AIR-FUEL MIXTURE AND STOICHIOMETRIC RATIO

AIR-FUEL MIXTURE AND STOICHIOMETRIC RATIO

CHEMICAL COMBUSTION OF FUEL

Subhankar Karmakar
Assistant Professor; SGIT
Jindal Nagar; Ghaziabad

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Chemical Combustion is basically a rapid oxidation process of hydro-carbon fuel inside thekjm combustion chamber in the presence of air. The oxidation of fuel is basically a Exothermic or heat liberating chemical process.

In SI engines generally we use volatile hydrocarbon as fuel. The intermixing of fuel with air takes place outside the engine and the device that prepares air-fuel mixture of required mixture strength is called CARBURETION and the device is known as CARBURETTOR.

Estimation of air quantity needed for complete combustion of a given fuel

We know that any chemical reaction can be represented by the corresponding chemical equation like
CH₄ + 2O₂ = CO₂ + 2H₂O
here molecular weight of CH₄
μ_CH4 = 12 + 4x1 = 16
μ_2O2 = 2x16x2 = 64
μ_CO2 = 12 + 2x16 = 44
μ_2H2O = 2x(2+16) = 36

For complete combustion of CH₄
16 kg CH₄ needs 64 kg O₂ therefore,
1 kg of CH₄ needs (64/16) = 4 kg of O₂
For 23 kg of O₂ air needed is 100 kg
hence, for 4 kg of O₂ air needed is (100/23)x4 = 17.39 kg of air.
Air-fuel ratio will be 17.39 : 1

The ratio of air fuel mixture, needed for the complete combustion of the fuel or the chemically correct ratio of air fuel mixture required for complete combustion of the fuel is called " Stoichiometric Air fuel mixture. "

If the amount of air in the air-fuel mixture is less than the chemically correct amount of air, then the mixture is called rich mixture, where as if the quantity of air is more than the chemically corrected amount of air it is called lean mixture.

The strength of air-fuel mixture has a profound influences on the process of combustion. The required mixture strength for different operation conditions are different.

Different Operating Conditions	Required air-fuel Mixture Strength
For Max. Efficiency	17 : 1, 16.4% weak
For Max. Power	12 : 1 17.8% rich
For Starting, Idling, & Low load running	11 : 1 ~ 16 : 1 very rich mixture
For accelerated motion	13 : 1 rich mixture
For Part Load running Cruising Range	17 : 1 Lean Mixture Strength

THE CONCEPT OF VAPOUR LOCK IN IC ENGINES

VAPOUR LOCK

Vapour lock is a problem that mostly affects " Gasoline-fuelled internal combustion engine. " It occurs when liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburettor or fuel injection system resulting in transient loss of power or even complete stalling.

REASONS OF VAPOUR LOCK

The fuel can vapourise due to being heated by the hot engine or by the local hot climate or due to a low boiling point at high altitude.

In regions where higher volatility fuels are used during winter to improve the cold starting, the use of winter fuels during summer can cause vapour lock more easily.

Vapour lock occurs in older type gasoline fuel systems where a low pressure mechanical fuel pump driven by the engine is located in the engine compartment and feeding a carburettor. These pumps are typically located higher than the fuel tank, are directly heated by the engine, and feed fuel. directly to the float bowl or float chamber of carburettor. As in these pumps fuel is drawn from the feedline and fed into the fuel pump under negative pressure, it lowers the boiling temperature of the liquid fuel. As a result fuel gets evaporated fast and totally invades the fuel pump system and carburettor. As the carburettor becomes devoids of liquid fuel, the mixture it prepares will have less amount of fuel as the volume of vapour of fuel is larger than the equal amount of liquid fuel.

The automotive fuel pump is designed to handle a mixture of liquid and vapour phases of fuel, hence it should handle both the phases of fuel. But, if the amount of fuel evaporated in the fuel system is critically high, the fuel pump stops functioning as per the design and started to pump more vapours than liquid fuel and hence, less amount of liquid fuel will go to the engine. The vapours of fuel will invade the fuel pump delivery system which stops the flow of liquid fuel into the engine.

Most carburettors are designed to run at a fixed level of fuel in the flat bowl of carburettor and reducing the level will reduce the fuel to air mixture and hence, will deliver a lean mixture to the combustion chamber which translates into uneven running of the engine or even stalling while idling or sometimes momentary stalling when running.

VAPOUR LOCK AND (V/L) RATIO

The vapour liquid ratio or (V/L) ratio of a gasoline, defined as the amount of vapour released from a gasoline to the amount of liquid remaining at a given temperature directly correlates with the degree of vapour lock likely to be experienced with this gasoline in the fuel system of a car. At V/L ratio = 24, vapour lock may start and at V/L ratio = 36, vapour lock may be severe. Therefore, the volatility of the gasoline should be maintained as low as practical to prevent vapour lock.