jmdhuse

As the temperature has been getting higher in Phoenix (112F the last couple of days), I'm wondering what effect this has on the performance of my vehicle, particularaly when I compare it to the winter time when the temps might be in the high 20's or low 30's...

Certainly hot air is less dense, which must affect how much fuel charge gets into the cylinders. Also, I'm thinking there must be some thermodynamic law that relates efficiency of a heat engine with the difference between the temperature of the air in and exahust out.

Anyone with a technical mind out there wonder about the same stuff?

Cheers, Jon.

Member545

I'm sure Znod will be all over this one.

tachyon

Almost. That would be the Second Law of Thermodynamics.
The efficiency of an internal combustion engine can be expressed in terms of its Carnot cycle temperature ratio (T2-T1)/T2, where T2 is the temperature at which combustion occurs, and T1 is the temperature of the exhaust gases.
We are dealing with some high temperatures here (1,000+ degrees F at combustion), and the ambient air temps from 30 degrees F to 120F don't come into play much here.
I studied thermodynamics as a mechanical engineering student, so I think I have this correct, but I'm relying on some severly damaged brain cells from 25 years ago.
Any other nerds please jump in here.

odabo

I think I damaged by brain cells while studying that stuff! But yeah that's correct. I think the ambient temperature will have more of an effect on air density and hence the fuel charge than the operating temperature of the engine. If you have ever watched a drag race you'll notice the strip times generally get faster after sundown. On the other hand the pavement causes the tires to heat up and become stickier during the day which also has an affect on performance. Normally however street tires aren't that soft so I don't believe it makes that much difference.

BetterMakeWay

Good point. Only slicks and a very good pavement can really make a difference especially when the tarmac is quite hot. On the other hand indeed the hoter the air the less power (i think).

znod

There are lots of ways to go at these things. But, an easy way is to think in terms of density altitude (DA). DA can be calculated various ways. But, given what has been said a good way to think of DA is as a function of air density (D).

So, DA = 44.3308-(42.265*(D raised to the 0.234969 power))
.....DA = Densisty altitude in kilometers (km)
.....D = Density in kilograms / meters cubed (kg / m3)

In the US, we would tend to think of DA in terms of feet. So, DA in feet = DA in km(100/0.03048) since (100' = 0.03048 km). DA also can be calculated, for example, as a function of altitude (A), temperature (T), altimeter setting (AS), and dewpoint (DP). AS, which is the barometric pressure reading that, for example, pilots use, is the reading that one's barometer would give. So, using such values, for example, one can pay a calculator-use subscription and go to SMOKEmUp.com and use a calculator to calculate DA using A, T, AS, and DP. Then, one can use DA and another caculator to convert one's actual 1/4 mile results to what they would have been at zero altitude under standard weather conditions. Another calculator then can be used to convert these results to any desired A (or really DA). These are the procedures we have been using in reporting our adjusted for weather and altitude to 500' 1/4 mile results on the straight-line performance thread.

Once these adjustments are made, then it is easy to see the effects of weather and altitude (neither of which can be ignored for comparative purposes). So, for example, check out my adjusted and unadjusted results, in comparison with grogan545i's similar results, here.

I have an appointment in a short while, but I'll post more later.

znod

Yep, I'll be back soon.

madoon

well...you guys may be talking about the engine performace in these tempratures...ill tell you what happens where im from(40+ degrees for the past week)...

first...you say the air tempreature doesnt really effect well it does...dependent on where you are, the car comes with diffrent specs...here the thermostat of the engine is designed to start cooling the car at something like 90 or 92 degrees C...so when the engine temperature goes past that point the car start its cooling procedure...i think the car starts pumping more water in the radiator although im not sure...but what i am sure of is that the car starts using its cooling fan...
and just like when using the AC the fan is electrical equipment that takes some power from the engine...once you reach that point...that means the car is really feeling hot...there is quite a BIG performance slump....
im sure that also at that point the computer start restricting engine power and performance in order to maintain its life...
also another thing for you guys..when the temperature outside is over 40 Celsius...at noon...when the sun is high up in the sky the asphalt temperature is at high 50s...and believe me...at those point there is siginificatly less grip in the tarmac...

znod

To continue, note that if one looks only at our unadjusted results, then grogan545i's car will appear to be much faster. But, on the other hand, if one looks at our weather and altitude adjusted to 500' results, then it is easy to see that our cars should be closely matched on the same track on the same day. The comparisons between the two sets of results imply correctly that grogan545i has been running under more favorable weather conditions (Pennsylvania versus the Arizona low desert) and a more favorable altitude than I do (400' versus 1600').

znod

And, to continue more, to see the sorts of effects that weather can have, as suggested by jmdhuse, the results from a single car can be used. Below are altitude/weather averages for my last 48 passes.

Weather Averages:
A = Altitude (all passes) = 1600'
T = Temperature Fahrenheit = 52.753
AS = Altimeter setting in inches of mercury (Hg) = 30.024
DP = Dew Point Fahrenheit = 30.865
DA in Feet = 1533'

If my DA for a specific pass is < 1600' (> 1600'), then weather conditions were favorable (unfavorable). As it turns out, I have 26 passes with DA's > 1600' and 22 passes with DA's < 1600'. My average unadjusted 1/4 results for the "aboves," "belows," and overall are:

Averages for "Aboves:"
ET = 13.77
TV = 102.90

Averages for "Belows:"
ET = 13.72
TV = 102.79

Overall
ET = 13.74
TV = 102.85

So, one can see that the weather changes over the conditions that I encountered over the 48 passes have affected my results very little. However, the related weather ranges have not been extreme. Here are my weather ranges for T, AS, and DP:

T = 43 F to 70 F
AS = 29.4 Hg to 30.7 Hg
DP = 17.1 F to 44 F

One would expect to see much more of a difference given the current Phoenix weather conditions. I intend to get around to some really hot weather passes to get a feel for how much difference our extremes can make. I'll post back somewhere if, and when, I get around to the passes.

But, here is an example that may help.

Here is the DA calculation given my average weather conditions.

Density Altitude Calculation

Your results:
Air Temp 52.753 (?F)
Altimeter Setting 30.024 (in)
Dew Point 30.865 (?F)
Altitude 1600 (Feet)
Density Altitude 1532.8 (feet)

From here, I correct my overall average results to zero sea level under "standard" weather conditions.

Correct 1/4 mi. Timeslip to Sea Level

Your results:
Density Altitude 1533 (feet)
Uncorrected ET 13.74 (sec)
Uncorrected MPH 102.85 (mph)
Corrected ET 13.507 (sec)
Corrected MPH 104.656 (mph)

Now, I can eliminate the altitude effect by converting to 1600'.

Correct 1/4 mi. Timeslip to New Density Altitude

Your results:
E.T. 13.507 (sec)
Trap Speed 104.656 (mph)
Measured DA 0 (feet)
Corrected to 1600 (feet) DA
Corrected ET 13.752 (sec)
Corrected Trap Speed 102.755 (mph)

These results clearly imply the minimal effects of weather, on average, for my 48 passes. These effects are minimal because my average DA is so close to my A.

But now, suppose that I made another 48 passes with the average early morning, before dawn temperature being 100 F--which can happen here in August. In addition, I am going to asssume 20% relative humidity (i.e., a dew point of about 50 F) and AS of 30 Hg. Note that these conditions also are highly possible in August in Phoenix. Under these conditions, my DA is:

Density Altitude Calculation

Your results:
Air Temp 100 (?F)
Altimeter Setting 30 (in)
Dew Point 50 (?F)
Altitude 1600 (Feet)
Density Altitude 4594.5 (feet)

Next, I use this DA to convert my performance averages to zero sea level and standard weather conditions and subsequently to 1600' to rid myself of the altitude effect.

Correct 1/4 mi. Timeslip to Sea Level

Your results:
Density Altitude 4595 (feet)
Uncorrected ET 13.74 (sec)
Uncorrected MPH 102.85 (mph)
Corrected ET 12.96
Corrected MPH 109.065

Correct 1/4 mi. Timeslip to New Density Altitude

Your results:
E.T. 12.96 (sec)
Trap Speed 109.065 (mph)
Measured DA 0 (feet)
Corrected to 1600 (feet) DA
Corrected ET 13.195 (sec)
Corrected Trap Speed 107.084 (mph)

Now consider these ratios:

13.752/13.195 and 102.755/107.084,

and multiply as follows to yield:

(13.752/13.195)(13.74) = 14.32 sec.
and
(102.755/107.084)(102.85) = 98.69 mph.

Thus, my prediction is that, under the assumed weather conditions, my average unadjusted ET and TV would be 14.32 and 98.69, respectively. These values clearly show the potential effects of weather as suggested by jmdhuse.

Adjustment of the above predictions for weather and altitude to 500' yields:

Correct 1/4 mi. Timeslip to New Density Altitude

Your results:
E.T. 14.32 (sec)
Trap Speed 98.69 (mph)
Measured DA 1600 (feet)
Corrected to 500(feet) DA
Corrected ET 14.139 (sec)
Corrected Trap Speed 99.979 (mph)

--which doesn't sound quite so bad.