Internal Combustion Engine

Facts:

Key operating facts:

• Power control
  • More air $\Rightarrow$ More Power
• Fuel Injection
  • Intake Stroke

Combustion:

• Constant air/fuel ratio
  • Close to stoichiometric
• Premixed mode
  • Start of combustion controlled by spark ignition
  • Front propagation

Pros/Cons:

• Subject to knocking $\Rightarrow$ need high octane # fuel

Ideal Otto Cycle:

Process:

Intake Stroke:

• $0\to 1$: constant pressure

Compression Stroke:

• $1\to 2$: isentropic
• $2\to 3$: combustion, constant volume

Power Stroke:

• $3\to 4$: isentropic

Exhaust Stroke:

• $4\to 1$: constant volume
• $1\to 0$: contant pressure

System:

• Unit mass inside combustion chamber
• Intensive/specific quantities
• Compression ratio $r=\frac{V_1}{V_2}$

First Law of Thermodynamics:

• $1\to 2$: $\mathrm{\Delta }{u}_{12}=-w_{12}$
• $2\to 3$: $\mathrm{\Delta }{u}_{23}=C_v(T_3-T_2)=q_{in}$
• $3\to 4$: $\mathrm{\Delta }{u}_{34}=-w_{34}$
• $4\to 1$: $\mathrm{\Delta }{u}_{45}=C_v(T_1-T_4)=q_{out}$

Second Law of Thermodynamics:

• $1\to 2$: $T_2=T_1{r}^{\gamma -1}$ $P_2=P_1{r}^{\gamma }$
• $3\to 4$: $T_3=T_4{r}^{\gamma -1}$ $P_3=P_4{r}^{\gamma }$

Specific heat added:

• Heat released by combustion $q_{in}=C_v{T}_1(\frac{T_3}{T_1}-r^{\gamma -1})$

Specific work produced:

• Power Stroke - Compression Stroke $w_{net}=w_{12}+w_{34}={\int }_1^{2}Pdv+{\int }_3^{4}Pdv=C_v{T}_1(\frac{T_3}{T_1}-r^{\gamma -1})(1-r^{\gamma -1})$

Thermal Efficiency:

• Not a function of any temperature
• Independent of any heat added

Sources of Irreversibility:

• Combustion: $2\to 3$
• Uncontrolled expansion: $4\to 1$

Facts:

Key operating facts:

• Power control
  • More air $\Rightarrow$ More power
• Fuel Injection
  • End of compression stroke

Combustion:

• Air/fuel ratio not constant
  • From overall lean to very lean
• Non-premixed
  • Start of combustion controlled by fuel injection

Pros/Cons:

• Higher compression ratio $\Rightarrow$ higher efficiency
• Higher emissions due to non-homogeneous mixtures