I've had this question for a long time.

I know the percent will differ based upon the throttle position and will be either by stoichiometric ratio or rich mixture.

The stoichiometric ratio (AFR) for petrol (gasoline) is 14.7 : 1 right?

So if it is, when 1ml of petrol is used, then 14.7ml of air is used, right?

So, if I open the throttle more and more:

  1. The air part will be reduced more and more?
  2. Up to which point can the air be reduced. (I mean in the AFR ratio like 10: 1 , 8: 1) ?
  3. So, in the carbureted engine, the butterfly valve will do this and in a fuel injected engine, ECU will determine this one right?

Are there any visuals or videos for these points, so I can understand it more?

  • Not exactly sure what you're asking. The ratio should be more or less maintained, why would the air part be reduced? May 20, 2016 at 8:24
  • In certain circumstances the fuel will be reduced or even cut completely, for example coming downhill in gear, many electronically controlled fuel injected cars cut the fuel completely. At full throttle, the air is not reduced, it is still increased but is dependent on vacuum from the back of the inlet valves whereas fuel is "pushed" into the inlet side so the amount increases faster and the mixture becomes richer. May 20, 2016 at 10:32
  • 3
    Your usage of milliliters is incorrect. The fuel is much denser than air, and the ratio is concerned with the mass of each component. If you wanted to use anything, you could say 1 gram of fuel to 14.7 grams of air (but air is notoriously hard to keep on a triple-beam balance).
    – JPhi1618
    May 20, 2016 at 12:30

3 Answers 3


Let me see if I can clear some of this up for you.

The stoichiometric ratio (AFR) for the petrol is 14.7 : 1 right?

This is correct, and it's important to understand the concept of stoichiometric ratio. It means that when the fuel and air are burned, they must be in this ratio if you want them to combine completely, with nothing left over (well, none of the original compounds).

So it is, If 1ML of petrol is used,then 14.7ML of air is used, right?

No, this is not correct. The ratio is concerned with the amount of compounds on a molecular level. Since the liquid fuel is much, much more dense than air, you can't use a measurement of volume like milliliter (ml). If you want to imagine the quantities side by side, you have to atomize the fuel into a gas, then combine the 1ml of fuel-gas with the 14.7ml of air. Or you could weigh the fuel and air and combine them.

So, if I have given the throttle more and more, the air part will be reduced much and much?

The Wikipedia article has a good explanation of the air-fuel ratio and how it changes:

A stoichiometric mixture unfortunately burns very hot and can damage engine components if the engine is placed under high load at this fuel–air mixture. Due to the high temperatures at this mixture, detonation of the fuel–air mix shortly after maximum cylinder pressure is possible under high load (referred to as knocking or pinging). Detonation can cause serious engine damage as the uncontrolled burning of the fuel air mix can create very high pressures in the cylinder. As a consequence, stoichiometric mixtures are only used under light load conditions. For acceleration and high load conditions, a richer mixture (lower air–fuel ratio) is used to produce cooler combustion products and thereby prevent detonation and overheating of the cylinder head.

So, the answer to your question is, yes the air part of the ratio is reduced. You could also say that the fuel is increased, but the result is a ratio with too much fuel. The extra fuel is not burned, but has a needed cooling effect.

In case I misunderstood your statement, when you step on the throttle the overall amount of air increases. The throttle is basically an air control - more throttle, more air (and more fuel). The ratio is decreased as explained above, but to overall quantities of each go up.

Up to which point this air can be reduced?

I could use some help on these numbers, but I believe that an AFR range of 12:1 (rich) - 16:1 (lean) can be found in an engine.

When the mixture is way to rich or lean, getting the mixture to ignite will be a problem, and even if combustion happens, it won't be very powerful.

So, in the Carburetor, the butterfly valve will do this and in the FI, ECU will determine this one right?

In a carbureted engine, the carburetor is responsible for mixing the right amount of fuel with air. The butterfly valve allows more air in, and more air flowing through the carb causes it to use more fuel.

In a car with EFI (Electronic Fuel Injection), the computer (ECU) uses an air flow meter (MAF) to determine the amount of air going in to the engine, and then calculates how long each fuel injector should be fired to give the correct ratio. There is still a butterfly valve that controls the amount of air.

  1. Upto to which point this air can be reduced. (I mean in the AFR ratio like 10: 1 , 8: 1) ?

Realize, if you were to open the butterflies and nothing else were to occur (no additional fuel), you'd be going lean (higher air to fuel ratio ... 16:1, 18:1, and much higher).

The computer or carburetor in most vehicles will control the amount of fuel going in to keep it at the specific ratio which is actually a bit below stoich, in the 13:1 range. It will hardly ever be perfectly stoich. The reason it is run a little bit rich is two reasons. First, it controls detonation. Detonation or pre-ignition happens much easier when the air/fuel (A/F) ratio is lean. Detonation is the bane of the gas engine and causes damage. Secondly, it also helps reduce emissions, most notably Nitrogen Oxides (NOx), which is the nastiest of the three most civilized nations try to control. It is one of the contributors to acid rain, is the brown hazy crap you see in polluted cities, and burns the lung tissues in us humans.

To keep the A/F ratio close to stoich, a carburetor relies on air flow to draw fuel in. The greater the air flow, the more fuel is drawn in. The ports for this are just above the butterflies. This means, when you open the butterflies, it increases air flows through the venturis, which then increases fuel flow. If the butterflies are opened too fast, the ports in the venturis will find it hard to match the air flow at first. Most carburetors have an acceleration circuit which pumps extra fuel into the venturis to compensate, keeping the A/F ratio closer to the desired.

As for fuel injection (FI), the butterflies themselves just control the air flow. Depending on the FI design, the incoming air can be measured in several different ways. It can be measured with:

  • Manifold Absolutely Pressure (MAP) sensor: Provides pressure difference between inside the intake manifold as compared to outside the intake manifold. This difference can be related to intake vacuum reading.
  • Mass Airflow (MAF) sensor: Provides the computer with how much air is coming into the intake manifold directly. This is used with the MAP sensor to get a more accurate reading and thus a better response.
  • Throttle Position Sensor (TPS): Directly connected to the butterflies (throttle plates), tells the computer the driver's intention. Open further, go faster: open less, slow down.
  • O2 Sensor (or Lambda sensor): Provides the computer with how well the air fuel mixture is burning, thus allows micro-adjustments of the fuel to get closer to the desired air fuel mixture.
  • Air Intake Sensor: Measures air temperature in the intake tract before the air gets inside the engine. Cooler air will mean denser air, which means more oxygen, which will affect the A/F ratio if not accounted for.

There are other sensors which play a roll. Basically what happens inside the engine computer (and this is in a very basic sense), is the computer gets input from each of these devices. Inside the computer there are "lookup tables" which tells it, under "these circumstances" allow the fuel injectors to stay open xx amount of time. This is usually measured in milliseconds. The input is used to cross-reference the amount of fuel which it should be allowing to be input into the engine by each fuel injector. As the computer gets new information and the conditions are noted as changing, the computer adjusts accordingly. The computer also "learns" over time, knowing via the O2 sensor what is working and what isn't. It will put small modifiers onto the reference tables to fine tune it. This allows for variation over time and for subtle differences each vehicle (even ones which are otherwise identical) will have.


The ratio is typically maintained. About 14:7 for idle, cruise, part throttle, and a bit richer (12-13:1) for WOT .

The butterfly valve controls how much air goes into the engine. The carburetor or fuel injection system's job is to match the amount of fuel going into the engine to the amount of air going in.

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