1. BTE 1013 ENGINEERING SCIENCEs
6. ENGINE POWER
NAZARIN B. NORDIN

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

3. Torque & power

4. 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.

5. 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.

6. Imep
Example 6.1
An indicator diagram taken from a single-cylinder 4-stroke engine has an effective area of 600 mm2. If the base length of the indicator diagram is 60 mm and the constant is 80 kPa/mm calculate the indicated mean effective pressure.

7. Indicated mean effective pressure (i.m.e.p)
Solution
Indicated mean effective pressure or
(imep)= (effective area of indicator diagram/base length of the diagram)×constant
imep = (600/60)×80 kPa/mm
imep = 800 kPa = 8 bar

8. Calculation of indicated power
Indicated power is calculated from the formula
Indicated power (ip) = (P ×l×a×N)/1000
Unit as kW
where P = mean effective pressure in N/m2, l = length of engine stroke in m, a = cross sectional area of cylinder bore in m2, N = number of working (power) strokes per second.

9. 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 cross-sectional area of the cylinder bore is m2. Calculate the indicated power of the engine in kW.

10. Calculation of 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 kPa=500,000 Pa = 500,000 N/m2;
l = 0.12 m;
a = m2.
Substituting these values in the formula gives
ip = [( ×0.12×0.008×25)/1000] kW
= 12 kW

11. 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

12. 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 m2 and the length of the stroke is 120 mm. Calculate the indicated power of the engine in kW.
Solution

13. 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 the Figure 14.1 on next slide:

14. Simple dynamometer

15. Simple dynamometer Work done per revolution of the flywheel
= force × distance
= (W−S)×2R 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×2π joules

16. Brake power
Brake power = work done per second = 2π × Torque× number of revolutions per second =2π x T x n ie. 2π.T.N This is normally written as brake power (bp = 2πTN where N = number of revolutions per second.) When the torque is in newton metres (N m), the formula bp = 2πTN 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 = 2π.T.N/1000 kW

17. Brake power
Example 6.4:

18. Volumetric efficiency

19. Volumetric efficiency
Example 6.5
A 4-cylinder 4-stroke petrol engine with a bore diameter of 100 mm and a stroke of 110mm 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

20. 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.

21. Thermal efficiency Example 6.6
During a 10-minute dynamometer test on a petrol engine, the engine develops a brake power of 45 kW and uses 3 kg of petrol. The petrol has a calorific value of 43 MJ/kg. Calculate the brake thermal efficiency.

22. Mechanical efficiency
The mechanical efficiency of an engine is defined as brake power, b.p. indicated power, i.p. X 100 % An engine develops a brake power of 120 kW at a speed of 3000 revs/min. At this speed, the indicated power is 140 kW. Calculate the mechanical efficiency of the engine at this speed.

23. THANK YOU