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Chapter: Mechanical - Automobile Engineering - Steering, Brakesa and Suspension Systems

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Loss of Traction

One undesirable side effect of a differential is that it can reduce overall torque - the rotational force which propels the vehicle.

Loss of Traction

 

One undesirable side effect of a differential is that it can reduce overall torque - the rotational force which propels the vehicle. The amount of torque required to propel the vehicle at any given moment depends on the load at that instant - how heavy the vehicle is, how much drag and friction there is, the gradient of the road, the vehicle's momentum and so on. For the purpose of this article, we will refer to this amount of torque as the "threshold torque".

 

The torque on each wheel is a result of the engine and transmission applying a twisting force against the resistance of the traction at that wheel. Unless the load is exceptionally high, the engine and transmission can usually supply as much torque as necessary, so the limiting factor is usually the traction under each wheel. It is therefore convenient to define traction as the amount of torque that can be generated between the tire and the ground before the wheel starts to slip. If the total traction under all the driven wheels exceeds the threshold torque, the vehicle will be driven forward; if not, then one or more wheels will simply spin.

 

To illustrate how a differential can limit overall torque, imagine a simple rear-wheel-drive vehicle, with one rear wheel on asphalt with good grip, and the other on a patch of slippery ice. With the load, gradient, etc., the vehicle requires, say, 2000 N-m of torque to move forward (i.e. the threshold torque). Let us further assume that the non-spinning traction on the ice equates to 400 N-m, and the asphalt to 3000 N-m.

 

 

If the two wheels were driven without a differential, each wheel would push against the ground as hard as possible. The wheel on ice would quickly reach the limit of traction (400 N-m), but would be unable to spin because the other wheel has good traction. The traction of the asphalt plus the small extra traction from the ice exceeds the threshold requirement, so the vehicle will be propelled forward.

 

With a differential, however, as soon as the "ice wheel" reaches 400 N-m, it will start to spin, and then develop less traction ~300 N-m. The planetary gears inside the differential carrier will start to rotate because the "asphalt wheel" encounters greater resistance.

 

Instead of driving the asphalt wheel with more force, the differential will allow the ice wheel to spin faster, and the asphalt wheel to remain stationary, compensating for the stopped wheel by extra speed of the spinning ice wheel. The torque on both wheels will be the same - limited to the lesser traction of 300 N-m each. Since 600 N-m is less than the required threshold torque of 2000 N-m, the vehicle will not be able to move.

 

An observer simply sees one stationary wheel and one spinning wheel. It will not be obvious that both wheels are generating the same torque (i.e. both wheels are in fact pushing equally, despite the difference in rotational speed). This has led to a widely held misconception that a vehicle with a differential is really only "one-wheel-drive". In fact, a normal differential always provides equal torque to both driven wheels (unless it is a locking, torque-biasing, or limited slip type).

Traction control system (TCS),

 

A traction control system (TCS), in German known as Antriebsschlupfregelung (ASR), is typically (but not necessarily) a secondary function of the anti-lock braking system (ABS) on production motor vehicles, designed to prevent loss of traction of driven road wheels. TCS is activated when throttle input and engine torque are mismatched to road surface conditions.

 

Intervention consists of one or more of the following:

 

·                   Brake force applied to one or more wheels

 

·                   Reduction or suppression of spark sequence to one or more cylinders

 

·                   Reduction of fuel supply to one or more cylinders

 

·                   Closing the throttle, if the vehicle is fitted with drive by wire throttle

 

·       In turbocharged vehicles, a boost control solenoid is actuated to reduce boost and therefore engine power.

 

Typically, traction control systems share the electro hydraulic brake actuator (which does not use the conventional master cylinder and servo) and wheel speed sensors with ABS.

 

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