Transcript Thermodynamics I

QUESTIONS
Performance
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Typical thermal efficiency: ~25%
Mechanical friction
Rolling friction
Exhaust
Radiator
Driving auxiliaries: fan, water pump,
etc
Practice problem
 A four-cylinder, four-stroke engine
has 3.75-in cylinder. Clearance
volume of 17% of cylinder volume at
BDC. Stroke is 3.45 in. Crankshaft
rotates at 2600rpm. Otto cycle
begins at 14.6psi and 100oF.
Maximum cycle temperature is
5200oR. What horsepower developed
by the engine?
Diesel Cycle
 Rudolph Diesel
• Improve efficiency of power
generation vs steam engine: new
approach
• Overcome deficiency of SIIC
• CIIC
Diesel cycle
 Higher molecular weight fuel; less
volatile. Less refined fuel.
 Air compressed before fuel addition;
no autoignition
 No spark plug
 Higher pressures; sturdier
construction
Diesel cycle
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Air standard
Constant specific heat
Variable specific heat
Otto cycle: compression ratio - 1/2
Diesel cycle
Compression ratio - 1/2
Cut-off ratio - 3/2
Diesel cycle
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1→2 isentropic compression
2→3 constant pressure heat addition
3→4 isentropic expansion
4→1 constant volume heat rejection
Performance
 Approximate “rule”
• >300hp SIIC preferred for
performance
• < 300hp efficiency & costs favor CIIC
 Thermal efficiency
• SIIC 25 – 30%
• CIIC 35 – 40%
Cycle analysis
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Heat
Work
Thermal efficiency: related to ratios
Specific heats: fixed or variable
Practice Problem
 The compression ratio of an air standard
Diesel cycle is 17.At the beginning of
compression, conditions are 14.0 psi, 2ft3,
& 520oR. Maximum cycle temperature is
4000oR.
 Find
• Cycle work, Btu
• Thermal efficiency
• mep, psi
• Cut-off ratio
Dual cycle
Dual cycle analysis
 Heat
 Work
 Thermal efficiency
Two stroke engine
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Simple; inexpensive; light weight
No valves
Power stroke every revolution
Any orientation
Two stroke engine
 No lube system; mix with oil. More
costly fuel. More wear.
 Purging of exhaust inefficient; some
mixing with fresh fuel.
 Noise & pollution; local regulations?
Wankel
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Rotary; smooth torque curve
Relatively simple & light-weight
High power:weight ratio
Chambers “rotate” in engine casing
Wankel
 Difficult to seal compartments
 Mixing of fuel & exhaust
 Fuel consumption; short duration of
combustion
 Low compression ratio
 Replace vs repair
Cycles
 Stirling
 Ericsson
Stirling
Ericsson