This document provides information about the Otto cycle, which is the ideal thermodynamic cycle that models the processes in a spark-ignition internal combustion engine. It includes: - A flow diagram and PV diagram of the Otto cycle processes - Equations for calculating temperature, pressure, heat transfer, work, efficiency, and mean effective pressure at each state point - Two example problems applying the Otto cycle equations - Key parameters like compression ratio, heat added, expansion ratio, and state variables The goal is to analyze the thermodynamics of the ideal Otto cycle as a basis for comparing spark-ignition engines. Sample calculations are provided to illustrate applying the cycle equations.

1. LECTURE UNIT NO. 2
I. Otto Cycle (SVSV)
- Basis of comparison for spark-ignition engines (Gasoline engines)
Flow Diagram:
QA QA QR QR
V=C V=C
ma 2 ma 3
TDC
ma
ma ma
L
S=C S=C
1 4 5
BDC
PV diagram:
VC TDC L BDC
P
3
2 4
1
Pm
V
V2=V3 VD=V1 – V2
V1 – V4
VC = Clearance Volume = V2 = V3 = cVD
Where: c = % clearance
Clearance Ratio = c = V2 / VD = V3 / VD = VC / VD
TS diagram:
3
4
2
1
Have Courage
Depending on what your specific success is, it may take courage to arrive at your desired destination.

2. EQUATIONS:
1. PVT Relationships:
Process: 1 – 2 Isentropic Compression Process (S = C)
T2 / T1 = (P2 / P1) k-1 / k = (V1 / V2) k-1
(T2 / T1) 1/ k-1 = (P2 / P1) 1 / k = V1 / V2 = rk (S=C)
Isentropic compression ratio
rk (S=C) = V1 / V2 = (V2 + VD) / V2 = (cVD + VD) / cVD
rk (S=C) = (c + 1) / c
Process: 2 - 3 Constant Volume Heat Addition Process (V = C)
P3V3 / T3 = P2V2 / T2
T3 / T2 = P3 / P2 = rP (V=C)
Process: 3 - 4 Isentropic Expansion Process (S = C)
T4 / T3 = (P4 / P3) k-1 / k = (V3 / V4) k-1
(T4 / T3) 1/ k-1 = (P4 / P3) 1 / k = V3 / V4
(T3 / T4) 1/ k-1 = (P3 / P4) 1 / k = V4 / V3 = re (S=C)
But: V3 = V2 and V4 = V1
re (S=C) = V4 / V3 = V1 / V2 = rk (S=C)
Isentropic Expansion Ratio = Isentropic Compression Ratio
re (S=C) = rk (S=C)
Process: 4 – 1 Constant Volume Heat Rejection Process (V = C)
P1V1 / T1 = P4V4 / T4
T4 / T1 = P4 / P1 = rP (V=C)
2. Heat Added, QA = Σ + Q
Process: 2 – 3 (V=C)
Q2-3 = mCV ΔT
Q2-3 = mCV (T3 – T2)
Note: CV air = 0.7186 kJ/kg-K
3. Heat Rejected, QR = Σ - Q
Process: 4 – 1 (V=C)
Q4-1 = mCV ΔT
= mCV (T1 – T4)
Q4-1 = - mCV (T4 – T1)
Be Excited To Learn
Referring back to the analogy of Edison, when asked about his failures by a young boy, Edison commented, "Young man, I
didn't fail 9,999 times, I discovered 9,999 ways not to invent the light bulb." As you work toward your specific success, always
enjoy opportunities to learn, even if it takes longer than you think it should.
better, more educated decisions from the lessons learned.

3. 4. Network, Wnet
Wnet = | QA | - | QR |
= | mCV (T3 - T2) | - | - mCV (T4 – T1) |
Wnet = mCV(T3 – T2 – T4 + T1)
Or by cyclic integration of W = PdV,
Wnet = (P2V2 – P1V1) /1-k + (P4V4 – P3V3) /1-k
Wnet = mR(T2 – T1)/1-k + mR(T4 – T3)/1-k
5. Otto Cycle Thermal Efficiency, eoc
eoc = Wnet / QA x 100 %
= mCV(T3 – T2 – T4 + T1) / mCV(T3 – T2) x 100%
= (T3 – T2 – T4 + T1) / (T3 – T2) x 100%
eoc = 1 - (T4 – T1) / (T3 - T2) x 100%
Or by knowing that:
T4 = T3 (1/rkk-1) and T1 = T2 (1/rkk-1), substituting
eoc = 1 - __1___ x 100%
rkk-1
6. Otto Cycle Mean Effective Pressure, Pm
Pm = Wnet / VD
Pm = eoc x QA / (V1 – V2)
SEATWORK:
1. An Otto Cycle of the beginning of the compression stroke has a pressure of 100 kPa and temperature is 15 °C.
The heat transfer to air per cycle is 1800 kJ/kg air and the adiabatic expansion ratio is 8. the rate of air flow is
10 kg/s. Determine
a. The pressure and temperature at the end of each process of the cycle
b. The thermal efficiency ans.56.47 %
c. The mean effective pressure. ans. 1405.20 kPa
2. An air-standard Otto Cycle has a compression ratio of 8.0 and has air conditions at the beginning of
compression of 100 kPa and 25°C. The heat added is 1400 kJ/kg. Determine
a. The four cycle state points
b. The thermal efficiency ans. 56.47 %
c. The mean effective pressure ans. 1056.41 kPa
Share Your Success
Although this may be more at the end of the process, it is important. When you finally do reach your success, use your
experience to teach, guide, and mentor others so that they too might succeed.