[Question] Generators

[g20313225]

Hey everyone..

I'm new to this but i have a few questions.
If you have an average 4kw Generator, what is the maximum "torque" needed to keep running the gen on ful power and at what RPM??

Thanks

[AndyD]

Hi g20313225, welcome to TFSA.

Can you give some more info about your question? You say you have a 4 KW generator, this would normally be labeled as a 4 KVA generator. Can your generator give a 4KVA output constantly or would this be the maximum power for a short duration?

Also you are looking for the maximum torque (τ max) in newton meters (Nm) is this a school or university type calculation? Torque is a simple product of a force being applied at an angle but can be a very complex calculation.

Are you perhaps looking for a required shaft power in Kilowatts so you can specify a correctly sized engine to drive the generator?

[g20313225]

Hey thanks for the reply..

This would be the Maximum power.. Yes I'm looking for the shaft power in Kilowatts so that i get the correct engine thanks very much..

[AndyD]

Okay, I'm going to make a few assumptions. Single phase genset, 4KVA constant output with max short delivery of 5KVA driving an AC 1or2 load, (not large induction motor start current rated). I'm also assuming direct drive from the primary drive on the crank so minimal transmission losses. If you have a gearbox or belt and pulley system you'll need to upsize by around 20% to compensate.

Pure drive kW power I would work on double the electrical output to allow for genset inefficiency, also a substantial cooling fan for the engine plus a safety margin so you're looking for about 8 kw engine output at around 3500-3700rpm. I would use a 4-stroke petrol engine for economy and suitability at constant rpm. Maybe an overhead valve configured engine for flatter torque characteristics.

Depending on how restrictive the exhaust scilencer is and the stage of tune the engine is set at I think you loking at a 4-stroke petrol engine between 250 and 350cc, maybe nerarer the 350cc end of the scale. There's portability and fuel economy to consider if you want to use a larger engine, much bigger than 350cc they start getting heavy to move around as well as heavy on fuel.

*Edit*
I just did some digging around for specs of 'off the shelf' generators and the figures look reasonable. It looks like short stroke singly cylinder engines are a firm favourite, I'm not sure if this is to make them compact of if they give better performance in this particular application. Who knows, they might just be cheaper to manufacture. Here's a typical offering of a 4 KVA generator. At 82 kilos dry weight it's a bit Rubinesque, I wouldn't like to have to carry it far. Also you wouldn't mistake it for economical at nearly 2 litres per hour consumption.
Detailed Specifications:
Model XR5500
Generator AC Rated Voltage 220
Frequency (Hz) 50
Max. AC Output 4.8 Kva
Rated AC Output 4 Kva
Noise Level at 7m 72 dB
Engine Model XR 182F
Type Air-cooled,1-cylinder,4-stroke,OHV
Displacement (cc) 340
Max. Power output 8.1kw/3600r/min
Starting system: Recoil/Electric
Ignition System Transistor Magneto
Fuel Tank Capacity 25 L
Continuous operating time(h) 10
Fuel consumption 1.9 L /H
The Whole Dry Weight 82 Kg
Dimension 700x540x 530 mm

[Dave A]

The diesel alternative makes for some interesting comparisons. Over double the weight for starters.

Fuel consumption wasn't listed, but to save anyone hauling out the calculator - works out at about 1.685 litres per hour.

[AndyD]

I never really considered a diesel option. At small sized engines you tend to have to put up with the disadvantages of diesel without realizing any of the advantages you get with the larger diesel engines. Also 3600 rpm isquite high for a low-tech diesel engine, I doubt the diesel would be any more reliable at this rev range.

I was surprised to see the sound figures were lower for a diesel but when I scrutinised the two side by side it's because the diesel version comes in a sonic attenuating enclosure whereas the petrol is open frame. This would also account for some of the extra weight on the diesel version coupled with the fact the diesel is 406cc whilst the petrol is 340cc.

I have to say that in this day and age the fuel consumption of both versions is shocking when compared to their actual mechanical power output. Both versions are guzzlers of note.