DESIGN AND ANALYSIS OF TWO WHEELER CARBURETOR
ABSTRACT Carburettor is the most important component in fuel feed system of spark ignition engines. It is connected between the fuel filter and induction manifold. It supplies air-fuel mixture of varying proportions to suit engine operating conditions. There are two laws which allow the carburettor to function. They are Vacuum and Venturi Effect.
INTRODUCTION SI engines generally use volatile liquids. The preparation of the fuel-air mixture is done outside the engine cylinder. The fuel droplets that remain in suspension also continue to evaporate and mix with air during suction and compression processes also. So carburetion is required to provide a combustible mixture of fuel and air in required quantity and quality. The literature survey reveals a lot of work has been carried out in the area of designing and analyzing the carburettor using CREO and ANSYS. The analysis of carburetor has been carried out by applying structural and thermal loads. The present work particularly deals with the making of carburettor by using Rapid Proto Typing to increase mass production, to achieve cost reduction, to achieve weight reduction and to prevent the component from getting corroded or undergoing corrosion.
1.1 CARBURETTOR: Carburettor is the most important component in fuel feed system of spark ignition engines. It is connected between the fuel filter and induction manifold. It supplies air-fuel mixture of varying proportions to suit engine operating conditions.
1.1.1 Definition: The carburettor is a device for atomizing and vaporizing the fuel and mixing it with the air in varying proportions to suit the changing conditions of spark ignition engines. The air-fuel mixture so obtained from the carburettor is called the combustible mixture. The process of mixing the gasoline fuel with air to obtain the combustible mixture is called carburetion. Vaporization is the change of state of fuel from liquid to vapour. Atomization is the mechanical breaking-up of the liquid fuel into small particles (but not actually breaking-up into atoms, as the name implies) so that every particle of fuel is surrounded by the air. In order to produce very quick vaporization of the liquid fuel, it is sprayed into the air passing through the carburettor. Spraying of the liquid turns into many fine particles, so that the vaporization occurs almost instantly
1.1.2 Working Principle of Carburettor: There are two laws which allow the carburettor to function. They are Vacuum and Venturi Effect. Vacuum is simply suction, or air pressure below that of local atmospheric conditions. When a piston drops on its intake stroke, the intake valve above it opens and the resulting suction from the cylinder causes a vacuum to be created in the intake tract. The throttle plate controls how much of the vacuum created in the intake tract gets to which of the carburettor’s fuel circuits. While idling the engines intake vacuum is held downstream of the throttle plate, isolating the main fuel system and giving the pilot circuit the full effect of intake vacuum, because its fuel delivery downstream of the throttle plate(between the throttle plate and the cylinder heads). The Venturi effect means that when a gas (air in this case) passes through a reduction in diameter of tube, the velocity of the gas increases and its pressure drops. The area of the carburettor throttle containing the slide is called venturi because it has tapered reduction in diameter and, with the controlled movement of the slide , it uses the venturi effect to create a controlled low pressure above the needle jet. Any pressure below ambient conditions acts as a vacuum, or source of suction. The faster the air flow under the slide the greater the drop in pressure above the needle jet. At idle condition, the throttle plate nearly closes off the bore of the carburettor, which results in a strong vacuum downstream of the throttle plate and with the slide at its lowest position, nearly closing off the needle jet with the thickest part of the needle, the pilot circuit regulates the fuel flow. As the throttle is opened, allowing more air into the engine, the needle and slide begins to rise, intake vacuum weakens (causing a drop in a pilot fuel delivery) and air flow above the needle jet speeds up. The Venturi effect creates a low pressure causing suction above the needle jet and fuel is drawn up from the midrange /main circuit. As the needle and slide continue to rise, they expose an ever-greater amount of needle jet orifice to the air flow above it, causing a progressive rise in fuel delivery with the increasing air flow.
1.1.3 Air-Fuel ratio: (mixture requirements)
Fig: 1.1. Shows the variation of mixture requirements from no-load to full-load in a S.I Engine. 1. Idling and low speed : (From no-load to about 20% of rated power) Idling refers to no power demand. During idling air supply ids throttled and residual gases make up a large fraction of charge at the end of the suction period. In addition, during overlap period some exhaust gases are drawn back into the cylinder. The result is chemically correct (stoichiometric) mixture of air and fuel (~ 15: 1) would be so diluted by residual gases that combustion would be erratic or impossible. A rich mixture is supplied during idling (say A/F ratio 11: 1 or 12: 1). The richness should gradually change to slightly lean for the second range as shown in the above figure. 2. Cruising or normal power :(From about 25% to about 75% of rated power) In the normal power range the main consideration is fuel economy. Because mixture of fuel and air is never completely homogeneous the stoichiometric mixture of fuel and air will not burn completely and some fuel will be wasted. For this reason an excess of air, say 10% above theoretically correct (~ 16.5: 1), is supplied in order to ensure complete burning of fuel.
3. Maximum power : (From 75% to 100% of rated power)
Maximum power is obtained when all the air supplied is fully utilized. As the mixture is completely homogeneous a rich mixture must be supplied to assure utilization of air.
1.2 Factors influencing carburetion: 1. The engine speed; the time available for preparation of mixture 2. The vaporization characteristics of fuel. 3. The temperature of the incoming air. 4. The design of carburettor.
In case of modern high speed engines, the time duration available for formation of mixture is very small and limited. The time duration for mixture formation and induction may be of the order of 10 to 5 milliseconds.
Atomization, mixing and vaporization are the processes which require a finite time to occur. The time available for mixture formation is very small in high speed engines(For example in an engine running at 3000 r.p.m.., the induction process lasts for less than 0.02 second). For completion of these processes in such a small period a great ingenuity is required in designing the carburettor system.
In order to achieve high quality carburetion within such a short time requires good vaporization characteristics of the fuel which are ensured by presence of high volatile hydro-carbons in the fuel.
The temperature is a factor which effectively controls vaporisation process of the fuel. If the temperature of incoming air is high, it results in higher rates of vaporization. The mixture temperature can be increased by heating the induction manifold but it will result in reducing power due to reduction in mass flow rate.
For a S.I engine, the design of carburetion system is very complicated owing to the fact that the air-fuel ratio required by it varies widely over its range of
operation, particularly for an automotive engine. For idling as well as for maximum power rich mixture is required. Carburetor is required to provide the best economy mixture. In the peak power operation the engine requires a much richer mixture (i) During peak power operation some parts of the cylinder gets heated up. So enrichening the mixture reduces the flame temperature and the cylinder temperature and the cooling problem is solved. (ii) Since high power is required the cruising setting must be transferred to a setting in which the mixture will deliver maximum power or to a setting in the air-fuel ration lies in the range of 12:1.
1.3 Compensating Devices of Carburetor: The automobile has to run on different roads on different loads and conditions. The main metering system of the carburetor alone will not be able to take care of the needs of the engines. Therefore, compensating devices are provided. The important compensating devices are 1.3.1 Air-Bleed Jet The air bleed jet is present in the main nozzle. The flow of air through the orifice is restricted. Initially, when the engine is not operating both the jets are filled with fuel. When the engine starts fuel comes out from both the nozzles but gradually t5he engine picks up and after that only air comes out of the air-bleed jet and mixes with the fuel coming out from the main nozzle and forms the fuel-air emulsion.
1.3.2Emulsion Tube
The main metering is jet is generally kept 25mm below the fuel level in the float chamber so as to avoid the overflow of the fuel. A jet is placed at the bottom of a well having holes which are connected to the atmosphere. When the throttle is opened fuel starts to flow from the well and the holes get uncovered and the air-fuel ratio increases i.e. the richness of the mixture decreases when all the holes get uncovered. The air is drawn through these holes and the fuel gets emulsified and the differential of pressure across the column of fuel is not as high as that of the simple carburetor. 1.3.3 Compensating Jet The main purpose of the compensating jet, which is connected to a compensating well, is to make the mixture leaner as the throttle valve opens gradually. The compensating well is vented to the atmosphere and is also connected to the main fuel chamber through a restricting orifice. With the increase in air flow rate, the fuel level in the compensating well decreases so the fuel supply rate through the compensating jet also decreases. Thus the compensating jet tends to lean the mixture whereas the main jet tends to richen the mixture. So the sum of the two jets tends to keep the mixture to the required ratio. 1.3.4 Back Suction Control Mechanism In this device, the top of the fuel float chamber is connected to the entry part of the body of the carburetor by means of a long vent line fitted with a control valve. Another vent is connected from the top of the chamber to the venturi of the throat. When the control valve is completely open then pressure at the float chamber is same as that of the air inlet. So there lies a pressure difference between the float chamber and the venturi and fuel from the float chamber flows into the venturi. But when the control valve is closed the pressure at the venturi and the float chamber are same and there is no fuel flow. Thus by proper control of the control valve a proper differential between the float chamber and the throat can be maintained and hence the quality of the mixture. 1.3.5 Auxiliary Valve
When the engine is in idle conditions the pressure at the top of the auxiliary valve is atmospheric. With increase in load, the vacuum at the throat of the venturi increases. So a pressure differential is created between the throat and the spring and this pressure difference raises the valve against the spring force. And as a result more air flows and the mixture becomes leaner.
1.3.6 Auxiliary Port The auxiliary port connects the air entering part above the throat with the air leaving part below the throat by means of a long vent containing a butterfly valve. If the butterfly valve is opened then some additional amount of air passes through this vent and thus the flow of air across the venturi decreases.
1.4 Survey of Carburetor: 1.4.1 Introduction: The Design and analyzation of carburetor has been done using Pro- E. The analysis of carburettor has been carried out by applying structural and thermal loads.
1.4.2 Existing system: The Existing Carburettor installed in the two-wheeler uses a metallic component made out of Cast-iron, Aluminum, Zinc. The make of the carburetor is from a 100c.c Hero Honda splendor bike.
1.4.3 Disadvantages of Existing Carburetor: 1. It weighs more, so assuming /expecting weight reduction is quite un-natural.
2. The initial cost of the component is high. 3. The component probably has a chance to get corroded. 4. The heat conduction capacity is more.
1.5.4 Proposed system: The proposed system is related with the plastic conversion of carburettor using CAD, CAE and RPT Technologies. The component is designed , analized and then it is manufactured in RPT process, the material used in this process is polyamide.
