Parasitic load

Parasitic load is a term used with regard to electrical appliances, railway locomotives and internal combustion engines. With regard to electrical appliances, it represents the power consumed even when the appliance is shut off, that is standby power. With regard to railway locomotives, it is any of the loads or devices powered by the prime mover not contributing to tractive effort, such as an air compressor, traction motor blower, or radiator fans. With regard to internal combustion engines such as those used in automobiles, it refers to devices that take energy from the engine in order to enhance the engine's ability to create more energy or convert energy to motion.
Electricity production.
With regard to electricity production, parasitic loss it is any of the loads or devices powered by the generator, not contributing to net electric yield found by subtracting productive yield from gross yield or:
 GY - PY = PL where
 "GY" is gross electric yield (the output of the generator);
 "PY" is productive yield (the electricity which is made available to external electric loads)
 "PL" is parasitic load.
Parasitic loss in internal combustion engines.
The term "parasitic loss" is often applied to devices that take energy from the engine in order to enhance the engine's ability to create more energy or convert energy to motion. In the internal combustion engine, almost every mechanical component, including the drivetrain, causes parasitic loss and could thus be characterized as a parasitic load. 
Examples.
Bearings, oil pumps, piston rings, valve springs, flywheels, transmissions, driveshafts, and differentials all act as parasitic loads that rob the system of power. These parasitic loads can be divided into two categories: those inherent to the working of the engine and those drivetrain losses incurred in the systems that transfer power from the engine to the road (such as the transmission, driveshaft, differentials and axles). 
For example, the former category (engine parasitic loads) includes the oil pump used to lubricate the engine, which is a necessary parasite that consumes power from the engine (its host). Another example of an engine parasitic load is a supercharger, which derives its power from the engine and creates more power for the engine. The power that the supercharger consumes is parasitic loss and is usually expressed in kilowatt or horsepower. While the power that the supercharger consumes in comparison to what it generates is small, it is still measurable or calculable. One of the desirable features of a turbocharger over a supercharger is the lower parasitic loss of the former.
Drivetrain parasitic losses include both steady state and dynamic loads. Steady state loads occur at constant speeds and may originate in discrete components such as the torque converter, the transmission oil pump, and/or clutch drag, and in seal/bearing drag, churning of lubricant and gear windage/friction found throughout the system. Dynamic loads occur under acceleration and are caused by inertia of rotating components and/or increased friction.
Measurement.
While rules of thumb such as a 15% power loss from drivetrain parasitic loads have been commonly repeated, the actual loss of energy due to parasitic loads varies between systems. It can be influenced by powertrain design, lubricant type and temperature and many other factors. In automobiles, drivetrain loss can be quantified by measuring the difference between power measured by an engine dynamometer and a chassis dynamometer. However, this method is primarily useful for measuring steady state loads and may not accurately reflect losses due to dynamic loads. More advanced methods can be used in a laboratory setting, such as measuring in-cylinder pressure measurements, flow rate and temperature at certain points, and testing of individual parts or sub-assemblies to determine friction and pumping losses.
For example, in a dynamometer test by "Hot Rod" magazine, a Ford Mustang equipped with a modified 357ci small-block Ford V8 engine and an automatic transmission had a measured drivetrain power loss averaging 33%. In the same test, a Buick equipped with a modified 455ci V8 engine and a 4-speed manual transmission was measured to have an average drivetrain power loss of 21%.
Laboratory testing of a heavy-duty diesel engine determined that 1.3% of the fuel energy input was lost to parasitic loads of engine accessories such as water and oil pumps.
Reduction.
Automotive engineers and tuners commonly make design choices that reduce parasitic loads in order to improve efficiency and power output. These may involve the choice of major engine components or systems, such as the use of dry sump lubrication system over a wet sump system. Alternately, this can be effected through substitution of minor components available as aftermarket modifications, such as exchanging a directly engine-driven fan for one equipped with a fan clutch or an electric fan. Another modification to reduce parasitic loss, usually seen in track-only cars, is the replacement of an engine-driven water pump for an electrical water pump. The reduction in parasitic loss from these changes may be due to reduced friction or many other variables that cause the design to be more efficient.