Torque vs HP to run a 10kw generator 3

[BSCS-EE]

I am building my own 10kw portable generator. The generator manufacture recommends an 18hp engine.
I want to run the 18hp engine at 2500rpm not at 3600rpm (longer engine life, less fuel usage). I chose 2500 rpm because that is the max torque output value for the engine. I will use different size pulleys to obtain the 3600 rpm at the generator to achieve its max output. The question is will the max engine torque at 2500 rpm be able to turn the generator at full load (10kw) or will it need the peak hp from the engine at 3600 rpm?

 

[BrianPetersen]

That depends on the engine's torque curve, which you haven't supplied us.

In old English units, HP = torque x rpm / 5252

It also depends on the efficiency of the generator, which we also don't know. You'll need more than 10 kW shaft input (engine output) to get 10 kW electrical output.

I'm sure the generator manufacturer is recommending an engine with a little more than the theoretical limit, in the interest of having the generator be able to produce its rated output no matter (within reason) the temperature, altitude, etc and to provide a bit of wiggle room to allow for engine wear, fuel properties, slightly clogged air filter, etc all of which would affect the engine's real world power output.

 

[FreddyNurk]

Given that output power is effectively torque multiplied by rotational speed, there well may be an issue in attempting to run the engine at 2500rpm rather than 3600rpm.

The other thing to consider is how to regulate the engine speed under load to run at 2500rpm, as most engines in that size will likely not have an adjustable governor to allow it to run at the desired speed, rather than the fixed speed 3600rpm that is typical. Generator application engines will be sold with a fixed speed governor capable of running at 3000rpm or 3600rpm, a 2500rpm unit may not be as common, or easily able to be procured.

EDMS Australia

 

[itsmoked]

Have you done your due diligence?

12kW Multi-fuel

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

So what is the significance to you of the max torque rpm? Some engine could have a max torque at zero rpm. That would not be good for running a generator.

 

[FreddyNurk]

I agree with itsmoked.

EDMS Australia

 

[ScottyUK]

Max torque value isn't usually co-incident with max power output and is usually lower down the speed range. Operation at a reduced speed to achieve max torque may not actually give you a longer life as some forces within the engine will be higher at the reduced speed than the higher speed typical of a direct drive generator.

 

[waross]

If this was a good idea you would see a lot of generators using belt reduction to run at reduced speeds.
The normal torque increase of an engine at reduced speeds is one factor which helps the motor withstand and recover from electrical overloads.
UFRO helps also.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

If you're really going to go to the bother I'd use a water cooled engine so you can cut out about 80% of the (damn) noise in one fell-swoop. Pick one bigger so you can run it slower as you want.

I used a belt to run a two bearing 10kW using a nearly new 42HP Renault. It was a cogged timing belt. I stacked them on top of each other to shorten the whole thing nicely. Worked well!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A couple of things.
1. This is a site for engineers. As an engineer we shouldn't have to be explaining to you the relationship between HP, torque and speed. Even a computer engineer should have been exposed to the relationship between HP torque and speed somewhere in his education.
2. If you had bothered to look at some typical performance curves of small engines you would be aware that the HP of small engines drops off with speed. A 3600 RPM motor develops about 75% to 80% of max HP at 2500 RPM.
For 18 HP at 2500 RPM you should be looking at an engine in the range of 22 HP to 25 HP.
If you really want to run slower, use a 35 HP or 36 HP motor at 1800 RPM. This is commonly done to limit noise and wear.
Above 15 kW 1800 RPM sets are quite common and above 25 kW 1800 RPM sets are almost universal.
The idea of running slower is a good idea.
Trying to run slower without direct drive may not be so good an idea,
Trying to get rated HP out of an engine at 77% rated speed may not be possible.
This wheel has been invented.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BSCS-EE]

Great replies... I will use the approach recommended by waross and itsmoked ... bigger engine (an old VW air cooled 4 cyl engine), with direct drive.

 

[waross]

How much running time do you expect?
If you anticipate a lot of run time hours, the, subject to budget constraints, consider an 1800 RPM generator.
If you anticipate only occasional use, then 3600 RPM may be cheaper.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BrianPetersen]

Why would you use a VW air-cooled flat-four to make 10 kW instead of just buying a commercially-available generator with a modern engine, which will surely be more efficient and lighter and with no tomfoolery needed in order to make it work?

I have a Yamaha EF2000i inverter generator (2 kW) that I use on camping trips and the like, and "just in case" to keep the fridge and the furnace running in the event the grid power gets knocked out for an extended period. You can buy bigger ones, and you can daisy-chain these generators together to get more power out of them.

I'm sure Yamaha (and Honda, who make similar products) put a reasonable amount of thought into optimizing the engine size and calibration for this application. It doesn't use much fuel, and it's quiet, and it starts on the first pull of the pull-cord.

Given that the Yamaha (and similar Honda) use an inverter to generate the AC, the engine is free to spin at whatever RPM it needs to in order to be efficient. I don't even know what RPM the Yamaha engine runs at. (It varies, depending on the applied load.) I know it's a tiny engine (I'd guess 100 - 150cc) spinning fast (I'd guess 5000-ish rpm at full load - which isn't much for an engine of that size)

 

[electricpete]

...and you can daisy-chain these generators together to get more power out of them.

I'm not that familiar with these machines, but it doesn't strike me as trivial to place small diesel generators in parallel.

fwiw I do agree with the point that buying an off the shelf diesel generator seems a lot more sensible than trying to put buy separate engine and generator and cobble something together yourself.

=====================================
(2B)+(2B)' ?

 

[FreddyNurk]

electricpete, the specific OEMS have kits to allow their small inverter based generators in parallel, from what I can recall at rather a great cost, although not as expensive as buying the next size up generator. The generators in question are petrol (gasoline?), not diesel.

They are, as far as I have looked into it, specific to particular makes and models of the generator, rather than a generic kit that one can use with any synchronous generator.

EDMS Australia

 

[waross]

These are inverter machines Pete. Generate DC and invert to AC. There are sync cables to tie the inverters together.
Not the same as sync'ing AC generators.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Yes as the sync is happening inside an inverter board.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

thanks Freddy Bill and Keith. I haven't heard of that before, but makes sense that a unit with inverter and the right controls is more flexible than a standard diesel generator. That might provide an attractive low cost entry point for consumers which can easily be expanded later.... gonna look into that for my home one of these days.

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(2B)+(2B)' ?

 

[catserveng]

I have to wonder what you really need. You have sized a 10kW generator, is that your max required power? What kind of load profile do you expect? Long term what you are driving and how you run it will be a big driver in how much fuel you use.

I have three generators I regularly use on my jobsites, a Honda EU2200i, a EU3000iS and a Generac XD5000E diesel unit. The Honda's are nice inverter units, and most of the contractors i work with have similar units, so I keep a parallel cable with each one and it meets most of my shorter term needs. The 2200 is easy to move and provides good power, and runs most of the day for smaller jobs. The other Honda is heavier and is used for longer or larger jobs. For bigger or longer term jobs I use the diesel Generac. Selection is based on what I'm actually driving and for how long. Sometimes issues like noise are a factor, such as during a nighttime outage, the 3000 watt Honda is actually quieter than the smaller unit, and even though may not need the extra power I'll use it to keep from pissing off the neighbors.

If what your end use of your generator is not that important then a home built belt drive unit operated once in a while will likely be fine, but if your gonna run larger equipment for longer periods of time, a commercial product may be more suited to your needs.

An old friend last year started building a cabin up North, he is an old time heavy equipment mechanic and has doe a fair amount of generator work in his career, he decided to "roll his own" generator for construction and then to have as a backup once he got his off the grid cabin all online. The belt drive ended up being a pretty constant PITA, especially when starting his sawmill, at lower loads the fuel consumption was higher than he planned, after about 6 weeks of messing with it he was down for a visit and asked to borrow my Generac 5kW diesel unit, did everything he wanted and more and in the end he just bought me a new one and kept mine, while at first it seemed too expensive, in the long run it was the better choice.

My 2.5 cents, MikeL.