chap 2 theory

8/2/2019 Chap 2 Theory

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Engine theory in a pill

(based on H.Heywood Internal combustion engine fundamentalsand DieselNet website)

This Chapter describes the operation of two-stroke and four-stroke designs and explains some

of the parameters affecting engine performance.

2.1 The first and second laws of thermodynamics in engines

The First law of thermodynamics simply states that energy can not be created nor destroyed

but has to be converted from one form to another. For instance, a mass of engine fuel

contains chemical energy that is converted to mechanical energy within the cylinder of an

internal combustion engine. Theoretically, if this process was ideal and no losses were

incurred, this energy conversion would be 100% efficient. Yet in reality, converting energy

from one form to another involves many losses resulting in an overall loss in efficiency. This

fact is what the Second law of thermodynamics expresses, as it states that the useful work

from a combustion system should be less than the energy input. In general, the ratio between

useful work and the thermal energy added to the cylinder (control volume) represents the

brake thermal efficiency of the system.

Considering the cylinder and piston arrangement, the combustible mixture of fuel and air

is burned in the control volume, producing heat that results in the expansion of the volume

causing the piston to move. Motion of the piston creates friction against the cylinder walls

leading to friction heat loss. Another source of loss results from the temperature associated

with the heat generated by the combustion process itself. As the combustion temperature

increases, the cylinder material approaches its limitation in mechanical strength. Therefore,

cylinders are cooled, by water or air, to move heat away from the material, thus preserving

its mechanical strength. Heat transferred away from the cylinders material is another lossadded to the balance between the energy received by that cylinder and the energy it delivers

back.

Another major source of loss in this energy conversion system is exhaust gases flowing out

of the control volume. Exhaust gases exit the control volume with heat energy delivered to

the outside air without any benefit. They also exit with a great deal of potential energy

(pressure) as well as kinetic energy (speed). With respect to the system under consideration,

therefore, we can think of a control volume where fuel and air are supplied and in return

piston motion (work) is delivered, but the work delivered is much less than the value of

energy supplied to the control volume. The difference between the energy supplied to the


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Internal combustion engines

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control volume and what it delivers is the sum of losses including, but not limited to, cooling

and exhaust losses.

While the description given so far for the energy exchange through a control volume is

generally correct it may not be very complete. A better account of all the energies entering

and leaving the control volume will have to include other types of energies not yet

mentioned. For instance, any mass entering the control volume brings with it several forms of

energy:

- internal energy; mainly due to its temperature which is generally very small,- kinetic energy; mainly due to injection characteristics which usually leads to

important interactions between the fuel and air within the control volume,

- potential energy; generally associated with pressure admitting mass into the controlvolume,

- flow energy; principally associated with the inter-relation between the controlvolume and its pressure.

Revisiting the first law of thermodynamic and considering the various forms of energy we

are now acquainted with, one could think of the energy balance in a control volume as

follows:

Net Output = Energy Supplied to the Control Volume - Total Energy Loss

Fig. 2.1. Overview of energy input and losses. (DieselNet)

In other words, a careful account of the energies supplied to the control volume and those

delivered by the same volume can assist in assessing the system conversion efficiency.

2.2 The operation of reciprocating internal combustion engines

2.2.1 Two-stroke engines


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Chapter 2. Engine theory in a pill

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Principles of operation

By definition, two-stroke engines require two strokes to complete their combustion cycle.

Figure 2.2 below gives details of the two-stroke combustion cycle (note, however, that the

two-stroke engine represented in the Figure is not the only design possible).

With the transfer and exhaust ports open, air under slight pressure in the crankcase flows

into the cylinder. The moving piston eventually covers the transfer ports, thus trapping the

inducted air in the cylinder. Further upward motion toward top dead centre (TDC) compresses

the air where fuel is injected at the appropriate timing. Heat absorbed from the surrounding

hot compressed air causes the fuel to evaporate and mix with the air. Once the auto-ignition

temperature is reached, combustion begins and causes the working fluid (combustible

mixture) to expand thus applying pressure on the surface of the piston thus producing useful

work at the engine output crankshaft. Meanwhile, fresh air flows into the crankcase to be

compressed by the descending piston on its way to bottom dead centre (BDC). While

descending, the piston uncovers the exhaust port starting the scavenging of the cylinder and

causing a slight increase in crankcase pressure. This increase in crankcase pressure causes

induction of fresh air into the cylinder through the transfer port and the cycle resumes once

again.

Fig. 2.2. Two-stroke combustion Cycle (DieselNet)

Scavenging in two-stroke engine

The process of purging exhaust gases from a previous cycle and filling the cylinder with fresh

air for a new cycle is referred to as scavenging. The main method for scavenging two-stroke

engines is by using the pressure of the inducted fresh air to purge or displace the burned

gases from the previous cycle. Generally, the greater the incoming air pressure, the more

complete the scavenging process. Therefore, better scavenging in two-stroke engines is

achieved, in part, by raising the pressure of fresh air being inducted into the cylinder. Thisprocess is accomplished by using various devices such as blowers, compressors, or pumps.


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Scavenging in two-stroke engines is performed mainly by one of three methods:

cross-scavenging,

loop-scavenging,

uniflow-scavenging.

Fig. 2.3. Scavenging Methods in Two-stroke engines (DieselNet):(a) Cross-Scavenging; (b) Loop-Scavenging; (c) Uniflow-Scavenging

The actual scavenging in two-stroke engines is far less than ideal. In fact, during

scavenging not only does the fresh charge exchange heat with the residual gases, it also mixes

with it and changes its chemical composition in the process. The final chemical make up of

the mixture at the end of the scavenging process plays an important role in the combustion

quality as well as its resultant emissions. An inherent loss in two-stroke engines results when

some fresh charge escapes through the exhaust ports during scavenging. This phenomenon is

often referred to as short-circuiting which leads to lower volumetric efficiency.

A two-stroke is usually smaller in size than a four-stroke engine having the same power

output and tends to have higher specific power (power output for a given engine

displacement) than its four-stroke counterpart. Two-stroke engines are generally less fuel

efficient than four-stroke engines. The main reason for this relative fuel inefficiency in two-

stroke engines is poor scavenging and relatively low volumetric efficiency.

2.2.2 Four-stroke engines

Principles of operation

The four-stroke engine takes four strokes to complete the combustion cycle. Figure 2.4 below

shows in schematic form the four-stroke combustion cycle as applied to a diesel engine. In

the first stroke, the intake stroke, the piston moves from its position at TDC toward BDC.

During most of the intake stroke, air is inducted into the cylinder. In the second stroke, air iscompressed by the piston moving back to TDC from its starting position at BDC.


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Fig. 2.4. Four-stroke Diesel engine operation (DieselNet)

This second stroke is known as the compression stroke where air in that cylinder heats up

to a temperature usually above the auto-ignition temperature of the fuel which is injected

into the cylinder near TDC. As the fuel burns, heat energy is released raising the pressure

inside a greatly reduced volume near TDC. This energy release produces pressure that is

applied to the top surface of the piston thus pushing it back toward its BDC. This stroke is

known as the expansion stroke since it is through that expansion that power is imparted to

the piston and causes it to move to BDC. The expansion stroke is also known as the power

stroke for obvious reasons. It is also referred to by some as the work stroke since theexpanding gases are producing work by applying their pressure to the top of the piston. The

last of the four strokes is the exhaust stroke where combustion by-products are sent into the

exhaust system for evacuation into the atmosphere.

Changes in pressure and volume are very often shown using the pressure-volume diagram

in Figure 2.5 below. In this Figure, called a close diagram, pressure and volume changes for

a naturally-aspirated diesel engine are illustrated.

Fig. 2.5. Pressure-volume diagram for four-stroke naturally-aspirated engine (DieselNet)


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In Figure 2.5 intake and exhaust valve events are marked by points 1 through 4, where

point 1 is the point at which the intake valve opens, point 2 is the intake valve closing,

point 3 is the exhaust valve opening, and point number 4 is the exhaust valve closing.

It is important to note that both intake and exhaust valves remain open during the time

between points 1 and 4 as well as its equivalent crank angle duration. This period is

referred to as valve overlap and plays very important role in engine performance and

emissions.

The intake valve closing occurs a few (crankshaft angles) degrees beyond BDC to improve

cylinder filling and, therefore, the volumetric efficiency of the engine. Effective and rapid

compression of the charge begins after closing of the intake valve as the piston travels from

BDC to TDC. In naturally aspirated engines, pressure inside the cylinder during the intake

stroke is below atmospheric pressure. Restrictions through the air intake filter, air inlet

piping, intake manifold, intake port, and intake valve contribute to pressure loss and help

reduce cylinder pressure to below atmospheric. Shortly following combustion, the expansion

stroke begins and is marked by a number of chemical reactions and heat transfer processes

while the piston travels from TDC to BDC. At point 3, the exhaust valve opens thus allowing

some of the combustion products to go through a blowdown process as a result of the

pressure differential between the cylinder and the exhaust system. The remainder of the

exhaust gases are expelled from the cylinder by virtue of the piston motion from BDC to TDC

during the exhaust stroke.

Another way to illustrate the four-stroke cycle is through the pressure-crank angle

diagram shown in Figure 2.6 below, which is also called an open diagram.

Fig. 2.6. Pressure-crank angle diagram for a four-stroke diesel engine (DieselNet)ID - ignition delay; EVC - exhaust valve closing; IVC - intake valve closing; TDC - top dead centre;

BDC - bottom dead centre; EVO - exhaust valve open; IVO - intake valve open

The pressure-crank angle diagram highlights the point at which fuel is injected (I) as well

as ignition delay. During this delay fuel injected into the cylinder evaporates using heat from

the charge that has been compressed. The result of the heat transfer from the compressed air

to the fuel is a reduction in the rate of pressure rise that is illustrated in Figure 2.6. Followingthe start of combustion, the rate of pressure rise increases dramatically and the combustion


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pressure peaks a few crank angle degrees past TDC. Factors controlling the rate of pressure

rise include: the ignition delay, fuel quality, and the rate of injection. In many designs, the

engine noise, vibration, and harshness characteristics are often tied to the rate of pressure

rise in the cylinder. Together with the rise in cylinder pressure, cylinder temperature also

increases and reaches its peak. The maximum combustion temperature depends on several

factors including: fuel rate, fuel injection timing, fuel quality especially its calorific value and

cetane number, initial cylinder pressure at intake valve closing, and charge temperature.

2.3 Important engine parameters and performance features

The following definitions are commonly used:

1. brake torque (Mo) engine torque is normally measured with a dynamometer

bF=oM

2. engine power(Ne)oMn2=eN

Where n is engine speed in rpm

3. maximum rated power the highest power an engine is able to develop for shortperiods of operation.

4. normal rated power the highest power an engine is able to develop in continuousoperation.

5. rated speed the crankshaft rotational speed (revolution) at which rated power isdeveloped.

6. compression ratio () a basic parameter which defines the geometry of areciprocating engine

Where Vs is the swept volume (displaced volume)Vk is the clearance volume

volume_cylinder_mimimum

volume_cylinder_imummax=

kV

+sV=

kV


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7. ratio (L) of connecting road length(L) to crank radius (r)

Crank radius (r) is related to stroke (S) as follows

8. indicated pressure (pi) equivalent to the constant pressure value, delivering thesame work to the piston as real pressure. It describes the reality of an engine cycle.

9. indicated work (Li1) work in one cylinder according to indicated pressure

StAip=sVip=1iL

Wherepi is the indicated pressure

Vs is the swept volume

At is the area of piston crown

S is the stroke

10. indicated work (Li) work according to indicated pressure for the whole engineisVip=iL

Where i is the quantity of cylinders

11.mean effective pressure (pe) this factor indicates the engines ability to work

or

Where m is the mechanical efficiency

r

L=L

2

S=r

ime pp

Tie ppp


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pT is the mean friction pressure gives information about friction losses,

caused by: piston and piston rings against cylinder liner, friction resistance in

bearings, losses through the fan, valve systems, cooling system etc.

12. indicated power(Ni) the theoretical power of an engine if it were operating at 100%efficiency; i.e. with no frictional losses

kW

Where Vs is the swept volume of one cylinder - dm3,

i is the quantity of cylinder,

pi is the indicated pressure - kPa,

n is the engine speed - rpm,

is the quantity of stroke: for 2-stroke engine =1, for 4-stroke engine =2.

13.useful power(Ne) power to take at the crankshaft

kW

Where pe is the mean effective pressure kPa

the remainder as above.

14.theoretical efficiency(t) ratio between theoretical work (Lt) of the thermodynamiccycle and total quantity of heat (Q) added to the engine during one cycle.

In a real engine this gives information about heat losses.

11

1

1

11

k

k

kt

Where is the compression ratio, kadiabatic index,

is the rise in pressure during heat added,

is the increase in volume while heat is adde.

15. indicated efficiency(i) relation between indicated (Li) and theoretical (Lt) works,as follows:

60

npiVN isi

60

npiVN es

e

Q

Lt

t

t

i

iL

L


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It shows the losses caused by different properties of ideal and real charge (mixture of

fuel and air), unburned mixture process, cooling and flow resistance.

Where Ge is the fuel consumption per hour - kg/h,

W is the caloric value of fuel kJ/kg.

16.thermal efficiency(c) ratio between indicated work (Li) and total quantity of heat(Q) added to the engine during one cycle.

It can also be shown by:

17.mechanical efficiency (m) ratio of heat changed to useful work (Le) and heatchanged to indicated work (Li). It can be formulated as a ratio between pressures or

power, as follows:

Mechanical efficiency takes ino account friction and drive losses.

18.volumetric efficiency ( v) volume flow rate of air into the intake system (Vair)divided by the rate at which volume is displaced by the piston (dV s). An alternativeequivalent definition of volumetric efficiency is by the mass adequate to volumes.

sdm

airm=

sdV

airV=v

19.total (useful) efficiency (o) ratio between useful work (Le) and total quantity ofheat (Q) added to the engine during one cycle.

Q

Lic

itc

WG

N

e

i

c

3600

WG

N

et

i

i

3600

i

e

i

e

i

em

N

N

p

p

L

L

QLe

o


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Another formula of useful efficiency which gives information about all sorts of losses in

engines is:

For a laboratory tested engine:

Figures 2.7-2.9 below show the relations between efficiencies and relative engine speed

(speed to maximum speed).

Fig 2.7. Relations between efficiencies and relative engine speed

20.excess air number (air factor)() ratio of real quantity of air (L rz) (added to burnatomic mass of fuel) and theoretical amount of air (Lteo) to burn one fuel unit.

teoL

rzL=

Fig. 2.8. Relation air factor versus theoretical efficiency and compression ratio

mcmito

WG

N

e

e

o

3600


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21.air/fuel (A/F) andfuel/air(F/A) ratios ratios between mass of flow rates of air andfuel. The mass is measured during engine testing.

22.fuel consumption per hour (Ge) quantity of fuel added to the engine during onehour. It can be volumetric fuel consumption (dm3/h) or weighted (mass) unit (kg/h).

Ge =t

Vpp3600

Ge =t

Gp3600

Where Vp is the volume in dm3of fuel consumed by engine in time of t,

p is the fuel density - kg/dm3,

t is the time of measure - s,

Gp is the mass of fuel (kg) consumed by engine in time of t

23.specific fuel consumption (ge) gives information about how many grams of fuel anengine needs to give 1 kilowatt of power during one hour, g/kWh :

eN

eG=ge

Typical operating data for internal combustion engines are presented in Table 2.1 below.

Type ofengine

Operatingcycle

Compressionratio

Bore

m

Stroke/Bore

Speed

rpm

bmep

atm

Powerper unitvolumekW/dm3

Weight/powerratio

kg/kW

Approx.bsfc

g/kWh

Spark ignition engines

Small(e.g.

motorcycle

2S, 4S 6-11 0,05-0,085

1,2-0,9 4500-7500

4-10 20-60 5,5-2,5 350

Passengercars

4S 8-10 0,07-0,1

1,1-0,9 4500-6500

7-10 20-50 4-2 270

Trucks 4S 7-9 0,09-0,13

1,2-0,7 3600-5000

6,5-7 25-30 6,5-2,5 300

Large gasengines

2S, 4S 8-12 0,22-0,45

1,1-1,4 300-900 6,8-12 3-7 23-35 200

Wankelengines

4S ~9 0,57 dm3 perchamber

6000-8000

9,5-10,5 35-45 1,6-0,9 300

Diesel engines

Passengercars

4S 17-23 0,075-0,1

1,2-0,9 4000-5000

5-7,5 18-22 5-2,5 250

Trucks 4S 16-22 0,1-0,15

1,3-0,8 2100-4000

6-9 15-22 7-4 210


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Locomotive,industrial,

marine

4S, 2S 12-18 0,15-0,4

1,1-1,3 420-1800

7-23 5-20 6-18 190

Largeengines andstationary

2S 10-12 0,4-1,0

1,2-3 110-400 9-17 2-8 12-50 180

Table 2.1. Typical engine parameters (Heywood)

The relation between some engine parameters can be shown as graphic diagrams. These

are called engine maps or characteristics.

The most popular maps give relations between some parameters against engine speed e.g.

Ne=f(n), Mo=f(n), ge=f(n). The gross indicated power map shown in Figure 2.9 is obtained

when, in the spark ignition engine the throttle is wide-open, or in a diesel engine the fuel

system is giving a maximum dose.

When the revolutions have a constant value, it is possible to derive a load map of the

engine. The performance map in Figure 2.10 shows, in a three-dimensional diagram, the

relations between some indicated engine parameters (e.g. brake specific fuel consumption,

emission, exhaust temperature etc.) and mean effective pressure (mean load of the engine)

and engine speed.

Other maps, showing other characteristics (such as a map of the ignition momentum

control) can also be drawn.

Fig. 2.9. Gross indicated power map


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Fig. 2.10. Performance map with contours of constant bsfc

Review questions

Explain how the Second law of thermodynamics affects the working of the combustion

engine.

What are the differences between two- and four- stroke engines?

What kind of Indicated and useful parameters of engine operation do you know?

chap 2 theory

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