Diesel Dictionary
Sometimes even gear heads can get lost in translation. Equipment operators know much more about machine technology and terminology than the average Joe or Jane, but when it comes to deciphering the lexicon of certain specialized systems (like a diesel engine) the phraseology can still be confusing. Do you really know what the engine intake manifold does or what diesel displacement actually means? Do you understand the difference between diesel emission regulation Stages (I, II, III, IV) and Tiers (1, 2, 3, 4)?
Even the most mechanically-inclined contractors need a resource to fall back on when it comes to precise terms and exact definitions, so John Deere Power Systems researched its diesel databanks to come up with the essential engine dictionary that follows. Before navigating the vocabulary below, let’s first clarify what makes a diesel engine different from a gasoline engine. Rudolf Diesel developed his namesake engine in 1892, because he wanted to make an engine that was more efficient than gasoline engines. Diesel engines achieve higher efficiency because they compress air at a much higher ratio, up to 25:1 for diesel vs. up to 12:1 for gas.
A diesel engine fills its cylinders with air, compresses the air and then injects diesel fuel into the compressed air, usually near top dead center. The properties of diesel fuel are such that the heat of the compressed air ignites the air/fuel mixture with no spark — this is called autoignition. A spark is needed for a gasoline engine.
Diesel fuel is also different. Aside from the different smell and feel, diesel has higher energy density than gasoline. That is to say, 1 gal of diesel contains more BTUs than 1 gal of gasoline. That’s why, combined with their better efficiency, diesel engines get better mileage than equivalent gas engines. Now that you have a general background of how diesels developed, let’s detail a nuts and bolts glossary.
Engine Basics
Aspiration: This means “breathing in.” In reference to an engine, this is how the air makes it into the cylinder. A naturally-aspirated engine draws in ambient air using the vacuum created by the downward movement of the piston in the cylinder. Most diesel engines today use forced air induction, either turbocharging or supercharging, to improve engine performance.
Crankshaft: This component is a critical element of the power cylinder portion of the engine. This device translates the up and down axial motion of the pistons into rotational energy (torque).
Cylinder: This is the cylindrical chamber in which the piston travels and where combustion takes place.
Cylinder Head: The cylinder head is located directly above the cylinders and is bolted to the engine block to withstand the extreme pressures experienced during combustion. It contains the valves and passageways (ports) needed to introduce fresh air, maintain compression and remove exhaust gases. A four-valve cylinder head will have more efficiency, power and torque than a comparable two-valve cylinder head.
Displacement: An engine’s displacement is the total volume of air/fuel mixture an engine can draw in during one complete engine cycle. It is calculated by determining the product of the cross-section area of the cylinder, the piston stroke and the number of cylinders. Displacement is usually measured in liters. For example, a John Deere 9.0L engine has a displacement of nine liters spread over six cylinders.
Fuel Injection: This is the manner by which the fuel is introduced into the cylinder at the proper time during the compression cycle, resulting in combustion. Some engines use multiple injections to improve engine performance and reduce emissions.
Glow Plugs: Sometimes in cold temperatures, the compressed air cannot get hot enough for autoignition. Glow plugs heat the air, helping it reach the appropriate temperature. Glow plugs are wires that heat by resistance. Today’s electronically controlled engines generally do not require glow plugs.
Horsepower: Every equipment operator knows that horsepower is the measure of an engine’s power. But what does horsepower really mean? James Watt estimated that a horse could do 33,000 ft-lbs of work in one minute and that arbitrary measurement stuck. (See the torque entry for more information on foot-pounds.) So, one horsepower equals 33,000 ft-lbs per minute, and one horsepower (over the course of an hour) is equivalent to 2,545 BTU. Horsepower is a measure of the power an engine can produce against a given load.
Intake Manifold: This part of the engine delivers air to the cylinder head for the compression cycle.
Piston: Pistons are lubricated shafts that move up and down inside the engine cylinders. Modern piston design includes various “bowls” in the top of the piston for the purpose of improving combustion efficiency.
Stages (I, II, III, IV): The European Union’s (EU) term for the phased-in implementation of increasingly stringent diesel engine emissions regulations.
Stroke: Diesel engines are either two- or four-stroke. The piston traveling its full length in one direction is a stroke. The piston has an intake stroke and a compression stroke. Ignition forces the piston down on the power stroke and the final stroke, when the piston pushes the exhaust gases out of the cylinder, is the exhaust stroke.
Tiers (1, 2, 3, 4): The Environmental Protection Agency’s (EPA) term for the phased-in implementation of increasingly stringent diesel engine emissions regulations.
Torque: Torque is turning force or rotational force, measured in foot-pounds or newton-meters. It’s the force you apply when you turn a wrench. To calculate torque, you multiply the force by the distance from center. For example, you can generate 200 ft-lbs of torque by putting 200 lbs of force on a wrench that’s 1 ft long or by putting 100 lbs of force on a wrench that’s 2 ft long. The power output of an engine is torque multiplied by rotational speed. A torque curve plots torque as a function of engine speed (rpm).
Transient-Response Time: The time that is required for an engine to recover the set speed after a load is imposed. This is an engine’s “zero to 60” factor.
Turbocharging: This is a form of aspiration. A turbocharger, which consists of a turbine and a compressor, puts air into the cylinder more forcefully. The exhaust spins the turbine, which is joined by a shaft to the compressor, thereby pressurizing the intake air. A turbocharged engine typically has better performance than a comparable naturally-aspirated engine.
Current and Emerging Emissions Technologies
Below is some terminology you will probably encounter as Tiers 3 and 4 engines go into production. Some of these technologies are in use today; others will be used to meet Tier 4 requirements.
Diesel Particulate Filter (DPF): DPFs are through-the-wall flow devices that trap and hold particles in the exhaust (soot). This technology will likely be used to meet Tier 4 requirements. With the help of a catalyst, the DPF cleans or regenerates by burning the collected soot. Regeneration requires high exhaust gas temperatures. “Active” DPFs maintain proper filter performance by periodically raising the exhaust temperature to ensure regeneration.
Cooled Exhaust Gas Recirculation (EGR): Lowering an engine’s peak combustion temperature is one way to lower oxides of nitrogen (NOx), one of the emissions regulated by the EPA. EGR re-circulates exhaust gas by mixing it with incoming fresh air. This process lowers oxygen content, reducing NOx. Cooled EGR includes the cooling of the exhaust gas by passing it through a heat exchanger prior to recirculation. This lower-temperature, re-circulated exhaust gas reduces NOx more than non-cooled EGR.
Diesel Oxidation Catalyst (DOC): A DOC is a flow-through aftertreatment device that is effective at reducing carbon monoxide, hydrocarbons and some particulate matter (PM). The DOC oxidizes both gaseous (volatile) hydrocarbons and the semi-volatile portion of PM. DOCs operate at peak efficiency when the sulfur content in the fuel is 500 ppm or lower. DOCs typically reduce carbon monoxide by 40 percent, hydrocarbons by 50 percent and PM by 20 percent.
High-Pressure Common-Rail Fuel System (HPCR): HPCR distributes fuel to the injectors via a high-pressure accumulator or rail. Pressure, timing, rate and duration are electronically controlled. Multiple injections — a method for reducing engine noise and NOx — are also possible with HPCR.
NOx Adsorber (NAC): An NAC, sometimes called a Lean-NOx trap (LNT), is an aftertreatment device that acts as a molecular sponge, trapping NOx. When the trap is full, the NAC must be cleaned or regenerated. A clean NAC can reduce NOx by up to 90 percent.
Selective Catalytic Reduction (SCR): This aftertreatment technology reduces NOx emissions, typically by using a urea-based solution. The urea solution is mixed with engine exhaust gases in the catalytic converter. The ammonia in the urea decomposes and reacts with the NOx in the converter, producing oxygen, nitrogen and water.
Variable Geometry Turbocharger (VGT): A VGT places variable angle vanes in an annular ring around the turbine. Varying the angle of these vanes can boost compressor rotation at low speeds or prevent overspinning at high speeds. Coupling VGTs with electronic controls enables improvements in engine performance and emissions reductions.
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