Choking Device | Functions of Choking Device

Choking device

Starting an engine in cold weather is somewhat difficult. Choking device makes engine starting easier. The choke valve is butterfly valve, similar to the throttle valve. This valve is situated between the air intake and the venture.

01 - choke valve - carburetor

At the time of starting, the choke valve is turned to close almost the inlet passage. This is called choking. Then, during suction stroke, a greater depression is created in the inlet passage and is felt on the fuel jet which is situated at the throat of the venture. This causes more fuel to be ejected by the fuel jet. Choking restricts airflow and provides an over supply of fuel. It should be remembered that only the lighter fractions of the supplied fuel evaporate at lower temperatures and form a combustible mixture in the cylinder.

During initial cranking of the engine, the choke valve is almost completely closed. Once the engine fires consistently, the choke valve is opened slightly to keep the engine running. As the engine warms up to its normal temperature, the choke valve is opened gradually to its full extent. At all other times of engine operation, the choke valve is kept wide open. Thus the inlet passage is unrestricted.

01 - choke valve - POSITIONS OF CHOKE VALVE

The choke valve may be operated either manually or automatically. Manual choke operation is usually effected by a flexible cable connected to a knob in the dash board/instrument panel. Automatic choking is accomplished by a thermostatic element. The tension of the thermostat spring keeps the choke valve in a nearly closed position. As soon as the engine begins to operate, the exhaust gases heat the thermostat casing. This decreases in the tension of the spring and causes the choke valve to open to wide open position.

During choking, only the lighter fractions of the supplied fuel evaporate and form the combustible mixture in the cylinder. The unvapourized heavier fractions of the fuel mxi with lubricating oil film on the cylinder wall. The contaminated lubricating oil may run down into the crankcase as the piston rings scrape the oil. Thus, the crankcase oil gets diluted. This lowers the lubricating characteristics of the oil and may cause greater wear of the engine parts. It is evident, that during choking, fuel is wasted. Hence, choking should be done only when the engine fails to start and should be limited to the minimum period necessary to realize starting.

Unloader or dechoker If, for any reason during starting period, the engine is flooded, it becomes necessary to clear the excessive gasoline out of the intake manifold. This is accomplished by an arrangement of the throttle lever and chokes lingage. In one arrangement depressing the accelerator pedal to the floorboard forces the choke to open sufficiently to allow the engine to clean out the intake manifold. This device is called unloader or dechoker.

Propeller Shaft | Functions of the Propeller Shaft

Propeller shaft

The propeller shaft connects the transmission shaft to the pinion shaft at the wheel axle. The propeller shaft is also called driveline shaft or drive shaft.

01 - PROPELLER SHAFT - LAYOT OF FOUR WHEELER

The propeller shaft carries the power from the engine, clutch and transmission unit to the driving wheels of the vehicle, through the final drive and differential unit.

Functions of the propeller shaft

In most of the automotive vehicles, the engine is located at the front and the rear wheels of the vehicle are being driven. This arrangement stipulates a longer propeller shaft to be used. In some arrangements two or three propeller shafts are used to make up the length.01 - PROPELLER SHAFT - PROPELLER SYSTEM

In some vehicles, the engine is kept at the front and the front wheels of the vehicle are being driven. In some other vehicles, the engine is at the rear and the rear wheels are being driven. For such arrangements a short propeller shaft is used to drive each wheel.

The engine and the transmission unit are attached to the vehicle frame with some flexible mounting. The rear axle housing with differential and wheels are attached to the vehicle frame by suspension springs.

Due to the above arrangement, the transmission output shaft and the input shaft to the rear axle housing are in different planes. This compels the propeller shaft that connects these two shafts to be kept inclined.

Further, whenever the rear wheels encounter irregularities in the road, the rear axle housing moves up and down, compressing and expanding the suspension springs. As this happens, the angle between the transmission output shaft and the propeller shaft changes. Further, the length to be occupied by the propeller shaft also changes.

The variation in the length of the propeller shaft happens because the propeller shaft and the rear axle housing rotate on arcs with different points as their centres of rotation.

The rear axle housing moves in the shorter arc than that of the propeller shaft. This is because the centre of the rear axle housing arc is the point of attachment of the rear spring or control arm to the vehicle frame. This aspect causes a reduction in the length occupied by the propeller shaft as the angle between the transmission and the propeller shaft increases.

01 - PROPELLER SHAFT - COMPONENTS OF PROPELLER SHAFT

To sum up, the propeller shaft does the following functions:

1. It transmits rotary motion of the gearbox output shaft to the differential and then to the wheels through the axle shafts.

2. It transmits motion at an angle which is varying frequently.

3. It accommodates changes in length between gear box and rear axle.

In the case of cars, where the overall length of the vehicle is not too much, the propeller shaft is of single length. On the other hand, the distance between the transmission shaft and the pinion shaft of the differential is more in the case of trucks, buses and long chassis cars. In such cases, on or more intermediate propeller shaft is connected to the gearbox main shaft and the other end to be main propeller shaft. The intermediate propeller shaft is supported in a bearing unit. The bearing unit consists of a bracket, a rubber pad and a ball bearing. The bearing bracket is attached to the cross member of the vehicle frame. The intermediate shaft arrangement reduces the length of the main propeller shaft.