Figure 4 shows some of the equipment used to investigate the thermal and mechanical efficiencies of a single-cylinder four-stroke petrol engine - AQA - A-Level Physics - Question 3 - 2020 - Paper 5

The indicated power can be calculated using a pressure measurement taken from the sensors above the piston. The formula for indicated power (IP) is given by:

IP = rac{P_m imes V_{s}}{60}

where:

• $P_m$ is the mean pressure inside the cylinder,
• $V_{s}$ is the swept volume of the piston.

Brake power (BP) can be determined by using the dynamometer to measure the torque it transmits to the output shaft. The formula for brake power is given by:

u}{60} $$ where: - $\tau$ is the torque in Newton-meters, - $\nu$ is the rotational speed in revolutions per minute.

Thermal efficiency can be calculated by comparing the useful output power to the input power. The formula for thermal efficiency ($\eta_{th}$) is:

$\eta_{th} = \frac{BP}{P_{in}}$

This gives us the percentage of input power that is converted into useful work.

Mechanical efficiency is calculated by comparing brake power to indicated power. The formula is:

$\eta_{mech} = \frac{BP}{IP}$

This indicates how effectively the engine converts indicated power (from combustion) into brake power (useful output).