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

To find the compression ratio, we can use the relation:

P_1 V_1^eta = P_2 V_2^eta

Rearranging gives:

$\frac{V_1}{V_2} = \frac{P_2}{P_1}$

Substituting values:

$\frac{V_1}{V_2} = \frac{67 \times 10^5}{1.0 \times 10^5}$

Calculating yields:

$\frac{V_1}{V_2} = 67$

Diesel engines require a higher compression ratio to achieve the high temperatures necessary for fuel ignition. In contrast, petrol engines rely on a spark to ignite the air-fuel mixture, which allows them to operate effectively at lower pressures and temperatures.

In drawing the p-V diagram, ensure that the curve closely resembles that seen in ideal cycles, but includes features such as flatter regions near the points where the work is done, to account for the time taken for valve operations.

(X placed on the diagram near the end of compression stroke.)

1. The ideal cycle assumes constant volume processes, while the indicator diagram shows varying pressure and volume due to losses and inefficiencies.

2. The ideal cycle represents an instantaneous process, whereas the indicator diagram reflects the actual time taken due to mechanical constraints such as valve timing and engine inertia.