Tailpipe: Component of the exhaust system after the silencer, which allows the exhaust gases to go to the atmosphere.

Top Dead Centre (TDC): The highest point to which the piston travels in the cylinder of an I.C. engine.

Swirl ratio: The ratio of the angular rotational speed of air about the cylinder axis to the angular rotational speed of the crankshaft.

Swirl: Rotational flow of charge within the engine cylinder, about its axis.

Swept volume or Displacement: Also known as 'displacement', it is the maximum volume of air-fuel mixture which can be induced into a cylinder in a single intake stroke.

Surge: A driving problem where the vehicle engine on its own appears to gain prover for a small moment and then falls back down to the previous speed. The reason for this is very lean air-fuel mixture (which may be due to wrong carburettor setting, clogged air filter, blocked main jet, etc.) or retarded ignition timing.

Surface discharge plug: This is a special type of spark plug where the space between the plug electrodes is filled with some semi-conductor material.

Supercharging: The process of supplying to the engine, air-fuel mixture at a pressure above the atmospheric.

Stratified charge: A charge containing strains or layers progressively becoming leaner from one layer to the other. With such a charge very lean air-fuel mixtures can he used, but the NOx emissions are increased.

Stoichiometric mixture: The air-fuel mixture which would achieve complete combustion of fuel under standard conditions. It is 14.6 parts of air and 1 part of petrol by mass. By volume, this would be about 60 parts of air to 1 part of petrol.

Starter: A high-torque electric motor used for starting the engine, typically through a high ratio geared drive connected to the flywheel ring gear.

Square engine: This is an I.C. engine having stroke equal to its bore size.

Spark plug: A device screwed into the combustion chamber of a spark ignition engine wherein electrical charge from the distributor jumps a preset air gap to a grounding point(s). The resultant spark ignites the fuel/air mixture in the combustion chamber.

Solid state ignition: The modern ignition system with no mechanical contact breakers and conventional distributor replaced by a crankshaft-actuated electronic trigger, making the computer fire high voltage ignition coils in a predetermined sequence.

Smart coolant pump: A coolant pump which is controlled directly by the engine management system so that precise engine temperatures can be maintained by varying the rate of coolant flow as per requirements.