ASE A9 Light Vehicle Diesel Engines key terms

The process in which diesel fuel ignites spontaneously when injected into air that has been heated to a high temperature by compression inside the cylinder, with no spark plug involved.

The short interval between the start of fuel injection and the beginning of combustion, during which the fuel mixes with hot air and prepares to burn. Long delay causes rough, noisy combustion.

A rating of a diesel fuel's readiness to self-ignite under compression. Higher numbers ignite sooner and shorten ignition delay, giving smoother running and easier cold starts.

A diagnostic procedure that feeds regulated compressed air into a cylinder set at top dead center and measures how much escapes, helping locate leakage past rings, valves, or the head gasket.

A measurement of cranking pressure in each cylinder, compared against specification and against the other cylinders, used to judge ring, valve, and head gasket sealing.

Combustion gases that escape past the piston rings into the crankcase during running. Excessive blowby points to worn rings or cylinder walls and often shows as heavy crankcase vent flow.

The casting bolted to the top of the block that closes the cylinders and houses the valves, injectors, and often the camshaft, coolant passages, and intake and exhaust ports.

The sealing layer between the cylinder head and block that contains combustion pressure, coolant, and oil. Failures show as coolant loss, white exhaust smoke, or gases in the cooling system.

The small clearance designed into the valve train between the rocker or cam follower and the valve stem, allowing for heat expansion. Incorrect lash affects valve timing, sealing, and noise.

The gradual sinking of a valve deeper into its seat from wear and repeated pounding, which reduces valve lash and can hold valves open. Common on hard-used diesel valve seats.

The machined ring in the cylinder head that the valve face presses against to seal the combustion chamber. Worn or pitted seats cause compression loss and misfire.

A lever in the valve train that transfers motion from the pushrod or camshaft to the valve stem, opening the valve. Its adjustment point sets valve lash on many engines.

A rigid rod that carries motion from the camshaft lifter up to the rocker arm in overhead-valve engines. Bent pushrods indicate valve-to-piston contact or over-revving.

A replaceable cylindrical liner pressed or cast into the block that forms the cylinder bore and its wear surface. Wet liners contact coolant directly; dry liners do not.

A heat exchanger that transfers heat between engine oil and engine coolant or air, keeping oil temperature in a safe range under heavy load.

A spring-loaded valve in the pump or oil gallery that opens to bypass oil back to the sump when pressure exceeds a set value, protecting seals and the pump.

The component that draws oil from the sump and forces it under pressure through the galleries to bearings and other moving parts. Types include gear, rotor, and vane designs.

A temperature-controlled valve that blocks coolant flow to the radiator until the engine warms, then opens to regulate operating temperature.

A passage that lets coolant circulate through the engine without passing through the radiator while the thermostat is closed, promoting even warm-up.

A check that samples air from the cooling system for combustion gases, often using a chemical block-test fluid that changes color, to confirm a leaking head gasket or cracked casting.

An exhaust-driven compressor that forces extra air into the engine, increasing the mass of oxygen available so more fuel can be burned for greater power.

A valve that routes exhaust gas around the turbine once target boost is reached, limiting turbine speed and protecting the engine from overboost.

A turbocharger whose turbine inlet vanes change angle to match exhaust flow across the speed range, reducing lag at low speed and controlling boost at high speed.

The amount by which intake manifold pressure exceeds atmospheric pressure when a turbocharger or supercharger is forcing air into the engine, usually measured in psi or kPa.

A heat exchanger that cools compressed intake air after the turbocharger, making it denser so more oxygen enters the cylinder and combustion temperatures stay lower.

An exhaust device that traps soot particles from the exhaust stream. It is cleaned periodically by a regeneration event that burns the collected soot away.

The process of raising exhaust temperature to burn soot trapped in a particulate filter, restoring flow. It may be passive during normal driving or active using extra fuel.

A catalytic converter that uses oxygen to convert carbon monoxide and unburned hydrocarbons in diesel exhaust into carbon dioxide and water. It also helps generate heat for filter regeneration.

An aftertreatment system that injects a urea-based reagent into the exhaust, where it reacts over a catalyst to convert nitrogen oxides into nitrogen and water.

A urea-and-water solution stored in a separate tank and injected into the exhaust for selective catalytic reduction. Low level or poor quality can trigger power derate.

A system that routes a measured portion of exhaust gas back into the intake to lower combustion temperature and reduce nitrogen oxide formation.

A heat exchanger that lowers the temperature of recirculated exhaust gas before it enters the intake, further reducing peak combustion temperature and nitrogen oxide output.

Pollutants formed when nitrogen and oxygen combine at high combustion temperature and pressure. Diesel aftertreatment and EGR strategies target them specifically.

A fuel system design in which a high-pressure pump feeds a shared rail that supplies all injectors, allowing injection pressure and timing to be controlled electronically per event.

An injector that combines a high-pressure pump and nozzle in one unit driven by the camshaft, with a solenoid or piezo actuator controlling injection timing and quantity.

The crankshaft position at which fuel injection begins. It strongly affects power, emissions, noise, and cold starting, and is set by the engine control module.

A small preliminary fuel charge delivered just before the main injection to start combustion gently, reducing ignition delay, noise, and nitrogen oxide formation.

An electrical heating element in the combustion chamber that warms intake air and surrounding metal to aid cold starting in indirect and some direct injection diesels.

A control strategy that advances injection timing and adds fuel during low-temperature starting to compensate for long ignition delay and help the engine fire promptly.

A system that routes crankcase gases back into the intake instead of the atmosphere, often with an oil separator, to control blowby emissions and prevent seal pressure buildup.