Idle - Why is it bad?
 

I've heard that simply idling isn't good for your engine (in any vehicle). Why is this?

I'll leave it to the experts to give a technical explanation, but this reminds me of a funny story. A friend of mine has an ancient Audi that he turned on in the late afternoon following a recent ice storm. The plan was to melt the ice rather than go at it with a pickaxe. So he goes inside and works on remodelling his basement. Five and a half hours later, he remembers that something is amiss, races outside and turns the poor thing off. "it smelled real funny," he said. Nothing seemed to be broken, though -- at least no more broken than it was before.

Lol...

If I had to guess, I would say heat. At idle you have no airflow through the radiator and your water pump is surely not pumping at any reasonable rate. I don't know if it would be enough heat to do any harm as there would be some natural convection flow through the radiator once the T-stat opens(in addition to pumped flow), and the radiator will transfer some of the heat regardless of airflow through it.

stress on the engine. believe it or not, cars that are in motion (not under heavy load) is easier on the engine than idle.

Actually, I think that DANomite's explanation is nearer the mark. Cars do have fans fitted to overcome the problem of lack of airflow at idle or low speeds. ;)

Long warm-ups at near idle were the norm years ago, but I believe that it's now recommended that you use a short warm up followed by moving off gently, with the engine under light load until it warms up fully. Warming the RX8 up a bit before moving is a good idea though, as the flooding issue can apparently catch you if the engine is still too cold when you first move off. :o

Apart from the wear thing, I think that there are also issues of increased internal deposits from excessive idling ("gumming up the works..") plus pollution concerns.

There are a lot of compromises to be made at the high and low ends of engine speed and power demand. For example, the cooling capacity was mentioned. You can't have everything. This is why the gas-electric hybrids can achieve such high mileage despite the inefficiencies inherent in converting mechanical energy to electrical and back again. It's because the design allows the gasoline engine to be optimized for a much narrower range of rpm and power output instead of the idle-to-redline performance that conventional engines must provide.

Just to back up the load-on-the-engine-at-idle theory, the calculated engine load displayed by WDS at idle is around 50%, at light throttle around 3,000 rpm it is about 20 % (not driving the car, just revving the engine).

Long idle can be a problem for the spark plugs as they are designed for the heat generated while driving, not idling. Warm up is the other issue, especially for emmisions.

I disagree with the comments on high stress at idle. Not if the thermostat keeps the engine at the proper temp and the plugs don't foul.

Besides these two points I really feel from many years of technical work that idle is not a problem for the rotary engine. The 2 stroke outboards we sell at Yamaha are commonly idled for hours when fishing. The main problem is the plugs fouling. Second is a increased wear on some crankshaft seal rings only used in the 115 & up that do not get enough oil and gravity tends to pull them downward. This is only a problem for a vertical crank and 2 stroke, not a RX8 motor. Then the outboards can have the thermostat stick open from sand or other trash in the water (no radiator, but outboards use the lake or ocean water to cool the motor) with resulting cold engine problems.

the heating problem was more notorious in the FD models.