Sharing power


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Posted by David Sherman on Tuesday, April 01, 2003 at 10:53AM :

In Reply to: Re: OT Weird Vintage 4x4 posted by Clint Dixon on Tuesday, April 01, 2003 at 0:21AM :

This is basically the mechanical version of the situation you have when connecting several generators or several power supplies to the same electrical load. It's done all the time, and so long as the output characteristic of the sources is such that the voltage drops with increasing load, they will automatically share to a greater or lesser extent. It's only when the sources are extremely "stiff" that the controls get more tricky. In the case of gas or diesel engines, instead of an electrical output resistance, we have the torque/rpm curve of the engine. Since loading down the engine always slows it down for a given throttle setting, two or more similar engines in parallel ought to automatically share the load reasonably well, depending on how closely their torque curves matched for all throttle positions. A squishy torque converter would only help with this, but would not be necessary.

I do think you'd have trouble with trying to share two diesel engines that each had their own governor. The effect of the governor is to make the rpms of each engine less dependent on its load, which means that whichever engine is stronger or has a slightly stiffer governor will end up taking most of the load. In the worst case scenario, one engine will end up always running at max power and the other one will only contribute when even more power is needed. I'd be curious to see how Cat handled that. I suspect the two governors are interconnected somehow, or perhaps one engine has only an injector pump without a governor, and is controlled by the "master" engine's governor.



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