It would probably be more energy efficient to put a slightly larger engine in the car to begin with, and would not require a second fuel injector, etc.

If you already figure that the AC option is priced at near $1000, then you know that the original parts-cost must be at least half of that.

Add the cost of a 5-horsepower engine with an electric starting system and some kind of valving to get gas from the regular gas-tank, electronics, etc, you can see that the cost of it would probably be prohibitive. And now you've got two engines to change oil, sparksplugs, etc for.

On the other hand, the value is small--just a little more passing power--most of the cars I have cut the AC compressor out when the throttle is in the 90% or further range.

Now if you were proposing the A/C be changed to a hermitically sealed-compressor-electric-motor unit like most homes and refrigerators use, I'd be more apt to agree--on the basis that these seem to last forever with no maintenance. Fan-belt driven compressors are only as good as the fanbelt.

I know it is prohibitive to have two engines in a car - Only high end modelscan afford this - Maintaining two engines is not going to be a easy task.I just want to isolate the belt driven compressor from the engine - I amnot sure if a additional engine would help - Or if you can think of analternative way of running the a/c without putting strain on the engine.A small electric motor which charges a battery to run the a/c so that youdo not have much strain the engine when you are cruising.

An electric A/C that can run with the engine off would need a heavy duty alternator, plus alot of storage batteries, and space for those batteries.

The engine would be loaded charging the battery set, and by the extra weight of hauling the battery pack around.

Another possibility is to have the compressor connected to the engine, but store "cold" in a container of eutectic salts, which absorb heat in a controlled fashion as needed when the engine is off. I admit though, I am not too sure of the physics of such a material, or the practicalities of installing such a system in an automobile.

As for specifically a second engine for A/C, I know of a long-haul trucker that had one on his truck. It also provided heat (engines do that) and 120V electricity to power appliances. I don't know how one could be fitted into a typical motor vehicle (A fifth wheel semi-tractor has plenty of room to install one), or how anti smog/idling laws will be compatible with them, especially when they are only for something as superficial as comfort.