motor bakar torak

PEMBEKALAN MATERI PRAKTIKUM

LAB KONVERSI ENERGIJurusan Teknik Mesin Fak Teknik UNSRI

MOTOR BAKAR TORAK

M Zachri Kadir

MOTOR BAKAR(Combustion Engine)

INTERNAL COMBUSTION

ENGINE

EXTERNAL COMBUSTION

ENGINE

IGNITIONWORKING CYCLE

(STROKE)

SPARK IGNITION(MOTOR BENSIN/ OTTO)

COMPRESSION IGNITION

(MOTOR DIESEL)

4 STROKES( 4 TAK )

2 STROKES( 2 TAK )

TURBIN GAS

TURBIN UAP

MESIN UAP

External Combustion EngineTurbin Gas

External Combustion EngineTurbin Uap

External Combustion EngineMesin Uap

STEAMFrom BOILER

Internal Combustion EngineMotor Bakar Torak

OTTO Engine ( Spark Ignition - 4 Strokes )

Diesel Engine ( Compression Ignition – 4

Strokes )

Four Stroke EngineIntake Compression Power Exhaust

1. Intake Stroke piston moves from TDC to BDC, drawing in fresh air-fuel mixture.

2. Compression Stroke piston moves from BDC to TDC, compress air-fuel mixture.

3. Power Stroke piston at TDC, spark plug ignite the air-fuel mixture. the combustion occur very fast that, in theory, the piston still at TDC. After that the piston is pushed to BDC.

4. Exhaust Stroke piston moves from BDC to TDC, pushes the combustion gases out.

Two Stroke Engine PowerCompressionIntake & Exhaust

1. Compression Stroke piston moves from BDC to TDC, compress air-fuel mixture.

2. Power Stroke piston at TDC, spark plug ignite the air-fuel mixture. After the piston is pushed to BDC. Meanwhile, about half way, combustion gases are discharged and fresh air-fuel mixture is drawing in .

BoreStroke

TDC

BDC

Intakevalve

Exhaustvalve

Over View on Reciprocating EnginesTop Dead Center (TDC) : Upper most position

Bottom Dead Center (BDC) : Lower most position

Stroke : Length of piston travel

Bore : Diameter of the cylinder

Clearance Volume (Vc) : V where piston is at TDC

Displacement Volume (Vd) :Swept Volume (Vmax-Vmin)

Compression Ratio (rv) = (Vmax/Vmin) = (VBDC/VTDC)

Mean Effective Pressure (MEP) :

Wnet = (MEP) x (Displacement Volume)

Reciprocating Engine is INTERNAL COMBUSTION ENGINE, and is Classified into 2 types:

1. Spark Ignition: Gasoline Engine, Mixing air-fuel outside cylinder, ignites by a spark plug

2. Compression Ignition: Diesel engine, fuel is injected into the cylinder, self ignited as a result of compression.

Equivalent

v

P

Wnet

vmin vmax

Actual Processes

Wnet

v

P

vmin vmax

MEP

Equivalent by MEP

Mean Effective Pressure, MEP Concept

TDC BDCWnet = (MEP) x (Displacement Volume)

= (MEP) x (Vmax-Vmin)

Air Standard Otto CycleIdeal cycle of spark ignition engine, comprises of 4- Process:Process 1-2 Isentropic CompressionIsentropic Compression (piston moves from BDC to TDC)

Process 2-3 v = constant, heat addedv = constant, heat added (piston stays still, represents combustion)

Process 3-4 Isentropic expansionIsentropic expansion (piston moves from TDC to BDC gives POWER)

Process 4-1 v = constant, heat rejectionv = constant, heat rejection (piston stays still, represents EXHAUST and INTAKE stroke)

s

T

s1=s2 s3=s4

2

1

4

3qin

qoutv = const.

v = co

nst.

v

P

v2=v3v1=v4

2

1

4

3

wout

win

Pv k = c

Pv k = c

There are only 2-stroke of all 4-processes,

TDC BDC

Analysis of Air Standard Otto CycleReview of equations used:

).....(6.19

...(6.18).......... and

constant

gases Ideal of Process Isentropic

: gas Ideal

0.

q system closed :law1st

ferheat trans olumeConstant v

1

2

1

/)1(

1

2

1

2

2

1

2

1

1

2

2211

2332

2332

32

322332

kkk

kk

kkk

v

v

v

v

P

P

T

T

V

V

v

v

P

P

vPvPPv

)T(TCq dT C RT, du Pv

uuqwconstv

wuu

)(Pressure EffectiveMean

11

,1

1

,

efficiency Thermal

21

32

14th

1221

112221

4321

th

vvMEPw

q

q

q

qor

k

TTRw

k

vPvPw

C

CkcPv

Pdvw

wwwq

w

net

H

L

v

pk

net

H

net

2

1

2

1

min

max

1Ottoth,

3

4

1

4

3

1

1

2

2

1

232

141

23

14

23

14th

2332

1414

32

14th

11

1/

1/1

1

1

1

efficiency Thermal Otto

v

v

V

V

V

Vr

r

T

T

v

v

v

v

T

T

)T(TT

)T(TT)T(T

)T(T)T(TC

)T(TC)T(TCq)T(TCq

q

q

v

kv

kk

v

v

v

v

1. The higher rv the higher thermal eff.2. The higher rv cause Self-Ignition

engine knock3. Higher Octane Number of fuel used retard

the self-ignition4. Typical rv of gasoline engine ~ 9.0 – 10.05. Thermal efficiency of actual spark

ignition engine ~ 25-30%

Questions

1. What is the difference between the clearance volume and thedisplacement volume of reciprocating engines?2. Define the compression ratio for reciprocating engines.3. How is the mean effective pressure for reciprocating enginesdefined?4. Can the mean effective pressure of an automobile engine in operation be less than the atmospheric pressure?5. As a car gets older, will its compression ratio change? How about the mean effective pressure?6. What is the difference between spark-ignition and compression ignition engines?7. Define the following terms related to reciprocating engines: stroke, bore, top dead center, and clearance volume.

Otto Cycle1. What four processes make the ideal OTTO cycle?

2. How is the rpm (revolutions per minute) of an actual four-stroke. gasoline engine related to the number of thermodynamic cycles? What would your answer be for a two-stroke engine?

3.Are the processes which make up the Otto cycle analyzed as closed-system or steady-flow processes? Why?

4. How does the thermal efficiency of an ideal Otto cycle change with the compression ratio of the engine and the specific heat ratio of the working fluid?

5. Why are high compression ratios not used in spark-ignition engines?

6. An ideal Otto cycle with a specified compression ratio is executed using (a) air, (b) argon, and (c) ethane as the working fluid. For which case will the thermal efficiency be the highest? Why?

7. What is the difference between fuel-injected gasoline engines and diesel engines?

1a. Indicated Power.

Indicated Power (IP) : Power obtained at the cylinder. Obtained from the indicator diagram. Given by:

IP = PiLANn/60x in Watts

where Pi is the indicated mean effective pressure, in N/m2, L is the stroke length, in m

A is the area of cross section of the piston, m2,

N is the engine speed in rev/min, n is the number of cylinders and x =1 for 2 stroke and 2 for 4 stroke engine.

1b. Brake Power

Brake Power (BP) : Power obtained at the shaft. Obtained from the engine dynamometer.

Given by:BP = 2NT/60 in Wattswhere T is the brake torque, in Nm, given by T = W.Lwhere W is the load applied on the shaft by the

dynamometer, in N and L is the length of the arm where the load is

applied, in m N is the engine speed, in rev/min

1c. Friction Power

Friction Power (FP) : Power dissipated as friction. Obtained by various methods like Morse test for multi-cylinder engine, Willan’s line method for a diesel engine, and Retardation test and Motoring test for all types of engines. Given in terms of IP and BP by:

FP = IP – BP in Watts

2. Mean Effective Pressure.

Indicated Mean Effective Pressure (IMEP). This is also denoted by Pi and is given by

Pi = (Net work of cycle)/Swept Volume in N/m2

The net work of cycle is the area under the P-V diagram.Brake Mean Effective Pressure (BMEP). This is also

denoted by Pb and is given byPb = 60.BPx/(LANn) N/m2 This is also the brake power per unit swept volume of the

engine.Friction Mean Effective Pressure (FMEP). This is also

denoted by Pf and is given byPf = Pi - Pb N/m2

3. Efficiencies.Indicated Thermal Efficiency (i) given by

i = IP/(mf . Qcv)mf is the mass of fuel taken into the engine in kg/s Qcv is the calorific value of the fuel in J/kg

Brake Thermal Efficiency (b) given byb = BP/(mf . Qcv)

Indicated Relative Efficiency (i,r) given byi,r = i/ASE

ASE is the efficiency of the corresponding air standard cycle

Brake Relative Efficiency (b,r) given byb,r = b/ASE

Mechanical Efficiency (m) given by m = BP/IP = Pb/Pi = b/i = b,r/I,r

Specific Fuel Consumption (sfc or SFC)

This is the fuel consumed per unit power. Brake Specific Fuel Consumption (bsfc). This is given by

bsfc = mf/BP kg/J

if BP is in W and mf is in kg/sbsfc is usually quoted in kg/kWh. This is possible if BP is in kW and

mf is in kg/h.Indicated Specific Fuel Consumption (isfc). This is given by

isfc = mf/IP kg/J

if IP is in W and mf is in kg/sisfc is also usually quoted in kg/kWh. This is possible if IP is in kW

and mf is in kg/h.Mechanical Efficiency in terms of the sfc values is given by

m = isfc/bsfc

Specific Energy Consumption (sec or SEC).

This is the energy consumed per unit power.

Brake Specific Energy Consumption (bsec). This is given by

bsec = bsfc.Qcv

We can similarly define indicated specific energy consumption (isec) and based on the two quantities also we can define mechanical efficiency.

Air Capacity of Four-stroke cycle Engines

• The power, P, developed by an engine is given by

• Power will depend on air capacity if the quantity in the bracket is maximized.

• Plot of power versus air flow rate is normally a straight line.

ca QFMP

Volumetric Efficiency

Indicates air capacity of a 4 stroke engine. Given by

Mi is the mass flow rate of fresh mixture. N is the engine speed in rev/unit time. Vs is the piston displacement (swept volume). ρi is the inlet density.

is

iv

V2

NM

NV

M2

si

i

Volumetric Efficiency

Can be measured:

At the inlet port

Intake of the engine

Any suitable location in the intake manifold

If measured at the intake of the engine, it is also called the overall volumetric efficiency.

Volumetric Efficiency Based on Dry Air

Since there is a linear relationship between indicated output (power) and air capacity (airflow rate), it is more appropriate to express volumetric efficiency in terms of airflow rate (which is the mass of dry air per unit time).

Since fuel, air and water vapor occupy the same volume

Va = Vf = Vw = Vi

Thus we have:

aM

ii

i

a

aaaa V

MMvMV

Here ρa is the density of dry air or the mass of dry air per unit volume of fresh mixture.Thus, since

id

iv

V2

NM

ad

av

V2

NM

Also Vd = ApL

s = 2LN

L2

sN

L is the piston stroke and s is the piston speed.

sA

M4

LAL2

sM2

pa

a

ap

av

Measurement of Volumetric Efficiency in Engines

The volumetric efficiency of an engine can be evaluated at any given set of operating conditions provided and ρa can be accurately measured.

Measurement of Air FlowAirflow into the engine can be measured with

the help of a suitable airflow meter. The fluctuations in the airflow can be reduced with the help of surge tanks placed between the engine and the airflow meter.

.

aM

Measurement of Inlet Air Density

By Dalton’s Law of partial pressures:

pi = pa + pf + pw

In this case pi is the total pressure of the fresh mixture,

pa is the partial pressure of air in the mixture,

pf is the partial pressure of fuel in the mixture,

pw is the partial pressure of water vapor in the air.

Since each constituent is assumed to behave as a perfect gas, we can write

wfa

a

i

a

ppp

p

p

p

a

aa

oa V

MT

RpSince

29

f

ff

f

of V

MT

m

Rp

w

ww

ow V

MT

18

Rp

iwfa TTTTNow

wfa VVV

1829

29

w

f

fa

a

i

a

M

m

MM

M

p

pHence

M indicates mass of the substance, 29 is the molecular weight of air,

mf is the molecular weight of the fuel, and 18 is the molecular weight of water vapor.

182929

1

1

a

w

fa

fi

a

M

M

mM

Mp

p

h6.1m

29F1

1

fi

Fi is the ratio of mass of fuel vapor to that of dry air and h is the ratio of mass of water vapor to that of dry air at the

point where pi and Ti are measured.

io

a

ao

aa TR

p

TR

pNow

29

29

hm

FTR

p

fi

io

ia

6.129

1

129

This indicates that the density of air in the mixture is equal to the density of air at pi and Ti multiplied by a correction factor, that is, the quantity in the parentheses.

The value of h depends on the humidity ratio of the air and is obtained from psychrometric charts.

For conventional hydrocarbon fuels, the correction factor is usually around 0.98, which is within experimental error. For diesel engines and GDI engines, Fi is zero.

In practice, with spark ignition engines using gasoline and with diesel engines the volumetric efficiency, neglecting the terms in the parentheses, is given by

4

sA

TR

p29

M

p

io

i

av

If we do not neglect the terms in the parentheses we get the following relation for volumetric efficiency:

hm

F

sA

TRp

M

fi

p

io

i

av

6.129

1

14

29

If the humidity is high or a low molecular weight fuel is used in a carbureted engine, the correction factor cannot be ignored. For example, with methanol at stoichiometric conditions and h = 0.02, the correction factor is 0.85.

Volumetric Efficiency, Power and Mean Effective Pressure

Since

and

ca QFMP

sA

M4

pa

av

cavp QFsA4

1P

For an engine, the mean effective pressure, mep, is given by

221

NV

P

VV

Pmep

s

sA

P4

p

cav QF

Ways to increase power and mep

• The mean effective pressure may be indicated or brake, depending on whether η is indicated or brake thermal efficiency. Thus, the mean effective pressure is proportional to the product of the inlet density and volumetric efficiency when the product of the thermal efficiency, the fuel-air ratio, and the heat of combustion of the fuel is constant.

• From the preceding two expressions we can figure out ways to increase the power and mep of an engine.

Background on the Otto Cycle• The Otto Cycle has four basic

steps or strokes:– 1. An intake stroke that draws a

combustible mixture of fuel and air into the cylinder

– 2. A compression stroke with the valves closed which raises the temperature of the mixture. A spark ignites the mixture towards the end of this stroke.

– 3. An expansion or power stroke. Resulting from combustion.

– 4. An Exhaust stroke the pushes the burned contents out of the cylinder.

• To the right is an idealized representation of the Otto cycle on a PV diagram.

• http://www.rawbw.com/~xmwang/javappl/ottoCyc.html

http://www.rawbw.com/~xmwang/javappl/ottoCyc.html

http://www.rawbw.com/~xmwang/javappl/ottoCyc.html

Comparing Engines….

• mep= work done per unit displacement volume– Or average pressure that results in the same

amount of indicated or brake work produced by the engine

– Scales out effect of engine size– Two useful types: imep and bmep

• imep: indicated mean effective pressure– -the net work per unit displacement volume done by the gas

during compression and expansion

• bmep: brake mean effective pressure– -the external shaft work per unit volume done by the engine

BMEP

• Based on torque:

dVbmep

4

Vdbmep

4

(4 stroke)

(2 stroke)

dVbmep

2

Compare…

• Brake specific fuel consumption (bsfc)– Measure of engine efficiency– They are in fact inversely related, so a lower

bsfc means a better engine– Often used over thermal efficiency because

an accepted universal definition of thermal efficiency does not exist

N

fm

bW

fmbsfc

2

bsfc

• bsfc is the fuel flow rate divided by the brake power

• We can also derive the brake thermal efficiency if we give an energy to the fuel called heat of combustion or, qc

N

fm

bW

fmbsfc

2

N

fm

bW

fmbsfc

2

qcbsfcqcfm

bW

1

Compare…

• Volumetric Efficiency, ev

– The mass of fuel and air inducted into the cylinder divided by the mass that would occupy the displaced volume at the density ρi in the intake manifold

– Note it’s a mass ratio and for a 4 stroke engine

– For a direct injection engine

NV

mme

di

fav

)(2

0fm

2-stroke premixed-charge enginehttp://science.howstuffworks.com/two-stroke2.htm

http://science.howstuffworks.com/two-stroke2.htm

2-stroke premixed-charge engine

• 2-strokes gives ≈ 2x as much power since only 1 crankshaft revolution needed for 1 complete cycle (vs. 2 revolutions for 4-strokes)

• Since intake & exhaust ports are open at same time, some fuel-air mixture flows directly out exhaust & some exhaust gas gets mixed with fresh gas

• Since oil must be mixed with fuel, oil gets burned

• As a result of these factors, thermal efficiency is lower, emissions are higher, and performance is near-optimal for a narrower range of engine speeds compared to 4-stroke engines

2-stroke Diesel engine• Used in large engines, e.g. locomotives• More differences between 2-stroke

gasoline vs. diesel engines than 4-stroke gasoline vs. diesel– Air comes in directly through intake ports,

not via crankcase– Must be turbocharged or supercharged to

provide pressure to force air into cylinder – No oil mixed with air - crankcase has

lubrication like 4-stroke– Exhaust valves rather than ports - not

necessary to have intake & exhaust paths open at same time (but may do this anyway)

– Because only air, not fuel/air mixture enters through intake ports, “short circuit” of intake gas out to exhaust is not a problem

– Because of the previous 3 points, 2-stroke diesels have far fewer environmental problems than 2-stroke gasoline engines

2-stroke Diesel engine• Why can’t gasoline engines use concept similar to 2-stroke

Diesel? They can in principle but fuel must be injected & fuel+air fully mixed after the intake ports are covered but before spark is fired

• Also, difficult to control ratio of fuel/air/exhaust residual precisely since relative amounts of exhaust & air leaving exhaust ports varies from cycle to cycle (due to turbulence) - ratio of fuel to (air + exhaust) critical to premixed-charge engine performance (combustion in non-premixed charge engines always occurs at stoichiometric surfaces in overall lean mixtures anyway, so not an issue for non-premixed charge engines)

• Some companies have tried to make 2-stroke premixed-charge engines operating this way, e.g. http://www.orbeng.com.au/, but these engines have found only limited application

http://www.orbeng.com.au/

Engine design & performance parameters• See Heywood Chapter 2 for more details

• Compression ratio (rc)

Vd = displacement volume = volume of cylinder swept by piston (this is what auto manufacturers report, e.g. 5.2 liter engine means 5.2 liters is combined displacement volume of ALL cylinders

Vc = clearance volume = volume of cylinder NOT swept by piston

• Bore (B) = cylinder diameter

• Stroke (L) = distance between maximum excursions of piston

• Displacment volume of 1 cylinder = πB2L/4; if B = L (typical), 5.2 liter, 8-cylinder engine, B = 9.4 cm

• Power = Angular speed (N) x Torque () = 2πN

rc maximum cylinder volume

minimum cylinder volume

Vc Vd

Vc

P (in horsepower) N (revolutions per minute, RPM) x (in foot pounds)

5252

Classification of unsteady-flow engines

Piston at bottom of travel

Piston at top of travel

Bore

Stroke Displacement volume

Clearance volume

Engine design & performance parameters• Engine performance is specified in both in terms of power

and engine torque - which is more important?– Wheel torque = engine torque x gear ratio tells you whether you

can climb the hill– Gear ratio in transmission typically 3:1 or 4:1 in 1st gear, 1:1 in

highest gear; gear ratio in differential typically 3:1• Ratio of engine revolutions to wheel revolutions varies from 12:1 in

lowest gear to 3:1 in highest gear

– Power tells you how fast you can climb the hill– Torque can be increased by transmission (e.g. 2:1 gear ratio

ideally multiplies torque by 2)– Power can’t be increased by transmission; in fact because of

friction and other losses, power will decrease in transmission– Power really tells how fast you can accelerate or how fast you

can climb a hill, but power to torque ratio ~ N tells you what gear ratios you’ll need to do the job

Engine design & performance parameters• Indicated work - work done for one cycle as determined by the cylinder P-V

diagram = work acting on piston face

Note: it’s called “indicated” power because historically (before oscilloscopes) the P and V were recorded by a pen moving in the x direction as V changed and moving in the y direction as P changed. The P-V plot was recorded on a card and the area inside the P-V was the “indicated” work (usually measured by cutting out the P-V and weighting that part of the card!)

• Net indicated work = Wi,net = ∫ PdV over whole cycle = net area inside P-V diagram

• Indicated work consists of 2 parts– Gross indicated work Wi,gross - work done during power cycle

– Pumping work Wi,p - work done during intake/exhaust pumping cycle

• Wi.net = Wi,gross - Wi,pump

• Indicated power = Wi,xN/n, where x could be net, gross, pumping and n = 2 for 4-stroke engine, n = 1 for 2 stroke engine (since 4-stroke needs 2 complete revolutions of engine for one complete thermodynamic cycle as seen on P-V diagram whereas 2-stroke needs only 1 revolution)

Engine design & performance parametersAnimation: gross & net indicated work,

pumping work

Gross indicated work

Gross indicated workPumping workPumping work

Net indicated work

Net indicated work(+)(+)

(-)(-)

Engine design & performance parameters

• Brake work (Wb) or brake power (Pb) = work power that appears at the shaft at the back of the engine

• Historically called “brake” because a mechanical brake [like that on your car wheels] was used in laboratory to simulate the “road load” that would be placed on an engine in a vehicle)

• What’s the difference between brake and indicated work or power? FRICTION– Gross Indicated work = brake work + friction work (Wf)

Wi,g = Wb + Wf

– Note that this definition of friction work includes not only the “rubbing friction” but also the pumping work; I prefer

Wi,g = Wb + Wf + Wp

which separates rubbing friction (which cannot be seen on a P-V diagram) from pumping friction (which IS seen on the P-V)

– The latter definition makes friction the difference between your actual (brake) work/power output and the work seen on the P-V

– Note the friction work also includes work/power needed to drive the cooling fan, water pump, oil pump, generator, air conditioner, …

– Moral - know which definition you’re using


• Mechanical efficiency = (brake power) / (indicated power) - measure of importance of friction loss

• Thermal efficiency (th) = (what you get / what you pay for) = (power ouput) / (fuel heating value input)

• Specific fuel consumption (sfc) = (mdotfuel)/(Power)

units usually pounds of fuel per horsepower-hour (yuk!)

• Note also

th Power output (brake or indicated)

Ý m fuelQR

isfc Ý m fuel

indicated power;bsfc

Ý m fuel

brake power

th,i 1

(isfc)QR

;th,b 1

(bsfc)QR


• Volumetric efficiency (v) = (mass of air actually drawn into cylinder) / (mass of air that ideally could be drawn into cylinder)

where air is at ambient conditions = Pambient/RTambient

• Volumetric efficiency indicates how well the engine “breathes” - what lowers v below 100%?

– Pressure drops in intake system (e.g. throttling) & intake valves– Temperature rise due to heating of air as it flows through intake system– Volume occupied by fuel– Non-ideal valve timing– “Choking” (air flow reaching speed of sound) in part of intake system having

smallest area (passing intake valves)

• See figure on p. 217 of Heywood for good summary of all these effects

v Ý m air (measured)

airVd N /n


• Mean effective pressure (MEP)

• Power could be brake, indicated, friction or pumping power, leading to BMEP, IMEP, FMEP, PMEP

• Note Power = Torque x 2πN, thus

Brake torque = BMEP*Vd/2πn

• MEP can be interpreted as the first moment of pressure with respect to cylinder volume, or average pressure, with volume as the weighting function for the averaging process

MEP Work per cycle

Displacement volume

PdVcycle

Vd

(Power)n /N

Vd

(Power)n

Vd N

MEP (Work per cycle)/m

(Displacement volume)/mintake(Work per cycle)/m


• MEP is useful for 2 reasons– Since it’s proportional to power or work, we can add and

subtract pressures just like we would power or work– (More important) It normalizes out the effects of engine

size (Vd), speed (N) and 2-stroke vs. 4-stroke (n), so it provides a way of comparing different engines and operating conditions

• Typical 4-stroke engine, IMEP ≈ 120 lb/in2 ≈ 9 atm - how to get more? Turbocharge - increase Pintake above 1 atm, more fuel & air stuffed into cylinder, more heat release, more power


• Pumping power = (pumping work)(N)/n = (P)(V)(N)/n

= (Pexhaust - Pintake)VdN/n

but PMEP = (pumping power)n/(VdN), thus PMEP = (Pexhaust - Pintake)

(wasn’t that easy?) (this assumes “pumping loop” is a rectangle)• Estimate of IMEP

• Typical engine at wide-open throttle (Pintake = Pambient):

th,i,g ≈ 30%, v ≈ 85%, f = 0.068 (at stoichiometric),

QR = 4.5 x 107 J/kg, R = 287 J/kg-K, Tintake = 300K

IMEPg / Pintake ≈ 9.1

• In reality, we have to be more careful about accounting for the exhaust residual and the fact that its properties are very different from the fresh gas, but this doesn’t change the results much

IMEPg (Gross indicated power) n

Vd N

(th,i,g Ý m fuelQR )n

Vd N

(th,i,g Ý m air[ f /(1 f )]QR )n

Vd Nth,i,g (vair,ambientVd N /n)QR n

Vd N

f

1 f

th,i,gvQR

Pambient

RTambient

(1 f )

f

1 f

IMEPg

Pintake

th,i,gv fQR

RTambient

Pambient

Pintake


• Emissions performance usually reported in grams of pollutant emitted per brake horsepower-hour (yuk!) or grams per kilowatt hour (slightly less yuk), e.g.

Brake Specific NOx (BSNOx) = mdotNOx / (Brake power)

• One can also think of this as (mass/time) / (energy/time) = mass / energy = grams of pollutant per Joule of work done

• …but Environmental Protection Agency standards (for passenger vehicles) are in terms of grams per mile, not brake power hour, thus smaller cars can have larger BSNOx (or BSCO, BSHC, etc.) because (presumably) less horsepower (thus less fuel) is needed to move the car a certain number of miles in a certain time

• Larger vehicles (and stationary engines for power generation) are regulated based on brake specific emissions directly

Four-Stroke Diesel Engine

• Intake stroke– Intake valve open, exhaust valve shut– Piston travels from TDC to BDC– Air drawn in

• Compression stroke– Intake and exhaust valves shut– Piston travels from BDC to TDC– Temperature and pressure of air increase


• Power stroke– Intake and exhaust valves shut– Fuel injected into cylinder and ignites– Piston forced from TDC to BDC

• Exhaust stroke– Intake valve shut, exhaust valve open– Piston moves from BDC to TDC– Combustion gases expelled


• Strokes– Intake– Compressio

n

– Power– Exhaust

Two-Stroke Diesel Engine

• 1 power stroke every crankshaft revolution (vice every two w/ 4-stroke)

• Uses pressurized air to simultaneously supply new air and expel combustion gases

• Scavenging– Exhaust valve open, inlet port exposed– Pressurized air enters, expels combustion

gases– Piston near BDC


• Compression– Intake and exhaust valves shut– Piston travels from BDC to TDC– Temperature and pressure of air increase

• Power stroke– Intake and exhaust valves shut– Fuel injected into cylinder and ignites– Piston forced from TDC to BDC


• Strokes– Compression

– Power– (Intake/Exhaust)

Two vs. Four-Stroke Engines

• Two-stroke advantages– Higher power to weight ratio– Less complicated valve train

• Four-stroke advantages– More efficient burning process– As size increases, power-to-weight ratio

improves

Gasoline vs. Diesel Engine

motor bakar torak

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