Air Fuel Ratio at Starting, Warming up, Accelerating, Cruising, Heavy loads and Decelerating

December 6, 2016 | Engine Info


The air – fuel ratio is the ratio between the mass air and the amount of fuel. This ratio needed to burn well the fuel and air. If the ratio is richer or leaner, the engine will not burn well the air – fuel mixture that causing bad combustion stage. theoretical air fuel ratio is 14.7 amount of air needed to burn well 1 part fuel. But at some conditions not all fuel can be mixed and vaporized with air. Some of the various condition will explain on article below.

But, before we explain about air fuel ratio on various condition, let me first explain about types of air fuel ratio on the vehicle.

  • Rich air – fuel mixture mean, the amount of air less than theoretical air – fuel ratio
  • Lean air – fuel mixture mean, the amount of air greater than theoretical  air – fuel ratio
  • theoretical  air – fuel ratio is 14.7 : 1 (14.7 is air, and 1 is fuel)


  • Air – fuel mixture ratio, 13 : 1 = Rich
  • Air – Fuel mixture ratio, 15 : 1 = Lean
  • Air – Fuel mixture ratio, 14.7 : 1 = theoretical

After you read and understand the article above, now we are explain about the best air – fuel mixture on various conditions:

At starting Condition:

At the starting condition, all engine components such as cylinder head, cylinder block, intake manifold, are on the cold temperature. Then the injector will following condition to spray the fuel into combustion chamber. So, the situation of The air – fuel mixture in the combustion chamber becomes lean. That is mean Rich air – fuel mixture needed to get the best perform to starting engine.

At Warming up Condition:

At this condition, coolant temperature are still low, and make vaporized of the fuel becomes worse, and also causing of ignition worse. So in this case also, The rich air – mixture, still needed.

At Accelerating Condition:

At this condition the accelerator pedal is depressed, of course more amount of air comes into cylinder but the fuel late following air mass due to load change on engine condition. in this case more fuel needed to inject to the mixture or still rich air fuel mixture needed.

At Cruising or Constant Speed Condition

In this condition, engine is already warmed up, and the fuel – mixture ratio is near to the theoretical  air – fuel ratio.

At Heavy Loads Condition:

At the heavy loads, engine produce more power to run the vehicle, and little bit richer fuel – mixture ratio supplied into the cylinder.  in this case, why only little bit richer air – fuel ratio needed at heavy loads condition, because to reduce the burning temperature and make sure that all of the air is comes and burn well on the combustion chamber.

At Decelerating Condition:

At this condition, the accelerator pedal going to release, that is mean no output power needed from the engine. So the fuel will cut off and decreased the fuel supply to the combustion chamber. In this stage, will clean the exhaust gases as well.

Read also: Common Causes of Engine Hard Starting Especially When Cold

What the common symptoms if air – ratio always on rich or lean condition?

  • The first symptom is engine will produce black smokes blowing to the exhaust manifold
  • Engine will jerking
  • At acceleration, the vehicle will get poor power
  • High fuel consumption

What the common causes of bad air – ratio in the car?

  • First possible cause is defective injector
  • Clogged on intake air cleaner and air filter
  • Malfunctions of EFI relative sensors, Such as: Air fuel ratio sensor, oxygen sensor, MAF sensor, ECM, Camshaft position sensor and so on.

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