall unfortunately, I only have 65mm from bed to bottom of the mount bracket! It looks like the R&Ds might be the only option, I remember looking around a lot when selecting new ones and the height was the limiting factor.

Yes, that was one of them.

 

A soft mount with less than 65mm height is going to be a challenge... 😉

[Tracy D'arth]

You can use shim washers instead of the bottom nuts to give you more space.

[cheesegas]

15 minutes ago, IanD said:

Yes, that was one of them.

 

A soft mount with less than 65mm height is going to be a challenge... 😉

Yeah...I've ordered the AV Mounts, will update in December when I'm back!

 

9 minutes ago, Tracy D'arth said:

You can use shim washers instead of the bottom nuts to give you more space.

Thanks, good idea but unfortunately the alternative mounts are magnitudes too high, higher than the engine leg!

[Oldturn around guy]

Our last boat had the 3KR1 engine. It broke the original hard CT type mounts, together with one of the back feet, and wrecked the Aquadrive. TW marine refurbed the Aquadrive and recommend very squishy mounts. The engineer used shim washers to stop the bottom nuts undoing. Kept it for several years after with no issues, although it did wobble like a jelly at tickover. It was marinised by Beta who used to have a useful chap called Neil who knew these engines. Failing that Engines Plus can get spares.

[cheesegas]

3 minutes ago, Oldturn around guy said:

Our last boat had the 3KR1 engine. It broke the original hard CT type mounts, together with one of the back feet, and wrecked the Aquadrive. TW marine refurbed the Aquadrive and recommend very squishy mounts. The engineer used shim washers to stop the bottom nuts undoing. Kept it for several years after with no issues, although it did wobble like a jelly at tickover. It was marinised by Beta who used to have a useful chap called Neil who knew these engines. Failing that Engines Plus can get spares.

Thanks for the reply, the 3KR1 is the same as mine but about 200cc bigger I think. Definitely vibe-y engines, although mine's dead smooth at tickover, it's just slightly above is the issue. Another vote for soft mounts then!

 

Mine isn't marinised at all, it's got a dry exhaust and no other modifications from the stock industrial engine I reckon. I didn't know Beta did a version of it though, I'll try them for spares. Engines Plus don't have many spares and the ones they do are very expensive - lift pump diaphragm £800 (well actually the whole pump as they don't sell the part), water pump £600 etc etc. It was also fitted into Hitachi EX series mini diggers but plant equipment has a fairly short life so parts aren't stocked by Hitachi dealers any more.

[IanD]

I don't think any of the Beta engines are exactly the same as yours, but they're similar and also Kubota-based so you might be lucky with spares... 😉

[Oldturn around guy]

In the early years Beta marinised Isuzu engines before switching to Kubota.

[IanD]

15 hours ago, cheesegas said:

Yeah...I've ordered the AV Mounts, will update in December when I'm back!

 

Thanks, good idea but unfortunately the alternative mounts are magnitudes too high, higher than the engine leg!

These Aquadrive mounts are soft and will easily fit in 65mm height:

 

https://www.peachment.co.uk/engine-mountings-couplings/

 

"It is essential to select the correct rubber grade and mount for the engine or generator in question. For reference, the chart above shows the level of deflection
(or squash) that will occur on each model of mount for a given loading.
In general you should aim for 3mm on the 50210, 4mm on the 50220, 5mm on the 50230 and 6mm on the 50240. The numbering system is simple: A 50223
mount is the 50220 model with number three rubber hardness and so on. Some engines, particularly those with less than four cylinders, require particular care and
your distributor will help with these."

 

[dmr]

19 hours ago, IanD said:

3-pots are balanced vertically but have a strong rocking couple end-to-end i.e. crank nose and tail going in opposite directions at the rotation speed, which is about 20Hz in your case. If the mounting feet aren't soft enough and the resonance of the engine on the feet coincides with this, you get a *lot* of movement. It's why 3-cyl suppliers like Yanmar supply extra-soft feet, to move this resonance down below idle speed.

 

You mention the load rating of your engine feet, but not what the static deflection under load is -- soft feet suitable for 3-cyl this is normally something around 4mm. Do you know what the weight of the engine/gearbox is, and what the rated deflection of the feet is?

 

The resonant frequency for a rocking mode (likely culprit for 3-cyl) doesn't depend so much on the static load as the inertia -- when the resonance happens can you tell whether end-to-end rocking is the problem?

 

But do note that many threes will be fitted with an out of balance pulley and flywheel to turn half of the pitch into a yaw.

 

In an automotive application the designers/engineers will know if the body is more sensitive to pitch or yaw and select the out of balance accordingly, but an off the shelf engine will likely just go for 50:50

[IanD]

8 minutes ago, dmr said:

 

But do note that many threes will be fitted with an out of balance pulley and flywheel to turn half of the pitch into a yaw.

 

In an automotive application the designers/engineers will know if the body is more sensitive to pitch or yaw and select the out of balance accordingly, but an off the shelf engine will likely just go for 50:50

 

True, but this doesn't solve the problem that the 3-cyl vibration is at half the frequency of a 4-cyl, which makes it much more likely to cause a resonance problem with the feet.

 

This is true even if they're "soft", because to drop the resonant frequency by half needs feet which are a quarter as stiff, which means 4x the static deflection.

[Peugeot 106]

19 hours ago, cheesegas said:

Got an ongoing issue with engine RPMs and resonances which I'm hoping someone can advise on.

 

I've got a 3 cylinder Isuzu 3KC1 diesel in my narrowboat, coupled to a PRM Delta 30 (basically an older 150) hydraulic box with a Centaflex coupling, about 1' of shaft to the conventional stern gland. Engine is mounted on four AV Mounts' marine series, all brand new as of last year. The rear mounts are in line with the flywheel so the gearbox overhangs and the front ones in line with the belts roughly. The left side of the engine is around 30kg heavier so these mounts are 100kg rated, the other two are 75kg.

 

When I got the boat the idle speed was set to 800rpm (Isuzu state 1000rpm in the manual), one of the mounts was sheared and one of the brackets had worked loose so it all bounced around. Ever since replacing the mounts, I haven't been able to get things quite right. 

 

Currently for faultfinding purposes I have the propshaft disconnected at the gearbox flange, engine idle is set to 1000rpm (verified by a handheld laser tach), alternator regulator switched off so there's no load. Between 1100rpm and 1250rpm there's a really bad resonance which is characteristic of 3 pot diesels - the whole engine jumps around, everything bounces and shakes. If I cruise at this RPM, it moves enough to vibrate the propshaft against the stern tube and cause the gland to leak excessively...and it'll shear the bolts that hold the air filter bracket on! Above 1250rpm it's perfectly smooth.

 

Last summer I set the idle to 1300rpm to avoid the resonance band as it's difficult to go from idle straight to 1300rpm when cruising without the engine trying to shake itself out the boat. However - and it may be coincidence - the splines on the drive plate sheared after a couple of months. The gearbox never sounded very happy engaging drive at 1300rpm but the PRM manual doesn't mention an idle speed. https://www.prm-newage.com/media/File/delta man.pdf

 

Is this normal for 3 cylinder diesels? And what's the consensus on the high idle speed causing drive plate failure and/or being bad for the gearbox?

 

This engine was also used in mini diggers, generators and reefers - all applications where it's either idling or at full power RPMs with no variation. I wonder if it was never intended for variable speed use? There's quite a few around in narrowboats though, including its bigger brother the 3KR1.

Have you sent this to R&D technical. My experience with them is that they can be abrupt but this us after all their bread and butter. Their opinion could be helpful even if their mounts aren’t cheap and they probably have direct experience of your problem

you have clearly explained the problem so they should have an answer

[IanD]

8 minutes ago, Peugeot 106 said:

Have you sent this to R&D technical. My experience with them is that they can be abrupt but this us after all their bread and butter. Their opinion could be helpful even if their mounts aren’t cheap and they probably have direct experience of your problem

you have clearly explained the problem so they should have an answer

If you think the R&D mounts aren't cheap then the Aquadrive ones might come as a nasty shock, going by comments I had from Finesse -- they're what's under the generator inertia frame on my boat...