A Comparative Study on Mechanical Slide (Variable Venturi) and Constant Velocity (Butterfly type) Carburetor

Abstract

Motorcycle engines are mostly categorized as SI-small engine since it does not require the torque and power profile like passenger car or other sort of automobiles. Carburetor is a mechanical device which supplies the required air-fuel homogeneous mixture as per requirements of engine various load condition. In the context of domestic market there are largely two types of carburetors is being used for long time. Mechanical sliding piston type and butterfly as constant velocity type. Both genre of carburetor has its specific advantage and disadvantages. The study shows us the advantages and disadvantages of both this carburetor hence helps to decide which one is preferable in terms of performance and economical aspects.

Introduction

S.I engines are the power house of motorcycles. It normally uses volatile liquid fuels for producing power to move. The fuel air mixture preparation is done outside of the engine cylinder. The process of mixture preparation is extremely important for spark-ignition engines. For efficient operation of engines, it is required to provide a combustible mixture through out proper carburetion by carburetor.

Carburetion & Carburetor

Carburetion is a process which form combustible mixture by mixing appropriate amount of fuel with air before entrance to engine cylinder.

The machine or device which does this carburetion process is known as carburetor.

Carburetion Affecting Parameters

There are factors or parameters which vary the quality of carburetion such as:

1. Engine speed
2. Vaporization characteristic of fuel
3. Intake air temperature
4. Carburetor design

Since the main purpose of carburetion is to prepare homogenous appropriate combustible mixture, it always focusses and mechanically programmed to deliver mixture as per the demand of engine speed and load. Modern engines are designed for high speed, the time available for mixture preparation is very limited. As an example, an engine running at 3000 rpm has only about 10 milliseconds(ms) for mixture induction during intake stroke. When the speed becomes 6000 rpm, the time available is only 5 ms. For achieving high quality carburetion, air velocity at the point of fuel injection has to be increased.

Fig 01: Venturi (Throat)

The carburetor has a minimum sectional area called venturi section where the fuel gets mixed with air stream shown in the fig. 01 B-B section is the narrowest section called venturi (Throat)

One of the other factors that ensures high quality carburetion within a short period time is high volatility of fuel. Fuel with good volatility property can mix within short time with air thus prepare a high-quality carburetion. The temperature of the intake air has a great contribution for efficient carburetion. High atmospheric air temperature maximizes the vaporization of fuel and helps to prepare a good homogeneous mixture.

The design of carburetor, intake air system and combustion chamber also the important key factor of efficient combustion thus better power output and mileage.

Air – Fuel Mixture

There are three types of air fuel mixture forms for various engine operations:

1. Stoichiometric Mixture
2. Rich Mixture
3. Lean Mixture

Stoichiometric Mixture: It is considered as chemically correct mixture. The mixture where enough air is mixed with fuel for complete combustion. For example, 1 Kg of Octane ( 8 18) required 15.12 kg of air to burn completely. Here chemically correct or stoichiometric air-fuel ratio for ( 8 18 ) is 15.12:1 or approximated to 15:1.

Rich Mixture: Mixture which contains less air than stoichiometric ratio is a rich mixture (For example air fuel ratio of 12:1, 10:1 etc.)

Lean Mixture: Mixture which contains more air than stoichiometric ratio is a lean mixture (For example air fuel ratio of 17:1 or 20:1)

Requirement of Mixture at Various Load and Speeds

Air fuel mixture influences on engine performance. Air fuel ratio contributes to affect on both power output and brake specific fuel consumption. Indicated by the curve fig. 02 the mixture at maximum engine output is considered as the best mixture for power with an air fuel ratio of 12:1. The mixture indicating the minimum point of brake specific fuel consumption curve is considered as the best economy mixture with an air fuel ratio of 16:1. It should be noted that the best power is achieved when the mixture is richer than stoichiometric ratio and best economy is available when the mixture is leaner than the stoichiometric ratio.

Fig 02: Power output and bsfc variation with air fuel ratio

Air Fuel Mixture Requirements

The requirements in an engine is shown in the fig. 03 (ABCD) with the help of a curve. It indicates the three general ranges of throttle operation. The engine requirements vary in each of these. For this reason, carburetor must be able to fulfil the requirement air-fuel ratio to meet these demands. These are:

1. Idiling (Rich Mixture)
2. Cruising (Lean Mixture)
3. High Power (Rich Mixture)

Fig 03: Carburetor performance to meet engine requirements

Mechanical Slide Piston (Variable Venturi) Carburetor

Mechanical slide piston type carburetor is widely used in various motorcycle. It consists of a sliding piston operated by a cable with manual control. When the rider twists the throttle, it pulls the piston upward with the help of cable. Thus, air stream flows through the carburetor venturi section. The intake air enters with the help of engine suction pressure alone itself which varies with the movement of the slide piston. The piston movements affect the venturi affect, as the piston goes up it create more space to flow air and when the piston moves downward it restrict the air flow. This action gives a variation in venturi section thus it called variable venturi carburetor. As the piston controls the air flow by its movement, and engine speed does not have any control over the piston motion. Air fuel supply is less synchronized with engine load and speed. Thus, it leads to unsmooth throttling and less tends to be economical. But in terms of sudden power and acceleration, carburetor has its own independence as the piston is out of control of engine speed. Rider can easily rev up the engine within short period of time. This type of carburetor is very helpful in terms of performance riding with high power. But as the throttling is less smooth and does not synchronize with the engine speed and vacuum it is hence less economical in terms of other type of carburetor.

Fig 04: Mechanical Sliding Piston Type Carburetor

Constant Velocity (Butterfly Type) Carburetor

Unlike mechanical sliding piston type carburetor, CV does not consist of a sliding piston that operates by a cable. The piston is operated by vacuum pressure of engine by a vacuum chamber above the piston. And a butterfly throttle valve is used to control the intake air flow. Thus, the velocity of air is controlled by engine itself, and that’s how it named after constant velocity type carburetor. As because the engine speed it self operates the sliding piston, it has better synchronization of engine speed with air fuel supply. Thus, it does not supply excessive fuel apart from engine requirements. So, the fuel economy is much better in comparison with mechanical slide piston type carburetor. Also, the throttling experience is much smoother than the mechanical slide piston type carburetor. In addition, in terms of high-power performance the carburetor is less friendly than the other one. As from previous discussion engine mixture requirements in idling, cruising, power condition. The carburetor is more friendly to supply lean mixture in cruising range.

Fig 05: Constant Velocity Carburetor

Comparison Data of Domestic Market on Using Types of Carburetors

Economy Segment

Performance Segment

Conclusion

From the above analysis the conclusions are:

1. In the perspective of fuel economy and smooth engine throttling the C.V carburetor is much more economical and comfortable as it mainly operates by engine speed thus works in better synchronization of air fuel and engine speed.
2. C.V carburetor are less friendly to performance-oriented motorcycle as it does not allow excessive fuel flow (rich mixture) or less riche mixture in compare with mechanical piston slide type carburetor.
3. C.V carburetor is less modification able due to the synchronized operation with engine speed.
4. Mechanical piston slide carburetor is manually controlled thus it has more independence to rider throttling behavior thus ensure sudden riche mixture (high power). In terms of rich mixture supply within a short time is accessible for mechanical slide piston type carburetor.
5. Mechanical piston slide carburetor is easily re-adjustable and modification able.

References:

1. Internal combustion engines, V Ganesan 4th Edition
2. CFD analysis of flow through venturi of a carburetor, Deepak Ranjan Bhola, NIT,Rourkela, India
3. Design and CFD analysis of a flow through venturi of a carburetor, Pelimali Ravichandra, Mr. G. Mahadev, Dr. K. Vijay Kumar, Dr. M. Sreedhar Reddy, Malla Reddy College of Engineering, Telangana, India – International Research Journal Of Engineering & Technology-www.irjet.net
4. www.bikebd.com
5. https://www.youtube.com/watch?v=3GviSuDGm5k
6. https://support.shoptjbc.com/article/100-is-it-a-cv-or-mechanical-carb