Car Parts and Accessories

The driving force

Pistons, powered by the expansion of burning petrol/air mixture in the combustion chambers, provide the driving force of the engine.

In a medium-sized car, when the engine is running at maximum speed, each piston may be sliding up and down inside its cylinder as many as 100 times a second. Because of this rapid succession of stops and starts, the pistons must be strong yet light in weight. In most modern cars they are made of an aluminium alloy.

Heat generated by the combustion of the fuel causes expansion both in the pistons themselves and in the cylinders, which are of steel or cast iron.

Piston rings seal the gap between the piston and the cylinder bore. Compression rings, usually two, prevent gases escaping past the piston into the crankcase; and a `scraper’ ring removes excess oil from the cylinder wall and returns it to the sump.

Power is transmitted from the pistons to the crankshaft—and at the same time converted to rotary motion—by the connecting rods and crankshaft. The connecting rods are usually steel forgings.

The top end of each connecting rod. called the small-end, is fitted inside the piston to the gudgeon pin, which allows the rod to pivot as it moves up and down with the piston. The gudgeon pin, also known as the wrist pin, is usually hollow to save weight and is often located in the piston by two spring clips called circlips.

The bottom end of the connecting rod, called the hi g-end, is bolted to the crankshaft and follows a circular orbit, while the small-end follows the up-and-down movement of the piston.

A big-end may be split horizontally or obliquely. An oblique split, by reducing the width of the rod at its widest point, allows a larger connecting rod to be withdrawn upwards through the cylinder bore.

Crankshaft and Car Engine

Transmitting the power

The crankshaft, revolving in most cars at up to 6 000 times a minute, transmits power to the gearbox, and so to the wheels. It is cast or forged in a single piece and in places is machined to an accuracy of less than 0,025 of a millimetre.

The main parts of the shaft are the journals. The main journals rotate within bearings which are supported in the crankcase. The crank-pins rotate within bearings in the big end of the connecting rod, thus allowing power to pass from the piston to the connecting rod and then the shaft. The journals are joined by balancing cranks, or webs, for smoother running.

The flywheel, a heavy and carefully balanced disc fitted at the gearbox end of the crankshaft, helps smooth running by maintaining a steady rate of turning.

With thedownward thrusts of the pistons giving repeated sudden thrusts to the crankshaft as the flywheel maintains its momentum, the shaft is subject to slight twisting and untwisting, known as torsional vibration. A damper—a metal disc incorporating a ring of rubber—is fitted to the front end of the crankshaft to help control torsional vibration.

The firing order of the cylinders is also important in making the crankshaft rotate smoothly. Counting the cylinder nearest the fan as No. 1, the firing order in a 4- cylinder engine is usually 1, 3, 4, 2 or 1, 2, 4, 3, so as to give reasonably even turning of the crankshaft.

On their firing strokes, the pistons push the crankshaft down; and on their other three strokes they are forced up or pulled down by the continuing rotation of the crankshaft. The crankpins are set at different angles to the shaft so as to give a uniform spacing of the firing impulses.

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