Cummins N855 alternator upgrade

[woolf]

We have swapped the standard Delco alternators on our Cummins N855 for larger APS 55i units, which work nicely. Only the standard double V belt setup is now operating on the edge of slipping, due to the increased power demand.

We would like to change to multi-groove pulleys, but could not find a source for a conversion kit for this engine. Any idea for a source where I might look - before having to place a custom order with a machine shop?

Thanks!

 

[DavidM]

Post you question on the Sbmar.com forum, Tony Athens site for all things related to Cummins marine.

But I am a little concerned that two belts are slipping. Usually one belt is good for 100 amps and two almost double that.

Are they matched and adjusted right?

David

 

[woolf]

I checked on Tony Athens‘ web page, but did not post the question on their forum – might do that now, thank you.

The alternator takes nominal 28V @ 300A = 8.4KW or [FONT=&quot]~[/FONT]11.5hp plus conversion losses - at peak charge we saw 260A so far. We mounted with new top quality Gates belts, pulleys are o.k. and properly aligned. Still, we had to tension the belts quite hard to eliminate slip.

The alternators will work hard under way, as they are replacing a generator. For the cruising season, I will take an extra 2 sets of spare belts, but still multi-groove belts might be the safer choice.

On the PTO side we should be fine, as my Cummins guy told me that we can take 20hp safely off that PTO.

 

[sunchaser]

I checked on Tony Athens‘ web page, but did not post the question on their forum – might do that now, thank you.

The alternator takes nominal 28V @ 300A = 8.4KW or [FONT=&quot]~[/FONT]11.5hp plus conversion losses - at peak charge we saw 260A so far. We mounted with new top quality Gates belts, pulleys are o.k. and properly aligned. Still, we had to tension the belts quite hard to eliminate slip.

The alternators will work hard under way, as they are replacing a generator. For the cruising season, I will take an extra 2 sets of spare belts, but still multi-groove belts might be the safer choice.

On the PTO side we should be fine, as my Cummins guy told me that we can take 20hp safely off that PTO.

8.4 kw is a big load. Belt and pulley design is an art for bigger loads. As Dave suggested contacting SBar is a good start. Do you have some good photos as well as pulley dimensions, belt type and size and contact angles to send to Tony? BTW, he is a business so bigger consults may well be a cost. Do you have a good marine or belt shop in the Med to check with?

Will you keep the genset for at anchor use? Does the 855 have a smaller alternator for charging batteries?

 

[woolf]

The mechanics at the shipyard swapped the pulleys from the original Delco to the new APS alternator - one unit only per engine. It fitted perfectly, so mechanically it is still the original belt + pulley setup – see fotos old + new. Charging of house banks is directly from these alternators. Start battery bank is charged via a separate charger.

There are a number of good mechanical workshops around, that might build a custom pulley set, the boat is currently in Holland. But there is not much know how and upgrade parts available locally for the Cummins N855 engines. A multi-groove replacement kit would be much preferred, if available.

The boat came with twin generators, and one would run basically 24/7 when away from the dock. We added 2 x 300A alternators to the mains for power under way, and 2 x 900Ah battery banks for 24h silent time at anchor (under perfect conditions). Kept the 2 x NL generators for charging, and to boost limited shore power if needed, as they were already installed and in very good condition.

 

[sunchaser]

Woolf
Can you dial down output from 8.4 kw to a lower number to assess what draw is acceptable for belt life and slippage? Are you concerned about too tight a belt and cruising gen bearing problems?

 

[woolf]

My main concern is running through lots of belts, with the associated slippage, dust mess, and loss of power at inconvenient times. The bearings and seals of the PTO should be fine with that extra mechanical load. Ideally, the new set-up should also include a spring-loaded belt tensioning arrangement.

We can regulate the alternator charge, via the MV AlphaPro regulator. We have programmed a zero charge rate at engine start, to ramp up to full load after 30 seconds, to put some load on the engine during warm-up.

However, I would like to have a full charge rate available when cruising. That is approx. 10-15% below the nominal 300A charge rate established by the manufacturer in their lab tests. The boat is all electric, except diesel heat and gas BBQ, with significant electric loads like galley, appliances, watermaker, …. that are fed under way by the alternators via inverter.

 

[C lectric]

I would also suggest use the phone and call Seaboard Marine. Forums are great but sometime actually talking to someone helps a lot more, at least for the initial contact. Then a forum or Email can sort out the rest.

 

[Simi 60]

Timely post that has turned me off going a considerably larger alt on our nta855m

Extensive solar, for us negates the need for anything more than 100amp, 45amp was working fine

As the OP is in Spain would solar perhaps have been the easier route?
No need to burn diesel for power.
Almost to late now of course.

 

[Simi 60]

What is it that you are running where the red thing is?

 

[O C Diver]

What is it that you are running where the red thing is?

Looks like a flexible coupling to a hydraulic pump. Buddy of mine has that same setup for his bow thruster on a KK 42. His includes an electric clutch.

Ted

 

[O C Diver]

Regarding the alternator, one of the other options is to replace the pulley on the alternator, at least for the summer. The pulley is just under a half circle where the belt is in contact with it. As you increase the pulley diameter, you will increase the pulley contact area and hopefully eliminate any chance of slippage. The downside is that it will require more engine RPM for the same alternator RPM and full output. This is all pretty simple math. Do a test to determine minimum engine RPM for full alternator output and then consider your normal cruising RPM to calculate how much bigger a pulley you can use on the alternator. Spinning the alternator faster is more optimal for the same amps, as it increases cooling with a faster fan speed.

Ted

 

[woolf]

Timely post that has turned me off going a considerably larger alt on our nta855m

Extensive solar, for us negates the need for anything more than 100amp, 45amp was working fine

As the OP is in Spain would solar perhaps have been the easier route?
No need to burn diesel for power.
Almost to late now of course.

We are getting the boat ready specifically for higher latitude cruising, not counting on much sunshine up there. Different use case than yours.

The plan is to operate high load systems like watermaker, washer, dryer, and get max charge to the batteries, while the main engines are running. At cruise, they are operating at low load anyway. We expect to make more miles/year than average.

That should extend quiet time at anchor significantly, which is important to us, coming from a sailboat.

 

[woolf]

Looks like a flexible coupling to a hydraulic pump. Buddy of mine has that same setup for his bow thruster on a KK 42. His includes an electric clutch.

Ted

Exactly, that's a flex coupling for the pumps on the front end of the engines. Not clutched, but running permanently to power the central hydraulic system.

 

[woolf]

Regarding the alternator, one of the other options is to replace the pulley on the alternator, at least for the summer. The pulley is just under a half circle where the belt is in contact with it. As you increase the pulley diameter, you will increase the pulley contact area and hopefully eliminate any chance of slippage. The downside is that it will require more engine RPM for the same alternator RPM and full output. This is all pretty simple math. Do a test to determine minimum engine RPM for full alternator output and then consider your normal cruising RPM to calculate how much bigger a pulley you can use on the alternator. Spinning the alternator faster is more optimal for the same amps, as it increases cooling with a faster fan speed.

Ted

Good idea - I'll include that for my test schedule, before ordering custom parts, in case I cannot find a conversion set.

I will also have to measure the actual rpm of the PTO at our usual 1200rpm cruising speed, as I could not find that information in the handbook.
That will have to wait until the Covid travel restrictions are lifted, unfortunately.

 

[FF]

Perhaps simple automatic load shedding on some of the AC devices would allow less than max output , still with an enjoyable life style.

 

[woolf]

Perhaps simple automatic load shedding on some of the AC devices would allow less than max output , still with an enjoyable life style.

We discussed that option in detail with the guys who designed the upgrade of the system for us. Automatic load shedding would have been surprisingly complex and expensive to retrofit, in our particular system.

It was decided to separate only the AirCo for use exclusively with a generator running, and switch on/off other large consumers manually, based on the alarms and data provided by the DC system monitor.

 

[sunchaser]

Woolf
Load shedding need not be automatic. Breaker or, switch flipping manually is common for all vessels big and small. For non genset time prioritize the must haves such as primary battery charging, instruments, lights, ER fans, induction cook top (Vs standard high draw stove top), RO, hydronic heating etc.

Run the gensets for heavier loads like washer dryer, all kitchen stuff, secondary charging, AC etc.

I spend time on a large vessel (100'+) and the crew is very cognizant of manual load management to minimize time on second genset. Until you have a bullet proof drive setup it would seem that minimizing load on your cruise gen to improve reliability is a worthy goal. As an aside, what type of house batteries do you have?

Good luck, it seems you have things very well thought out. Thanks for asking us neophytes for ideas.

 

[twistedtree]

Two thoughts to add to the mix

First, alternators are around 50% efficient, so the input power at the pulley will be about double the electrical output. So I think your pulley load is more like 17kw or 21hp. This will impact your load calculations

Gates has a program that you can download to model and calculate belt loading. You input the geometry of your pulley system, loads, and belt selection, and it will tell you what tension is required, and whether you have enough belt size. It might be worth the time to run it on your setup.

When I ran the program on my pulley setup, I learned something that was very counterintuitive. The amount of belt wrap makes a different up to about 60-70 deg, but beyond that the amount of belt wrap makes very little difference. You would think 90 deg would be much better than 70, and that 180 would be even better yet. But it actually makes very little difference. It was a real eye opener to me. The real issue is belt tension, and one of the great aspects of a serpentine belt is the typical use of a spring tensioner to maintain correct tension.

 

[woolf]

Woolf
Load shedding need not be automatic. Breaker or, switch flipping manually is common for all vessels big and small. For non genset time prioritize the must haves such as primary battery charging, instruments, lights, ER fans, induction cook top (Vs standard high draw stove top), RO, hydronic heating etc.

Run the gensets for heavier loads like washer dryer, all kitchen stuff, secondary charging, AC etc.

I spend time on a large vessel (100'+) and the crew is very cognizant of manual load management to minimize time on second genset. Until you have a bullet proof drive setup it would seem that minimizing load on your cruise gen to improve reliability is a worthy goal. As an aside, what type of house batteries do you have?

Good luck, it seems you have things very well thought out. Thanks for asking us neophytes for ideas.

Thank you for your comments, I have worked that load-shedding game before 24/7 on our sail boat. Looks like we’ll be in for it again, to a certain extent, now on the power boat [FONT=&quot]��[/FONT].

We have taken all possible mechanical options out of that DC power equation, like hot-fill on washer and dishwasher, heat-pump dryer, induction cooking, hot water and hydronic heating. Nominal DC, we have 600A available under way, vs. 300A charge on shore power. In theory, theory corresponds to reality – in reality, often no so much.

The refitted system works nicely so far, and optimizing the drive set-up for the alternator is just that, optimizing. The 855s are terrific low-rev commercial duty engines, no electronics, no turbos, just not very common here in Europe. We’ll work that PTO detail out, I’m confident, and the combined wisdom of the forum is a great help.

We used to have deep-cycle 2V telecom cells on board, but have now installed Firefly batteries for the first time. The tech specs were convincing, and we could import directly from the factory in India at a price point below the 2V alternative. I understand lithium, but as we have no weight restrictions, and space was available, the bang-for-the buck equation was clear for us. We’ll see how that works out over time.

 

[woolf]

Two thoughts to add to the mix

First, alternators are around 50% efficient, so the input power at the pulley will be about double the electrical output. So I think your pulley load is more like 17kw or 21hp. This will impact your load calculations

Gates has a program that you can download to model and calculate belt loading. You input the geometry of your pulley system, loads, and belt selection, and it will tell you what tension is required, and whether you have enough belt size. It might be worth the time to run it on your setup.

When I ran the program on my pulley setup, I learned something that was very counterintuitive. The amount of belt wrap makes a different up to about 60-70 deg, but beyond that the amount of belt wrap makes very little difference. You would think 90 deg would be much better than 70, and that 180 would be even better yet. But it actually makes very little difference. It was a real eye opener to me. The real issue is belt tension, and one of the great aspects of a serpentine belt is the typical use of a spring tensioner to maintain correct tension.

I'll check out that lead to the Gates calculator. Looks like I'll be just within the 20hp load recommendation for my Cummins PTO.

Your note is also in line with the recommendation received before from my engine guys: look for a serpentine belt set-up with an integrated tensioner, for a long term trouble free installation.

Fortunately, spare belts are not a prohibitive cost, until I have found that belt-pulley set.

Thank you.

 

[O C Diver]

When I ran the program on my pulley setup, I learned something that was very counterintuitive. The amount of belt wrap makes a different up to about 60-70 deg, but beyond that the amount of belt wrap makes very little difference. You would think 90 deg would be much better than 70, and that 180 would be even better yet. But it actually makes very little difference. It was a real eye opener to me. The real issue is belt tension, and one of the great aspects of a serpentine belt is the typical use of a spring tensioner to maintain correct tension.

I wonder how that varies based on pulley diameters and loads. Many smaller pulleys on diesel engine alternators seem to be wrapped 120 to 180 degrees by the belt. On the below linked picture (it's protected), it seems an idler pulley was added mainly to increase belt degrees on the pulley.

https://www.deere.com/en/industrial-engines/tier-3-stage-iii-a/powertech-e-4-5l-tf285/

Ted

 

[twistedtree]

I wonder how that varies based on pulley diameters and loads. Many smaller pulleys on diesel engine alternators seem to be wrapped 120 to 180 degrees by the belt. On the below linked picture (it's protected), it seems an idler pulley was added mainly to increase belt degrees on the pulley.

https://www.deere.com/en/industrial-engines/tier-3-stage-iii-a/powertech-e-4-5l-tf285/

Ted

I agree it looks that way. There is also lots and lots of guidance "out there" suggesting 90 deg or more wrap. I think Balmar has such guidance, and I've heard it from a lot of credible sources. That said, the engineering analysis from gates doesn't bear it out.


In many ways it actually does makes sense. I remember a physics problem many moons ago comparing the friction of two items sliding on a surface. Both were the same weight (equivalent to belt tension), but each had different contact areas (equivalent to belt wrap). You would expect the larger contact area to yield more friction, but they are both exactly the same. All that matters is the coefficient of friction, and the contact force.


Now that's theoretical physics, but the principal applies just the same with a belt.

 

[O C Diver]

Since you did a lot of research on belts, how does length of belt between these pulleys effect performance? When looking at this specific application, any stretch or slackening of the belt by the pulleys, shafts and bearings not being perfect, could unload the tension somewhat and possibly cause slippage. It would seem to me, if the pulleys were further apart, the length that the belt stretches could absorb more of the imperfections. Wondering if there's a guide or calculation to figure the required spacing between pulleys.

Ted

 

[High Wire]

It may be an optical illusion but in the second picture of post 5, the outer belt appears to have a small arc between the pulleys meaning not fully tensioned.

 

[woolf]

I zoomed in on the original hi-res foto, and there it looks o.k. Anyway, that is the old Delco unit which worked fine, the new 55i is mounted in the first picture.

 

[twistedtree]

Since you did a lot of research on belts, how does length of belt between these pulleys effect performance? When looking at this specific application, any stretch or slackening of the belt by the pulleys, shafts and bearings not being perfect, could unload the tension somewhat and possibly cause slippage. It would seem to me, if the pulleys were further apart, the length that the belt stretches could absorb more of the imperfections. Wondering if there's a guide or calculation to figure the required spacing between pulleys.

Ted

Interesting question, and I don't know the answer. I was working with a fixed geometry, so didn't experiment with different belt lengths.

 

[Ski in NC]

Good chance Cummins made some of those N series engines with serp belt aux drive pulley. Did a little sniffing around but ones I saw were v-belt.

Also it is true that a bigger diameter pulley on alt will help with traction, but will also reduce rpm and output. If output stays ok at dead idle, this might be worth looking into. A bigger aux drive pulley would help too.

Also counter that statement that alts are 50% efficient (with regards to aux drive limit calcs). Sure some small cheap units might be 50%, but larger more refined units such as this should be in the 80-90% range. Alt mfr should have that data. I would not do the calc assuming 50%.

 

[twistedtree]

Also counter that statement that alts are 50% efficient (with regards to aux drive limit calcs). Sure some small cheap units might be 50%, but larger more refined units such as this should be in the 80-90% range. Alt mfr should have that data. I would not do the calc assuming 50%.

Wow, that's a big difference. I'll need to check that out. I'm going based on a paper I believe published by Prestolite.

 

[woolf]

Wow, that's a big difference. I'll need to check that out. I'm going based on a paper I believe published by Prestolite.

The APS spec sheet claims Efficiency: 87% typical for the 55i, but that obviously does not include losses from the pulley + belt drive.