1. Internal Combustion Engine Theory
ME403
Internal Combustion Engine Theory

2. 4-Stroke Spark-Ignition Engine Cycle

3. Idealize Otto Cycle

4. 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.

5. 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:
The work done per cycle:

6. 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.

7. 4-Stroke Diesel Engine Cycle

8. 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:
The work done per cycle:

9. Summary: 4-Stroke Engine Cycles
Otto Cycle
(Spark-Ignition)
Diesel Cycle
(Compression-Ignition)
Compression
Const-s Compression
Expansion
Const-v Combustion
Const-s Expansion
Const-P Combustion
Exhaust
These 2 strokes are simply idealized by a
Const-v Cooling process
Intake

10. 2-Stroke Spark Ignition Engine http://science. howstuffworks
2-Stroke Spark Ignition Engine Compression +Exhaust stroke, Combustion, Expansion + Exhaust + Intake stroke

11. 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 (p/4)B2 = N (Vmax – Vmin)
= Engine Displacement Volume
rv = Vmax/Vmin = Compression Ratio B

12. 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:

13. Example: Performance of Ideal Otto Engine

14. Example: Performance of Ideal Diesel Engine

15. 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: 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.