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Thermodynamic Principles All internal combustion
Open cycle, heated engine Gasoline (Otto) engine
Spark ignitionCompresses air-fuel mixture
Diesel engineCompressed ignitionCompresses air only
INTERNAL COMBUSTION ENGINE:
AN ENGINE THAT PRODUCES POWER BY BURNING FUEL INSIDE A COMBUSTION CHAMBER WITHIN THE ENGINE
Gas CyclesCarnot Cycle
T2
T1
s1 s2
Work W
1
2 3
4
1-2 - ADIABATIC COMPRESSION (ISENTROPIC)2-3 - HEAT ADDITION (ISOTHERMAL)3-4 - ADIABATIC EXPANSION (ISENTROPIC)4-1 - WORK (ISOTHERMAL)
Heat Q
Carnot Cycle
Carnot cycle is the most efficient cycle that can be executed between a heat source and a heat sink.
However, isothermal heat transfer is difficult to obtain in reality--requires large heat exchangers and a lot of time.
2
1
TT-1
Carnot Cycle
Therefore, the very important (reversible) Carnot cycle, composed of two reversible isothermal processes and two reversible adiabatic processes, is never realized as a practical matter.
Its real value is as a standard of comparison for all other cycles.
Gas cycles have many engineering applications Internal combustion engine
Otto cycleDiesel cycle
Gas turbines Brayton cycle
RefrigerationReversed Brayton cycle
Terminology
Bore = d Stroke = s Displacement volume =DV = Clearance volume = CV Compression ratio = r
4ds2
CVCVDVr
TDC
BDCVV
Mean Effective PressureMean Effective Pressure (MEP) is a fictitious pressure, such that if it acted on the piston during the entire power stroke, it would produce the same amount of net work.
minmax VVWMEP net
The net work output of a cycle is equivalent to the product of the mean effect pressure and the displacement volume
Otto Cycle Derivation
Thermal Efficiency:
For a constant volume heat addition (and rejection) process;
Assuming constant specific heat:
QQ - 1 =
QQ - Q =
H
L
H
LHth
T C m = Q vin
1-TTT
1 - TTT
-1 =)T - T( C m)T - T( C m - 1 =
2
32
1
41
23v
14vth
T C m = Q v Rej
For an isentropic compression (and expansion) process:
where: γ = Cp/Cv
Then, by transposing,
TT =
VV =
VV =
TT
4
3
3
41-
2
11-
1
2
TT =
TT
1
4
2
3
Otto Cycle Derivation
TT-1 =
2
1thLeading to
The compression ratio (rv) is a volume ratio and is equal to the expansion ratio in an otto cycle engine.
Compression RatioVV =
VV = r
3
4
2
1v
1 + vv = r
vv + v =
volume Clearancevolume Total = r
cc
sv
cc
ccsv
where Compression ratio is defined as
Otto Cycle Derivation
Then by substitution,
)r(1 - 1 = )r( - 1 = 1-v
-1vth
)r( = VV =
TT -1
v1
2-1
2
1
The air standard thermal efficiency of the Otto cycle then becomes:
Otto Cycle Derivation
Summarizing
QQ - 1 =
QQ - Q =
H
L
H
LHth T C m = Q v
1-TTT
1 - TTT
-1 =
2
32
1
41
th
)r( = VV =
TT -1
v1
2-1
2
1
)r(1 - 1 = )r( - 1 = 1-v
-1vth
TT =
TT
1
4
2
3
2
11TT th
where
and then
Isentropic behavior
Otto Cycle Derivation
Heat addition (Q) is accomplished through fuel combustion
Q = Lower Heat Value (LHV) BTU/lb, kJ/kg
Q AF m =Q
fuelain cycle
Otto Cycle Derivation
T C m = Q vin also
Sample Problem – 1The air at the beginning of the compression stroke of an air-standard Otto cycle is at 95 kPa and 22C and the cylinder volume is 5600 cm3. The compression ratio is 9 and 8.6 kJ are added during the heat addition process. Calculate:
(a) the temperature and pressure after the compression and heat addition process(b) the thermal efficiency of the cycleUse cold air cycle assumptions.
Carry through with solution
kg 10 x 29.6RTVPm 3-
1
11
Calculate mass of air:
Compression occurs from 1 to 2:
ncompressio isentropic VVTT
1
2
112
k
11.42 9K 27322T
K 705.6T2 But we need T3!
Get T3 with first law:
23v23 TTmcQ Solve for T3:
2v
3 TcqT K705.6
kgkJ0.855kg6.29x10kJ8.6 3
K2304.7T3