What effect does an AC have on the car engine?

At idle? That might be a problem long term. If it's at cruise speed, it shouldn't hurt a bit unless you are in some really extreme temperatures, like climbing a ten mile hill out of Death Valley in August. If the A/C is designed for your car/engine combination, it shouldn't get into an overheating condition. The only other noticable effect would be fuel consumption.

Most modern AC systems can handle extended idling with no problem. Look at police cruisers. It helps to have a big V8 but now a days even 4 cylinders with factory engineered AC can idle for long periods with out stressing the cooling system.