Why are alternators on marine engines so low geared 1:1,63-2 while alt can do 1:4

[CaptainRivet]

HI all,


why are the alternators on marine engines are so low geared?
Volvos D1,2,3,6 all alternators are geared 1:1,63 till max 1:2 means at rev limit 3000RPM its 6000RPM.
The 12V alternators used 115A Mitsubishi or the 80A Hitachi or the 140A Biosch have a orange line of 16000RPM so it won't be a problem at 1:3 or even 1:3,5....
So more amps low down and more cooling if it turns faster, 1:3 is 9000RPM and 1:4 is 12000RPM, both far away from the orange line 16000RPM.

[RaymondR]

V belt loading??

[rslifkin]

Probably a balance between "good enough" output at low RPM and longer alternator life by spinning it slower. Marine engines are more likely to spend a lot of time at sustained high RPM than an automotive application, so the desired alternator RPM at max engine RPM may be different.

[RaymondR]

The curve in the image is characteristic of alternators and shows that above about 2,000-3,000 rpm there is little increase in current output.

An increase in speed would result in greater cooling air flow however this would also increase the loading on the drive belt.

[Jim Cate]

An increase in speed would result in greater cooling air flow however this would also increase the loading on the drive belt.

Ray, it seems to me that the loading contributed by the fan drag must be way less than the loading from the electrical generation. I doubt if that is a significant factor. Bearing losses increase with RPM too and likely wear goes up, so that higher speeds do have
"costs", but we all know that overheating is a big factor in alternator failure and output reduction, so the benefit of air flow may well outweigh the various downsides of higher speed operation. (Obviously an amateur's thoughts!).

Jim

[Pete7]

Originally Posted by RaymondR

V belt loading??

The modern ones have serpentine belts don't they?

[RaymondR]

Originally Posted by Jim Cate

Ray, it seems to me that the loading contributed by the fan drag must be way less than the loading from the electrical generation. I doubt if that is a significant factor. Bearing losses increase with RPM too and likely wear goes up, so that higher speeds do have
"costs", but we all know that overheating is a big factor in alternator failure and output reduction, so the benefit of air flow may well outweigh the various downsides of higher speed operation. (Obviously an amateur's thoughts!).

Jim

Hi Jim,

The fans provided on the average alternator are a piece of crap from an air pumping viewpoint but excellent from a ease and cost of manufacture viewpoint and and with a well designed fan one could probably achieve multiples of cooling effect with a reasonable amount of design effort.

Modern roller bearing design has reached a plateau and I suspect there's nothing further to be achieved there.

Overheating is the single largest factor in both design and durability as those of us who choose to adopt the cruising lifestyle and it's concomitant requirement for a modern lifestyle - whereas our maritime forbears could survive with a wood stove and oil lighting and three meals a day we require electricity and all powered by it (if you want to dispute this assertion just go over to the extant thread on coffee makers)

There are a number of salient threads running at the moment, one of the more promising being that on 48 volt systems, but we live in interesting times technology wise and since cost tends to drive us to borrow heavily from the mass produced, road transport industry, which appears to be in a transition from hydrocarbon to electrical motive power who knows where it will take us.

And finally, whilst we are amateurs we are skilled amateurs, who, wilst we may at time be lacking in technical skills most certainly tend to develop superior conceptual skills.

Merry Xmas to both yourself and Ann mate.

[CaptainRivet]

My guess is emission norms and hp consumption.
Take a small D1-30hp common in 40-45ft monos or two in a 40ft cat eg 13t Lagoon.
If you gear that 1:4 it can deliver at idle 600rpm=2400RPM already 80% at 1000RPM=4000Rpm=100%
Means at 1000RPM I use 5hp to produce around 80A of my already borderline 30hp for a 40-44ft mono which can be to much power draw for maneuvering in tough conditions in tight spots, eg Harbour. Mass production builders owned by private Equity squeeze the last bit of lemon and just uses the smallest engine they can, so have 3hp more and get away with smaller engine is their choice.
2nd this early load is also affecting emissions, at idle and low rev operation. If you see what on some sweet water lakes like Lake Constance is done to that engines (detune,additional filters…), to comply with emission regulations that’s an easy solution to avoid a lot emissions.

To gear it higher aim is to get earlier more amps and to have it spin as fast as possible to get as much as cooling possible at typical engine rpm like 1400 for motor sailing or 1800-2200 for long motoring session but don‘t overheat for seldom WOT or 80% WOT at 2700-3000. so gearing would be 1:3,5 till 1:5 depending on the engine to achieve that.

Looking at the curve what do I want as boat owner: 80% at idle and 100% at 1400RPM because that is the typical RPM for motorsailing where the engine runs long time and as much cooling as possible at 1800-2200 as much cooing as possible. Above it shouldn‘t die, means has to be operational save. That has to be matched to the alternator curve and orange line rev limit.
Mark Gasser eg gears all his kits at 1:2.5 but these are all heavy duty low reving alts by default.

And looking at eg the Mitsubishi 115A that is developed for light commercial cooling trucks and running in the Mitsubishi Canter Cooling truck at max engine RPM 15000RMP, typical driving RPM is 9000RPM for alt for million kilometer at least.
So the bearings and belt can do that and wear is not a topic here.

Fans forget: we tried that already 20years ago in the car stereo scene..doesn‘t really work. And if you look at modern alternator they rarely have a fan, most don’t eg the 115A Mitsubishi or the high current ones. And on low rpm engines that’s even less of an effect then 8000rpm redline car engines.

If this is all true, then changing the gearing would be a no brainer, especially if you run the optional engine or are even overpowered (like me)

[roryboy]

QUOTE The 12V alternators used 115A Mitsubishi or the 80A Hitachi or the 140A Biosch have a orange line of 16000RPM so it won't be a problem at 1:3 or even 1:3,5.... QUOTE

Please run your alternator at 10,500 RPM for a season and get back to us with the results.

[HeywoodJ]

Another possibility, the pulleys on most alternators are already as small as you can go and still provide the turning radius required for the belts they use. That only leaves increasing the crankshaft pulley diameter to raise the ratio. Large crankshaft pulleys then restrict installation location, fitting of other auxiliary gear, etc. Since very few of these engines were actually designed for recreational boat use charging a big house bank in a short period of time I doubt that the engine industry has much interest.

[rslifkin]

Originally Posted by HeywoodJ

Another possibility, the pulleys on most alternators are already as small as you can go and still provide the turning radius required for the belts they use. That only leaves increasing the crankshaft pulley diameter to raise the ratio. Large crankshaft pulleys then restrict installation location, fitting of other auxiliary gear, etc. Since very few of these engines were actually designed for recreational boat use charging a big house bank in a short period of time I doubt that the engine industry has much interest.

That's a good point. Going too small on the alternator pulley also reduces the amount of power that can be transferred, as there's less belt contact surface on the pulley.

[CaptainRivet]

Originally Posted by HeywoodJ

Another possibility, the pulleys on most alternators are already as small as you can go and still provide the turning radius required for the belts they use. That only leaves increasing the crankshaft pulley diameter to raise the ratio. Large crankshaft pulleys then restrict installation location, fitting of other auxiliary gear, etc. Since very few of these engines were actually designed for recreational boat use charging a big house bank in a short period of time I doubt that the engine industry has much interest.

That's exactly the 3rd additional point, they look for overall efficency.

But why is there no one make pulley kits? If you are getting Balmar or Grass alternators they all (on Grass, smaller Balmar no) change gearing to 1:2,5 which is optimum for the low rpm Heavy duty ones.


The 115A Mitsubishi is 62mm, you can go down to 50mm, to raise Cranckshaft pully to 1:3 is 1cm more diameter and you need a bigger pulley to for the waterpump, so its not getting a foam maker. Grass does so its possible and you would acctually just need a turning machine or maybe just bigger crankshaft pully from another volvo engine and make the waterpump accordingly.

[CaptainRivet]

Originally Posted by roryboy

QUOTE The 12V alternators used 115A Mitsubishi or the 80A Hitachi or the 140A Biosch have a orange line of 16000RPM so it won't be a problem at 1:3 or even 1:3,5.... QUOTE

Please run your alternator at 10,500 RPM for a season and get back to us with the results.

With the 115A mitsubishi any time, make me a pulley kit and I run it on 10500 max and promise to get back with the results. most likely it will turn 90% of the time with 4500-6000RPM then.

Reorganised Mitsubishi worldwide, so I have still acccess to a lot information there if I need
As I wrote the Mitsubishi Fuso and Canter cooling trucks (if compressors running in the big version they need 50-70A) running them geared to 9000RPM in the 80-100kmh range this trucks are allowed and most commonly drive...a 1 Mio. km is nothing for them, they run in 3rd world contries for 2,5Mio. km+
Volvo engine overhaul between 7000-10000hx60kmh (very high average speed for a truck) makes 600000km equivalent. maybe once replacing alternator, its an wear&tear part for 350 Euro or 200 for an overhauled one.

[Bowdrie]

Interesting thread.
So, after reading thru it I went and measured the pulleys.
It appears to be 1:1.5, no problem getting full output at a fairly low engine speed and have never had any heat issues.
I guess it's ok.

[Jammer]

Originally Posted by CaptainRivet

HI all,


why are the alternators on marine engines are so low geared?
Volvos D1,2,3,6 all alternators are geared 1:1,63 till max 1:2 means at rev limit 3000RPM its 6000RPM.
The 12V alternators used 115A Mitsubishi or the 80A Hitachi or the 140A Biosch have a orange line of 16000RPM so it won't be a problem at 1:3 or even 1:3,5....
So more amps low down and more cooling if it turns faster, 1:3 is 9000RPM and 1:4 is 12000RPM, both far away from the orange line 16000RPM.

Generally because there is no overall benefit and it's hard to do. The reason higher pulley ratios are used in other applications -- trucks, buses, tractors -- is to allow for good alternator output at idle. Usually this is achieved by using higher belt speeds, i.e. a larger crank pulley.

Any overall improvement in charging performance is going to be marginal. Picking up even 20%, in certain infrequently used RPM ranges, doesn't change much.

Whether using a serpentine belt or not, the total amount of contact area between the belt and the pulley is a concern and reducing the pulley size below the standard 2.7" increases the amount of slip. The other way to go is to increase belt speed and that means upsizing the pulleys on the crankshaft and the water pump, which poses clearance problems in most sailboat applications especially if the size increase is large.