How Humid Air Affects Automotive Engine Performance

As relative humidity changes with a constant temperature, the density of air alters slightly but significantly. For instance, at 32°C, 40% relative humidity (RH) results in a density of 1.148532 kg/kg, while 75% RH at the same temperature gives a density of 1.141344 kg/kg. This change in density impacts the engine's performance due to the air intake volumes.

Reduction in Oxygen Molecules Per Liter

Selecting the intake air for an engine signifies a constant volume system. Therefore, when the air is more dense, the engine receives a greater mass of air under lower humidity conditions. However, as humidity increases under similar temperature conditions, the proportion of oxygen molecules in the air per square foot decreases. Higher humidity typically means fewer oxygen molecules available for combustion, negatively affecting engine performance.

Exception with Water Injection Systems

An exception to this rule exists in applications like knock-limited turbocharged engines. Here, water injection systems can help cool the incoming air and cylinders through evaporative cooling, preventing knock, and enabling more boost or ignition timing advance. Lower humidity increases the efficiency of the water vapor evaporation, intensifying the cooling effect. In such contexts, high humidity can actually enhance performance.

Smoothened Combustion and Improved Engine Performance

High humidity can smooth the combustion process, making the engine run more smoothly. This improvement is perceptible, leading to a more stable and efficient engine operation.

Impact on Modern Engine Performance

Modern engines are less affected by humidity compared to older ones, especially those with carburetors. These systems tended to run richer at higher altitudes, which provided more fuel to compensate for the lower oxygen levels. In contrast, modern engines with precise fuel management systems can adjust to humidity changes. Even in race conditions, the effect may be negligible unless the temperature and humidity are extremely high.

Effect on Maximum Horsepower

The reduction in maximum horsepower is proportional to the partial pressure of water vapor relative to atmospheric pressure. Modern engine controls must handle fuel management very closely to maintain stoichiometric ratios. Water vapor decreases the oxygen charge in each intake stroke, necessitating a proportional reduction in fuel. This is analogous to operating the engine at a higher altitude, where less air/fuel charge equates to less peak power. Even though this is noticeable, adjustments can be made to minimize the impact, as long as the engine is not operating at full throttle.