Thanks Paul, that answers my question. I suppose you will be able to observe what happens to your clutches over a period, and consider adding a speed control if it turns out to be necessary. Alternatively you could reduce your fixed motor speed, or change the final drive sprocket ratio. However because you have 4 batteries, a simple contactor-controlled selectable motor voltage system seems the easiest to implement. The down-side is that you would then have to charge the 4 batteries individually, requiring 4 regulated, high current 12 Volt battery chargers.
The Greenfield clutch system seems to work very well, provided you have a governed petrol engine with variable speed. The Briggs petrol engine has a minimum speed of 1,750 rpm and a maximum of about 3,200. Of course you can't get much torque out of the engine at idle, but from about 2,000 rpm upward it will pull quite vigorously. My current guess is that you are going to have to make a change of some kind in your electrical design to achieve something similar, but time will tell.
