Does a 120 amp alternator put enough load on diesel?

[Valmika]

Originally Posted by Paul Kelly

Could you try having shorter showers?

1.5 gallon showers but still cools as new cool water enters WH.

[Dockhead]

Originally Posted by Stu Jackson

1. Just in idle will do squat for charging. Period. You need to run at at least 1500 rpm to get the alternator to actually produce anything worthwhile.. . .

That depends entirely on the alternator, and on the gearing.






School bus alternators have steep output curves, as they are designed to put out power at idle.


Mine, for example, which is geared at 2:3.5, puts out more than 50% of its rated output when the engine is running at 1000 rpm. So at least 1.3kW.




I think the OP's idea was to get hot water from engine cooling loop and a calorifier, and turn on the electric hot water heater just to load the engine for health. I don't think it's a dumb idea at all, but depending on his alternator (and how it's geared), it might not be doing much to help his engine.



But I agree with what others have said -- idling the engine for 20 minutes or so to make a tank of hot water is not going to hurt it, if it's run under load afterwards. Or he could just put it in gear.

[Dockhead]

Originally Posted by Valmika

Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water

A lot of people put the boat in gear for this purpose. At anchor, just put it in reverse and at idle, with the helm centered. And someone on watch!!

[bradfordharley]

The info provided in previous threads is good math, however efficiencies are way off. There is an incredible amount of loss due to heat in an alternator. The general rule of thumb I’ve always heard is a minimum of 1 hp per 25 amps. If in doubt, do a google search and also look at the dc generators on the market. You usually get 100 amps from a 5.5 hp generator. It will need to be a lot more than idle to both generate rated power and cool itself.
If it were me, I would plan 7hp per 120amp alternator and plan on running above 1500 rpm (or 2000 for a 3k+rpm diesel). I myself have two valeo 125amp alternators and get about 70 amps from each when under load and have to throttle field to keep them from cooking.
Good luck!

I

[Woodland Hills]

Originally Posted by nhschneider

You can safely idle your engine for 45 minutes. You can safely idle your engine for 45 hours though you might develop some substantial carbon deposits. Have you ever seen a diesel-powered crane in operation? They’ll often idle for days on end with no ill effects. Your engine will be fine.

Fair winds and calm seas.

This could be an apples to oranges comparison if the crane engines are newer computer controlled units vs an older 40 hp mechanical engine in a boat. One can idle indefinitely, the other, maybe not so much.

[Cheechako]

I'm not sure I would heat water that way, but the load is fine. You dont need much load on the diesel anyway, contrary to what many believe.
But put a heat exchanger in your water heater and use the engine heat to heat it up. It stays warm for about 24 hours and works great!

[a64pilot]

This is yet another excellent use for the little Honda generator.
Yesterday our little Honda charged batteries significantly, made 100 gls of water, washed three loads of clothes and heated 12 gl of water for showers.
All using substantially less than a gallon of gas.
To heat 12 gls of water until the thermistor cuts power takes a little less than an hour and less than a liter of gasoline.
Our water heater draws 12 amps at 120 VAC, so 1440 Watts.

[Dockhead]

Originally Posted by Cheechako

I'm not sure I would heat water that way, but the load is fine. You dont need much load on the diesel anyway, contrary to what many believe.
But put a heat exchanger in your water heater and use the engine heat to heat it up. It stays warm for about 24 hours and works great!

That's what he's proposing to do!! He already has the heatx, as far as I understand.



He's just asking about how to load up the engine while he does so.

[hzcruiser]

Originally Posted by Valmika

Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water

Also keep in mind that, in general, an idling engine is not revving the alternator fast enough to put out max power. Unless you have a smart alternator and replaced the alt pulley with a smaller one. That's what some people do to get the most out of their diesel engine.


BTW, idling for an hour every now and then won't damage the engine (as in glazing the cylinder walls), as mentioned by someone else.

[a64pilot]

You really can’t go smaller with the average alt pulley, it’s as small as you can go without belt slip and bending the belt too tight, even a serpentine belt.

Can’t usually go larger on the crank pulley either, however what you can do is not easy and it’s not cheap, you have two belts with a jackshaft in the middle and compound the thing.
Mark Grasser offers a belt kit that speeds up an alternator, to begin with even at max RPM we spin alternators too slow for optimum speed for max current output, plus speed it up and you increase cooling significantly.

However for less money, buy a Honda, use it to heat water, make ice, charge batteries, all kinds of things and save the hours on your engine.

[hzcruiser]

Originally Posted by a64pilot

You really can’t go smaller with the average alt pulley, it’s as small as you can go without belt slip and bending the belt too tight, even a serpentine belt.

Can’t usually go larger on the crank pulley either, however what you can [...]

However for less money, buy a Honda, use it to heat water, make ice, charge batteries, all kinds of things and save the hours on your engine.

As always, there are pros and cons to every solution. A separate genset could make sense if you have the room for it and carry ULP (gasoline) anyway for the outboard.
If the OP thinks about a bigger alt, I would look into an externally regulated one with a smart regulator to get the most out of it (possibly without even changing pulley sizes).

[Pizzazz]

May be you got the picture already but the actual numbers are like this. A 40 hp diesel at idle makes around 2-3 hp just to turn (think about cranking it yourself at 1000 rpms with no load. If your alternator is geared correctly, it will start producing current immediately and at fast idle (1,300 rpms) it should be capable of close to full output. The efficiency would be about 55%. Therefore, if you manage to load your alternator fully through the inverter (say 90-100A @ 14V, this is about 2 hp electric or 4 hp mechanical).

The load on the diesel will be minimal but there is no issue to run the diesel at low rpm's, just do not do it all the time. In terms of fuel consumption, if you are making 7 hp altogether, burning around 240 g/hp (fairly constant) and burning around half a gallon an hour or around $2/hour. It is a good way to make hot water but make sure you are not actually discharging the batteries, this is not optimal because then you need to charge them again.

SV Pizzazz

[seabreez]

Originally Posted by donradcliffe

It wouldn't be a problem for the engine, but it would take about 50 amp-hours out of your batteries. The heater/inverter will be pulling about 140-150 amps with losses, and the 120 amp rated alternator will only put out about 90 amps if you are lucky.

I second the motion here... to answer your question... it is safe to run the engine 30 minutes, your batteries will not discharge to a point of not starting...