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?

  • Cold air is more dense and therefore contains more Oxygen by volume so the mixture ratio can remain the same. Sep 29, 2015 at 9:35
  • @SteveMatthews : While it is true that colder, denser air requires more fuel to maintain a certain AFR, the question here relates to why the air-fuel ratio is richer on cold startup
    – Zaid
    Sep 29, 2015 at 9:56
  • Because the engine is cold at startup, and by extension, so is the air. Therefore there is more oxygen. So without enriching the mixture, the resultant mixture will to too lean to support combustion. The mixture is therefore set to be more rich and the net result is that the overall resultant mixture is kept in balance. Sep 29, 2015 at 10:10
  • @SteveMatthews : The question isn't about why more fuel is added. It's about why a richer AFR is used.
    – Zaid
    Sep 29, 2015 at 10:17
  • @Zaid Agreed, however the relative mass of cold air needs to be taken into account. When you pull the choke out it's for the two reasons, one of the air mass and one of the answers below regarding fuel drop out. Sep 29, 2015 at 11:05

3 Answers 3


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.

  • 1
    The Secondary Air Injection system burning fuel in the exhaust also has the added benefit of bringing the catalytic converter up to operating temperature quickly.
    – JPhi1618
    Sep 29, 2015 at 13:38
  • Additionally, ignition studies show that without a doubt operating temperature and viscosity have a higher net result regarding efficiency of burning due to the reduced viscosity of fuel at a higher temperature. Vaporization or atomization of fuel is in direct correlation with viscosity which in turn is in direct relation to the temperature of the liquid. This is a great answer. Dec 8, 2015 at 10:59
  • @Zaid, thank you. is this increase in fuel injection done by using the oxygen sensor data or using the coolant temperature sensor? (is there a common method or depend on the manufacturer?)
    – Mohammad M
    Mar 4, 2022 at 17:33

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.


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.

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