[Setec Astronomy]

It's not as simple as running rich, but that's part of it. The basic explanation is that you're converting the fuel to heat. The heat expands the combustion products, driving the pistons up and down. A vehicle is not very efficient at capturing this heat; in comparison to say a modern condensing boiler/furnace in your house, which captures so much of the heat for heating your house that it can use a plastic (PVC) exhaust pipe. In your car, a lot of the heat goes out the tailpipe, and a lot of it is absorbed by the various metal parts of your engine, and the coolant and oil. If you do a lot of short trips, you keep using the fuel energy to "re-heat" all those parts of your engine, especially during the winter when they cool down quickly.

The open loop/closed loop different is similar but different. The O2 sensors and the catalytic converter don't operate properly until they reach a certain temperature. Since the engine control computer can't read the O2 sensor it can't be sure the combustion process is tuned properly, and therefore uses a pre-programmed "open loop" fuel map based on throttle position, MAP, etc. As noted, this mixture is rich.

I think the easiest way to visualize this is to think about the heater in your car. How does this work? Hot coolant is run through a heat exchanger, and air is drawn over that heat exchanger (your heater core) to be warmed and then enters the passenger compartment. If you drive for 20 minutes, stop for a cup of coffee for 3 minutes, and get back in the car, your heat will still blow hot. If you get out for 2 hours to have dinner, it will blow cold. You need to put the heat back into the coolant in order to take it out to warm the air. The heat that you put into the coolant is energy from the gas in your tank, so the more times you do it the more energy you use. That's a little oversimplified, but whatever.