Although fuel injection systems have replaced carburetion on all passenger cars and light trucks, there are still vehicles on the road that have carburetors. Technicians must understand the principles of carburetion and how carburetors have been modified to make them more efficient. Carburetors have a venturi and work according to the principle of pressure differential. The amount of air and fuel delivered to the engine depends on the difference between the low pressure (vacuum) in the engine’s cylinders and the high pressure of the outside air (atmospheric pressure).
FUNCTIONS OF A CARBURETTOR
The main functions which a carburettor is required to perform are:
l. To keep a small reserve of fuel at a constant head.
2. To vaporise the fuel to prepare a homogeneous air fuel mixture.
3. To supply correct amount of the air fuel mixture at the correct strength under all conditions of load and speed of the engine.
MIXTURE STRENGTH REQUIREMENT
the air fuel ratio required at different engine speeds. For start the mixture required is very rich. because the engine is cold at that time due to which the fuel does not vapourize properly. only the light, votatile fractions of the fuel would vopourize at lower temperatures. Thus to get
enough volatile fuel fractions while starting a cold engine, more fuel is required. While idling there is no external load on the engine, which has to produce just the frictional horse power. However, since the engine is still not running hot at normal temperatures, and the airflow through the carburettor is slow, some liquid fuel drops out of the air-fuel mixture onto the manifold walls and condenses there. To compensate for this, a rich mixture is required, though not as rich as required for starting. For normal part throttle operation at cruising speeds a comparatively lean mixture, about l5: 1 air fuel ratio, will suffice. In fact 14.6: 1 mixture, i.e., 14.6 parts of air and I part of gasoline or petrol (by mass) is the one which would give chemically complete combustion of petrol under ideal conditions. Such a mixture is called the stoichiometric mixture. By volume this would be about 60 parts of air to 1 part of petrol. At cruising speeds, 15:1 mixture would provide fuel economy. In fact in the present day emission-controlled engines, mixtures as lean as 18:1 have been used at cruising speeds, when operating at sea level. A rich mixture on the other hand, has more energy to produce power. Therefore, at nearly full throttle, (at high speeds or even at normal speeds when going uphill), a rich mixture will have to be supplied. Apart from this, a rich mixture is required during the acceleration period to provide the necessary power for increasing the vehicle speed. Weaker mixtures than the optimum required under given conditions produce less power though affecting economy. Engine misfiring, overheating and incomplete combustion result if adequate ignition timing advance is not provided to compensate for the slow combustion of weak mixtures. Rich mixtures on the other hand provide more power but are uneconomical. Mixtures richer than the optimum required result in excess of carbon monoxide in the exhaust which is black in colour. Moreover continuous use of richer mixtures causes the carbon deposits to form on spark plug electrodes and the combustion chamber walls.
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