Another take on the rainy day/mileage improvement theory

[Christ]

Originally Posted by fud2468

But doesn't the extra water vapor in the air reduce the amount of oxygen available for combustion?--meaning a richer mixture?
Ray Mac

Thus the reason you can advance the timing, getting more power with the same amount of fuel.

Yes, the mixture is technically rich, but that doesn't mean you're using more fuel. If you can get better power out of the mixture by advancing the timing, without having a pinging (auto ignition) problem, you'll get better efficiency. (More power from same fuel = better efficiency) That doesn't mean it will affect your MPG, just that you can squeeze out more power without more fuel.

If you lean out your mixture and introduce water injection to the equation, you can safely burn less fuel in your engine. It's a common trick with boost applications, to add water injection in case of detonation at high boost levels, to help cool the mixture, and to take up extra space, essentially making the mixture "richer", which helps to quench auto ignition.

PS Liquid water does not compress. Too much of it will destroy your engine on the compression stroke, using a "hydraulic effect" i.e. It will transfer all the power of your piston's movement into the head, which, in most cases, will hold the force, causing the connecting rod to bend and wretch.

The only engine I've personally seen that stood up to hydro-lock at mid-high engine speeds (off idle, maybe ~2k RPM) was the Wankel Rotary engine. It cracked the apex seal, but would still run after the water was extracted.

Test was performed with donated engines, using a measured amount of water per engine (1.5x the volume in the chamber at relative TDC) (Rotaries don't have TDC, they have "full compression")

[Intrigued]

Originally Posted by fud2468

But doesn't the extra water vapor in the air reduce the amount of oxygen available for combustion?--meaning a richer mixture?
Ray Mac

Check your local meteorlogical blog at your favorite TV station about that, and think about what you're saying. The air can only hold about 4 percent water vapor, and the "relative humidity" is the percentage of that 4 percent. (4 percent water vapor = 100 percent relative humidity) Since the air we breathe only has something like 20 percent oxygen to begin with, changing that by 4 percent doesn't do that much "reducing the amount of oxygen available."

If I remember my tech schooling right, the main difference is that the air charge is denser, which will lead to either more fuel being pulled through the jets on a carb, or the MAF signaling to send more fuel on a computer car, which ends up with the "technically rich" scenario that Christ just mentioned...

...and how your car is set up/designed makes all the difference in whether or not you can use any of that to much advantage, Sooo... you're gonna either have to play with your car, or find a blog on your particular setup to see what others have found out...

[almightybmw]

Originally Posted by Christ

The only engine I've personally seen that stood up to hydro-lock at mid-high engine speeds (off idle, maybe ~2k RPM) was the Wankel Rotary engine. It cracked the apex seal, but would still run after the water was extracted.

Test was performed with donated engines, using a measured amount of water per engine (1.5x the volume in the chamber at relative TDC) (Rotaries don't have TDC, they have "full compression")

I love rotaries. Shame the design behind them doesn't match the engineering, otherwise they'd be much better FE wise. Still fairly impressive power output from such a small displacement.

[Christ]

Originally Posted by almightybmw

I love rotaries. Shame the design behind them doesn't match the engineering, otherwise they'd be much better FE wise. Still fairly impressive power output from such a small displacement.

You have to be really careful using the term "rotary" to describe Wankel engines... yes, they're rotaries, but there are quite a few rotary type engines in use in the world, and it's easily mistakable.

That said, wankels burn oil. LOL. That's why the Renesis engine from Mazda "needs" to use a special type of oil that has less negative effects on combustion. It's a finer oil, and costs (or used to cost) more per quart than the standard "quickie lube" oil change.

When I worked for Sears, we refused any RX-8 that came in for service, and my manager would only accept an RX-7 if I was working that day, since noone else knew anything about them.