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I know a rich mixture is more fuel added than air in the ratio required (14.7:1), but how can it affect anything?

I mean, if fuel is added but with no air it can't combust, so how can that affect the ease of starting an engine when cold?

Since it isn't burning fuel inside, does it evaporate and expand? Or what happens?

Zaid
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user3407319
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3 Answers3

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There are two issues with cold fuel:

  • cold fuel is harder to vaporize (as HandyHowie mentions)
  • cold fuel is likely to condense on cold engine parts, making its burning harder, less predictable

So more fuel is added than necessary - using a richer AFR - in anticipation that a decent proportion of fuel will not be burned. In other words, during cold starts the effective AFR is leaner than expected, so the mixture is enriched to compensate for that

The excess unburnt fuel is usually burnt through the addition of extra air (often called secondary air) in the exhaust, where the temperature of the combustion in the cylinder is sufficient to allow any fuel that remains in the exhaust gases to burn before reaching the catalytic converter.

To quote the Bosch Fuel Injection & Engine Management book:

Gasoline is less likely to vaporize when it is cold. Even if it is adequately vaporized, some fuel condenses on the cold parts of the engine before it can be burned. The engine requires extra fuel for starting so that, in spite of vaporization and condensation problems, the engine still receives a combustible air-fuel mixture.

Community
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Zaid
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When the engine is cold, the fuel won't evaporate properly, so what would normally be a correct mixture on a hot engine will actually be a weak mixture when cold. To overcome this, more fuel is added when the engine is cold to give a more ideal mixture for starting.

HandyHowie
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During cold start, the more fuel is injected so as to increase the compression ratio of the engine which has effect of increasing the pressure and temperature in combustion chamber. A large part of the fuel is thrown out in the form of white smoke which is why emission norms enforce tighter norms on emissions happening in cold start transients.

Suyash
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