engine or genny?

Grehan

Grehan

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Inland France + Oxon.
www.french-waterways.com
We have a Kipor 2kW generator which is pretty good, easy to start, fairly quiet. In an idle moment I am wondering about the relative efficiency and/or fuel consumption comparing the Kipor - delivering 240v into our shore power socket - with our Nanni/Kubota 50HP engine - delivering 12v and then an inverter producing 240v.
The Kipor manual says its Set Fuel Consumption SFC is 500 g/kW.h whereas the engine seems to be about half that (plus losses (?) for the inverter).
Anyone who knows what's what will see I'm floundering here, but on a rule of thumb basis, how do the two compare purely on efficiency/economy?
 
After 10 or 15 years if you have to throw the Kipor away you will think "not bad for the investment". However, after 10 or 15 years of gentle running the engine needs a rebuild because the bores are glazed and it wont' start easily you might think differently.


Pete
 
I would think the engine will take more than 500g/hr to tick over.
They tend to quote best cases figures, at most efficient load and RPM.

There were some graphs around for Yanmars of fuel use vs load at a few different RPM's.

Also, some inverters are not intended for heavy use so may not last long if used for long hours. I have a cheap eBay one which is great, but I would not rely on it long term for hard use. I expect you get what you pay for. Again the efficiency varies with load and they tend to quote either best case or full load.

But then again, not every yacht has space for a generator, so the inverter is a boon.

I think there is a lot to be said for keeping life aboard simple enough to run off 12V...
 
Use the Genny to provide 240v while charging the batteries then use invertor 'til batteries need recharged.
 
Having lived aboard for 10 years off and on, I appreciate the 12v comment, but I'm still interested in the 220/240v efficiency comparison.
This is a typical Kubota/Nanni power curve, which indicates a fairly flat ~250 g/kW.h
Kubota-V2203-M-PC.gif
 
cliff said:
Use the Genny to provide 240v while charging the batteries then use invertor 'til batteries need recharged.
Click to expand...
That is of course, exactly what I am doing, but . . .
[edit]
I'm also aware of the pertinent practical comments made in the other thread at http://www.ybw.com/forums/showthread.php?t=319100, but I am still interested to know the relative 'energy cost' -
  • Generator 240v into shore power socket and used direct
  • Generator 240v to charge batteries, then 12v used to power inverter to supply 240v
  • Engine to charge batteries, then 12v used to power inverter to supply 240v
?
 
Last edited:
Grehan said:
That is of course, exactly what I am doing, but . . .
[edit]
I'm also aware of the pertinent practical comments made in the other thread at http://www.ybw.com/forums/showthread.php?t=319100, but I am still interested to know the relative 'energy cost' -
  • Generator 240v into shore power socket and used direct
  • Generator 240v to charge batteries, then 12v used to power inverter to supply 240v
  • Engine to charge batteries, then 12v used to power inverter to supply 240v
?
Click to expand...
As you are in froggie land option 2 will be more economical in the mid to long term provided you have a good 240v charger (>60A) - a couple of hours or so should recharge your batteries - surplus 240v power canbe used for other items while charging the batteries (fridge/freezer, wine cooler immersion heater etc)
 
I would have thought that using the genny would be more efficient to produce 240V the genny will be sized to produce the 2kW and run efficiently when loaded.

Your engine will be sized to push your yacht along and run efficiently at hull speed (or should be). Therefore the engine driving the alternator to produce 12V to top up batteries and run an inverter will mean the engine is running very lightly loaded (unless you have several large alternators which can absorb most of the engines power). Diesel engines do not like to be run lightly loaded and they are pretty inefficient at tick over, depending on the engine the most economical point is 60%-80% power which you won't get producing 12V only.

For all the other options you will lose power everytime you convert from 12V DC to 240V AC and vice versa. Charging batteries is not particularly efficient either.

I suspect that the most efficient option is to run the genny when you need to use a lot of power...hoover, washing machine?:) and top up your batteries at the same time so that the genny runs at max efficiency. Then run off batteries for lower powered stuff.

Ross
 
Just as easy to do it empirically.

Disconnect the fuel line from the filter, put pipe in measuring jug, run engine. Measure time taken for an amount to be used.

Please let us know the result!
 
Going right back to my mechanical engineering degree days, a couple of figures that stuck in my mind (because engines have always been of interest) are that a petrol engine is at best about 25% thermally efficient, and a diesel engine about 40% at best. (i.e. about that proportion of the total energy in the fuel is converted to mechanical output). Things have improved a bit since then too.

Given that you are probably not running either at optimum efficiency my guess on relative efficiencies would be more akin to 22% Kipor and 35% Nanni/ Kubota.

I'll let you play with the numbers from there, remembering also that with the inboard diesel you also get hot water from the calorifier (I'm assuming), increasing overall efficiency considerably. Perhaps up to 50-65% ish? That's something you don't get from an air cooled genny.
 
Last edited:
Grehan said:
but what about efficiency?
Click to expand...

To get the specific fuel consumption figure quoted on the specs, you need to be able to use all the power that the engine can develop at a particular RPM.

SFC increases to an infinite number at no load.

The lowest speed on the 50 Hp Kubota V2403 (might be your engine, might not) graph is 1300 RPM, where it is already able to develop over 20kW. If you've got a 20kW alternator, then you could benefit from an SFC of ~235g/kWh.

A 20kW alternator would be putting something like 1400 amps into a 12V battery bank. Anything less than this will increase SFC as an increasing proportion of the fuel used will be just to overcome friction losses and keep the mechanical bits whirling around and the oil and water circulating.

Too many unknowns to be able to predict what is "most efficient".

Andy
 
misterg said:
The lowest speed on the 50 Hp Kubota V2403 (might be your engine, might not) graph is 1300 RPM, where it is already able to develop over 20kW. If you've got a 20kW alternator, then you could benefit from an SFC of ~235g/kWh.

A 20kW alternator would be putting something like 1400 amps into a 12V battery bank.
Click to expand...

Given that a big alternator is about 150A, and many boats have alternators that are between about 55A and 90A, and that batteries won't accept the full power of the alternator for long, the above scenario will only use about 5% of the available power.

I'm for using the genny to charge the batteries, (through a 70A charger), and provide 240v at the same time.... For a variety of the reasons given above.

If I need to run something over about 500W, I'll use the genny... Less than that, I'll use the inverter for a while.
 
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