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Transcript of History internal combustion - Faculty of Engineering...
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Dr. Primal [email protected]: (081) 2393608
Internal combustion Engines: History, engine types and operation of 2 & 4 stroke engines
History if internal combustion (IC) engines
• Both power generation and refrigeration are usually accomplished by systems that operate on a thermodynamic cycle po er cyclesby systems that operate on a thermodynamic cycle: power cycles and refrigeration cycles.
• Power producing devises: engines
• Refrigeration producing devices: refrigerators, air‐conditioners and heat pumps
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and heat pumps.
• Steam engine ‐ 1700 (external combustion engines)
History of IC Engines1860 Lenoir’s engine (a converted steam engine)
combusted natural gas in a double acting piston, using electric ignition. Efficiency = 5%
3
History ‐ continued
• 1876 Nikolaus Otto patented the 4 cycle engine, it used gaseous fuelfuel
• 1882 Gottlieb Daimler, an engineer for Daimler, left to work on his own engine. His 1889 twin cylinder V was the first engine to be produced in quantities. Used liquid fuel and Venturi type carburetor, engine was named “Mercedes” after the daughter of his major distributor
• 1893 Rudolf Diesel built successful CI engine which was 26% efficient (double the efficiency of any other engine of its time)
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efficient (double the efficiency of any other engine of its time)
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Classification of Engines
E l I l C b i• External vs Internal Combustion
• Spark Ignition SI or Compression Ignition CI
• Configuration
• Valve Location
• 2 Stroke or 4 Stroke
5
Engine Configurations
In Line(A bil )
V(Automobile)
(Automobile)
Horizontally Opposed (Subaru)
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Radial (Aircraft)Opposed Piston
(crankshafts geared together)
V Engine
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Wankel Rotary Piston Engine
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3
Rotary “Wankel” Engine
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Ref. Internal combustion engines and air pollution, E. F. Obert
10
Basic considerations in the analysis of power cycles
• Cycles encountered in actual devices are difficult to analyze because of the ypresence of complicating effects such as friction etc.
• Consider a cycle that resembles the actual cycle closely but it made up totally of internally reversible process (id l l )
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(ideal cycle)
in
net
in
netth q
w
Q
Wor ,efficiency Thermal
Idealizations and simplifications
• Cycle does not involve any friction: no pressure drop in the
ki fl idworking fluid.
• Expansion and compression process: quasi equilibrium.
• Pipes connecting various components are well insulated.
12
p
• Neglecting changers in KE and PE
4
Net work of the cycle
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Carnot cycle
• The Carnot cycle is the most efficient cycle
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• The Carnot cycle is the most efficient cycle that can be executed between heat a source and a heat sink.
H
LCarnotth T
T1,
Air‐standard assumption
• Gas power cycles (working fluid gas): spark ignition engines, diesel engines, conventional gas turbines, etc.
• All these engines energy is provided by burning a fuel within the system• All these engines energy is provided by burning a fuel within the system boundary.
• Working fluid (air) mainly contains nitrogen and hardly undergoing any chemical reactions in the combustion chamber and can be closely resembles to air at all times in the chamber.
– Assumptions: working fluid as air, behaves as ideal gas, internally
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p g , g , yreversible cycle, combustion process replace by heat addition process by a external source, exhaust process replace by heat rejection process that re‐stores initial state of working fluid, specific heat values determines at room temperatures (call cold‐air‐standard assumptions).
TDC
Intakevalve
Exhaustvalve
Reciprocating Engines
Top Dead Center (TDC) : Upper most position
Bottom Dead Center (BDC) : Lower most position
Stroke : Length of piston travel
Bore : Diameter of the cylinder
BoreStroke
BDC
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)
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รศ.ดร.สมหมาย ปรีเปรม
Reciprocating Engine is INTERNAL COMBUSTION ENGINE, and is Classified into 2 types:
1. Spark Ignition (SI): Gasoline Engine, Mixing air‐fuel outside cylinder, ignites by a spark plug (Auto cycle)
2. Compression Ignition (CI): Diesel engine, fuel is injected into the cylinder, self ignited as a result of compression (Diesel cycle).
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Parts of an engine
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P
Actual ProcessesP
Equivalent by MEP
Mean Effective Pressure, MEP Concept
Equivalent
v
Wnet
v v
Wnet
vv
MEP
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vvmin vmaxvvmin vmax
TDC BDCWnet = (MEP) x (Displacement Volume)
= (MEP) x (Vmax-Vmin)
Four Stroke Engine – spark ignition 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
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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.
Actual and ideal cycle in spark ignition engine
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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, b t h lf b ti
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about half way, combustion gases are discharged and fresh air‐fuel mixture is drawing in .
2‐stroke engines generally less efficient than 4‐stroke engines since partial expulsion of unburned mixture with exhaust gas. It has higher power/weight ratio.
Air Standard Otto Cycle (Nikolaus A. Otto 1876)Ideal cycle of spark ignition engine, comprises of 4- Process:
Process 1-2 Isentropic Compression (piston moves from BDC to TDC)
Process 2-3 v = constant, heat added (piston stays still, represents combustion)
Process 3-4 Isentropic expansion (piston moves from TDC to BDC gives POWER)
Process 4-1 v = constant, heat rejection (piston stays still, represents EXHAUST and INTAKE stroke)
T
2 4
3qin
P
2 4
3
wout
There are only 2-stroke of all 4-processes,
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ss1=s2 s3=s4
1qout
vv2=v3 v1=v4
1win
What is the different of Otto cycle from Carnot cycle, the most efficient cycle
TDC BDC
T
2 4
3qinEnergy balance – Otto cycle (I)
)/()()( kgkJuwwqq outinoutin
Neglecting changes in KE and PE
ss1=s2 s3=s4
1qout
P
2
3
wout
Heat transfer to/from the system is under constant volume (no work)
)( 23v23in TTcuuq
)( 14v14out TTcuuq
1T
T 4
Evaluate at room tem: called cold air standard assumption
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vv2=v3 v1=v4
2
1
4
win
in
out
in
netOttoth q
q1
q
w,
1
T
TT
1T
T
1TT
TT1
2
32
1
41
23
14
T
2 4
3qinEnergy balance – Otto cycle (II)
in
out
in
netOttoth q
q1
q
w,
1
TT
1T
TT
1TT
TT1
3
1
41
23
14
ss1=s2 s3=s4
1qout
P
2
3
wout
1T
T2
2
Processes 1‐2 and 3‐4 are isentropic and v2=v3and v4=v1 (Pv
k=constant)
3
4
1k
4
3
1k
1
2
2
1
T
T
v
v
v
v
T
T
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vv2=v3 v1=v4
2
1
4
win
Compression ratio
2
1
2
1
v
v
V
V
V
Vr
min
max
1kOttoth r
11 ,
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Thermal efficiency of a Otto cycle (I)
1kOttoth r
11 ,
• High compression ratios: temperature of air/fuel mixture rises above auto ignition temperature (premature ignition)‐produces audible noise is called engine knock.
• Improvement of thermal efficiency was obtained utilizing higher
i ti ( t 12) li
k=1.4
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compression ratios (up to 12) gasoline blend with tetraethyl lead (improving octane rating) but it has been prohibited to use since the hazardous of lead to health. Octane rating = measure of fuel
quality (measure of engines knock resistance)
Thermal efficiency of a Otto cycle (II)
• Most compression ratios are around 10:1, meaning that the gas let into the cylinder is
Monatomic gas (He, Ar)
airmeaning that the gas let into the cylinder is compressed to 1/10 times its original size.
• Efficiency is better with a higher compression ratio but only to the limits of the fuel quality.
Molecular weight of the working fluid increases
CO2
k=1.2 ethane
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• Thermal efficiency of actual spark ignition engine ~ 25‐30%
Compression Issues
• Problems can occur during a cycle if there is:
– Lack of Compression caused from gasses leaking past the piston, a hole in the piston, or the intake or exhaust valves are not sealing properly.
– Lack of Spark caused by malfunctioning spark plugs, dirty spark plugs, mistimed firings, or bad connections between l d th b tt
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plugs and the battery.
How Fuel is Handled
• Structure of Gasoline
– Is mostly comprised of hydrocarbon molecules having from six to ten carbon atoms.o si to te ca bo ato s.
– Octane is a measure of the resistance to detonation. The octane number assigned to gasoline (87,89, 93, 100, 114, 120) represents the ratio of heptane, which easily detonates, to isooctane, which does not want to detonate (better to say octane number above 100 as “performance number”. It is calculated by different way. Often itʹs done
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y yby pure extrapolation. ) . Eighty‐seven‐octane gasoline is gasoline that contains 87‐percent octane and 13‐percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane).
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Chemical Energy of Gasoline
• The chemical energy of one gallon of gasoline is, on the average, 125 000 BTU per gallon (132×106 J per 3 8 L)125,000 BTU per gallon (132×10 J per 3.8 L).
• Only about 25% of chemical energy in gasoline is converted to mechanical energy.
• Basically out of a one dollar gallon of gasoline, 75 cents is
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wasted.
Cylinder Configurations
Straight Configuration V Configuration
Displacement refers to the volume inside each
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Flat Configuration
the volume inside each piston chamber. For example: a 3.0 Liter engine with 6 cylinders will have 0.5 liters per cylinder.
Diesel cycle: The ideal cycle for compression ignition (CI) engine (Rudolph Diesel 1890)
• Similar to spark ignition engine differing mainly in the method of initiating combustion.
• In spark ignition (SI) engines (gasoline engines), air fuel mixture compressed below auto ignition temperature of the air/fuel mixture and combustion starts by firing spark plugs.
• In combustion ignition (CI) engines (diesel engines) air compressed above the auto ignition temperature of the air fuel
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p g pmixture and then fuel inject into the air.
• SI engines has a carburetor and diesel engine has a fuel pump.
• The compression ratio of diesel engines typically higher (12 ‐24)
Diesel engine
• The fuel injection starts when the piston reaches to TDC.
• Combustion process takes place over longer interval.
• Because of this longer period the heat addition process can be approximated as constant pressure heat addition process.
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pressure heat addition process.
• Other parts are common for both SI and CI engines.
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Energy balance – Diesel engine (I)
)/()()( kgkJuwwqq outinoutin
)()()( TTchhuuvvPq )()()( 23p2323232in TTchhuuvvPq
)( 14v14out TTcuuq
in
out
in
netDieselth q
q1
q
w,
1T
T 4
33
1
T
TkT
1T
T
1TTk
TT1
2
32
11
23
14
)(
)(
Energy balance – Diesel engine (II)
in
out
in
netDieselth q
q1
q
w,
1
T
TkT
1T
TT
1TTk
TT1
2
32
1
41
23
14
)(
)(
2
3
2
3c v
v
V
Vr Define new quantity; cutoff ratio
Utilizing definition of isentropic ideal‐gas relations
1r1
1k
cDi lh
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)(, 1rkr
1c
1kDieselth
r is the compression ratio
Otto vs. Diesel
)(, 1rk
1r
r
11
c
kc
1kDieselth1kOttoth r
11 ,
• Limiting value of rc=1; when efficiencies of both Otto and Diesel cycles are identical.
• Diesel cycle operates much higher compression ratios, therefore thermal efficiency of Diesel engines are usually higher than SI engines (35 to 40%).
ratio) ncompressio same the on operate cycles both (when DieselthOttoth ,,
35
e gi e ( o %)
• Diesel engines burns fuels more completely than gasoline engines.
Energy content of 1 gallon of diesel on average, 147,000 BTU per gallon (155×106 J per 3.8 L).
Dual cycle
• More realistic way to model:Combination of heat transferCombination of heat transfer processes in gasoline and diesel cycles.
• The relative amount of heat transfer during each process can be adjusted to approximate
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j ppactual cycle more closely.
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Brayton Cycle: The ideal cycle for gas turbine engines (George Brayton 1870)
• Used in aircraft propulsion and electric power generation.
• Gas turbines usually operate on an open cycle.
• Fresh air ambient condition compressor (temperature and pressure raised) combustion chamber (fuel burn) turbine (work produce) expands to atmospheric pressure.
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• However, open gas turbine cycle can be modeled as a closed cycle.
Open and closed cycle gas turbine
open‐cycle gas turbineclosed cycle gas turbine
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• Combustion and exhaust processes are constant pressure processes.
• Exhaust propels craft or used to generate steam.
closed‐cycle gas turbine
Simple Ideal Brayton Cycle
1‐2: isentropic compression
Modeled as a closed cycle. Air standard assumptions are applied. Air is the working fluid.
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1‐2: isentropic compression
2‐3: constant pressure heat addition
3‐4: isentropic expansion
4‐1: constant pressure heat rejection
Energy balance for steady flow process
)/()()( kgkJhwwqq outinoutin
)( TTchhq )( 14p14out TTchhq )( 23p23in TTchhq )( 14p14outq
1
T
TT
1T
TT
1TT
TT1
q
q1
q
w
2
32
1
41
23
14
in
out
in
netBraytonth )(
)(,
P 1 2 d 3 4 i t i d P P d P P
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Processes 1‐2 and 3‐4 are isentropic, and P2=P3 and P4=P1
4
3k
1k
4
3k
1k
1
2
1
2
T
T
P
P
P
P
T
T
k
1k
p
Braytonth
r
11,
Pressure ratio
1
2p P
Pr
11
Typical pressure ratios and highest temperatures
• Common pressure ratios 11 to 16.
• Highest temperature occurs at the end of the combustion process and limited pmaximum temperature of turbine blades can withstand.
• For a fixed inlet temperature, the net work output increases with pressure ratio, reaches to a maximum value and then decreases.
Ai /f l ti b 50 i t
k=1.4
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• Air/fuel ratio above 50 is not uncommon.
• Higher back work (usually more than one‐half of turbine work output).
Advantageous and disadvantageous
Gas turbine engines have a great power‐to‐weight ratio compared to reciprocating engines. That is, the amount of power you get out of the engine
d t th i ht f th i it lf i dcompared to the weight of the engine itself is very good.
Gas turbine engines are smaller than their reciprocating counterparts of the same power.
The main disadvantage of gas turbines is that, compared to a reciprocating engine of the same size, they are expensive. Because they spin at such high speeds and because of the high operating temperatures, designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpoint
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turbines is a tough problem from both the engineering and materials standpoint.
Gas turbines also tend to use more fuel when they are idling, and they prefer a constant rather than a fluctuating load. That makes gas turbines great for things like transcontinental jet aircraft and power plants, but explains why you donʹt have one under the hood of your car.
Gas Power Cycle ‐ Jet Propulsion Technology
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In jet propulsion, gas turbine produces little power and the high velocity exhaust gas responsible for producing essay thrust for moving.
Gas Turbine Improvements
• Increase the gas combustion temperature (T3) before it enters the turbine.
Limited by metallurgical restriction: ceramic coating over the turbineLimited by metallurgical restriction: ceramic coating over the turbine blades
Improved intercooling technology: blow cool air over the surface of the blades (film cooling), steam cooling inside the blades.
• Modifications to the basic thermodynamic cycle: intercooling, reheating, regeneration
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• Improve design of turbomachinery components: multi‐stage compressor and turbine configuration. Better aerodynamic design on blades (reduce stall).
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Deviation of actual gas‐turbine cycles from idealized ones.
Isentropic efficiency of the
1a2
1s2
a
sc hh
hh
w
w
compressor
Isentropic efficiency of the turbine
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s43
a43
s
aT hh
hh
w
w
EXAMPLE PROBLEM
The pressure ratio of an air standard Brayton cycle is 4.5 and the inlet conditions to the compressor are 1004.5 and the inlet conditions to the compressor are 100 kPa and 27C. The turbine is limited to a temperature of 827C and mass flow is 5 kg/s. Determine
a) the thermal efficiency
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b) the net power output in kWc) the BWR (back work ratio)
Assume constant specific heats.
Draw diagram
P 2 3inq
1 4outq
47
v
Start analysis
Let’s get the efficiency:
k1kpr
11
From problem statement, we know rp = 4.5
48
349054
11
41141.
. ..
13
Net power output:
Net Power:
Substituting for work terms:
compturbnetnet wwmwmW
)h(h)h(hmW 1243net
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)T(T)T(TcmW 1243pnet
Applying constant specific heats:
Need to get T2 and T4
Use isentropic relationships:
;p
p
T
T k
1k
1
2
1
2
k
1k
3
4
3
4
p
p
T
T
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T1 and T3 are known along with the pressure ratios:
Solving for temperatures:
K4615.4300T 4.14.02 T2:
4140T4: K7.715222.01100T 4.14.0
4
Net power is then:
netW (5 kg/ s)(1.0035 / ( ))kJ kg K
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(1100 715.7) (461 300) K
kW1120Wnet
Back Work Ratio
12comp hhw BWR
43turb hhw BWR
Applying constant specific heats:
TT
52
42.07.7151100
300461
TT
TT
43
12
BWR
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The Brayton cycle with regeneration (I)
• In gas turbine engines the temperature of the exhausted gas leaving the turbine is often considerably higher than the
f h i l i htemperature of the air leaving the compressor.
• Therefore, counter‐flow (call regenerator) heat exchanger can be used to capture portion of heat from the exhaust gas.
• This improves the efficiency of the cycle.
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The Brayton cycle with regeneration (II)
25actregen hhq ,
25regen
hh
hhq 'max,
24 hh
24
25
regen
actregen
hh
hh
q
q
max,
,
With cold‐air standard assumption
TT
54
)(. practicein850TT
TT
24
25
Under cold air assumption with regeneration
k
1k
p3
1regenth r
T
T1,
The Brayton cycle with regeneration (III)
k
1k
p3
1regenth r
T
T1,
• Therefore, the thermal efficiency of Brayton cycle with regeneration is depends on minimum/maximum
lltemperature ratio as well as pressure ratio.
• Re generation also most effective at lower pressures.
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