Basic Question About Idling

[averona]

Why does the car have such high rpm's when just started? What is happening as the rpm's "settle" down to 500 or so?

I know this is a rookie question, but I've always been curious.

[curiousB]

ECU is controlling engine to run a little faster at startup to promote lubrication and for engine to warm up.

[SRK]

That's a good question, and the answer is a bit complex. When the engine is cranked for starting the initial rpm is "flared" to ensure that all cylinders fire quickly without misfire, reducing emissions. After start-up the idle is held high to allow for leaner mixtures during cold running, and to help bring the catalytic converters to temperature faster, and to heat up the O2 sensors faster. This is all done to get then engine through the warm-up phase more quickly, as that phase is when emissions tend to be highest.

Once the engine and sensors are warm of course the computer goes into closed loop, and emissions are far lower.

[averona]

You guys are good

That's a good question, and the answer is a bit complex. When the engine is cranked for starting the initial rpm is "flared" to ensure that all cylinders fire quickly without misfire, reducing emissions. After start-up the idle is held high to allow for leaner mixtures during cold running, and to help bring the catalytic converters to temperature faster, and to heat up the O2 sensors faster. This is all done to get then engine through the warm-up phase more quickly, as that phase is when emissions tend to be highest.

Once the engine and sensors are warm of course the computer goes into closed loop, and emissions are far lower.

[killerFatty]

After start-up the idle is held high to allow for leaner mixtures during cold running, and to help bring the catalytic converters to temperature faster, and to heat up the O2 sensors faster.

Dont you have that backwards? It is running rich to warm up the cats and O2 sensors. Similer to the way a lawn mower or other small engine idles higher when they are chocked, because less air means more fuel to burn so it has to speed up rpms to burn the extra gas with less air per combustion cycle.

[SRK]

No, what I mean is a "leaner" mixture, less rich than would otherwise be needed, not "lean". It is still slightly rich, but less so as the higher rpm will allow it to be. The cats are warmed by exhaust gas, as are the O2's, along with their heater circuit. The important thing is to prevent a misfire, either rich or lean, as that's what causes a great deal of emissions. Higher rpm makes it easier to avoid a misfire.

[curiousB]

Until the O2s are heated the ECU runs open loop. Anytime the car runs open loop it intentionally runs rich. There is no concern about emissions as the cars are not regulated (EPA) for emissions during warm up. Only after the engine reaches normal operating temps. Therefore avoidance of misfires due to reasons of emissions is not the case. It is the case misfires would be very customer unsatisfactory however. High RPMs is to get engine oil flowing everywhere as quick as possible.

[SRK]

Cold start emissions are most certainly measured and controlled - it remains one of the most difficult aspects of vehicle emission control. When running in open loop engines derive their fuel mixture adjustments from "program values" stored in memory, which are initially base values set by the manufacturer, and then modified by long and short term fuel trims as the car is driven ( compensates for altitude, ambient temperatures, etc..)

The move to direct gasoline injection has been spurred in part by cold start performance, as it's much better than port injection in that regard.