Actual net power available at the engine's crankshaft is called the power output or brake power and is commonly expressed in the English system as brake hp (bhp). This term is derived from the fact that the power output of an engine can be measured by absorbing the power with a brake.

Torque (T) is a measure of the force of rotation. It consists of the product of the force applied to a lever and the perpendicular distance from the line of action of the force to the axis of rotation. It is most commonly expressed in lbf-ft or N-m. For a given torque, there are numerous combinations of amount of force and length of a lever arm. The lever arm is the distance between the centers of the crankpin and the main journal and is, therefore, a radius (r). Torque can thus be expressed as F x r. If, for example, in English units, the force on the crankpin is 500 lbf and the lever arm (length of crank) is 2 ft, then the torque rotating the crankshaft is 1,000 ft-lbf.

While power is the rate at which an engine does work, torque is the capacity of an engine to do work. Torque does not vary in proportion to speed of the engine as does brake power, but depends primarily on volumetric efficiency and friction losses. Substituting torque for the product of force and radius (Fr) in the equation for power yields the following relationship between power and torque.

where W represents the cyclic work and Vswept is the swept volume (Va—Vb), with the distinction that the two-stroke cycle requires one revolution and the four-stroke cycle requires two revolutions of the crankshaft.

Mep is the constant averaged pressure on the piston over the length of the piston stroke. It reduces the varying pressure on the piston to a single averaged value. Mep is useful in evaluating the ability to produce power. Since the product of piston travel and piston area is piston volume (displacement), the relative power-producing capability of two pistons can be established by comparing the product of mep times displacement in each case. The force on the piston is equal to the pressure times the area (A). Since mep is equivalent to a constant force on the piston over the length of the piston stroke, the work done per power stroke is F times L, where L is the length of the stroke. Thus, for a given displacement, mep is useful in evaluating an engine's ability to produce power due to the following relationship:

Engine power = displacement x speed x mep (9-22)

In accordance with Equations 9-20 and 9-22, if the displacement of the engine is doubled, torque also doubles.

Mep cannot be measured directly, but it can be calculated from the power equation if the power output, displacement (LA) and speed are known. The relationship of mep to power is shown by rearranging Equation 9-22 as follows:

In English units, when mep is in psi, power is in hp, LA in cf, and n is in power strokes per minute (in a four-stroke-cycle engine, n = N/2; in a two-cycle engine, n = N, where N is the crankshaft rotational speed, in rpm), Equation 9-23 becomes:


Note that when LA is expressed in in3, 33,000 becomes 396,000.

In SI units, when mep is in kPa, power is in kW, LA is in liters (l), and n is in power strokes per minute, Equation 9-23 becomes:

Given the relationship between power and torque, the relationship of mep and engine torque can be expressed, in English units, as:

where mep is in psi, torque in lbf-ft, and LA in in3 for a four-stroke cycle. For a two-stroke cycle, 150.8 would be replaced with 75.4.

In SI units, the relationship of mep and engine torque can be expressed as:

where mep is in kPa, torque in N-m, and LA in l for a four-stroke cycle. For a two-stroke cycle, 12.56 would be replaced with 6.28.

Bmep is a variable independent of the capacity of the engine. As shown by Equation 9-24, for a given set of engine operating conditions, the torque that is developed is proportional to the brake mep. Typically, torque and bmep curves peak at about half that of brake power. Since brake power is proportional to the product of torque and speed, and torque is controlled by the capacity of the engine, higher brake power arises from higher speed. However, with increased speed, friction losses increase at a faster rate than does power, thereby resulting in a relative decrease in mechanical efficiency. Thus, while power increases with increased speed, it does so at a decreasing rate.

Typically, reciprocating engine bmep values will range from 100 to 250 psi (6.9 to 17.2 bar), depending on engine type, design, aspiration type, and operating condition. Generally, bmep will be greater with super-charged/turbocharged engines than with naturally aspirated engines. Bmep values are highest at the speed where maximum torque is achieved and somewhat lower at maximum rated power.

The following example illustrates the measurement of actual brake power and torque for a four-stroke-cycle engine with the following characteristics:

Engine brake power -

Engine torque =

Engine Aspiration Systems (Intake and Exhaust)

The engine air intake system supplies clean air for combustion. The exhaust system forces exhaust gases remaining from the previous power stroke from the combustion chamber.

Valve assemblies open and close the intake and exhaust ports that connect the intake and exhaust manifolds to the combustion chamber. Valves, which are usually made from forged alloy steel, are subjected to the direct pressure and temperature of the combustion occurring within the cylinders. The valve stem moves in a valve guide, which can be an integral part of the cylinder head or may be a separate unit pressed into the head.

In a four-stroke-cycle engine, about two-thirds of the way through the power (expansion) stroke, the exhaust valve starts to open. The exhaust gas flows through the valve into the exhaust port and manifold until the cylinder pressure and the exhaust pressure reach equilibrium. This piston then displaces, or sweeps, the gases from the cylinder into the manifold during the exhaust stroke. The intake valve opens just before TDC, while the exhaust valve remains open until, or just after, TDC.

Figure 9-19 illustrates the admission process for an air-fuel mixture in a spark-ignited engine, detailing the location of the spark plug between the intake and exhaust valves. In a Diesel engine, a fuel pump-nozzle assembly is used to atomize and inject the fuel into the combustion chamber after air compression is complete.

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