Photo AI
Explain what is meant by an adiabatic change - AQA - A-Level Physics - Question 3 - 2022 - Paper 6
Question 3
Explain what is meant by an adiabatic change. Figure 4 shows the p–V diagram for an ideal diesel engine cycle. In this cycle, air is compressed adiabatically from ... show full transcript
Worked Solution & Example Answer:Explain what is meant by an adiabatic change - AQA - A-Level Physics - Question 3 - 2022 - Paper 6
Step 1
Explain what is meant by an adiabatic change.
Answer
An adiabatic change is a process in which there is no exchange of heat or energy between a system and its surroundings. In this context, any change of state occurs without heat entering or leaving the gas. This means that the internal energy of the system changes only due to work done on or by the system.
Step 2
Calculate the compression ratio V₁/V₂.
Answer
The relationship for adiabatic processes can be described by the equation: P_{1} V_{1}^{eta} = P_{2} V_{2}^{eta} where β is the adiabatic index. Therefore, we can rearrange for V₁/V₂: V_{1}/V_{2} = (P_{2}/P_{1})^{ rac{1}{eta}} Substituting the given values:
= (0.67)^{ rac{1}{1.4}} \ = 0.811$$ Thus, the compression ratio is approximately 1.23.Step 3
Explain why the compression ratio for a diesel engine must be greater than the compression ratio for a petrol engine.
Answer
Diesel engines require a higher compression ratio compared to petrol engines to achieve a sufficient rise in temperature for ignition. In a diesel engine, air is compressed to a point where its temperature becomes high enough for fuel to auto-ignite, which occurs at higher pressure and temperature. In contrast, petrol engines rely on a spark to ignite the air-fuel mixture, allowing for a lower compression ratio. Therefore, higher compression ratios in diesel engines facilitate self-ignition at optimal pressures and temperatures.
Step 4
Draw, on Figure 5, a typical indicator diagram for a real four-stroke diesel engine with the same values of V₁ and V₂.
Answer
The indicator diagram for a real four-stroke diesel engine generally demonstrates lower efficiency and different characteristics compared to the ideal cycle. The diagram would resemble a loop that includes the compression stroke, power stroke, exhaust, and intake phases, along with significant areas representing work done. It is important to note the shapes are rounded rather than sharp due to real engine inefficiencies.
Step 5
Mark with an X on your diagram the point where the injection of fuel starts.
Answer
The fuel injection point can be marked with an 'X' on the diagram at the beginning of the power stroke, where the pressure starts to rise significantly after the compression phase. This indicates the moment when the fuel is injected into the cylinder to initiate combustion.
Step 6
Explain two differences between the ideal cycle and the indicator diagram for real engines.
Answer
-
Shape of the Curve: The ideal cycle typically shows sharp, distinct corners in the pressure-volume diagram, indicative of instantaneous processes. In contrast, real engine diagrams exhibit rounded curves due to the finite time taken for various processes like combustion and exhaust.
-
Work Output: The ideal cycle assumes constant efficiency and no heat losses, meaning all work done in the cycle is fully converted to useful work. Real engines, however, experience heat losses and inefficiencies (friction, heat rejection, etc.), resulting in lower effective work being produced.