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Lecture 28. Internal Combustion Engine Models The Otto Cycle The Diesel Cycle. IC Engine Terminology. Finally … here is one of the reasons we spent so much time analyzing piston-cylinder assemblies in the early part of the course!. IC Engine Terminology. Fuel-Air ignition Spark

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lecture 28

Lecture 28

Internal Combustion Engine Models

The Otto Cycle

The Diesel Cycle

ic engine terminology
IC Engine Terminology

Finally … here is one of the reasons we spent so much time analyzing piston-cylinder assemblies in the early part of the course!

ic engine terminology1
IC Engine Terminology
  • Fuel-Air ignition
    • Spark
      • Gasoline engines
    • Compression
      • Diesel engines
  • 4-Stroke Engine
    • Four strokes (intake, compression, power stroke, exhaust) are executed for every two revolutions of the crankshaft, and one thermodynamic cycle
  • 2-Stroke Engine
    • Two strokes (intake, compression, power stroke, and exhaust) are executed for every one revolution of the crankshaft, and one thermodynamic cycle
ic engine performance
IC Engine Performance

Thermal Efficiency

Mean Effective Pressure

The mep provides a way to compare two engines that have the same displacement volume

modeling the ic engine
Modeling the IC Engine
  • Air Standard Analysis (ASC or hot ASC)
    • The working fluid is a fixed mass of air treated as an ideal gas
      • No intake or exhaust
    • The combustion process is replaced with a heat transfer from a high-temperature source
    • The exhaust process is replaced with a heat transfer to a low-temperature sink
    • All processes are internally reversible
  • Cold Air Standard Analysis (cold ASC)
    • All of the above
    • Heat capacity of the air is assumed to be constant at the ambient temperature
si engine otto cycle
SI Engine - Otto Cycle

3

1

4

2

3

2

TDC

4

BDC

1

TDC

BDC

  • 1-2 Isentropic compression from BDC to TDC
  • 2-3 Isochoric heat input (combustion)

3

v = const

4

2

v = const

1

si engine otto cycle1
SI Engine - Otto Cycle

3

1

4

2

3

2

TDC

4

BDC

1

TDC

BDC

  • 3-4 Isentropic expansion (power stroke)
  • 4-1 Isochoric heat rejection (exhaust)

3

v = const

4

2

v = const

1

otto cycle performance
Otto Cycle Performance

3

Compression Ratio

2

4

1

TDC

BDC

Thermal Efficiency

3

v = const

4

2

v = const

1

otto cycle performance1
Otto Cycle Performance

3

Mean Effective Pressure

2

4

1

TDC

BDC

3

Cold ASC values (Table C.13a) ...

v = const

4

2

v = const

1

ci engine diesel cycle
CI Engine - Diesel Cycle

2

3

1

4

2

3

TDC

4

BDC

1

TDC

BDC

  • 1-2 Isentropic compression from BDC to TDC
  • 2-3 Isobaric heat input (combustion)

3

P = const

2

4

v = const

1

ci engine diesel cycle1
CI Engine - Diesel Cycle

2

3

1

4

2

3

TDC

4

BDC

1

TDC

BDC

  • 3-4 Isentropic expansion (power stroke)
  • 4-1 Isochoric heat rejection (exhaust)

3

P = const

2

4

v = const

1

diesel cycle performance
Diesel Cycle Performance

2

3

Compression Ratio

Cutoff Ratio

4

1

TDC

BDC

Thermal Efficiency

3

P = const

2

4

v = const

1

diesel cycle performance1
Diesel Cycle Performance

2

3

Mean Effective Pressure

4

1

TDC

BDC

3

Cold ASC values (Table C.13a) ...

P = const

2

4

v = const

1

cycle evaluation
Cycle Evaluation
  • Strategy
    • Build the property table first, then do the thermodynamic analysis
  • Real fluid model
    • EES (fluid name = ‘air_ha’)
  • Air standard model
    • Ideal gas with variable heat capacities
      • Table C.16 (Air Tables)
      • EES (fluid name = ‘air’)
  • Cold air standard model
    • Ideal gas with constant heat capacities evaluated at the beginning of compression
      • Atmospheric conditions
ic engine performance1
IC Engine Performance
  • Known Parameters
    • Number of cylinders in the engine
    • Enough information to determinethe mass of the air trapped in thecylinder
    • Engine ratios (compression and cutoff)
    • Rotational speed of the engine (rpm)
    • Engine type
      • All cylinders complete a thermodynamic cycle in either two or four strokes
    • P and T at the beginning of compression
    • P or T at the end of combustion
ic engine performance2
IC Engine Performance

The power developed by the engine can be determined by

Crankshaft rotational speed

Number of cylinders

From the Otto or Diesel Cycle analysis

conversion factor

Crankshaft revolutions per cycle