NOx: A general chemical formula to represent a number of nitrogen oxides. e.g., NO and NO2. These tend to form in exhaust gases of an engine when combustion temperatures are high or the air-fuel mixture is lean.

Naturally aspirated engine: An engine using only the atmospheric pressure and vacuum in the intake manifold to fill the cylinder. Supercharged and turbo-charged engines are not naturally aspirated.

Muffler: A component to reduce the noise of engine exhaust; also called silencer.

Morse Test: Simple test for determining the mechanical efficiency of an engine.

Misfire: Absence of or inadequate firing of the air-fuel mixture in an engine due to fault in the electrical and/or fuel systems.

Multi-Point Fuel Injection (MPFI): A petrol fuel injection system using one injector per cylinder. Only air enters the inlet manifold. As this air approaches the inlet valve, the injector opens in the valve port, spraying fuel into the air stream.

Micro-hybrid: A vehicle using a "start-stop system", where regenerative braking technology helps to stop the engine when the vehicle pulls to a stop, is in neutral or is idling and to restart it when the driver presses the accelerator pedal.

Mean effective pressure: This is the average effective pressure throughout the whole power stroke. It is expressed in bars or kilo Pascals (1 bar = 100 kPa).

Manifolds: Sets of pipes carrying the air-fuel mixture and the exhaust gases to and from the engine.

Main bearings: The bearings supporting the crankshaft. These are located in the lower portion of the engine block.

Magneto: A self-contained device which generates and distributes electricity to the spark plugs from the mechanical energy of the engine. Now obsolete in four-wheelers, but still used in many two-wheelers.

Lambda sensor or probe: Lambda sensor or probe is an oxygen sensor which is an essential part of a closed loop computer-controlled fuel injection system. The sensor is placed in the exhaust manifold. Lambda control is used only in spark ignition engines. It is not possible in a diesel engine.

Lambda (λ): It is the excess air factor defined as the ratio of the actual air consumed in the engine to the theoretical air required. For a stoichiometric air-fuel mixture. λ = 1.0. For a rich mixture, λ < 1.0 and for a lean mixture, λ > 1.0.

Liquefied Petroleum Gas (LPG): Comprises of petroleum gases (mainly propane and butane) liquefied under pressure and used as an alternative fuel.

Liquefied Natural Gas (LNG): Alternative fuel comprising of natural gas liquefied at -160°C.