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

[vpbarkley]

You certainly should increase the drive ratio of your alternator so it turns faster. A serpentine belt drive system is what you need.

Serpentine belts can overate at smaller driven pulley diameters so greater speedup ratios are available, without greatly increasing the diameter of the driver pulley.

Serpentine belts also offer higher capacity with less slippage and belt wear. This is the ideal belt arrangement for an alternator.

[RaymondR]

I am going to rebuild my gas powered battery charger and will be using toothed instead of friction belt drive on it.

[CaptainRivet]

Originally Posted by dalemann

Marine alternators are for the most part well engineered. If you want better output you need to install an intelligent controller. The alternators that come mounted on engines are designed to give a large and short boost to bring the start battery back up after startup. Then they drop the output. If you want to charge the house batteries you need an intelligent controller. Check out Sterling in the UK. Great stuff. Makes night and day difference in the alternator's ability to charge the house bank.

If it’s a marine alternator….on most small 3 and 4 cylinder leisure engines for sailing boats you will mostly find low geared car alternators from stock.
The widespreaded Mitsubishi 115A alternator found on newer D1,2,4,6 engines is already an exceptionally good Stock alternator being a re-enforced version for light commercial truck with high power demands but still a high reving model as you can see from spec sheet I uploaded.
And compared to a car alternator it has some hidden potential, that is not used. If you look at the technical drawing, you will also find it is rated to deliver 111A at 5000rpm, so it reaches the 115A plateau at around 5500RPM.
Geared with 1:1,63 and volvos limiter at 3000RPM=4800RpM alternator speed it is not even reaching his rated output at WOT. Geared to 1:3 typical motor cruising rpm for me is 1800 till 2000rpm so 5400-6000rpm alternator which means it just reaches his max. Rating or the plateau of the charge curve, turning it faster will improve cooling but not much more current output so optimum being on the point. Factory geared it 1:1,63 as this will be most likely the highest efficiency, gearing it 1:3 will be maximum output with reduced efficiency and slightly reduced life span. Overhauled oem are available for 200Euro, so I can life with that.
orange line is 16000rpm so far away at WOT with 9000rpm and well within specs.

[Trawlerman]

Jammer has made the key point. For an alternator to output more amperage, it will present more reluctance to rotate at its pulley. The ability of the drive belt to drive the alternator will depend on the friction constant and the surface area of the belt in contact with the driven pulley. For the typical amperage available, say 110 amps max, the manufacturer has pretty well established the minimum size the pulley diameter can be to achieve sufficient friction to drive at a reasonable belt tension, and without slipping and belt wear. You could speed up the rotational speed with a larger pulley on the engine (with concomitant issues), but for most practical purposes, the pulley sizes are already optimized for typical use. Also, it is worth noting that there is very little extra amperage payback for spinning the alternator armature much faster. This sort of discussion occurs very often, wherein it is assumed that the engine manufacturer engineers did not do their homework. Almost invariably, it turns out they did.

[CaptainRivet]

Originally Posted by Trawlerman

Jammer has made the key point. For an alternator to output more amperage, it will present more reluctance to rotate at its pulley. The ability of the drive belt to drive the alternator will depend on the friction constant and the surface area of the belt in contact with the driven pulley. For the typical amperage available, say 110 amps max, the manufacturer has pretty well established the minimum size the pulley diameter can be to achieve sufficient friction to drive at a reasonable belt tension, and without slipping and belt wear. You could speed up the rotational speed with a larger pulley on the engine (with concomitant issues), but for most practical purposes, the pulley sizes are already optimized for typical use. Also, it is worth noting that there is very little extra amperage payback for spinning the alternator armature much faster. This sort of discussion occurs very often, wherein it is assumed that the engine manufacturer engineers did not do their homework. Almost invariably, it turns out they did.

The manufacturer engineers did their job very well but what was the target list that was given to them?
Definitely not give me the most that alternator can deliver to recharge a Lithium bank.
Their target list was: charge a lead starter Batterie and provide the amps the big mass manufacturer boats use in stock config with as small as possible power drain and load on the engines to pass strict emission norms and enable them to put as small as possible engine into that boat.
That they hit spot on, bullseye.

We have a different target list and are a very small customer target group they just ignore. Our goal give me all that alternator can deliver to charge my LFP bank permanently. With most car alternator they put on small leisure diesels like the 80A Hitachi spare yourself that, you are correct.
but the 115A Mitsubishi is different here. Loads hidden unused potential, see above and the gap between highest efficiency and highest output operation is big.

Facts:
D4-220 has a 48,6mm pulley from stock and is geared 1:2 compared to D2-50F with 62mm and 1:1,63.
Data sheet and Mitzi engineers confirmed it can do 100A constant for an 1h, then 90A but need to spin at around 8000-10000 as they do in fuso/canter cooling trucks=highest output setup, because engine has loads of reserves for heavy bulk goods like sand, dirt which a cooling truck never needs.
With 1:1,63 at 2000RPM engine or 3300 alternator on marine engine it can do 80A for about an hour and real 50-60A constant in a well ventilated engine room as cooling missing. Most efficient setup.
If you see above, the current delivery at typical cruising RPM 1400, 1800, 2200 increases significantly with 1:3 gearing as cooling capability rises.
If you could get additionally the rectrifier externally you successfully have converted it to a permanent 100A high output alternator.

Possible solution would be drive the alternator with a timing belt which allows for a very small pulley.
Actually done that on my classic mini with motorbike engine using timing belt to connect engine and differential which also Harley use as chain replacement to get a 1:3,5 due to space limitation.

[Trawlerman]

Going back to your original question:
"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...."
You have pretty well answered your own question, when you comment that they designed for a target customer. Having said that, it is not very reasonable to expect them to design for a relatively small target group (us boaters) and especially for an even smaller group using lithium based batteries. Having said all of this, there is nothing stopping any of us from redesigning our system, perhaps changing the flywheel pulley diameter and type, and using a large capacity alternator rotating faster. I would point out however, that I would like to see a curve showing how much more current is available above about 6000 RPM armature speed. I suspect that the red-line specs they tend to give have more to do with the alternator self destructing point rather than it's maximum optimal output RPM. Attached is some info from Bosch on their 210 amp alternator, and as you can see on the left curve, there is very little to be gained above an armature speed of 4500 RPM.


There is no doubt that as Lithium batteries become more common, that there will be manufacturers who design for that purpose.

[Statistical]

One aspect is that large pulleys take up more space. The alt pulley can only be so small meaning you would need to increase the drive pulley. Across all the variety of engines and engine bays it is simply more universal to have a low gear ratio. The other aspect is that for this weight class there are no marine engines. There are industrial diesel engines with marine kits. 99.9%+ of the users are not smallcraft boaters and they have zero need for higher output at lower engine speeds.

Like a lot of smallcraft marine stuff the market isn't that big so a lot of what is done isn't done that way because it is ideal it is done that way because it is economical and shoehorn it from larger markets.

For a "modern" sailboat with a large LFP battery bank yes a much higher gear ratio is probably better for both usable output and cooling at idle. It isn't going to come that way stock though.

[CaptainRivet]

@trawlerman:
There is not many more amps additionally but a lot of cooling above 5000RPM mark to help the alternator keep its thermal balance regarding your charge curve. We are talking about permanent capability for >1h motoring.
What Bosch is stating 2000RPM at engine idle which is typically 500-800RPM depending on the engine, so 1:2,5 till 1:4 they suggesting to get full alt output at idle but if you look at left chart that shows the effect of the self cooling and it does max 280A at 6000.
My typical motor cruising is 1800-2000, so there I wanna have the maximum output with best efficiency achievable. In the case of your Bosch 280A 4650RPM plus some RPM that supports the cooling, so the 5000RPM is your target. Means the 1:2,5 ratio Mark Gasser also chooses on the big case alternators. Above 2000 engine rpm you don‘t get many more amps but cooling and your target here is that your alternator can turn that fast without getting damage but not to harvest more amps, means WOT is orange line max. With 7000 On alternator.
On the 115A Mitzi, that’s 6000RPM so 1:3 gearing and WOT 9000, orange is 16000 so far away. Small case, needs more rpm for cooling