Horse wrote:How much difference does ambient weather condition make to engine performance?
e.g. Hot-Dry, Cool-Damp, High-Low air pressure, etc.
Cold air is more dense and thus contains more oxygen molecules, this allows the engine to burn more efficiently and generate a little more power. This comes at the cost of slightly more fuel consumption as a little more fuel needs to be injected to maintain the optimism fuel/air mixture.
Air pressure does also effect engines (for the same reason - it affects the number of oxygen molecules in a given volume of air), it is well known that engines generate more power at sea level than they do at high altitude, and this changes in air pressure will affect performance. Of course air pressure changes due to weather are generally small compared to those due to altitude and as with temperature the differences are minute.
In road driving an attentive driver will notice the difference in fuel consumption well before they notice any difference in performance.
Moisture does, perhaps surprisingly, impact power and consumption too. It does this in three ways.
Firstly water has much more mass than air, and thus damp air will have far fewer oxygen molecules than dry air (which reduces power and consumption).
Secondly water is incompressible so as it is heated in the combustion chamber the combustion pressure in the cylinders is increased which increases power without increasing fuel consumption. Most drivers will respond to this by using a slightly lighter foot leading to a reduction in consumption.
Finally in turbocharged engines the wet air will react to the intercooler by reducing in pressure to a greater extent (which sucks in greater amounts of air) than dryer air leading to an increase in power and consumption.
Overall the combined effect is that small amounts of moisture will increase power, especially in turbocharged engines. It has long been noticed in racing circles that lap times in the morning (when it is more likely to be cold and damp) are often better than in the afternoon.
Larger amounts, in contrast will have the opposite effect as the first factor (water molecules displacing oxygen molecules) comes to dominate the others, and of course if the water levels get too high it will cause
hydrolock and destroy the engine.