need to run richer on the warm up cycle?
I have always wondered this and not seen much of an explanation anywhere. Reading how engines work on howstuffworks.com didn't shed any light either. I can understand them needing a bit of extra fuel to start but I don't know why extra fuel is needed while the engine is cold. The only possible reason I can come up with is that cold oil has more resistance so you need more fuel & more power for the same amount of work as everything in the engine is battling against thick oil rather than thin runny oil. But this doesn't explain why diesel engines don't need extra fuel as they would have the same problem with the resistance of oil while it is still cold.
Anyone know the answer to this vexing question?
teabelly
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In a cold engine the fuel does not atomise as well - the petrol tends to 'puddle' in the inlet manifold and also condense on the cold surfaces in the cylinders. This is much less of a problem in a modern injection engine than in older carb. engines that had long inlet tracts.
Diesel engines don't suffer so much because the fuel is atomised by the injectors and in IDI designs there are glow plugs to warm the prechambers and assist atomisation at start-up.
Additionally, when the engine is cold there is additional friction and oil drag, so extra torque is needed to overcome these forces - hence 'fast idle' when cold.
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Perhaps glow plugs for petrol engines might be the answer :-)
teabelly
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I think direct injection engines, such as traditional diesels, don't actually need richer mixtures at all after firing. The fuel goes straight into the combustion chamber and has no opportunity to get lost on the way through the inlet manifold.
Old carburetor engines would often idle perfectly happily with the choke in, but hesitate a bit when accelerating from cold.
My Triumph doesn't need the choke to idle, but I give the choke a brief pull when the car starts to pull hard if it is a cold day. It is much more economical than simply leaving it out until the engine has warmed up.
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No, DI's don't need enrichment.
Behaviour on choke has a lot to do with the inlet manifold. A long manifold tends to give lots of opportunities for the fuel to 'puddle'.
Manifold 'wall wetting' is also the reason why petrol engines require enrichment on acceleration - extra fuel is needed to stabilise the liquid fuel layer on the inlet tract.
Engines that use gaseous fuel (e.g. petrol engines modified to run LPG) don't require post-start enrichment ("choke").
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Aprilia: If I might add a little to that: in a petrol engine, despite atomisation, the process requires the fuel to also be vapourised to burn. In a cold engine there isn't enough vapour mixed with the air for the spark to ignite the mixture reliably. Therefore a greater quantity of fuel is provided temporarily to ensure the required amount of vapour in the spark-plug vicinity. Some of the excess doesn't burn, condensing on the inlet tract etc, and exiting the exhaust port, to be burnt off by the cat., when its warmed up. With a diesel, fuel is injected into air already heated by compression, which is therefore at about 260c. At this temperature the auto-ignition temperature (about 210c) of the oil mist is exceeded, and complete combustion is inevitable. At very low temperatures the internal temperature might be less than the auto-ignition temperature, hence glow plugs are needed to warm the combustion chamber. Feel free to correct any error!
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>so you need more fuel & more power
There is a commonly held misconception that more fuel = more power. Maximum power is generated with _just the right_ amount of fuel, unless I am much mistaken.
-Mark
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