MPG and weather?

[IanInCO]

Hi All. Been reading ecomodder on and off for the past year and finally registered.

I have a question.

I've been using the MPGuino on my 1994 Honda Civic LX, 5 speed for over a year. Works great and am able to drive and consistently get around 45 mpg, and upwards of 57 mpg.

So my daily drive to work I get right at 45 MPG as I pull into the lot, at 36 miles.

Today, it's overcast, bit more humid than usual. I got 51 MPG on my drive to work.

My question is why? From experience the past year, this has occured before. And the only commonality I find is the weather. Overcast, etc.

I know every hill, stop light, when to coast, etc. I even hit typically more lights than usual today.

Perhaps some good gas? Just curious to see if I can reproduce these results every day.

Thanks for reading.

[scivicblu83]

first off welcome. As i understand it less dense air such as humid, hot, air contains less oxygen and requires less gas to burn and make exhaust that is acceptable to your ecu. more simply put the ecu is looking for a certain mix of exhaust gasses and will use less gas when there is less oxygen in the incoming air. this being the reason for people building warm air intakes.- not claiming to be right, just sharing what i understand.

[basjoos]

There are two mechanisms involved in improving your mileage when the humidity is high. In humid air, water molecules (which have a much lower mass than nitrogen, oxygen, or carbon dioxide molecules) displace some of the N, O, and CO2 in the atmospheric mix, which reduces the total tonnage of gas molecules that your car is shoving aside as you drive down the road (reduced aero drag).

Secondly, the extra water molecules also reduce the amount of oxygen available in the intake air feeding your engine, so the engine controller reduces the amount of fuel injected to maintain the proper stoch ratio, which (derates) reduces the maximum engine power available. This reduced power output also forces you to maintain wider throttle (for reduced pumping losses) to produce the same power output that you would get with low humidity air.

These two factors; reduced aero drag and reduced engine power is also why you get better mileage in the thinner air at higher elevations, something aircraft pilots have been taking advantage of since the dawn of aviation.

The reduced oxygen level in humid air is also why people with reduced lung capacity sometimes have problems breathing when the humidity is high.

You get your best mileage under hot, humid conditions and at higher altitudes, i.e. lower air densities, and the worst mileage under cold temps, low humidity, and at lower elevations, i.e. higher air densities.

[JRMichler]

On a good cold Wisconsin winter day, the air is about 30% more dense (heavier) than on a hot summer day. I don't have a psychrometric chart handy, so can't give exact numbers.

Air drag is proportional to the density, so is 30% higher on that cold day. You don't notice it because the engine has 30% more power. That extra power needs more fuel.

Result is gas mileage drops in colder weather. The MPG of my truck changes almost exactly 1 MPG per 10 degrees F.

Wind also has a large effect. I normally see trip mileage change by as much as 5 MPG depending in wind strength and direction.

[Diesel_Dave]

You might find these two threads interesting:
http://ecomodder.com/forum/showthrea...her-17819.html
http://ecomodder.com/forum/showthrea...cts-20561.html

I've been keeping weather data in my daily log for about 18 months now. Temperature has been the biggest factor that I've noticed (about 1% difference for every 2 deg F temperature change). So far I haven't been able to notice a strong dependence on humidity. That being said, I haven't gone through all the statistics in a while. Hmm, might be time for me to do some more number crunching...