AutoWeb defines some common terms used with diesel vehicles.
(See Diesel Exhaust Fluid.)
A type of diesel fuel made from sources other than petroleum, such as soybean oil or filtered fryer oil. Biodiesel is often sold in blends such as B20, which consists of 20% biodiesel and 80% petroleum diesel.
A measurement of the quality of diesel fuel. The higher the cetane number, the more easily the fuel ignites. Cetane rating is often seen as analogous to the octane rating of gasoline, though in fact it measures a different attribute. Petroleum-based diesel fuel sold in the US generally has a cetane rating of 40 or higher.
A type of diesel fuel injection in which diesel fuel is fed under extremely high pressure to individual injectors from a common pipe, known as the fuel rail. Common rail injection allows more precise control of the amount of fuel injected than the older injection pump system (see definition below), which uses a single (and rather complicated) engine-driven pump to pressurize, meter and distribute fuel to individual cylinders.
A process of igniting fuel using only the heat of compression. Diesels are compression-ignition engines, as opposed to gasoline engines, which use spark ignition.
Diesel exhaust fluid (DEF)
A urea-based solution used in the emissions systems of diesel engines with Selective Catalytic Reduction (see definition below). Diesel exhaust is high in nitrogen oxides (NOx); in order to reduce it, DEF is sprayed into the exhaust just upstream of the catalytic converter, where it vaporizes and releases ammonia. The ammonia reacts with the NOx to form water and nitrogen. The EPA requires that DEF-capable diesels be disabled if the supply of DEF runs out. DEF is carried in a separate tank, usually with enough capacity to last as long as the oil change interval. DEF should never be mixed with diesel fuel or poured directly into the fuel tank; doing so can cause extensive (and expensive) damage.
Diesel particulate filter
A device that traps soot (particulate matter) in the diesel engine’s exhaust. The soot is burned off by increased exhaust temperatures which occur when the car is driven at higher speeds. Burn-off can also be initiated by manipulating the fuel injection system to increase the exhaust temperature.
A form of fuel injection in which fuel is sprayed directly into the cylinder to improve efficiency. Most modern diesels feature direct injection; some older diesels use indirect injection, in which diesel fuel is sprayed into a “pre-chamber” just outside the main combustion chamber. (This differs from indirect injection in gasoline engines, in which fuel is mixed with air in the intake manifold or throttle body.)
A device that provides engine braking in a diesel vehicle by closing a valve in the exhaust to create backpressure. Because diesels do not have a throttle valve to limit the amount of air ingested by the engine, they do not have the same engine-braking characteristics as gasoline engines; therefore an auxiliary device such as an exhaust brake, compression-release brake, or driveline retarder is needed. Exhaust brakes are the simplest type of diesel engine brakes, and unlike compression release brakes (often known as “Jake Brakes” after the Jacobs brand name), they operate silently. Most diesel pickups now offer an exhaust brake as an option, and they should be considered a must-have for safe towing.
An electrical device that to pre-heats the diesel engine for quicker cold starting. Because diesels have no spark plugs, they rely on compression to generate heat to combust the fuel, which can make for slow starting in cold weather; glow plugs speed up the process. Older diesels required long glow plug cycles (10 seconds or more), but modern diesels pre-heat almost instantly, even in winter.
A mechanical device that pressurizes, meters and delivers fuel to the individual cylinders in a diesel engine to control its speed and power output. The injection pump combines the functions of the gasoline engine’s throttle valve, carburetor/fuel injection system, and distributor/ignition system. In modern diesels, the injection pump has largely been replaced by electronic common-rail injection.
A German mechanical engineer who invented the diesel engine. He received his first patent for the engine in 1892 and built the first working example in 1897.
Selective catalytic reduction (SCR)
An emissions control system used on modern road-going diesel engines to reduce nitrogen oxides (NOx). SCR uses ammonia and a specially-designed catalytic converter to convert NOx to nitrogen and water. The source of the ammonia is usually diesel exhaust fluid (see definition above), which must be replenished from time to time.
An exhaust-driven turbine that pumps air into the engine’s combustion chamber. A diesel engine’s power output is limited by how much air can be delivered to the cylinders. The turbocharger increases the volume of air and therefore the engine’s power potential. Non-turbocharged diesel produce lots of low-end torque (pulling power) but little high-end horsepower; the turbocharger increases high-RPM power, making the engines better suited to cars and light trucks. Virtually all modern road-going diesel engines have a turbocharger.
Ultra low sulfur diesel (ULSD)
A type of diesel fuel with reduced sulfur content. Sulfur is a key element in the formation of particulate emissions (soot), and the use of ULSD allows engines to employ diesel particulate filters (see definition above). ULSD has a maximum sulfur content of 15 parts per million (PPM), as opposed to 500 PPM in standard diesel fuel. As of 2010, all diesel fuel sold in the United States for use in road vehicles is ULSD.
(See Diesel Exhaust Fluid.)