29 degrees F in Texas--near sea level elevation

[86-tundra]

Originally Posted by strat61caster

I used this one:
Engineering Fundamentals of the Internal Combustion Engine (2nd Edition): Willard W. Pulkrabek: 9780131405707: Amazon.com: Books

So basically went along the route of finding the difference in density between 25 degrees and 75 degrees, made a ton of assumptions and shortcuts: mainly that the engine operates identically at both temperatures (losses and fluid flow effects), whatever heat added to the intake charge from ambient temperature has linear effect on density (definitely not true), and whatever else I'm forgetting. I also looked up a bunch of stuff that I didn't need to illustrate this.

Basically for every kilogram of air in the chamber there's X kgs of fuel which is combusted for energy which the pistons turn to work, since all we're doing is varying the temperature and consequently the density all you really need to do is calculate the two densities and find the ratio between them to get the theoretical power gains by operating in a cooler environment.

So, using this formula from my textbook:

(keeping 101 kPa because I'm lazy)

I calculated the density of 25 degree air to be: 1.307 kg/m^3
&
75 degree air to be: 1.185 kg/m^3

You can go ahead and figure out the volume of air sucked in, throttling losses, volumetric efficiency losses, air fuel ratios, heating values and such but really you're just multiplying the two numbers above by relatively similar values, the ratio of energy created in the chamber will be roughly the same before and after those calculations, which by my hand is about 10.4% for the temperatures I chose.

Wow, 10.4% power gain over a week, I admit I am surprised, I didn't expect it to be so high. Maybe the OP really was seeing a +20hp gain at the crank, my apologies for assuming it was the error of the butt dyno.

I must state that the 10% number I came up with is fake, it's the theoretical maximum you could gain from going -50 degrees on ambient temperature. The truth is the formula isn't linear, 25 vs. 75 is different from 75 to 125 (9.4% BTW) and the ambient air temperature passes through an environment that heats it up and expands the charge. So realistically you aren't seeing ambient temperatures at the chamber and the delta between the two environments won't be as large, warm air coming through a hot environment heats up a little, cold air going through a hot environment will heat up more. This reduces the gains you would expect based on my calculations.

So let's ballpark it, 25 degrees vs 75 degrees. We know the coolant temp runs hot, right around 180 degrees so lets assume the engine bay environment is 150 degrees. So lets guess that the 75 degree ambient air gets heated up by 25 degrees running through restrictive tubing that is heated up by the engine, 100 degrees as it fills the combustion chamber (which also heats up the air as it runs through the head past the valves). Eh, the number isn't real but the idea is.

Same engine operating at the same temperature (roughly speaking), you've got pretty much the same amount of heat being generated (within 10% right?) and a similar environment to cool it off. But since the 25 degree air is cooler it's more accepting of the heat from the engine bay, it's going to increase by more than 25 degrees, let's say it increases by 5 more on top of that, 55 degrees, all of a sudden our gains go from 10.4% to 8.7%, again these numbers aren't real but the principle is. You will never see that 10.4% gain in real life if all you're controlling is ambient temperature.

Feel free to build a spreadsheet based off the above equation and play with the numbers, maybe you'll agree with me when I'm skeptical of 'cold air intakes' when at best they're decreasing your inlet temperature by a couple of degrees while adding extra length of piping that will heat up the air as it passes through.

I'm sure my rambling is full of holes to be filled and attacked, I await contribution.

This is great, really informative stuff. 10% gain is pretty significant and would definitely show up on the butt radar Thanks for this. Now to shell out $160... for a book. I forgot what it's like to take college classes

[humfrz]

Originally Posted by strat61caster

..........

Basically for every kilogram of air in the chamber there's X kgs of fuel which is combusted for energy which the pistons turn to work, since all we're doing is varying the temperature and consequently the density all you really need to do is calculate the two densities and find the ratio between them to get the theoretical power gains by operating in a cooler environment...........

really you're just multiplying the two numbers above by relatively similar values, the ratio of energy created in the chamber will be roughly the same before and after those calculations, which by my hand is about 10.4% for the temperatures I chose.............

Wow, 10.4% power gain over a week,...........

I'm sure my rambling is full of holes to be filled and attacked, I await contribution.

Oh, my, @strat61caster ...... that is an impressive piece of work ....

However, you lost me when you correlated air density to power gains (loss) ...

Help me (and the guy from Texas) out here ......

Hey, me and the mrs, just got back from taking our granddaughters to see the movie "Frozen" ... in 3-D no less ..... so my brain is on a different level right now .....

humfrz

[u/Josh]

Originally Posted by humfrz

Oh, my, @strat61caster ...... that is an impressive piece of work ....

However, you lost me when you correlated air density to power gains (loss) ...

Help me (and the guy from Texas) out here ......

Hey, me and the mrs, just got back from taking our granddaughters to see the movie "Frozen" ... in 3-D no less ..... so my brain is on a different level right now .....

humfrz

Basically your engine is going to suck in the same volume of air no matter what the ambient temperature is (.5 L per cylinder), so if the air it sucks in is more dense, then the engine sucks a greater mass of air into that same volume. This allows more fuel to be injected and more power to be made. If the engine takes in 10% more air (by mass), then you can inject ~10% more fuel and make ~%10 more power.

[u/Josh]

Originally Posted by strat61caster

So basically went along the route of finding the difference in density between 25 degrees and 75 degrees, made a ton of assumptions and shortcuts: mainly that the engine operates identically at both temperatures (losses and fluid flow effects), whatever heat added to the intake charge from ambient temperature has linear effect on density (definitely not true), and whatever else I'm forgetting. I also looked up a bunch of stuff that I didn't need to illustrate this.

Basically for every kilogram of air in the chamber there's X kgs of fuel which is combusted for energy which the pistons turn to work, since all we're doing is varying the temperature and consequently the density all you really need to do is calculate the two densities and find the ratio between them to get the theoretical power gains by operating in a cooler environment.

So, using this formula from my textbook:
(keeping 101 kPa because I'm lazy)

I calculated the density of 25 degree air to be: 1.307 kg/m^3
&
75 degree air to be: 1.185 kg/m^3

You can go ahead and figure out the volume of air sucked in, throttling losses, volumetric efficiency losses, air fuel ratios, heating values and such but really you're just multiplying the two numbers above by relatively similar values, the ratio of energy created in the chamber will be roughly the same before and after those calculations, which by my hand is about 10.4% for the temperatures I chose.

Wow, 10.4% power gain over a week, I admit I am surprised, I didn't expect it to be so high. Maybe the OP really was seeing a +20hp gain at the crank, my apologies for assuming it was the error of the butt dyno.

I must state that the 10% number I came up with is fake, it's the theoretical maximum you could gain from going -50 degrees on ambient temperature. The truth is the formula isn't linear, 25 vs. 75 is different from 75 to 125 (9.4% BTW) and the ambient air temperature passes through an environment that heats it up and expands the charge. So realistically you aren't seeing ambient temperatures at the chamber and the delta between the two environments won't be as large, warm air coming through a hot environment heats up a little, cold air going through a hot environment will heat up more. This reduces the gains you would expect based on my calculations.

So let's ballpark it, 25 degrees vs 75 degrees. We know the coolant temp runs hot, right around 180 degrees so lets assume the engine bay environment is 150 degrees. So lets guess that the 75 degree ambient air gets heated up by 25 degrees running through restrictive tubing that is heated up by the engine, 100 degrees as it fills the combustion chamber (which also heats up the air as it runs through the head past the valves). Eh, the number isn't real but the idea is.

Same engine operating at the same temperature (roughly speaking), you've got pretty much the same amount of heat being generated (within 10% right?) and a similar environment to cool it off. But since the 25 degree air is cooler it's more accepting of the heat from the engine bay, it's going to increase by more than 25 degrees, let's say it increases by 5 more on top of that, 55 degrees, all of a sudden our gains go from 10.4% to 8.7%, again these numbers aren't real but the principle is. You will never see that 10.4% gain in real life if all you're controlling is ambient temperature.

Feel free to build a spreadsheet based off the above equation and play with the numbers, maybe you'll agree with me when I'm skeptical of 'cold air intakes' when at best they're decreasing your inlet temperature by a couple of degrees while adding extra length of piping that will heat up the air as it passes through.

I'm sure my rambling is full of holes to be filled and attacked, I await contribution.

Good post but I have one contention. I don't think 10.4% is the theoretical maximum gain between those two temperatures because I can imagine a situation where you are knock limited at the higher temperature but at the lower temperature you can (or the ecu can) advance timing closer to mean best torque. That means you would make your theoretical 10.4% more power because you are combusting that much more fuel, plus more power from running more aggressive timing.

[JonnyJames]

I've got to agree with the OP. Had a blast driving today. Car feels good in this weather.

[humfrz]

Originally Posted by u/Josh

Basically your engine is going to suck in the same volume of air no matter what the ambient temperature is (.5 L per cylinder), so if the air it sucks in is more dense, then the engine sucks a greater mass of air into that same volume. This allows more fuel to be injected and more power to be made. If the engine takes in 10% more air (by mass), then you can inject ~10% more fuel and make ~%10 more power.

hmmm......thanks for your input, u/Josh .... but, how does the engine "know" to inject more fuel ..... if the mass of the air is denser .. ??

Does the MAF sensor measure density as well as flow .. ??

humfrz

[drewbot]

Toronto: 500 ft above sea level, -4 degrees F = great feeling on the butt dyno

[kmbkk]

I'm in FL, 200 ft above sea level, and it was 25 this morning!

[u/Josh]

Originally Posted by humfrz

hmmm......thanks for your input, u/Josh .... but, how does the engine "know" to inject more fuel ..... if the mass of the air is denser .. ??

Does the MAF sensor measure density as well as flow .. ??

humfrz

MAF stands for mass air flow sensor, meaning the sensor measures the mass of air coming into the engine. Like I said before, more air mass into the engine allows for more fuel to be injected. I guess to answer your question, yes, the sensor responds to increased air density as well as increased flow (or velocity).

In our cars, we have hot wire mass air flow sensors. You can google around or check out this wikipedia link for more info.

http://en.wikipedia.org/wiki/Mass_fl...nsor_.28MAF.29

[dave77]

And I think the point is that the ECU keeps it's a/f ratio pretty constant, so it changes fuel amounts accordingly to the temp/mass air it records. At least that's what I think happens.

Originally Posted by u/Josh

MAF stands for mass air flow sensor, meaning the sensor measures the mass of air coming into the engine. Like I said before, more air mass into the engine allows for more fuel to be injected. I guess to answer your question, yes, the sensor responds to increased air density as well as increased flow (or velocity).

In our cars, we have hot wire mass air flow sensors. You can google around or check out this wikipedia link for more info.

http://en.wikipedia.org/wiki/Mass_fl...nsor_.28MAF.29

[dave77]

My butt dyno is pretty good. That's what she said.

[dave77]

Per the discussion, I have the "intuition" that when colder air comes into the engine via colder ambient air temperature, then this helps the whole "function" of the engine. So yea, I fully understand that "operating temperatures" of fluids and air are basically always the same in whatever environment (once the car gets warmed up). But it seems that the colder air at the intake just makes that part of the whole system a little better. And then it has its effects down the line somehow. And colder weather probably makes it easier on external engine components and cooling, so it's running "easier." But I don't really know what I'm saying.

[humfrz]

Originally Posted by dave77

.........But I don't really know what I'm saying.

Well, that sounds pretty good to me .....

humfrz

[strat61caster]

Originally Posted by humfrz

However, you lost me when you correlated air density to power gains (loss) ...

Help me (and the guy from Texas) out here ......

u/josh pretty much hit the nail on the head, let me know if there are anymore questions.

Originally Posted by u/Josh

I don't think 10.4% is the theoretical maximum gain between those two temperatures

You're right, there are gains I didn't account for, but there are a lot more losses that I didn't. I mentioned that the temperature of the air when it finally fills the chamber is MUCH warmer than ambient which knocks a couple of percent off, also note that when a fluid travels through a tube along with an increase of temperature due to friction there is also a loss of pressure further reducing the airs density (this is actually noted in the picture of the textbook I posted). Both of those facts reduce the % of air and consequently power "gained" over standard conditions.

Toss in the knowledge that the combustion process is not perfect, modern cars run lean for emissions and fuel economy, that stoichiometry is NEVER achieved, we don't know the parameters of the injection system (or at least didn't take them into account) and my little tirade of how fluids at lower temperature will rise in temperature more than fluids that are already warm I would be highly suspicious of any claims of power gains higher than the 10% I came up with.

Edit: At least with an OEM system designed to work across all parameters, you can definitely 'tune' to get better results in cold.

Don't forget that with the variability of ICE engines you can get as much as 5% variation on back to back dyno runs with identical conditions, humans have tried to make this stuff as much of a science as possible but there is a lot we can't control. Yet.