ME403

Internal Combustion Engine Theory

4-Stroke Spark-Ignition Engine Cycle

Idealize Otto Cycle

Otto Cycle

Idealization of the piston-cylinder internal combustion (IC) gasoline engine cycle which uses a spark plug to ignite the combustion process.

It consists of four reversible processes:

• Isentropic (no heat or friction loss) Compression• Isochoric (constant volume) Heating (combustion)• Isentropic Expansion• Isochoric Cooling

The last process replaces the actual intake and exhaust processes, which do not contribute to any work done.

Thermodynamics of Otto Cycle

There is only one isochoric heating process (2-3) and one isochoric cooling process (4-1). Assuming air is a perfect gas (ideal gas with constant specific heats and constant specific heat ratio k), for each kg of air:

k

H

L

H

netth v

v

T

T

TT

TT

q

q

q

w

1

2

1

2

1

23

14 11...11

)( 41014 TTCqq vL

netairnet wmW The work done per cycle:

Diesel Cycle

Idealization of the piston-cylinder internal combustion (IC) engine cycle utilizing the Diesel fuel, which ignites as long as the temperature reaches a critical point due to compression, and therefore does not require a spark plug.

It consists of four reversible processes:

• Isentropic Compression• Isobaric (constant pressure) Heating (combustion)• Isentropic Expansion• Isochoric Cooling

The last process replaces the actual intake and exhaust processes, which do not contribute to any work done.

4-Stroke Diesel Engine Cycle

Thermodynamics of Diesel Cycle

There is only one isobaric heating process (2-3) and one isochoric cooling process (4-1). Assuming air is a perfect gas (ideal gas with constant specific heats and specific heat ratio k), for each kg of air:

)(

)(1

)(

)(1

23

14

230

140

TTk

TT

TTC

TTC

q

w

P

v

H

netth

)( 41014 TTCqq vL

)()()()( 2302323223322332 TTChhvvPuuwuuqq PH

)()( 140230 TTCTTCqqw vPLHnet

netairnet wmW The work done per cycle:

Summary: 4-Stroke Engine Cycles

Stroke

Otto Cycle

(Spark-Ignition)

Diesel Cycle

(Compression-Ignition)Compression Const-s Compression Const-s Compression

ExpansionConst-v Combustion

Const-s Expansion

Const-P Combustion

Const-s Expansion

Exhaust These 2 strokes are simply idealized by a

Const-v Cooling processIntake

2-Stroke Spark Ignition Enginehttp://science.howstuffworks.com/two-stroke1.htm

Compression +Exhaust stroke, Combustion, Expansion + Exhaust + Intake stroke

Internal Combustion Engines: Geometry

Definitions

N = Number of Cylinders

S = Stroke B = Bore

TDC = Top Dead Center

BDC = Bottom Dead Center

Vmax–Vmin = Displacement Volume per cylinder

Vdisp = N S (/4)B2 = N (Vmax – Vmin) = Engine Displacement Volume

rv = Vmax/Vmin = Compression Ratio

B

IC Engine Performance

• Engine shaft rotation speed (rev/sec):

• Thermodynamic cycles per second:

For 2-stroke engine (1 cycle/rev):

For 4-stroke engine (1 cycle/2 rev):

• Total engine power (N cylinders):

• Heating rate required:

• Given heating value of fuel:

• Fuel consumption rate:

• Specific Fuel Consumption:

60

RPMf

2

fn

netWnNW

thH

WQ

fHV

f

Hf HV

Qm

W

mBSFCc f

fn

Example: Performance of Ideal Otto Engine

Example: Performance of Ideal Diesel Engine

Conclusion

Actual engine efficiency (typically 25% to 30%), is much lower than the ideal thermal efficiency, due to the following factors:

• Deviation of actual cycle from the idealize Otto or Diesel cycle• Air is not a perfect gas, especially at temperatures >500K• Mechanical (friction) losses• Heat loss to surrounding• Combustion of fuel is often incomplete

Typical BSFC: 0.4 to 0.5 lb/hr-hp (0.24 to 0.3 kg/hr-kw)

Engine power output is proportional to mass of air in cylinders, which is proportional to air density. Hence engine power available decreases proportionately with air density as flight altitude increases.