0.3.1 Explain what is meant by an adiabatic change - AQA - A-Level Physics - Question 3 - 2022 - Paper 6

To calculate the compression ratio for the adiabatic process, we use the formula for compression ratio: V_1/V_2 = rac{P_1}{P_2}^{rac{1}{ ext{γ}}} Substituting the values: V_1/V_2 = rac{1.0 imes 10^5 ext{ Pa}}{67 imes 10^5 ext{ Pa}}^{rac{1}{1.4}} Calculating this yields: 1.0 and 67. The answer should be $ext{approximately } 12.60$.

Diesel engines operate at higher compression ratios because they require higher temperatures to ignite the fuel through compression alone, avoiding the need for a spark. This high compression ratio is necessary to achieve the temperatures that facilitate self-ignition, in contrast to petrol engines, which ignite their fuel-air mixture via a spark plug at lower pressures and temperatures.

In your diagram, ensure that the curve reflects the ideal pressure-volume relationship for a four-stroke diesel engine, with the corresponding values of V1 and V2 maintained. The curve should illustrate the characteristic shape, indicating work done during expansion and compression.

On your drawn diagram, mark the point of fuel injection with an 'X.' This point typically occurs just before the start of the combustion process within the cycle.

Firstly, the ideal cycle assumes constant heat capacities and does not account for real gas effects or losses, while the indicator diagram captures the actual thermodynamic behavior of the engine, including irreversible processes. Secondly, the ideal cycle often shows sharp transitions between phases (e.g., instant expansion and compression), while the indicator diagram reflects smoother transitions over finite time intervals, capturing the delays due to actual valve operations and engine dynamics.