Sludge: Thick, black deposits in engine formed from dirt, oil, water, etc. It is usually formed in engines when oil changes are neglected.

Sensor: A device to record data for measurement and/or control.

Semi conductors: Materials having exactly four electrons in their outermost orbit. Germanium and silicon are two examples.

Secondary cell/battery: In this the metals and the electrolyte change when the battery supplies the current, the process being called 'discharging'. All these changes can be reversed by applying current to the battery in the opposite direction, the process being known as 'charging'. Lead-acid battery is the most common example of the secondary cell.

Run on or Dieseling: Tendency of an engine to continue firing even after the ignition is switched off; also called 'dieseling'.

Rotary engine or Wankel engine: An engine with a triangular shaped rotary piston moving inside an epitrochoidal housing. Invented by Dr. Wankel engine, it is also called Wankel engine.

Rocker arm: A valve train component that reverses the upward motion of the push rod to downward motion of the valve and vice-versa.

Reluctor: A wheel that rotates inside the distributor and triggers the release of voltage in an electronic ignition system.

Relay: An electrical switch whose movement is caused by the action of an electromagnet inside the box. When power is applied to the relay's coil, the electromagnet comes alive and pulls across the switching contacts. With a relay, a small current can energies a device consuming large current.

Relative Efficiency: Relative Efficiency of an engine is the ratio of its indicated thermal efficiency and its air standard efficiency.

Regulator: A device which maintains the current and/or voltage levels of a circuit within predetermined range.

Radiator: A component of the engine cooling system, which is in fact a heat exchanger. Its functions to dissipate the engine heat from the coolant to the atmosphere.

Radial engine: An engine in which the cylinders are arranged radially in a circle around the crankshaft.

Quench area: The area of the flat surface of the piston crown that opposes the flat surface of the cylinder head as the piston approaches TDC.

Push rod: A rod meant to transmit the reciprocating motion of the tappet to the rocker arm.