Week 2. Gas Power Cycles II
Objectives
1. Evaluate the performance of gas power cycles for which the working fluid remains a gas throughout the entire cycle
2. Develop simplifying assumptions applicable to gas power cycles 3. Discuss both approximate and exact analysis of gas power cycles 4. Review the operation of reciprocating engines
5. Solve problems based on the Otto, Diesel, Stirling, and Ericsson cycles 6. Solve problems based on the Brayton cycle; the Brayton cycle with
regeneration; and the Brayton cycle with intercooling, reheating, and regeneration
7. Analyze jet-propulsion cycles
8. Identify simplifying assumptions for second-law analysis of gas power cycles
9. Perform second-law analysis of gas power cycles
An Overview of Reciprocating Engines
• Basically a piston-cylinder device
• Top dead center (TDC): the position of the piston when it forms the smallest volume in the cylinder
• Bottom dead center (BDC): the position of the piston when it forms the largest volume in the cylinder
• Stroke: the distance between the TDC and BDC
• Bore: the diameter of the piston
• Clearance volume: the minimum volume formed in the cylinder when the piston is at TDC
• Displacement volume: the volume displaced by the piston as it moves between TDC and BDC
• Compression ratio (r)
TDC BDC min
max
V V V
V
r
An Overview of Reciprocating Engines II
• Mean effective pressure (MEP)
It is a fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle
• MEP can be used as a parameter to compare the performances of reciprocating engines of equal size
• The engine with a larger value of MEP delivers more net work per cycle and thus performs better
(kPa) V
V M EP W
Volume nt
Displaceme M EP
Stroke area
Piston M EP
min max
net min
max net net
v v
w W
The net work output of a cycle is equivalent to the product of the MEP and the displacement
volume
Otto Cycle: The Ideal Cycle for Spark-Ignition Engines
Actual and ideal cycles in spark-ignition engines and their P-v diagrams
Otto Cycle: The Ideal Cycle for Spark-Ignition Engines II
) (
) (
(kJ/kg)
)
( ) (
1 4 1
4
2 3 2
3
T T c u u q
T T c u u q
u w
w q
q
v out
v in
out in
out in
The energy balance for Otto cycle,
The thermal efficiency of the ideal Otto cycle
, 1
3 4 1
4 3 1
1 2 2
1
2 3 2
1 4 1 2
3 1 4 ,
1 1
) 1 /
(
) 1 /
1 ( 1
1
otto k th
k k
in out in
net otto
th
r
T T v
v v
v T
T
T T T
T T T T
T T T q
q q
w
T-s diagram of P-v diagram of the ideal Otto cycle
Otto Cycle: The Ideal Cycle for Spark-Ignition Engines III
Improvement of the thermal efficiency of gasoline engines 1. A higher compression ratios r
2. A higher specific heat ratio k
Monatomic gas (argon or helium) k=1.66 Biatomic gas (Hydrogen, Oxygen) k=1.4 Polyatomic gas (Water, Carbon dioxide) k=1.33
Ex 2-1) The Ideal Otto Cycle (constant specific heat)
The compression ratio in an air-standard Otto cycle is 10. At the beginning of the compression stoke, the pressure is 0.1 MPa and the temperature is 15oC. The heat transfer to the air per cycle is 1800 kJ/kg air. Determine
1. The pressure and temperature at the end of each process of the cycle.
2. The thermal efficiency
3. The mean effective pressure
