Port-fueled gasoline spark-ignited IC engines (hereafter referred to as “engines”) require an enriched mixture during a cold start; this is so that enough of the lighter fractions of the gasoline can evaporate to form a combustible mixture. In carburetted engines, this is accomplished by a choke plate in the carburetor that creates a bit of vacuum to draw in extra fuel. In a modern fuel-injected engine, the ECU knows the engine is cold via a temp sensor and adjusts the injection quantity accordingly.
Interestingly, IC engines also seem to idle poorly when cold, and various mechanisms are employed to open the throttle plate just a smidge past the idle position until the engine warms up. The most explicit example of this was a fuel-injected motorcycle I used to own: the ECU enriched the mixture during cold start, but the operator had to manually flip a fast-idle lever to open the throttle plates just a smidge. If that wasn’t done, the engine started poorly, and if it started at all, it usually stumbled and stalled in fairly short order. That fast-idle lever literally did not do anything except tug on the main throttle cable; it didn’t alter the fuel mixture at all by itself.
So what’s the deal? Why does an engine need to idle fast when cold?
Are gasoline direct-injection engines immune from this requirement? Can they idle at the warm-idle RPM when cold?
We got down to near 0°F here this morning and I noticed that my car was idling high - only about 1,000 RPM but higher than normal. Still this is less than the days of using a choke and its kickup linkage.
Even though the modern engines have better fuel/air mixing, the rest of the engine is still cold. Cylinder walls haven’t warmed up. Engine oil is still cold. Belts are still stiff. Even automatic transmissions are still cold and sluggish. Therefore it takes a little more horsepower to keep from stalling out.
The reason that the PCM or ECU does this is to bring the engine up to operating temperature faster. I realize that does not answer your question as to whether it is actually needed for the engine to run smoothly.
There is less wear and tear on the engine once it is warm. The other reason is to reduce emissions.
Some engines with catalytic converters use an air pump to inject air directly into the exhaust manifold to bring the converter up to temp fast. Sometimes called secondary air injection or smog pump. You may notice a sound the goes away after a few minutes when starting a cold car, that is the air injection pump heating up the exhaust.
Don’t know if this answers your question in that this refers to carburetors. The choke on a carbureted engine, in addition to creating a richer mixture, had a fast idle tab on the linkage. As the engine warmed up, and the choke started to release, so did the fast idle tab. As to why all this occurred, it was simply to warm the engine quicker.