Motor Vehicle Mechanics key terms

The sequence of intake, compression, power, and exhaust strokes that a piston completes in two crankshaft revolutions. Most automotive petrol and diesel engines operate on this principle.

The distance the piston travels from top dead center to bottom dead center inside the cylinder. It combines with bore diameter to determine engine displacement.

The ratio of cylinder volume at bottom dead center to the volume at top dead center. Higher ratios raise thermal efficiency but can cause detonation in petrol engines.

Abnormal, explosive combustion of the air-fuel mixture ahead of the flame front, producing a sharp knocking sound. It is caused by excessive pressure, heat, or low-octane fuel.

A rotating shaft with lobes that open and close the intake and exhaust valves in time with the crankshaft. It is driven by a timing belt, chain, or gears.

The precise moments, measured in crankshaft degrees, at which intake and exhaust valves open and close. Correct timing is essential for power, economy, and emissions.

A system that alters valve opening and closing points while the engine runs, improving torque, fuel economy, and emissions across the speed range. It is controlled by oil pressure and the ECM.

A toothed belt that synchronizes the rotation of the crankshaft and camshaft. It must be replaced at specified intervals to prevent catastrophic valve and piston damage.

An engine design in which pistons and valves occupy the same space at different times. If the timing belt or chain fails, valves and pistons collide and suffer severe damage.

The main rotating shaft that converts the reciprocating motion of the pistons into rotary motion. Its journals ride in main bearings and connect to rods via crankpins.

The link that joins the piston to the crankshaft crankpin, transmitting combustion force. Its big end carries a bearing shell and its small end holds the piston pin.

The total swept volume of all cylinders, calculated from bore, stroke, and cylinder count. It is commonly expressed in liters or cubic centimeters and influences power and tax class.

The casting that caps the cylinders and houses combustion chambers, valve seats, ports, and often the camshaft. It is sealed to the block by a head gasket.

A sealing layer between the cylinder head and engine block that contains combustion pressure and separates coolant and oil passages. Failure causes overheating, misfire, or fluid mixing.

An exhaust-driven turbine that spins a compressor to force more air into the engine, increasing power. It requires lubrication, cooling, and often a wastegate to limit boost.

A valve that bypasses exhaust gas around the turbine to regulate boost pressure. It may be controlled mechanically by a spring or electronically by the engine control module.

A heat exchanger that cools compressed intake air after the turbocharger or supercharger. Cooler air is denser, so more oxygen enters the cylinder for combustion.

The ducting that distributes air, or air-fuel mixture, from the throttle body to each cylinder's intake port. Its runner length and volume affect airflow and torque.

A housing containing a butterfly valve that controls how much air enters a petrol engine. Modern units are electronically actuated by the engine control module.

A system that delivers atomized fuel under pressure into the intake port or directly into the cylinder. It replaced carburetion for precise air-fuel control and lower emissions.

A diesel fuel system in which a high-pressure pump charges a shared rail feeding all injectors. The ECM controls injection pressure and timing independently of engine speed.

An electrically controlled nozzle that sprays fuel in a metered, atomized pattern. Clogged or leaking injectors cause rough running, misfire, and poor economy.

The proportion of air to fuel by mass entering the engine. Stoichiometric ratio for petrol is about 14.7 to 1; deviation affects power, economy, and emissions.

A sensor in the exhaust that measures residual oxygen and reports mixture richness to the ECM. Its signal enables closed-loop fuel correction and catalyst protection.

A device that measures the mass of air entering the engine, often using a heated wire or film. The ECM uses this signal to calculate fuel delivery and ignition timing.

The computer that manages fuel injection, ignition timing, emissions devices, and diagnostics. It reads sensor inputs and commands actuators to keep the engine running correctly.

The crankshaft angle at which the spark plug fires relative to top dead center. Advancing or retarding it affects power, efficiency, and the risk of detonation.

A device that screws into the combustion chamber and ignites the compressed mixture with an electric arc. Its heat range and gap must match the engine specification.

A transformer that steps battery voltage up to the high voltage needed for the spark plugs. Coil-on-plug designs mount one coil directly above each plug.

A piezoelectric sensor bolted to the block that detects detonation vibrations. The ECM retards ignition timing when knock is detected to protect the engine.

Failure of a cylinder to ignite its charge properly, causing rough running, vibration, and unburned fuel in the exhaust. It can damage the catalytic converter if prolonged.

An exhaust device coated with precious metals that converts hydrocarbons, carbon monoxide, and nitrogen oxides into less harmful gases. It requires correct mixture and no misfire to work.

A trap in the diesel exhaust that collects soot, then burns it off during regeneration at high temperature. Blockage causes high backpressure and reduced power.

A system that routes a small portion of exhaust back into the intake to lower combustion temperature and reduce nitrogen oxide emissions. It is closed at full load.

The network of radiator, water pump, thermostat, hoses, and coolant that removes engine heat. It maintains a stable operating temperature for efficiency and durability.

A temperature-controlled valve that blocks coolant flow to the radiator until the engine warms up. A stuck-open or stuck-closed unit causes overcooling or overheating.

A belt- or chain-driven pump that circulates coolant through the engine and radiator. Leaking seals or a failing bearing lead to coolant loss and overheating.

The oil pump, filter, galleries, and oil that reduce friction and carry heat away from moving parts. Oil pressure loss can destroy bearings within seconds.

A gear, rotor, or vane pump that pressurizes engine oil and sends it through the galleries. It is usually driven by the crankshaft or camshaft.

A replaceable half-cylinder of bearing material that supports a crankshaft or connecting rod journal. It relies on a pressurized oil film to prevent metal-to-metal contact.

An alphanumeric code stored by a control module when a monitored circuit or system fails. A scan tool reads the code, which guides systematic diagnosis.

A device that connects to the vehicle's diagnostic port to read codes, live data, and freeze-frame information. It also clears codes and runs functional tests.

Live sensor and actuator values displayed by a scan tool while the engine runs. Comparing them with specifications helps isolate faults that set no code.

A measurement of peak cylinder pressure during cranking, taken with a gauge in the spark plug hole. Low or uneven readings indicate worn rings, valves, or a head gasket leak.

A test that pressurizes a cylinder at top dead center and measures the rate of air loss. Listening for escaping air locates leaking valves, rings, or gaskets.

A regulated inspection that measures exhaust gases or checks onboard monitoring systems against limits. Vehicles failing it must be repaired before registration or use.