Air-Fuel Ratio

If you are an automobile enthusiast, or have little knowledge about engines, you must have listened to the word Air-Fuel ratio. We all know that for burning fuel inside the engine, we need Oxygen. This Oxygen is provided by supplying air. The proper mixing of air and fuel inside the combustion chamber is very necessary. Mixing increases the molecular contact between the air and the fuel particles. Better the mixing, better is the contact and more better the combustion. The proper combustion inside the cylinder is essential for optimum performance of the engine. This improves the power output and efficiency along with increasing the life of the engine. So let's discuss the Air-Fuel ratio and it's impact on engine performance parameters.

Stoichiometric Air - Fuel Ratio

To understand this term let's suppose that  'x' gram of air is required to burn 'y' gram of fuel completely. Now the ratio of x:y is called the Air-Fuel ratio. Technically the stoichiometric Air - Fuel ratio is defined as the ratio of mass of air (needed for complete combustion of a particular mass of fluid) to the mass of the fuel being burnt. It is also called Ideal Air-Fuel ratio , and such a mixture is called the Ideal Air-Fuel mixture. The Ideal Air- Fuel ratios for some of the common fuels is given below:
Petrol         14.7:1
Diesel         14.5:1
Ethanol         09:1

Types of Mixture Based on Air - Fuel Ratio

The Air- Fuel ratio for a petrol engine varies from 10:1 to 20:1 depending in the amount of throttle operation. We have three types of mixture based on the various air- fuel ratios. The mixture having stoichiometric A/F ratio is called the ideal mixture. It can be classified in to the Lean and Rich mixture depending on whether the the amount of fuel in the mixture is lesser or greater than the ideal amount. So the mixture having A/F ratio 6:1 is rich and the one having the A/F ratio 20:1 is lean. The ideal mixture is required for the complete combustion of the fuel . The complete combustion improves the efficiency and hence the mileage of the vehicle. It also causes less air pollution as the amount of carbon monoxide and Hydrocarbons in the exhaust gases is reduced. So, why not fix the A/F ratio to the ideal one. Why is the carburetor tuned to provide different A/F ratios for different throttle operation?

What is throttle operation?

Autocurious Throttle operation accelerator
Throttle Body

Many of you might not understand the meaning of the throttle operation. So let's first understand it. All of you must be aware of accelerators. These accelerators are connected to the throttle valves in carburetor or the throttle body. These valves regulate the amount of air entering the engine cylinder. As we accelerate or decelerate the vehicle this throttle body opens or closes controlling the air flow accordingly. There are three ranges of throttle operation:
1. Idling range: Throttle Valve is opened Partially
2. Cruising range : Throttle valve is opened around 50%
3. High power range ; Throttle valve is almost open.

A/F ratio required for different throttle operations.

Idling Range

During the idling range the throttle valve is partially open. This generally occurs at very low speeds. Due to partial opening of the throttle valve very less amount of air is supplied to the engine cylinder. Due to less air supply the pressure inside the cylinder remains low. This is because the intake stroke of the piston creates very low pressure inside the cylinder which could not be compensated by the low amount of air supply. Now, since the pressure inside the cylinder remains comparatively low, it pulls in the exhaust gases from exhaust pipe, causing the dilution of of the incoming charge. To burn the charge properly particle to particle contact between air and fuel is very necessary. So to overcome the effect of the charge dilution, rich mixture is required.

Cruising Range

Autocurious cruising motorcycle
A Cruising Motorcycle

At cruising range the vehicle's speed is kept constant at a comfortable and economical level. During this the throttle is about half open. This causes an appreciable amount of air to flow in . The increased air intake prevents the phenomenon of charge dilution. Hence the engine can perform well at A/F ratios closer to the ideal range. This improves the mileage and leads to proper combustion of fuel.

High power range

To produce more power, more fuel is required. As the burning of a greater amount of fuel will generate greater horsepower of energy. Hence we need a rich air fuel mixture. Further at high power operations, the temperature of the combustion chamber becomes very high. This causes many localised flame fronts to generate from the heated areas leading to knocking. For some high performance engines, the heating and knocking may become so severe that it may even damage the piston or valves. We all know that when fuel is releases in the combustion chamber it vaporizes. This vaporization causes cooling.  The higher the amount of fuel in the incoming charge, better the cooling. This is also an important reason for supplying rich mixture at high power operations.

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