Unit 2 - WordPress.com · Web viewA four stroke engine has a bore of 72 mm and a stroke of 102 mm. Its rated speed is 3200 rev/min and it is tested at this speed against a brake

ENGINE CRITERIA AND COMPARISON J3182 / UNIT5 / 1

Unit 5Title: Engine criteria and comparison (Part 1)

General objective:To understand the criteria of engine performance.

Specific objectives:At the end of this unit you should be able to:

1. define the indicated power and brake power.

2. define the thermal efficiency and brake power

efficiency.

3. calculate the torsion, fuel assumption and specific fuel

consumption

4. draw and discuss the graph of ;

4.1 torsion Vs speed and break power.

4.2 break power and indicated power Vs speed.


4.3 indicated power Vs break power.

4.4 fuel assumption Vs speed.

4.5 mechanical efficiency Vs speed and brake power.

Input This section introduces the subject matter that you are going to learn.5.0 Introduction

Over the past years, automotive manufacturers have built many sizes and

types of engines. These engines differ in the amount of power they produce. The

important factor when choosing an engine is the power and speed. The cost of

capital and cost of operation are also important .

5.1 The Indicator power (I.p)

Indicator power (i.p.)

This is defined as the rate of work done by the gas on the piston as evaluated

from an indicator diagram obtained from the engine. An indicator diagram has

the form shown in Figure.5.1 (power loop and pumping loop)

Indicator power represents the maximum power from the engine under ideal or

perfect condition. I.p is calculated on the basis of engine size, displacement,

operational speed and the pressure developed theoretically in the cylinder. Ip

will always be more than b.p.


Therefore indicator min effective pressure (imep) or (pi) is given as

(constant spring is given in bar per mm)

pi = area of diagram (mm 2 ) x cos tan t spring

large of diagram ( mm)

At the engine with only one cylinder

Work per cycle = pi x A x L

(with A is Area, L is Lange of displacement)

Work per minute = work per cycle x cycle per minute, so

Ip = piAL x (cycle/minute) or piALN

When an engine is four stroke, rpm is N/2, and when an engine is two stroke,

rpm is N. Therefore,

For four stroke engine, and for two stroke engine,

, n is the number of cylinder.

+ +

+ ++ +

+ +

+ ++ +

+ ++ +

+ ++ +

+ +

+ ++ +

--

--

--

--

-- -

---

-- -

---

--

Figure 5.1 : Ip at P-V Diagram

Power Loop

Pumping Loop


5.2 The Brake power (b.p)

Brake power is known as the indicated power output, bp is the power

developed inside the engine cylinder by the combustion of the charge. It is also

called brake power because a brake is used to slow down the shaft inside a

dynamometer. Brake horsepower is often used to compare engines and their

characteristics. Automotive manufacturers use brake horsepower to show the

differences between engines.

Example 5.2 (a)A 235 cubic inch engine will produce less b.h.p than a 350 cubic inch engine.

bp = 2NT,

N = rpm, T = torque = W x R, W = weight, R = radius from rotation point.

Example 5.2(b)One engine petrol 4-cylinder has 57mm, L =90mm, speed =2800 rpm, the

dynamometer radius is 0.356m, and brake load is 155 N

Calculate the brake power.

Piston area =

Stroke lange = 0.09m

Torque, T = WR 155N x 0.356m = 55.2Nm

b.p =


5.3 Friction Power and Mechanical Efficiency.

Friction power (fp) is defined as the horsepower being used to overcome

internal friction. Anytime two objects touch each other while moving, friction is

produced. Friction has a tendency to slow down the engine.

f.p = i.p – b.p

Mechanical efficiency, ηm = , ηm = 80% - 90% .

Example 5.3 (a)One engine petrol has i.p = 16.2kW and b.p = 19.2kW

Therefore,

Mechanical efficiency, m

power

Engine speed rpm

i.p

b.p

f.p

Figure 5.3: Power vs engine speed


5.4 Brake Min Effective Pressure (bmep)

from dynamometer, b.p = ηm x i.p

for four stroke engine,

bmep, (Pb) = ηm x Pi

Example 5.4 (a)

One engine petrol four stroke and four cylinder.

i. Area of piston = 0.0025mm2

ii. Lange of stroke = 0.09m

iii. Engine speed, = 2800 rpm

iv. b.p = 16.2 Nm


5.5 Thermal Efficiency,(ήbt ),Specific Fuel Consumption, (s.f.c) and Indicator min effective pressure, (i.m.e.p.)5.5.1 The power output of the engine is obtained from the chemical energy of

the fuel supplied. The overall efficiency of the engine is given by the brake

thermal efficiency, ήbt

ηbt = or

(mf is the mass of fuel consumed per unit time, and Qnet, v is the lower

calorific value of the fuel)

5.5.2 Specific fuel consumption (s.f.c) is the mass of fuel consumed per kW

develop per hour, and is a criterion of economical power production,

i.e

5.5.3 Indicator min effective pressure, i.m.e.p. Pm.


Example 5.5(a)One engine used 6.74 l/hour, density of fuel, =0.735 kg/m3 and Qnet.v =44200 kJ/kg

and brake power is 16.2 kw. From the b.m.e.p equation this engine has 7.55 bar and

84% of mechanical efficiency.

Calculate the brake thermal efficiency , (ηbt ) specific fuel consumption (s.f.c) and

Indicator min effective pressure, (i.m.e.p.) or ( Pm).

Mass of fuel, mf =

or


Activity 5AThis section tests your understanding of the subject matter. You have to complete this section by following the instructions carefully. 5.1 Define the meaning of Indicator power

5.2 Explain the function of brake horsepower in an engine.

5.3 Calculate the brake power based on the data below.

One engine has 60mm, L =92mm, speed =2850 rpm, the dynamometer

radius is 0.350m, and brake load is 158 N

5.4 How many percent of mechanical efficiency when one engine petrol has i.p =

187N and b.p = 158N ?

5.5 Calculate the thermal efficiency and specific fuel consumption when an engine

uses 6.5 l/hour, density of fuel, =0.735 kg/m3 and Qnet.v =44200 kJ/kg and

brake power is 18 kw.


Input This section introduces the subject matter that you are going to learn.5.6 Torque Vs speed and break power.

Torque is a good indicator of the ability of an engine to do work. It is

defined as force acting at a moment distance and has unit of N-m or Ibf-ft. CI

engines generally have greater torque than SI engine. Large engines often have

very high torque values with maximum brake torque MBT at relatively low speed.

Figure 5.6: Graph Torque Vs speed


Many modern automobile engines have maximum torque in the 150 to 200 N-m

range at engine speed usually around 3000 to 4000 rpm as shown in Fig 5.6.

5.7 Break power and indicated power Vs speed.

If the mean effective pressure (mep) and the mechanical efficiency of an

engine remains constant as the speed increases, then both indicator and the

brake power would increase in direct proportion to the speed, and the

characteristic curves of the engine would be the simple form shown in Fig. 1.15,

in which the line marked ‘bmep’ is the product of indicated mean effective

pressure (imep) and mechanical efficiency, and is known as brake mean

effective pressure (bmep). Indicated power and brake power usually increase

when speed engine increases.

Figure 5.7: Indicated Power and Brake Power Vs Speed


5.8 Mechanical efficiency Vs speed and brake power.

One way to show efficiency is by measuring the mechanical systems of

the machine. This method measures how efficient the mechanical system in a

machine. The machine we are concerned about is the gasoline engine.

4.3 Indicated power Vs break power.4.4 Fuel assumption Vs speed.

6 Thermal efficiency Vs speed and break power.

Figure 5.8: Mechanical efficiency Vs speed and brake power


Mechanical efficiency is a relationship between the theoretical (ip) amount

of work required to do a certain job and the actual (bp) amount of work required

to do the job. The mechanical efficiency increases when an engine is at low

speed, around 500 to 1500 rpm Fig: 5.8. When the speed increases, mechanical

efficiency decreases.

If friction horsepower can be reduced on the engine, the mechanical efficiency

will increase. If friction horsepower increase on an engine, the mechanical

efficiency will decrease.

5.9 Fuel Consumption Vs Speed of Engine

Specific fuel consumption is generally given in units of gm/kW-hr or

Ibm/hp-hr. For transportation vehicles, it is common to use fuel economy in

terms of distance traveled per unit of fuel, such as kms per litter (kml). To

decrease air pollution and depletion of fossil fuels, laws have been enacted

requiring better vehicle fuel.

Figure 5.9: Graph Power Vs Speed


Since early 1970s, most automobiles which were less than 15.7 L/100 km

used gasoline, and great steps have been made to improve fuel economy.

Specific fuel consumption decreases as engine speed increases, reaches a

minimum, and then increases at high speed (Fig 5.9). Fuel consumption

increases at high speed because of greater friction losses. At low engine speed,

the longer time per cycle allows more heat loss and fuel consumption goes up.


Activity 5BThis section tests your understanding of the subject matter. You have to complete this section by following the instructions carefully.5.6 Refer Figure A5.1, name the curves of A,B,C,D

Figure A5.1

A

B

C

D


5.7 Complete graph A 5.2 below which shows the graph of imep, bmep ip and bp

Figure A5.2 :Graph I.p and b.p Vs Speed

5.8 Define the mechanical efficiency when an engine is at low and high speed.

5.9 Explain how to increase the mechanical efficiency which affects friction

horsepower.

5.10 Define specific fuel consumption when engine speed is slow and then at high

speed.

.


Self –AssessmentSelf-assessment evaluates your understanding of each unit.5-1.

A four stroke engine has a bore of 72 mm and a stroke of 102 mm. Its rated

speed is 3200 rev/min and it is tested at this speed against a brake which has a

torque arm of 0.362 m. The net brake load is 165 N and indicator power is 24.5

kW and the fuel consumption is 6.78 l/h. The specific gravity of petrol used is

0.83 and it has a lower calorific value, Qnet, .v =44200 kJ/kg. Calculate the

speed, for the following :

i. engine torque, T

ii. brake power, b.p

iii. brake min effective pressure, b.m.e.p

iv. brake thermal efficiency, bt

v. specific fuel consumption, s.f.c


vi. mechanical efficiency, m

vii. and indicator min effective pressure, i.m.e.p

Documents

Unit 2 - WordPress.com · Web viewA four stroke engine has a bore of 72 mm and a stroke of 102 mm. Its rated speed is 3200 rev/min and it is tested at this speed against a brake