6 ENGINE POWER NAZARIN B NORDIN nazarinicam edu

6. ENGINE POWER NAZARIN B. NORDIN nazarin@icam. edu. my

What you will learn: • • • Torque and power Indicated power and brake power Volumetric efficiency Thermal efficiency Mechanical efficiency

Torque & power

Indicated power • Indicated power (ip) is the power that is developed inside the engine cylinders. It is determined by measuring the pressures inside the cylinders while the engine is on test, on a dynamometer. • Because the pressure varies greatly throughout one cycle of operation of the engine, the pressure that is used to calculate indicated power is the mean effective pressure.

Indicated mean effective pressure (Imep) • Because the pressure varies greatly throughout one cycle of operation of the engine, the pressure that is used to calculate indicated power is the mean effective pressure.

Imep • Example 6. 1 • An indicator diagram taken from a singlecylinder 4 -stroke engine has an effective area of 600 mm 2. If the base length of the indicator diagram is 60 mm and the constant is 80 k. Pa/mm calculate the indicated mean effective pressure.

Imep • Solution • Indicated mean effective pressure or (imep)= (effective area of indicator diagram/base length of the diagram)×constant imep = (600/60)× 80 k. Pa/mm imep = 800 k. Pa = 8 bar

Calculation of indicated power • Indicated power is calculated from the formula • Indicated power (ip) = (P ×l×a×N)/1000 • Unit as k. W • where P = mean effective pressure in N/m 2, l = length of engine stroke in m, a = cross sectional area of cylinder bore in m 2, N = number of working strokes per second.

Calculation indicated power • Example 6. 2 • In a test, a certain single-cylinder 4 -stroke engine develops a mean effective pressure of 5 bar at a speed of 3000 rev/min. The length of the engine stroke is 0. 12 m and the crosssectional area of the cylinder bore is 0. 008 m 2. Calculate the indicated power of the engine in k. W.

Calculation indicated power • Solution • The engine is single-cylinder 4 -stroke, so there is one working stroke for every two revolutions. N, the number of working strokes per second N = 3000÷ 60/2 = 25; P = 5 bar = 500 000 N/m 2, l = 0. 12 m; a = 0. 008 m 2. • Substituting these values in the formula gives ip = 500 000× 0. 12× 0. 008× 25/1000 k. W = 12 k. W

Number of working (power) strokes • As mentioned, each cylinder of a 4 -stroke engine produces one power stroke for every two revolutions of the crankshaft. In a multi-cylinder engine, each cylinder will produce one power stroke in every two revolutions of the crankshaft. A formula to determine the number of power strokes per minute for a multi-cylinder, 4 -stroke • engine is: • number of power strokes per minute • = (number of cylinders/2)×rev/min

Indicated power • Example 6. 3 • A 4 -cylinder, 4 -stroke engine develops an indicated mean effective pressure of 8 bar at 2800 rev/min. The cross-sectional area of the cylinder bore is 0. 01 m 2 and the length of the stroke is 120 mm. Calculate the indicated power of the engine in k. W. • Solution

Brake power • The engine power that actually reaches the output shaft or flywheel of an engine is known as the brake power. It is the power that is measured by a dynamometer and a dynamometer is also known as a brake, hence the term, brake power. • The simplest form of dynamometer is shown in Figure

Simple dynamometer

Simple dynamometer • Work done per revolution of the flywheel = force × distance = (W−S)× 2 R joules (W−S×R also = T the torque that the engine is exerting. By substituting T for W−S×R the) • work done per revolution becomes = T× 2 joules

Brake power = work done per second = T× 2×number of revolutions per second This is normally written as brake power (bp = 2 TN where N = number of revolutions per second. ) When the torque is in newton metres (N m), the formula bp = 2 TN gives the power in watts. As engine power is given in thousands of watts, i. e. kilowatts, the formula for brake power normally appears as: bp = 2 TN/1000 k. W

Brake power • Example 6. 4:

Volumetric efficiency

Volumetric efficiency • Example 6. 5 • A 4 -cylinder 4 -stroke petrol engine with a bore diameter of 100 mm and a stroke of 110 mm has a volumetric efficiency of 74% at an engine speed of 4000 rev/min. Determine the actual volume of air at STP that flows into the engine in 1 minute. • Solution

Thermal efficiency • The thermal efficiency of an engine is a term that is used to express the effectiveness of an engine’s ability to convert heat energy into useful work. Thermal efficiency is the ratio of energy output of the engine to energy supplied to the engine in the fuel.

Thermal efficiency • Example 6. 6 • During a 10 -minute dynamometer test on a petrol engine, the engine develops a brake power of 45 k. W and uses 3 kg of petrol. The petrol has a calorific value of 43 MJ/kg. Calculate the brake thermal efficiency.

Mechanical efficiency • The mechanical efficiency of an engine is defined as brake power indicated power. A certain engine develops a brake power of 120 k. W at a speed of 3000 rev/min. At this speed, the indicated power is 140 k. W. Calculate the mechanical efficiency of the engine at this speed.