gas mileage - questions that have bothered me lately

[260ZRED]

Some random stuff that I need to know from you guys! Please explain!

 

1)When it rains is it true you get worse gas mileage? If so, why?

 

2)Is it true that simply turning the LOWEST rpm possible while cruising will give you the best mileage? This seems to make sense as less RPM==>Less Air==>Less fuel?

 

3)How does humidity affect engine power output (and gas mileage)?

 

Thx,

Igor

[AK-Z]

I don't know if any of my explainations are true but this is my take on it.

 

1. From rain I see added weight and higher humidity.

 

2. This wouldbe true from my experience.

 

3. More humid air is less capabile to compress due to the added moisture. Also the added water vapor carries heat more easily than air and expands more than air when heated.

[EMWHYR0HEN]

"1)When it rains is it true you get worse gas mileage? If so, why?"

 

I would say Yes, your mile per gallon would decrease because there is slightly more resistance on all four tires. However, I don't think the rain has a substantial effect on MPG, unless that is your driving in a 1" puddle of rain.

 

"3)How does humidity affect engine power output (and gas mileage)?"

 

Humidity means that there is a tiny water molecules in the air. When driving in humidity, air, along with vapors, is drawn into the cylinders.

Water is denser than oxygen which means that there is less potential to make power because there is slightly less oxygen and the water has taken up more of the "space" in the cylinders.

 

Hope that makes sence, or correct in that matter!

[AK-Z]

thanx for clarifying my point on humidity. I can understand it in my head, but when I try to explain it, it all goes blah. I'm more of an example type of person than verbal.

[patzky1]

Water takes up oxygen's space a little more when it is humid. This means you get less oxygen in the cylinders per given amount of work to pump air in and out. Remember, an engine is just an air pump... Wouldn't the effect be kinda like having a slightly clogged air filter?

 

Also, I'm assuming modern fuel injected engines suffer less because they can adjust the mixture correctly, instead of a carburetor just seeing volume of air and not correcting for the water's presence.

 

Definitely more resistance moving the tires through water.

 

 

Pat

[AK-Z]

Also, I'm assuming modern fuel injected engines suffer less because they can adjust the mixture correctly, instead of a carburetor just seeing volume of air and not correcting for the water's presence.

 

Not necessarily. Injection and carburation are just means to atomize fuel into the air to enter the engine. Of course injustion has alot more sensors to monitor other reading to calculate timing and duration. EFI systems that I am I have seen do not have barometer sensors, which monitors humidity.

[EMWHYR0HEN]

Water takes up oxygen's space a little more when it is humid. This means you get less oxygen in the cylinders per given amount of work to pump air in and out. Remember' date=' an engine is just an air pump... Wouldn't the effect be kinda like having a slightly clogged air filter?

 

Also, I'm assuming modern fuel injected engines suffer less because they can adjust the mixture correctly, instead of a carburetor just seeing volume of air and not correcting for the water's presence.

 

 

 

 

Pat[/quote']

 

The effect of humidity on an engine isn't so much like a clogged filter, that's just an intake restriction. The way I see it it's more like running at a higher elevation because the air has a lower density. Just like moistior in the air, the water is more dense which gives the air a lower density.

 

No matter what, air with a low density dramatically effects an engines output ( which was 260ZRED' s orignial question).

 

As patzk1 stated, computer controlled engines do suffer less in humid air because the computer can compensate for the poor air quality and reduce the amount of fuel going in, opposed to a carbed engine that doesnt really have the ability to change a/f ratios without poping the hood.

 

give it a try, drive a carbed and efi car up passed 4000Ft. of elevation and youl see that the carbed car will run super rich and probably half the engine output, whereas the efi car will feel just a tad sluggish.

 

I remember watching the speed channel and seeing some pikes peak rally action and there was a car that was carbed and had a lever to lean out the a/f ratio at different elevations. Thats hardcore.

[Phantom]

There really is no easy answer to your questions.

Lower RPM does not always equal better fuel economy - it depends on where your engine is on it's torque curve. You can get slow enough to actually drop into a "weak" torque area and your fuel consumption will go up. Generally slower is better but you can go too far.

Water is H2O, Air is O2 and N2 primarily. Nitrogen is significantly heavier than hydrogen and is also heavier than oxygen. This means that vaporized water is less dense and delivers less oxygen than the equivalent volume of air - power goes down. HOWEVER - generally when it rains the air temperature drops - sometimes a lot - and the colder air is enough denser that even with the added moisture the power goes up.

[cygnusx1]

Yes, humid air IS less dense than dry air. That question tricked me in college and 'll never forget.

[AK-Z]

There really is no easy answer to your questions.

Lower RPM does not always equal better fuel economy - it depends on where your engine is on it's torque curve. You can get slow enough to actually drop into a "weak" torque area and your fuel consumption will go up. Generally slower is better but you can go too far.

Water is H2O' date=' Air is O2 and N2 primarily. Nitrogen is significantly heavier than hydrogen and is also heavier than oxygen. This means that vaporized water is less dense and delivers less oxygen than the equivalent volume of air - power goes down. HOWEVER - generally when it rains the air temperature drops - sometimes a lot - and the colder air is enough denser that even with the added moisture the power goes up.[/quote']

 

To add to your explanation of lower RPMs. It also matters on the load on the engine. Of course you are going to use more gas starting from a stop than at cruising, comparing rpms in both situations in the same gears.