With regard to engine performance, I have been led to believe that low air pressure, such as that experienced at high altitudes (for example, Johannesburg) and high air temperatures negatively affect engine performance.

Therefore, vehicles generally perform better at the coast than Johannesburg under similar ambient temperatures owing to higher ambient pressure at sea level. My question is: does high humidity in Durban during summer (sometimes in excess of 80%) also play a part in a vehicle’s engine performance as opposed to the drier conditions experienced in Johannesburg? CHRISTOPHER CHANDER, Durban

Answer: Firstly, the amount of moisture that can be suspended in air is highly influenced by the air temperature (the higher the ambient temperature, the more moisture can be absorbed). Still, the amount of water in the suspended state (gas form) compared with the mass of dry air (specific humidity) is very small. As an example, air at 25 degrees Celsius and
100 kPa at 100% humidity can hold only 0,02 kg of water suspended for every 1 kg of dry air. Therefore, the impact on performance is tiny. The increase in humidity does reduce the density of air slightly (less than 2% for the previous example when comparing dry air with 100% humidity), which in turn lowers the performance of the engine by a similar fraction. However, this is negligible when compared with the air-pressure difference between Johannesburg and Durban (standardised pressures are 83 kPa in Johannesburg and 101 kPa at sea level), which results in an estimated power loss at altitude of about 17% in a naturally aspirated engine. The performance of turbocharged engines is much less affected because the boost-control strategy attempts to achieve similar set points at altitude than at the coast, bar any turbine-speed limitation.