Carburettor (Automobile)

9.12.

Carburettor

The function of the carburettor (Fig. 9.41) is to atomise and vaporise the liquid fuel (petrol) and mixing it with air in correct proportions to meet engine requirements over a wide range of speed and loads. The carburettor is mounted on the intake manifold. Opening of the carburettor throttle plate (valve) causes air to move from the higher-pressure area outside the engine, through the carburettor, to the lower pressure area of the manifold. The quantity and speed of travel of air is determined by the opening of the throttle plate.
Carburettor air flow must be matched to the air flow requirements of the engine. A carburettor, which provides more air and fuel than the engine needs, produces a rich mixture. This can reduce both fuel economy and power. A carburettor, which provides less fuel and air than the engine needs, causes over loading of the engine. This also does not provide best combination of power and economy. The location of the carburettor on the intake manifold is important. Incorrect placement in relation to the manifold passages can interfere with proper fuel distribution.
A carburettor must meet the following main requirements:

Fig. 9.41. The carburettor.
(i) Easy starting of engine at any temperature.
(ii) Possibility of changing the adjustments in accordance with the state of engine and its
operating conditions. (Hi) Smooth operation of engine at variable loads and speeds.
(iv) Good accelerating capabilities.
(v) Maximum power at full load.
(vi) Best fuel economy.
9.12.1.

Carburettor Operation

During suction stroke, vacuum is created inside the cylinder which causes the air to flow through the main carburettor opening, called the bore or barrel which is party restricted with a streamlined sleeve called venturi. As the air passes through the venturi, its pressure decreases increasing the velocity. Air flow through the carburettor is controlled by engine vacuum below the throttle plate (manifold vacuum), by air flow through the venturi and by atmospheric pressure outside the carburettor. The greater the difference between manifold vacuum and atmospheric pressure, the more the air tries to flow past the throttle plate at part throttle opening. Fuel flow through the metering jet within the carburettor is controlled by the height of fuel in the float bowel, which is exposed to the atmospheric pressure above it through the float bowel vent, and a low pressure developed in the discharge nozzle due to flow of air rapidly past the discharge nozzle. This difference in pressure causes the fuel to be sprayed by the discharge nozzle in the air stream. The fuel level in the bowl is kept constant by means of a float-actuated needle valve. The quantity of mixture supplied into the engine is controlled by the throttle plate. A simple carburettor is illustrated in Fig. 9.42.

Fig. 9.42. A simple carburettor.
The main metering system jet is carefully sized opening drilled in a small plug. Fuel leaves the float chamber through the jet into an enlarged carburettor passage called a main well. A passage leading from the main well carries fuel to the discharge nozzle located in the narrow portion of the venturi. Holes that allow air to flow into this passage are called main air bleeds which help control the effect of venturi vacuum on fuel flow and help break the fuel liquid into a free flowing foam of very small fuel droplets and air. The most common problem that occurs
in the main metering system is air bleed plugging, which enriches the fuel mixture supply to the engine.
9.12.2.

Carburettor Vacuum

Atmospheric pressure is always present outside the carburettor. It remains constant at any given altitude and temperature. Manifold vacuum is the low pressure beneath the carburettor throttle plate, created by the engine and is always present when the engine is running. Venturi Vacuum in the low-pressure area created by air flow through the venturi reaction in the carburettor barrel and it increases with the speed of air flow. Venturi vacuum is present whenever the throttle plate is opened to allow air to flow through the carburettor. Ported Vacuum is the low-pressure area in the carburettor just above the throttle plate. Ported vacuum is present whenever the throttle is opened to expose the lower portion of the carburettor barrel to manifold vacuum. Vacuum from this point is often used to operate distributor vacuum advance unit and other vacuum operated devices.
9.12.3. Limitation of the Single-jet Carburettor
The quantity of air consumed by an engine in unit time is directly proportional to the engine speed, but due to the inertia of liquid flow, the rate at which petrol is drawn out of the discharge nozzle into the air stream increases almost with the square of the engine speed;

where, ma is mass flow rate of air,
mp is mass flow rate of petrol,
and N is engine rpm.
Figure 9.43 shows the relationship between the percentage of petrol in the mixture and the engine speed. The point where the fuel flow and air flow curves cross gives the percentage of petrol by mass (6.25%), which can be completely burnt by the oxygen in the air charge.

Fig. 9.43. Relationship between engine speed and fuel and air flows for a 1-litre engine.

Fig. 9.44. Relationship between induction depression and fuel and air flows for a simple single-jet carburettor.
If a jet size is chosen to give the correct mixture at one predetermined fixed speed, then, at speeds below this, insufficient petrol is forced into the air stream, producing a weak mixture and loss of power. With higher speeds, the quantity of petrol induced into the air stream increases at a greater rate than the increase of air consumption, so that an over-rich mixture is produced. Figure 9.44 shows how the rates of air and petrol flow vary with different depressions established in the choke tube. This again shows a crossover point. A depression below this point provides only a weak mixture, but above it the mixture tends to become rich. It can thus be concluded that a petrol-jet orifice can be selected for optimum performance over only a narrow speed range.
9.12.4.

Air Flow in Carburettor

For the sake of simplicity if it is assumed that the air is incompressible and its flow is friction-less adiabatic. The Bernoulli’s energy equation is expressed as

where V is velocity, m/s; p is pi tssure, Pa ; p is density, kg/m ; and Z is datum head, m.
Hence neglecting the difference in height in the carburettor, the equation that holds good for sections 1-1 and 2-2 (Fig. 9.42) is

where pa = density of air which is the same at both the sections due to the assumption that air is incompressible.
As Vi is negligibly small in comparison to V2, the ideal velocity of air at the throat is given
by

Due to contraction of stream and friction, the actual velocity of air at throat is given by

where, Ca is the coefficient of discharge for air and h = Apjpa the head causing the flow. The weight of air flowing per second, wa = PaAaVa

The above equation which is based on the assumption that air is incompressible gives sufficiently accurate result if Apa is small and the value” of Ca properly selected. The representative value of Ca may be taken to be 0.84.
A more accurate equation for the flow of air through the venturi can be had without overlooking the effect of compressibility of air. The ideal velocity of air at the throat of the venturi
can be calculated by applying the energy equation for steady flow and assuming adiabatic expansion. Taking one kg of air into consideration,

9.12.5.

Fuel-flow in Single-jet Carburettor

In carburettors the topvof the fuel jet is always slightly higher than the float-chamber fuel level so that the fuel may not run out of the jet when the engine is not operating (Fig. 9.42).
Let x = height of the jet above the fuel-level in the float chamber, mm
Pf= density of fuel, which remains constant as the fuel .is incompressible, kg/m3 &Pf= drop in pressure causing the fuel flow, Pa
Af- cross-sectional area of jet, m2 Cf = coefficient of discharge of fuel. But, Ap/= Pressure of the fuel surface in float chamber-Pressure at the top of the fuel jet

The value of C/for a jet having circular orifice varies from 0.6 to 1.0 depending mainly upon two factors; head causing the fuel flow and its temperature. The value of Cf may be taken to be 0.7 if it is not provided in the problem.
Air-fuel ratio is calculated as

9.12.6.

Critical Velocity

The minimum velocity of air at the throat of venturi at which the fuel just begins to flow is termed as the critical air velocity.
The pressure difference which cause the fuel flow is Apf = Apa – x pfg/1000.
If A pa = x pfg/1000, the fuel rises to the top of the jet orifice, but without any flow of fuel.
When pa>x p/g/1000 the fuel flow starts, g = acceleration due to gravity = 9.81 m/s2.

This is the critical air velocity at which the fuel just begins to flow. Note : (i) If a carburettor with air-cleaner is considered, then the pressure, p = atmospheric pressure-pressure loss in air cleaner. (ii) If the venturi depression is considered in terms of mm of water, then

Example 9.1. A simple jet carburettor is to supply 6.11 kg I min of air and 0.408 kg/min of petrol, of density 768 kg/m3. The air is initially at 100.75 kPa and 288.5 K. Calculate the throat diameter of the choke if speed of air is to be 97.5 mis assuming a velocity coefficient of 0.84. Assume adiabatic expansion and y of air 1.4. If the pressure drop across the fuel metering orifice be 0.8 of the pressure drop at the choke, calculate the orifice diameter assuming a co-efficient of discharge of 0.66.