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

To calculate the compression ratio, we use the formula for adiabatic processes:

$\frac{P_1 V_1^\beta}{P_2 V_2^\beta} = C$

Given that $\beta = \frac{1}{\text{γ}}$:

Let P_1 = 1.0 × 10^5 Pa and P_2 = 67 × 10^5 Pa, We can rearrange the above equation to find:

defining $\beta = \frac{1}{\text{γ}} = \frac{1}{1.4}$, Using the ideal gas law and the provided variables, we find V_1/V_2 = 20/1.529.

Diesel engines require a higher compression ratio to achieve a temperature high enough to ignite fuel without a spark plug. Unlike petrol engines, where a mixture is ignited by a spark, diesel engines compress the air to higher pressures and temperatures to facilitate ignition, offering greater efficiency in fuel combustion.

In the four-stroke diesel engine diagram, the curve should display two parallel lines on one side representing the compression and expansion strokes, reflecting the actual behavior of the diesel cycle.

Place an X at the appropriate point in the shaded area in Figure 5 to indicate where fuel injection occurs, noting that it takes place at the end of the compression stroke before the expansion stroke begins.

1. The ideal cycle assumes constant volume processes, whereas the real engine indicates variable volume behavior due to time delays and valve openings.
2. The ideal cycle shows no losses, while the indicator diagram accounts for actual thermal losses and inefficiencies in the engine process.