Work done by a gas

Internal Combustion Engine Basics

  • The internal combustion engine uses a mixture of fuel and air.

  • Ignition occurs via a spark plug (in gasoline engines) or through high pressure (in diesel engines).

  • The resulting explosion creates high pressure, pushing the piston down in the cylinder.

Work Done by an Expanding Gas

  • The process of gas expansion does work, converting chemical energy into mechanical energy (turning the crankshaft).

  • Key Equation: Work done by expanding gas is given by[ W = P \times \Delta V ]

    • Where:

      • W = work done

      • P = pressure of the gas

      • ΔV = change in volume

  • Understanding the Variables:

    • Delta (Δ) signifies a change, so ΔV is the difference in volume.

Example Calculation

  • Given: Pressure = 900 kPa, Volume change from 100 cm³ to 400 cm³.

  • Convert units:

    • Volume change: ( 400 cm^3 - 100 cm^3 = 300 cm^3 = 0.3 \times 10^{-3} m^3 )

  • Calculation:[ W = 900,000 \text{ Pa} \times 0.3 \times 10^{-3} m^3 = 270 \text{ Joules} ]

  • Assumption: Pressure remains constant during gas expansion.

Pressure-Volume Relationship

  • During expansion, pressure decreases as volume increases.

  • Graph of pressure versus volume typically shows a downward curve rather than a straight line.

The Otto Cycle (Engine Cycle Summary)

  1. Compression:

    • Gas volume decreases, pressure increases.

  2. Combustion:

    • Rapid expansion due to gas explosion, causing pressure to peak.

  3. Power Stroke:

    • Gas expands, doing work (pushes piston down).

  4. Exhaust:

    • Exhaust valve opens, expelling gases; fresh air intake occurs subsequently.

Adiabatic Processes

  • Definition of Adiabatic:

    • No heat energy is lost during the process (i.e., constant heat).

  • The work done results in changes in the gas's internal energy due to work being done on/by the gas.

Isothermal Processes

  • Definition of Isothermal:

    • Occurs at a constant temperature, meaning no change in thermal energy during certain stages of the cycle.

Key Terms to Remember

  • Adiabatic: No heat loss during expansion.

  • Isothermal: Constant temperature process.

  • These terms will be further discussed in relation to the second law of thermodynamics in future lessons.