Diesel injection glossary

Definitions of the terms that appear most often when discussing pumps, injectors, turbochargers, Common Rail and after-treatment systems in diesel engines.

Atomization: The way the injector breaks fuel into very fine droplets so it mixes well with air and burns completely.
Bar: Unit of pressure used in diesel injection. One bar is roughly atmospheric pressure at sea level; 1 bar is about 14.5 psi.
Boost pressure: Intake air pressure above atmospheric, generated by the turbo. It is measured in bar or psi.
Calibration map: Table in the ECU software that defines what value a parameter (for example, fuel delivery) should take under given operating conditions.
Carbon build-up: Carbon deposit formed by incomplete combustion that accumulates on nozzles, EGR valves and turbo vanes.
Cavitation: Formation and collapse of microbubbles in a liquid. In ultrasonic cleaning it is what removes deposits from parts.
Cetane number: Measure of how easily a diesel fuel ignites under compression. A low cetane number makes starting harder and increases noise.
Common Rail: Electronic injection system in which a high-pressure pump fills a common rail and the ECU opens each injector when needed. Because pressure does not depend on engine speed, it allows several injections per cycle.
Delivery (fuel volume): Amount of fuel delivered by an injector or pump in a given time or cycle. It is measured on a bench and compared with the manufacturer's specification.
DPF (diesel particulate filter): Filter fitted in the exhaust that traps soot. It is cleaned periodically through a process called regeneration, which burns off the accumulated soot.
Dynamic balancing: Process that balances the turbo's rotating assembly at a speed close to working speed to avoid vibration and bearing wear.
ECU: Engine control unit. It processes sensor data and governs injection, rail pressure, the turbo and other parameters.
EGR: System that sends part of the exhaust gas back to the intake to lower combustion temperature and reduce nitrogen oxide emissions.
Glow plug: Heating element that warms the combustion chamber to make a cold diesel engine easier to start.
In-line pump: Injection pump with one pumping element (plunger and barrel) for each engine cylinder. It is robust and used in large truck, bus and machinery engines.
Injection pump: Component that takes fuel at low pressure and delivers it to the injectors at the right pressure, volume and timing. In mechanical systems it also regulates injection timing according to engine speed and load.
Injection timing: The moment, relative to the piston's top dead center, at which injection starts. Incorrect timing causes smoke, noise and loss of power.
Injector: Part that sprays fuel into the combustion chamber at the right moment. It can be mechanical (opens by pressure) or electronic (opens on a command from the ECU).
Intercooler: Heat exchanger that cools the air compressed by the turbo before it enters the engine. Cold air is denser and allows better combustion.
Nozzle: Tip of the injector with calibrated holes through which atomized fuel exits. Its condition determines the spray shape.
Rail: Line that stores fuel at high pressure and distributes it to the injectors in a Common Rail system. It carries a pressure sensor and a regulator valve.
Return flow: Fuel that goes back to the tank through the return line of the injector or pump. Excessive return flow indicates internal leakage.
Rotary pump: Injection pump with a single pumping element and a rotating distributor that sends fuel to each cylinder. It is compact and was very common in light diesel vehicles.
SCR / AdBlue: After-treatment system that injects a urea solution (AdBlue) into the exhaust to convert nitrogen oxides into nitrogen and water.
Test bench: Equipment that reproduces the working conditions of a pump or an injector and measures its delivery, pressure and response, to compare them with the specification.
Top dead center (TDC): The highest position the piston reaches inside the cylinder. It is used as a reference to measure injection timing.
Turbocharger: Device that uses exhaust gas to drive a turbine that compresses the engine's intake air, so more air enters the cylinder and more fuel can be burned.
Unit injector: System in which each cylinder has a unit that combines pump and injector. Pressure is generated in the unit itself, driven by the camshaft.
Variable geometry (VGT): Turbo with movable vanes that adjust the flow of exhaust gas according to engine load. It improves response at low revs.
Wastegate: Relief valve that limits boost pressure by diverting part of the exhaust gas away from the turbine.
Water separator filter: Filter that retains water present in the fuel so it does not reach the pump or injectors. It must be drained periodically.