Compression ratio is a measure of the amount that the intake charge of an engine is compressed before combustion takes place.

The geometric compression ratio is the volume above the piston at bottom dead centre of the compression stroke divided by the volume above the piston at top dead centre.

In a forced induction engine, the intake charge going into the cylinder has already been compressed by the turbocharger or supercharger. When this charge is compressed inside the cylinder, the effect is the same as in a naturally aspirated engine with a higher compression ratio. This is known as the dynamic compression ratio.

Higher compression ratios are desirable because they increase the thermal efficiency of the engine. However, higher compression ratios can also lead to knocking.

Rosberg and Schumacher at one of Monaco's chicanes in 2011

Chicane is a motor racing term which describes a tight S-bend.

Chicanes are often added to circuits to slow cars down on what had previously been long straights. Famous examples include:

  • the Rettifilo, Roggia and Ascari chicanes at Monza
  • the Bus Stop chicane at Spa-Francorchamps
  • the Mulsanne and Ford chicanes at Le Mans
  • the Clark, Senna and Ostkurve chicanes at Hockenheim

A carburettor is a device which prepares the air/fuel mixture needed for combustion in an engine. Carburettors are now almost obsolete, and have been replaced by fuel injection.

A restriction or venturi is core to the operation of a carburettor. A fuel supply pipe emerges into the venturi, which carries intake air to the cylinders. As the air passes through the venturi its pressure drops, and this low pressure sucks fuel from the supply pipe. The fuel and air mix thoroughly in the intake system before they reach the combustion chamber.

The drawback is that it is very difficult to design a 'perfect' carburettor which supplies the ideal air/fuel mix for all engine operating conditions (in fact the carburettor was once defined as an instrument which provides the wrong mixture for all conditions). Fuel injection, which allows more precise control over fuelling, has largely taken over from the carburettor.

Goodyear tyre test

The contact patch is the part of a tyre tread which is in contact with the ground.

The test above shows a tyre being driven across a glass plate covered in coloured water. The black areas show the amount of tread in contact with the surface of the glass. As this demonstrates, the contact patch is a very small area.

A choke is a device which richens the air/fuel mixture during a cold start on vehicles fitted with a carburettor. They are now practically obsolete.

The choke or choke flap is fitted to the upstream end of a carburettor intake and restricts the amount of air entering the carburettor. This makes the air/fuel ratio richer, which is required for reliable cold starting.

Chokes are often controlled by a lever or knob on the dashboard, steering column or steering wheel hub, but an automatic choke is controlled by a temperature sensitive bi-metallic strip.

Excessive use of a choke can result in premature engine wear, as the large amount of fuel in the combustion chamber washes away lubricating oil from the bores and piston rings.