Untangling the terminology

Barry Lake continues the Automotive Post's series which aims to make everyday transport more understandable.

Q: What is ABS?

A: If the front wheels of a car lock during heavy braking, the tires can no longer steer the car and if the rear wheels lock, the car can go into a violent spin. The acronym ABS stands for Antilock Braking System -- a computer-controlled device for preventing wheel lockup.

Electronic sensors on each wheel detect when one or more wheels lockup. When this happens, the computer momentarily releases fluid pressure from the locked wheel's brake.

If it locks again, the computer releases it again -- and this can happen many times per second, so that near-maximum braking force can be maintained even while the system alternately releases and applies pressure. In this way, the vehicle remains stable while braking for the conditions.

ABS is a standard feature on premium cars in developed markets and is available as an option on many others.

Q: How does the clutch work?

A: In cars with manual transmission it is necessary to release contact between the engine and the gearbox when each gear change is made. Also, when the car has to be put into motion, there has to be some slippage allowed so the engine -- running at anything from 1500 revolutions per minute (rpm) upwards -- can begin driving the transmission, which is stationary. The clutch does both jobs.

The spring-loaded clutch pressure plate is bolted to the engine flywheel and clamps the clutch friction plate to it when the driver's foot is off the clutch pedal. This provides a solid drive between engine and transmission.

When the driver depresses the clutch pedal it releases the pressure from the pressure plate so the friction plate -- which is attached by a spline to the gearbox input shaft -- is completely freed from the engine's drive. When starting the car, the driver eases the pedal upwards, which allows the friction plate to slip at first, then gradually to take up full drive as the car builds up speed.

Q: What is a thermostat?

A: A thermostat, made of brass to resist rusting, is placed in the cooling system of a water-cooled car -- usually at the top- front of the engine where the heated water is directed to the radiator via the top radiator hose.

It operates like a valve, expanding to open when the water is hot and closing by spring pressure when the water temperature is cold.

Its purpose is to retain water in the engine (not passing through the radiator) during the warm-up process. When the engine reaches the optimum operating temperature, the thermostat is fully open, allowing full flow through the radiator for cooling.

Warming the car's engine quickly with the aid of a thermostat avoids prolonged running on the choke, which uses excessive fuel and washes oil from the cylinder bores, causing extra wear. It also allows a car's heater to become effective much sooner after engine start-up.

Q: What is the difference between water-cooled and air-cooled engines?

A: A water-cooled engine as used in most cars has a "water- jacket" or cavity containing water surrounding the cylinder bores within the engine -- as well as in the cylinder head. Unwanted heat is transferred from the cylinders to the water, which is circulated by a pump through the radiator at the front of the car. As the water runs through tubes within the radiator, air passes through the radiator core and past these tubes, as the car moves along. The air cools the water, which then exits from the bottom of the radiator into the bottom of the engine, beginning the cycle again.

An air-cooled engine -- more common in motorcycles -- contains no water. Instead of a water jacket, these engines have many "fins" cast onto the outside of the block and head to increase the area exposed to the air. As the vehicle travels along, air passing over the engine transfers heat away from it via these cooling fins.

When an air-cooled engine is enclosed within the body of a car -- such as an old Volkswagen Beetle -- a large mechanically- driven fan is provided to blow air through metal ducting directly onto the engine's cooling fins.