How to measure engine starting torque?

[Cantona156]

Hi

Couple of my friends would like to build our own onboard engine starter.
I guess I need the torque to overcome the engine compression @500-800revs ...

But i'm not sure how to determine the torque requirement of the engine.
Is there such a device to determine the starting torque of IC engine?

Thanks. :cool:

[Salto]

Get a spring balance, get an old prop, drill a hole in one prop blade at a known distance from the hub, hook the spring balance through the hole. With the engine primed (so the piston/bore/ring is lubricated) pull the spring balance and note the maximum force reached on the scale. You want to do this with as fast a rotational speed as you can. As you pull and the prop rotates, keep the line along the spring balance at right angles to the line along the prop blade.

The force that you read on the scale times the distance from the hub will give you the slow speed cranking torque. This will be about 70-80 % of the cranking torque at 500 - 800 rpm.

Graham.

[BMatthews]

Or you could measure the stalled torque of a common Sullivan electric starter running on 12 volts. They USUALLY start the motor but I've seen them stall if the engine kicks at all. Not perfect but a good starting point.

I'm not sure you'll have much luck with the 500 to 800 rpm thing. Way back when I had a second hand heli to play with I thought I'd save the money for the starter and just use my Makita battery drill. Speed was around 800 rpm or so. It was VERY hit and miss if it would start the engine (a Thunder Tiger 46). Oh, the engine spun and spun but the rpm just didn't seem to be high enough. It would catch and run now and then but it was far from reliable. Buying a Sullivan starter and heli adapter resulted in virtually instant starts.

THe moral is that RPM counts as well as torque. I would say that you're going to want to have a good 1500 or more RPM at the torque needed.

Good luck.