Crossf low engines have the inlet manifold on one side of the cylinder head and the exhaust system on the opposite side, so that during the four- stroke cycle, the inlet charge and exhaust gas flow across the combustion chamber—an arrangement that gives efficient cylinderfilling during the period of valve overlap.

Engines with the inlet and exhaust manifolds on the same side of the cylinder head have a reverse flow arrangment. This is a little less efficient during the valve overlap period, but means that the heat from the exhaust can be easily and cheaply used to warm the inlet manifold and so improve yap- orisation of the mixture inside. On a crossf low engine it is necessary to pipe water from the cooling system to heat the inlet manifold.

Smoothing out vibration

On a four-cylinder engine, if the firing order were 1,2,3,4, the crankshaft and the engine mountings would be subjectto considerable stress and vibra- tion. The stress is minim ised if the firing impulses are spread more evenly along the crank- shaft, and the firing order of four-cylinder units is either 1,3,4,2 or 1,2,4,3.

In addition to distributing the load as evenly as possible the engine designer adds webs to the crankshaft in order to coun- terbalance the mass of each piston and connecting rod assembly—this helps the engine to produce a smooth pow~&Qutput Engines are f1~ied to rubber mountings which reduce the amount of vibra1ioJa~p~ss~d on to the car body.

Pistons and connecting rods

The driving force that transmits the expansion of the burning fuel/air into halfaturn of the crankshaft is the piston. Since at maximum engine speed it may be sliding up and down the cylinder at 100 times a second, it must be light, yet strong enough to take the shock-loads of combustion and of sudden and frequent revers- als of direction.

In most cars the pistons are made of aluminium alloy, and have a number of hardened steel piston rings sitting in grooves round the upper por- tion of the piston. The rings fill the small gap between the pis- ton and the cylinder bore. Usu- ally two of them are compres- sion rings and seal the gap, while lower down an oil control ring scrapes excess oil from the cylinder walls and prevents itfrom being burned in the combustion chamber.

The piston’s up and down movement is converted to the circular motion of the crank- shaft by a forged steel connect- ing rod that pivots on a gud- geon pin (sometimes called a wrist pin) where it connects to the underside of the piston, and is bolted at its bottom end to the orbiting crankpin of the crankshaft.

The end of the connecting rod that embraces the gudgeon pin is called the small end, and the bottom end, which is larger, and is normally split so that it can be bolted round the crank- pin, is the big end. On most cars the big end encloses a two-piece plain bearing, a shell bearing, which has a soft metal surface in contact with the crankpin. It is this soft, relatively cheap bear- ing material, and not the finely ground crankpin, that is the first to fail if the lubrication system breaks down.

A few car engines, devel- oped from motorcycle units, have ball- or roller-type big end bearings, which cannot be split. As a consequence, these engines have multi-piece crankshafts which must be separated and re-assembled, using a powerful hydraulic press, when a connecting rod or bearings have to be changed.

Crankshaft and bearings

Most crankshafts are forged or cast in one piece. They have two sets of bearing surfaces— journals and crankpins— which are very accurately ground. The journals are spaced along the shaft and rotate in main bearings fixed to the bottom of the cylinder block. Crankpins revolve inside the connecting rod big-ends. The crankshaft is either hollow or drilled, allow- ing lubricating oil under pressure to flow from the jour- nals to the crankpins.