ADUK Archive

How much difference does ambient weather condition make to engine performance?

e.g. Hot-Dry, Cool-Damp, High-Low air pressure, etc.

presumably quantity / pressure of air, and temperature through the intake are the big ones - and the effect will be partly moderated by the ecu and car's map I know that when I had the RR tuned and altered at JE engineering a big part of the gain was that they ould chip to remap the engine to sit comfortably within UK weather parameters as I wasn't planning on driving through the Sahara / Arctic - however it did make the car more susceptible to issues in extremely hot weather...

Alasdair

Cold weather has several effects on the car: The engine intakes cooler air which is more dense and typically less humid, thereby increasing power, particularly for turbocharged cars. Lower friction on cold or damp roads means less rolling resistance to the drivetrain and colder, slightly stiffer dampers might contribute to that.

There are negative influences, though, including a longer time for the engine to heat up to an efficient temperature, thicker fluids (lubricants, coolant) that pumps powered by the engine have to move around and in a deep water film or through wind or a drizzle, rolling resistance might increase. Also, wipers and electric demisters require quite a lot of electricity from the alternator.

In my case, the advantages usually overweigh the disadvantages, but the Israeli summer is surprisingly hot and humid and other than the effect it has on the engine it usually requires operating the air conditioning which takes up a lot of power.

I don't know if this is correct but I was told years ago that an engine will run better on a foggy day due to the increased water/moisture in the air - Not sure what you can do with any increased performance if the visibility is poor

...this may be borne out by the fact that water injection can be used on engines but I'm unsure of it's applications/benefits.

TR4ffic wrote:Not sure what you can do with any increased performance if the visibility is poor

crash faster?!

Alasdair

TR4ffic wrote:I don't know if this is correct but I was told years ago that an engine will run better on a foggy day due to the increased water/moisture in the air - Not sure what you can do with any increased performance if the visibility is poor

...this may be borne out by the fact that water injection can be used on engines but I'm unsure of it's applications/benefits.

I don't know either...but a while back a motorcycling friend of mine said that the extra water passing through the carburettor could be a problem. Something to do with a Venturi effect (maybe on a cold day/icing?) I can't quite remember...

Answers on a postcard...

Ian

Carb icing used to be a problem, particularly on Kawasakis IIRC.

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.

I have noticed that my direct injection, NA petrol behaves differently depending on the weather. Cold, crisp days seem to improve matters.