Random question from a non-gearhead.
Why is it that with an aircraft engine I can start it up and set throttle so that it's at idle (1000rpm for me) immediately after startup. And if anything, bringing throttle down immediately after startup can result in RPMs dropping below idle.
Meanwhile on a car when I start it up, particularly in the cold, it'll run sometimes as high as 50%+ over idle RPMs until it's running for a minute and then it'll settle down to normal idle RPMs.
In short -- why is it that the car runs at higher idle RPMs until it's warm?
As stated, the current requirement for this from above is for catalyst light-off. This has to do with EPA requirements. Basically the purpose is to get heat (from the exhaust) into the catalytic converter that gets the catalytic process going faster which reduces tailpipe emissions.
The mechanism that allows this is the fact that your car has some form of idle air control. In the 80s and 90s (earlier days of electronic engine controls), this was primarily handled by some form of idle air control valve. These days where essentially every engine is throttle-by-wire (no physical cable between the gas pedal and the throttle butterfly) this mechanism is generally handled with the electronic control for the throttle body.
If you go back to older carbureted cars, the mechanism did vary to some degree as there were different designs. But your standard mechanical choke carburetor on an American car had the process of flooring the gas pedal twice to engage the choke. This would both enrichen the mixture by "choking" the carburetor, but then sometimes would also add a bit of air depending on the carburetor design. Start the car and it's at a high idle for a while, you hit the gas again to disengage the choke and it goes down.
With older carbureted engines, they often needed to run at a bit higher of an RPM for a while until they warmed up because the overall engine design, weaker ignition, and carburetor design sometimes wouldn't idle very well until it got some heat in it. Some of that was due to combustion, some of that was due to thick oil taking a whole lot more horsepower to flow. Modern engines really don't have this problem, we've come a long way.
Electronic ignition is not entirely an accurate statement, as this is really more a function of airflow than ignition.
When you're talking about aircraft engines, 1) you don't have emissions standards you have to deal with 2) you don't have electronic engine control generally (and when you do, at least the one I worked on didn't have any sort of electronic throttle/air control) 3) the fuel systems don't have any kind of choke mechanism, you just have the mixture knob and primer.
With that said, sometimes when it's especially cold out (the kinds of temperatures where you really should be preheating the engine) you might find that the engine is happier running in the 1000-1500 RPM range to get some heat into it so that it runs better and won't necessarily idle happily at first. I've started these things down into the 10F range without preheat and they generally aren't very happy. Coldest I started one was -25F with not nearly enough preheat and I got it going and it was very unhappy at first. The takeoff distance that day was roughly equal to that of a helicopter.
)
