Photo AI
Figure 5 shows the $p-V$ diagram for a theoretical petrol engine cycle compared to the indicator diagram for a real four-stroke petrol engine with the same maximum and minimum volumes - AQA - A-Level Physics - Question 4 - 2017 - Paper 6
Question 4
Figure 5 shows the $p-V$ diagram for a theoretical petrol engine cycle compared to the indicator diagram for a real four-stroke petrol engine with the same maximum a... show full transcript
Worked Solution & Example Answer:Figure 5 shows the $p-V$ diagram for a theoretical petrol engine cycle compared to the indicator diagram for a real four-stroke petrol engine with the same maximum and minimum volumes - AQA - A-Level Physics - Question 4 - 2017 - Paper 6
Step 1
State and explain the differences between the cycles
Answer
The theoretical cycle depicted in the diagram assumes ideal processes, such as isothermal and adiabatic expansions, which lead to maximum efficiency. The real cycle, however, experiences deviations due to real engine factors like heat losses, friction, and imperfect combustion.
In the theoretical cycle:
- Pressure increases linearly during combustion, resulting in higher overall efficiency.
- The shape of the curve is smoother as it represents ideal gas behavior.
In contrast, the real cycle:
- Shows pressure drops during the compression phase due to losses and the time it takes to transfer energy.
- Exhibits a less smooth curve, indicating real-time events like exhaust gas reversion and irregular combustion timings.
Step 2
Explain why the work output per cycle of the real engine is less than that predicted by an analysis of the theoretical cycle
Answer
The work output per cycle of the real engine is less than predicted due to various inefficiencies. In an ideal cycle, the maximum work is derived from the difference in area under the pressure-volume diagram during expansion and compression stages.
However, in a real cycle:
- Fuel does not burn completely, resulting in reduced thermal energy conversion.
- Friction and heat losses in engine components lead to additional energy loss, thus reducing the effective work output per cycle.
- The presence of processes like blow-by and residual gas in the cylinders further reduces the efficiency.
Overall, these factors contribute to a significant reduction in the work output, making it less than the ideal theoretical calculations.