Fuel Carburation and Distribution (Automobile)

9.11.

Fuel Carburation and Distribution

9.11.1

Fuel Carburation

The preparation of atomised and vaporised combustible homogeneous mixture of liquid fuel with air in correct proportions before ignition is called carburation. The process of carburation takes place in a jet carburettor. The following factors affect the process of carburation:
(i) Time. In modern high-speed engines, the combustible mixture is prepared in hundredths of second. Higher the engine speed, less time is available for mixture formation. In such a small time it is difficult to get high-quality carburation.
(ii) Temperature. The thermal state of the mixture before combustion is predetermined by the conditions of the surrounding air (temperature, pressure and moisture content), load on the engine, design features of the feed system, and the properties of the fuel. A higher temperature of mixture improves vaporization of fuel and mixture quality, and consequently increases indicated efficiency of the engine. But due to the reduced mass charge at higher temperature the power output drops.
(Hi) Design. The design of the carburettor with its elements, heating system for mixture, shape and cross-sectional area of intake pipe, and the shape of the combustion chamber have considerble effects on the uniform distribution of the mixture among the cylinders and in various operating conditions of the engine.
(iv) Quality of Fuel. Petrol is composed of various hydrocarbons having different volatility. An increase in light fractions in petrol favours better distribution of the fuel in air.
9.11.2.

Fuel Distribution

Fuel must be metered, atomised, and distributed to each cylinder in the form of a burnable mixture. In order to achieve this, a carburettor (Fig. 9.37) mixes the gasoline with air in the correct ratio and distributes the mixture as required by engine load, speed, throttle plate positon, and operating temperature. Proper fuel distribution depends on six factors as follows:
(i) Correct volatility of fuel.
(ii) Proper atomisation of fuel. (Hi) Complete vaporisation of fuel.
(iu) Design of intake manifold passage.
(v) Angle of carburettor throttle plate.
(vi) Location of carburettor on the intake manifold.

Fig. 9.37. Basic jobs of a carburettor.

Fig. 9.38. Atomisation and vaporisation of fuel.
9.11.3.

Fuel Atomisation and Vaporisation

In order to change gasoline from a liquid into a combustible form, first the liquid fuel enters the carburettor, where it is sprayed into incoming air and atomised i.e. reduced to mist (Fig. 9.38). The resulting air-fuel mixture then moves into the intake manifold, where the mist is changed into vapour. Vaporisation takes place only when the fuel is hot enough to boil. Since the intake manifold pressure is much less than atmospheric pressure, the boiling point of gasoline drops when it enters the manifold.
Heat from the intake manifold floor combines with heat absorbed from air particles surrounding the fuel particles to begin vaporisation. It is also helped by raising the temperature of the intake manifold, since the higher the temperature, the more complete is the vaporisation. This heated area in the intake manifold is called a hot spot.
When there is poor vaporisation, too much liquid fuel reaches the cylinders. Some of this extra fuel is given off as unburned hydrocarbons, and some wash away oil from the cylinder wall causing engine wear. The rest is carried past the piston rings blow by gases. Sometimes there is poor vaporisation, which may happen due to several factors as follows:
(i) A too low mixture velocity.
(ii) A cold manifold or low manifold vacuum. (Hi) Cold incoming air.
(iv) Insufficient volatility of fuel.
(v) Poor manifold design.
(vi) Low flow capacity of carburettor.
The design of an intake manifold has a direct bearing on mixture distribution and volumetric efficiency over the entire speed range of an engine. The location, size, and surface area of the hot spot on the manifold floor affect vaporisation. The hot spot is normally just under the carburettor (Fig. 9.39). Although the hot spot is usually heated by exhaust, engine coolant is sometimes circulated through passages between the carburettor base and manifold. Both velocity and heating are also affected by the size of the manifold passages through which the
mixture travels. If the passages are large, the mixture travels slowly, allowing fuel particles to cling to the manifold walls thereby reducing vaporisation. Small passages create a higher velocity and restrict the travel and distribution of the mixture. The angles of turning of internal manifold passages are also critical. When they are too sharp, fuel tends to separate out of the mixture (Fig. 9.40). The air-fuel mixture should be distributed as evenly as possible among the cylinders. If one of the cylinders receives lean mixture, an increase in the overall mixture strength is necessary for that cylinder to fire properly. This, intern, causes other cylinders to receive rich mixture.

Fig. 9.39. Manifold heat spot.

Fig. 9.40. Separation of fuel.