S.Govindarajan, Ucal Fuel Systems Ltd, Chennai

Introduction 

Two wheelers are characterized by simplicity in construction and low price. In India, the two wheelers are manufactured in different varieties such as mopeds, gearless scooterettes, geared scooters and motorcycles. All the mopeds and most of the scooterettes are powered by two stroke engine. All the scooters are powered by two stroke engines (with the exception of the latest Honda Activa which is 4 stroke powered). In the Motorcycles, both two stroke and four stroke engines are employed. The emission regulations were enforced from 1991 and progressively tightened. This has resulted in upgradation of engines, use of additional devices and in some cases, change of the engine from two stroke to four stroke. In this article, the design aspects of carburetors for two wheelers are discussed.

Mechanically Controlled Carburetor

Two Stroke Engine:

Two stroke engines employ mechanically controlled carburettor which is schematically shown in 
Figure 1.

Variable Depression Mechanically

Controlled Carburettor

Figure 1

Four Stoke Engine:

These vehicles also employ a caburettor of the mechanically controlled type and by properly optimizing the metering elements, it has been possible to meet the emission limits implemented from the year 2000 also. However, in some vehicles, additional controls in production are exercised for meeting the India 2000 emission norms. In some vehicles, add-on devices such as Secondary Air Valve are used to inject air into the exhaust, thus promoting oxidation of CO and HC.

Constant Depression Carburetor

Figure 2 shows the schematic diagram of the Constant Depression Carburettor. This carburettor is of very advanced design and is used in four stroke engines.

Constant Depression Carburetor

Figure 2

This carburettor has a butterfly valve to control the mixture flow to the engine as per the needs of the rider. A piston is provided which forms the venturi zone to discharge the fuel in the air stream. As the air flow is varied by operating the butterfly, the depression in the venturi zone, under the piston valve, also changes momentarily. This depression in the venturi zone (under the piston valve) is connected to the top of the piston. As the air flow increases due to the opening of the butterfly, the increased depression causes the piston to move up against the spring force till the proper position is achieved to maintain the same depression. When the butterfly opening is reduced, the decrease the air flow, the venturi depression decreases and the piston valve moves down to provide the same depression. By this design, the depression at the fuel delivery zone is always maintained constant thereby ensuring proper atomization of the fuel. The operating principle is one of variable area, constant depression in the venturi zone.

In the mechanically controlled carburettor, the venturi size is selected to achieve the maximum / target output at maximum engine speed at the same time having good torque at low speeds. If venturi selected is too big, then high end power any be achieved but at low speed, the air flow will be too small to create enough velocity to ensure proper atomization of the fuel. This results in low torque. On the other hand if a smaller venturi is selected to achieve good low speed torque at full load, then the venturi may be too small to provide sufficient air flow at high speed. Hence the selection of venturi size is a compromise. This situation is overcome by using the constant depression design. In this case, a bigger venturi size can be used to achieve maximum power without sacrificing low end performance.

Figure 3 shows the typical operating scheme of the mechanically controlled carburettor and the constant depression carburettor at part load, full load – low speed and full load – high speed.

Figure 3

The following may be noted :

1. Idling:

   In the variable depression carburettor, the piston valve is at the lower most position. In the CD carburettor, the butterfly is near close position and the piston is also at lower most position to admit air in the right quantity for idling.

2. Part Throttle operation:

   In the variable depression carburettor, the piston is positioned by the rider by operating the accelerator cable. In the case of CD carburettor, the butterfly is opened by the rider by means of accelerator cable. Piston valve finds its position in order to maintain constant depression at the venturi zone. This helps in promoting proper atomization of the fuel.

3. Full Throttle (Low speed) :

   This condition is characterized by full load low speed operation. As the speed is low, the quantity of air flow will also be small. In the VD carburettor, the venturi is opened fully. With this full opening, the venturi depression will be small. This is not conducive for good fuel atomization. In the case of constant depression carburettor, the butterfly is fully opened but the venturi opening is controlled by piston valve position. This results in smaller opening at the venturi zone and hence constant depression leading to proper atomization of the fuel. This helps in achieving good low speed full load torque.

4. Full Throttle (High speed):

   In the variable depression carburettor, the piston is at the high position providing maximum opening for air flow to deliver maximum power. In the CD carburettor, the butterfly is fully opened and the piston valve is also fully opened to provide maximum air flow at high speed to develop maximum power.

Figure 4 shows the torque characteristics that can be obtained with these two types of cabrurettors. Another advantage with the CD carburettor is that as the engine runs in and the vehicle accumulates distance, the venturi depression will still be maintained at the original value. This ensures proper mixture supply even when the engine wears out. With this design of carburettor, four stoke engines, of higher capacities are able to meet the Indian regulations of 2000, without after treatment devices.

Torque Vs Speed

Figure 4

Conclusion :

With the increasingly severe exhaust regulations, a definite shift is seen towards adopting four stroke engines in the two wheelers of even small engine capacities like 75cc. The constant depression carburettor helps in achieving the best out of the four stroke engine in terms of good fuel economy, reduced emissions and consistent performance over the age of the vehicle. As of today, vehicles upto 150cc engine capacity are able to meet the regulations of Year 2000 without any add-on device. As the regulations become more severe in the future, add-on device such as Secondary Air Valve and catalyst may have to be used. The choice of combination of systems – VD carburettor or CD carburettor with or without add-on device will be decided based on the engine design and the cost.

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