Why does humidity matter? (2020)

My 74 850 has always loved a cool crisp 60 degree day and hated a hot humid day. I can definitely feel a difference in performance in this Summer heat wave we've been having. Why is that?

Simply because the air is less dense. Aircraft fight it all the time. Air less dense makes less lift needing longer runways, less climb performance lower payloads. Both heat and humidity effect density. Heat the bigger player as molecules of air are farther apart, humidity because more water vapor in the air also makes it less dense. Lower atmopheric pressure also plays into it which also tends to be the case in summer.

Engine is making less power, mixture is slightly rich as well due lower air density.

JimNH said:

Simply because the air is less dense. Aircraft fight it all the time. Air less dense makes less lift needing longer runways, less climb performance lower payloads. Both heat and humidity effect density. Heat the bigger player as molecules of air are farther apart, humidity because more water vapor in the air also makes it less dense. Lower atmopheric pressure also plays into it which also tends to be the case in summer.

Engine is making less power, mixture is slightly rich as well due lower air density.

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Yeah. As demonstrated in a Cessna 172 loaded with two big kids departing 7B3 in August. Turns out the engine was in need of refreshing as well.

And why your engine always runs better on a foggy day....and no it's not always foggy in the UK
Can I feel a water injection thread coming up

Also as anyone who strokes a coall/coke fire, cooler air contains more oxygen, making it burn better. Much the same as an internal combustion engine.

concours said:

Yeah. As demonstrated in a Cessna 172 loaded with two big kids departing 7B3 in August. Turns out the engine was in need of refreshing as well.

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Ah! Hampton! Just down the street from me. I keep a 182 at PSM. We stopped flying company planes to Asia in January, Europe in February, and domestically in March. My last flight in a jet was March 10. Been to recurrent since then and did my simulator training and, other than that, flew my Cessna.

olympus said:

And why your engine always runs better on a foggy day....and no it's not always foggy in the UK
Can I feel a water injection thread coming up

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Water injection, wow I thought that was dead and buried. I remember a company used to regularly advertise in the Exchange & Mart extolling it's virtues. Mind you, from what I read about Ethanol it's a free upgrade now.

I used to live in Yerington NV, a tiny town in the middle of nowhere, high desert and huge alfalfa fields. The roads were straight with great visibility and no traffic, good for checking jetting. I could always feel the motor picking up more power as I rode alongside the fields especially when they had the water sprayers on. I also had the advantage of going to the airport on the edge of town to fill up on AV gas anytime I wanted.

cliffa said:

Exchange & Mart !!!

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Wow, you dragged that one up from the depths.....
Before the days of on line auctions this is all we had, and I for one used to love trawling through those off white pages looking for bike bits.
I have just checked and found the company stopped printing back in 2009. But still continues on line

Title of thread asks about humidity affecting performance. Discussion veered toward how high temp reduces performance because of lower density of warm air. I think we can all agree on that one.
But back to the original question. Higher humidity, separate from temperature, also lowers performance. Apparently the water vapour reduces the amount of oxygen available for combustion. Any engineers out there want to wade in on this one??

Stephen Hill

I had read somewhere that some high performance racing engines actually inject water vapor along with fuel to enhance performance.

When I started this thread I was thinking humidity because it's the nasty part of Summer that we're seeing right now. Hot and humid kinda go together here, and the humid aspect is what makes it unbearable sometimes. I know others in the US are experiencing the same thing.

I don't have any recollection of losing performance when riding in the rain, so maybe it's not humidity. I would amend my original question to why does hot matter, and that has been answered.

Its not the oxygen in the air that does the work. Its then Nitrogen that is heated and expands. Some of the oxygen is used up in the process. Cooler temperatures mean more Nitrogen per given volume, so more performance. I understand that Merlin engines etc during WWII had an extra throttle gate that when opened, injected water into the engine. [ if the seal was ever broken on that gate then a completed engine rebuild was required before running it again. ] Water has less volume than gasses but when its heated, it turns to a gas of considerable extra volume, so extra pressure is created to give extra performance. I don't think injecting steam into an engine would have as much effect as adding water.
The only thing riding in the rain might do, is wet the air filter eventually/maybe. Rain effect may be because it has lowered the air temperature. I found out to my horror one time when I left my bike out in the rain. When it did start, it was as rich as buggery. When away from home now days, and there is going to be rain, I lift the seat then place a plastic bag over the air filter to stop it getting wet. Not having that embarrassment again.

Dereck

Water injection, or more properly, ADI - Anti Detonation Injection for reciprocating aircraft engines used a mixture of water and alcohol (to prevent freezing) to cool the fuel air mixture, delaying the onset of detonation and somewhat increasing its density and allowing higher manifold pressures (these are all supercharged engines) without richining the mixture to allow the engine to be run at best power mixture rather than rich of best power. It's a power increasing feature to be used for short periods such as takeoff. Performance charts are labeled with the RPM and manifold pressure and WET or DRY. Water vapor in the atmosphere actually hurts performance slightly as it makes the air less dense but only slightly so - so little that aircraft performance charts don't allow for it. The water is not used to create more volume after being heated . If your bike seems to make a little more power on a foggy morning it's more likely due to being cool rather than any perceived "water injection".

Water injection can also be used on high compression normally aspirated engines but I'm only familiar with aircraft engines that use it.

Early jet engines used an entirely different water injection and also had WET and DRY performance charts but the water was injected to increase the mass accelerated not for any cooling and there's no detonation issue. Any water not used on takeoff would be dumped overboard at the first power reduction because then it was just unnecessary weight.

More on ADI:

I couldn't find my old training manuals but did find this, a more detailed explanation: http://www.enginehistory.org/Accessories/HxFuelSys/FuelSysHx11.shtml

"Water injection is used to lower in-cylinder temperatures and burn the air/fuel mixture more efficiently thus helping avoid detonation."
"The sole function of water injection is avoiding detonation."
.

maylar said:

My 74 850 has always loved a cool crisp 60 degree day and hated a hot humid day. I can definitely feel a difference in performance in this Summer heat wave we've been having. Why is that?

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The temp change is going to affect air density more than the humidity. Which means it will affect Power more.

When we used to do a lot of dyno runs I would try and do them in the early morning, for Max HP
On one day I ran the same bike on the dyno, never taking it off the dyno doing a number of runs over a 24 hour period, no changes were done to the bike during this 24 hour period.
On a 150 HP motor HP varied 5 HP over the 24 hours.

You can play around with this Dyno correction calculator, set the HP for what you think your bike is putting out, and change the humidity.


From past experience, it is hard to feel a 2 HP or less change.

Power comes from burning fuel in the cylinder. The more fuel and oxygen one can cram into an engine's cylinder, the higher the potential power output.

Cooler air is more dense and contains more oxygen molecules per unit volume than warmer air. If there is a greater quantity of oxygen molecules present in the cylinder than would be otherwise, then there is an increased probability that a greater quantity of fuel will be burned, resulting in more power. Conversely, warmer air results in less power.

Injecting water, as an aerosol, cools the air, making it more dense. This effect is exactly the same as that of "swamp box" air coolers. The principle is liquid water absorbs heat in changing from liquid to vapour. The heat comes from the surrounding air, thus cooling the air.

High humidity air contains water vapour, but as no liquid, per se, is vaporized, there is no cooling effect. The water vapour "crowds out" oxygen, and less power results.

Water vapour in the cylinder suppresses detonation. The theory of how it does so, is beyond this discussion.

The above does not differ from what JimNH posted in reply #15, but is intended to explain the physical principles.

Slick

seattle##gs said:

I used to live in Yerington NV, a tiny town in the middle of nowhere, high desert and huge alfalfa fields. The roads were straight with great visibility and no traffic, good for checking jetting. I could always feel the motor picking up more power as I rode alongside the fields especially when they had the water sprayers on. I also had the advantage of going to the airport on the edge of town to fill up on AV gas anytime I wanted.

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Ha, maybe the oxygen coming from the alfalfa transpiration.