Figure 4 shows the results of a test on an internal combustion engine which uses purified biogas - AQA - A-Level Physics - Question 3 - 2021 - Paper 6

Thermal efficiency can be calculated as follows:

$ext{Thermal Efficiency} = \frac{ ext{Overall Efficiency}}{\text{Calorific Value of Fuel}} \times 100$

Using the calorific value of biogas (32.3 x 10^6 J m³) and assuming ideal conditions, the calculation leads to:

$ext{Thermal Efficiency} = \frac{80}{32.3} \approx 2.47\%$

Mechanical efficiency is determined by assessing the ratio of brake power to indicated power:

$ext{Mechanical Efficiency} = \frac{ ext{Brake Power}}{ ext{Indicated Power}} \times 100$

Using the same indicated power:

$ext{Mechanical Efficiency} = \frac{40 kW}{50 kW} \times 100 = 80\%$

Understanding overall, thermal, and mechanical efficiencies is crucial for engineers because:

• It allows them to optimize engine performance and minimize fuel consumption.
• Engineers can make design adjustments based on efficiency data, enhancing the sustainability of the engine systems.
• Knowledge of these efficiencies can guide maintenance practices to improve the longevity and reliability of the engine.

Running the engine at speeds above 7000 rev min⁻¹ may lead to:

• A significant reduction in brake power as shown in Figure 4, where power begins to decline after the maximum speed.
• Increased fuel consumption without proportional power output, leading to inefficiencies.
• Potential wear and failure of engine components due to excessive operational stress, risking safety and reliability.