ENGINE PERFORMANCE TERMS

A number of common terms are used to describe an engine’s performance.

Power

The most common performance rating that has been applied to automobiles is a power rating of the engine. It usually is given in kilowatts or, in earlier times, in horsepower (note: .746 kilowatt = 1 horsepower). Power is the rate at which the engine is making useful work. It differs with engine speed and throttle angle. Power may be calculated at the drive wheels or at the engine output shaft. It is more convenient and valuable to the designer of an electronic engine control system to know the output power of only the engine than the power delivered to the wheels. This permits realistic comparisons of engine data as engine controls are varied. To create such measurements, an engine dynamometer is used.

The power delivered by the engine to the dynamometer is called the brake power and is designated Pb. The brake power of an engine is forever less than the total amount of power that is actually developed in the engine. This generated power is called the indicated power of the engine and is denoted Pi. The indicated power varies from the brake power by the loss of power in the engine due to friction between cylinders and pistons, and other friction losses. That is, Pb = Pi – friction and other losses

BSFC

Fuel economy can be calculated while the engine delivers power to the dynamometer. The engine is logically operated at a fixed RPM and a fixed brake power (fixed dynamometer load), and the fuel flow rate (in kg/hr or lb/hr) is measured. The fuel consumption is then given as the ratio of the fuel flow rate (rf ) to the brake power output (Pb). This fuel consumption is known as the brake-specific fuel consumption ( BSFC).

BSFC = rf/Pb

The units for BSFC are lb/hr/horsepower. By improving the BSFC of the engine, the fuel saving of the vehicle in which it is installed is also improved. Electronic controls assist to improve BSFC.

Torque

Engine torque is the twisting action produced on the crankshaft by the cylinder pressure pushing on the piston during the power stroke. Torque is created whenever a force is applied to a lever. The length of the lever (the lever arm) in the engine is determined by the throw of the crankshaft (the offset from the crankshaft centerline of the point where the force is applied).

The torque is expressed as the product of this force and the length of the lever. The units of torque are N·m (newton meters) in the metric system or ft lb (foot-pounds) in the U.S. system. (One ft lb is the torque created by one pound acting on a lever arm one foot long.) The torque of a typical engine differs with RPM.

Volumetric Efficiency

The variation in torque with RPM is strongly influenced by the volumetric efficiency, or “breathing efficiency.” Volumetric efficiency actually describes how well the engine functions as an air pump, drawing air and fuel into the different cylinders. It depends on a variety of engine design parameters such as piston size, piston stroke, and number of cylinders and is strongly influenced by camshaft design.

Thermal Efficiency

Thermal efficiency expresses the mechanical energy that is delivered to the vehicle relative to the energy content of the fuel. In the typical SI engine, 35% of the energy that is available in the fuel is lost as heat to the coolant and lubricating oil, 40% is lost as heat and unburned fuel in exhaust gases, and another 5% is lost in engine and drive train friction. This means that only about 20% is available to drive the vehicle and accessories. These percentages differ somewhat with operating conditions but are valid on the average.

Calibration

The meaning of engine calibration is the setting of the air/fuel ratio and ignition timing for the engine. With the latest electronic control systems, calibration is measured by the electronic engine control system.

ENGINE MAPPING

The improvement of any control system comes from knowledge of the system to be controlled. For the automobile engine, this information of the plant (the engine) comes primarily from a process called engine mapping.

For engine mapping, the engine is connected to a dynamometer and operated throughout its entire speed and load range. Measurements are prepared of the important engine variables at the same time as quantities, for example the air/fuel ratio and the spark control, are varied in a known and efficient manner. Such engine mapping is done in engine test cells that contain engine dynamometers and complex instrumentation that collects data under computer control.

From this mapping, a mathematical model is created that explains the influence of every measurable variable and parameter on engine performance. The control system designer must pick a control pattern, control variables, and control strategy that will satisfy all performance requirements (including stability) as computed from this model and that are within the other design limits such as cost, excellence, and reliability. To recognize a typical engine control system, it is instructive to consider the influence of control variables on engine performance.


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