Physics Puzzler

[johnc]

I was told today that a black race car is faster than a white race car because of the second law of thermodynamics. I thought about it for a second, realized it would take more than a second to understand, so I went back to welding parts.

 

Can someone explain this to me or do I have to find a physics book?

[Guest gprix1]

If there is some truth to this, I would imagine it has a very small effect in the grand scheme of things. I would compare it to saying "technically, a car is faster when you take 1 pound off of it". Well, technically that would be true but measuring it would be difficult.

[Guest dankinzle]

I think that someone has been reading physics books way too late at night...Although the black car would probably have higher energy because it would absorb more sunlight, I seriously doubt any significant amount of that energy would transfer to forward impetus in any way. The black car would just radiate more heat than the white.

[Guest Aaron]

A black car would absorb more light energy than the white car, however, virtually all of that energy would be converted to heat (some of the energy would probably work to break down the paint), not to any kind of forward motion. Essentially, the paint molecules in the black car would be moving faster than the paint molecules in the white car.

[Pop N Wood]

Alright, I'll delay writing a trip report a little longer.

 

Black objects radiate heat better than white objects. This means black objects should theoretically run cooler than light colored objects. Well known in racing. I have seen pictures of people who have painted their Z engines flat black to better dissipate heat.

 

Now for the 2nd law of thermodynamics business. Looking in a text book, the second law of thermodynamics goes on and on about "entropy" and "reversible processes". Simplifying things a bit, the 2nd law essentially says that it is impossible to build a 100% efficient heat engine. If you read a little further, they will talk about Carnot cycles etc. Details are unimportant but Carnot developed equations that let you determine the maximum efficiency of a heat engine. Weeding out even more details, the maximum efficiency of any engine is dependent upon the temperature difference across the engine.

 

I use to have to worry about this sort of thing when I was in the Navy working on a steam turbine. The larger the temperature change in the steam as it passed through the turbine the greater the efficiency of the turbine. Efficiency is good because that means you are converting more of the energy of the steam into mechanical energy.

 

So back to the statement. A car engine is a heat engine (a very inefficient one by the way). The cooler you can make the exhaust run, the greater the theoretical efficiency of the engine and hence greater HP.

 

So maybe what your friend was trying to say is this. If you paint a car black, it should run cooler. A colder car means a lower exhaust temperature, hence by the second law of thermodynamics you will generate more power.

 

A pretty nerdy argument if you ask me.

 

Two things to add. Icing down an intake or adding an intercooler is a different principle. These things cool down the intake charge allowing you to pack more fuel and air into the cylinders. This means you have more energy to begin with and is different from "efficiency".

 

Second thing. Just like cooling the exhaust increases the amount of power that can be extracted, raising the operating temperature inside the combustion chamber will realize even larger gains. That is why there was so much research in ceramic engines a few years ago. Ceramic engines can run with the heads glowing red hot. This gives (in theory) better gas mileage and lower emissions. I don't hink they were ever intended for performance gains though.

[jeromio]

How about this: Air flowing over the bodywork creates friction which creates heat. Black absorbs heat, white reflects it, right? So the black car is sort of going with the flow, while the white car is fighting it. Or, the black car is better able to dissipate that frictional heat than the white car.

 

I dunno - I did better in Physical Chemistry than Physics and I recall almost nothing of either at this late stage....

 

How about this, much more compelling argument: A multi colored car with lots of big, garrish sponsorship stickers will go much faster than any kind of plain white or black car since the owner will likely have received lots of dollars and put them towards expensive go-fast parts.

[Kevin Shasteen]

Originally posted by Jim Powers:

.., the 2nd law essentially says that it is impossible to build a 100% efficient heat engine..,

Impossible you say? Dont forget about the 1/2 dozen or so Super Carburetors that have popped up in the last 80 years or so.

 

Since Jim didnt want to give a Book Report; I guess I will.

 

Research will indicate that when the proper pressurizing of the fuel lines and the appropriate catalyst prior to fuel entering the engine is allowed to occur; that the thermo air pump we call an engine can be much more efficient; possibly approaching the 100% ratio.

 

When reading about the 80 years of Super Carb technology it appears that when one of these combinations were correctly "Stumbled Upon" mileage was in the neighborhood of 100, 150 and up to 200mpg. Also, when these technicians did happen to get the combination correct-they discovered that the exhaust was [COLD] to the touch; which implies "Close to Perfect Combustion", AKA: No Wasted Energy whereby nothing but air & "Cold" moisture was exiting the exhaust pipe(s).

 

It appears that the catalyst that was evident at the time was not actually understood. "ACCORDING TO THE WRITERS" on such illusive issues, the research I've read indicates that the Catalyst that was occurring was actually discovered in the mid to late 70's to early 80's and was termed "Thermo Catalitic Cracking" or T.C.C.

 

T.C.C theorizes that the Gas Molecule, when Cracked due to some Catalyst, will split the gas molecule into smaller molecules: namely Hydrogen. And the exhuast actually becomes moisture(?) and a little air, go figure. The exhaust is "Quiet" an almost unheard.

 

Futher reading on the T.C.C subject will lead the reader in understanding that the additives in the Fuel and Oil causes the "Catalyst" to be Neutralized, (Hmmm, sounds like a right wing conspiracy to me? ) even when using the appropriate pre-catalyst "Gimic" for the fuel before entering the Intake! In the earlier years 30's, 40's ect; the Catalyst that allowed these super carbs to function was the "Lead" that already existed in the make-up of the engine block.

 

It has been "suggested" by many that our Tank Brigade in WWII actually had super carb's on them & this alone is what helped us catch the German Tank Brigades, AKA: We didnt require "Refueling" as often as the Germans did. (Provable? The powers that be dont talk to me so I cant comment on its validity...., just passing along what I've read)

 

Anyway, this already existing lead in the cylinder block would work properly up until the lead was saturated as a catalyst; meaning, the catalyst would no longer occur and the supercarb was no longer super therefor returning the excessive mileage back gains back to a normal mileage. It was the T.C.C that was not understood in the earlier years.

 

If you research the issue you will also note that the Gas companies began adding additives to our fuel and oil somewhere in or about the 30's. As stated earlier, it is these additives that allegedly impeded any "Real" gains due to T.C.C. So to repeat the super efficiency mileage carbs one would require "Drip Gas" and "Pure Processed Oil" without any of the additives.

 

Well, there it is; you now have a basic book report on the Super Carb's.

 

Fact or Fiction, Real or Imagined, it definately triggers the imagination: What if I too could get 200mpg? Just think; with a supercarb our monster performance engines would be considered a dog if they only got 100mpg/Blasted Performance Engines!

 

Sorry if I got off topic; Back to the Black or White Car thing. I too doubt any gains from a black car would or could be positive in nature when factored in the "Unconfortability" of the driver in a black car verses a white car; especially if you have no A/C which many of us dont.

 

Also; the alledged minute inefficiency of a white car could easily be overcome with a little extra "Torque or HP" and/or "Gearing". Just my thoughts.

 

Kevin,

(Yea,Still an Inliner)

[Pop N Wood]

Don't want to rain on anyones parade, but the myth of a carburetor that gives a Cadillac 200 MPG have been around for years. They also get debunked every time around.

 

But even if a super carb did exist that completely consumed the gas and turned it into heat, none of that changes what I said above. You still have to turn that heat into HP. This is where the Carnot cycle and the 2nd law of thermodynamics come into play. They give the maximum theoretical efficiency. You can't do better than that, no matter what you do to the engine.

 

I don't remember numbers, but the Carnot efficiency of a gasoline engine is very poor. Probably less that 50%.

 

Now be careful with efficiency numbers. Since the Carnot efficiency is the maximum efficiency that can ever be obtained, a lot of people will consider this the "100%" number and state an efficiency that is a percentage of this max theoretical number. Thus you may approach "near 100% Carnot efficiency". But keep in mind the actual percentage of energy released when the gas is burned that gets turned into mechanical HP is very small.

 

To put numbers to it. Steam plants efficiencies typically run in the 33% to 40% range. Thus 60% of the energy released in burning coal or nuclear fission goes out into the river as waste heat. An automobile engine converts 6% or 7% of the heat from the gas into usable HP. The overwhelming majority of the waste heat is expended through the exhaust. That is why turbo charging is so lucrative. You recapture some of this otherwise wasted energy and put it to good use.

[Guest Anonymous]

Well being as I'm far from a brain surgeon in this category, let me just point out that while alot of color schemes are due to sponsorship, if this phenomenon carried any sort of real world measurable difference, then every race car you see out there would be black. I mean, if I'm in a NHRA series of races and I KNOW black means ET's, what color do I paint the car?

 

I think the measurable difference would be about as much or less than 'magnetically charged' fuel from one of those gizmo's, or the many cut up tin cans that cause your intake flow to be some sort of vortech tornado effect that supercharges your engine for free.

 

Bottom line, put down that big mac and fries for a while and you'll see real gains in ET, not hypothetical theoriums that are worth .000000001 et if you were in zero gravity... As they say, there really isn't such a thing as a free lunch.

 

Regards,

 

Lone

[Guest Anonymous]

It seems like the attack of the nerds

After looking at my Physics book, I took my College class 2 semesters ago. I see that Carnot effiecentcy is only the "IDEAL" effiecentcy and the normal effeicentcy is significantly lower.

 

Idont really remember that long ago but I do somewhat remember that car engines are like somewhere around 20% effeicent, meaning that 80% of energy is just waste. I really think that automobile manufactures NEED to create a relpacement or redo the internal combustion engine.

 

Just think about using some of the 80% of the waste. It means ALOT more power with MUCH better gas mileage.

 

The thing is to get the car manufactures to spend ALOT of money to redo a "good" engine, how long has the 350 been around, that engine was designed in the sixties. And what about all the jobs that would be lost if suddenly we did not need half as much oil we use now. It gets to be a complicated matter.

Mark

[Guest bang847]

This is turning into conspiracy theory...

from what I heard auto engines are about that 20% mark in mechanical efficiency...

but the efficency relative to the nessacary work is only about 1%... of that 20% we can harness only 5% of that is going to propel passengers.. that is SICK huh???

 

Well this wont make any of us feel any better but wanst it also said that

Einstein's brain operated at 10% of capacity? most of us are at 3-4%...

Im probably a little lower with all this smog breathing from the DAT...

[DavyZ]

Long Ta!!! Hey, Bud! Great to see you back on the board! Do me a favor and email me with a current email address or phone number. Thanks!

 

But back to the answers to the question:

 

So guys, is devil's food cake faster in your system than angel's food cake?

 

Tastes great or less filling?

 

Half full or half empty?

 

Davy

[Pop N Wood]

"The main advantage of a diesel engine is its high thermal efficiency. Diesel engines can achieve thermal efficiencies in excess of 50%. The best gasoline engines are only about 30% to 33% efficient, and then only at wide throttle openings. As a result, diesel engines have better fuel economy than gasoline engines."

 

"Thermal efficiency is defined as the amount of work produced by the engine divided by the amount of chemical energy in the fuel that can be released through combustion. This chemical energy is often referred to as net heating value or heat of combustion of the fuel."

 

"MARINE DIESEL ENGINES

Large marine diesels operate on the same principles as automotive DI diesels, but on a much larger scale. The pistons can be three feet (one meter) in diameter with a six-foot (two-meter) stroke. Because of the high mechanical stress involved with moving the large pistons, they operate at lower speeds, 70-100 rpm. Due to the slow speeds, these engines do not usually employ methods such as swirl to enhance mixing of fuel and air, as they are not necessary. These engines typically have compression ratios of 10:1 to 12:1 and can have thermal efficiencies of up to 55%."

[johnc]

Here's my take on it:

 

The black paint on a car works as a very inefficient heat engine. The sunlight (radiant energy) striking the black surface excites the dark colored molecules and some of the radiant energy is converted into kinetic energy (Brownian motion), thus raising the temperature of the surface of the car. This process is not very efficient so most of the remaining radiant energy is converted into heat due the the First Law of Thermodynamics (Conservation of Energy.) Remember, little of the radiant energy is reflected from the surface of a black car so that energy has to be accounted for in the system.

 

Because of the Second Law of Thermodynamics (Entropy) the output heat flows into the cooler heat sink - the surrounding air. Adding heat to the surrounding air increases the air temperature and Brownian motion of the molecules, which increases the distance between them and reduces the surface pressure in an unrestricted area.

 

Thus, the black car is moving through air that is less dense than the air a white car is moving through. Again, remember that most of the radiant energy is reflected off the surface of a white car.

 

And also remember that this is an academic discussion with no application in the real work...

[pparaska]

John, you crack me up!

[Pop N Wood]

No one told me this was a quiz...

[Guest Perry]

OK here's my take on it.

The black car will soak up more sunlight and have higher surface temps which will heat the air flowing into your ram air system causing a loss in horsepower that I dare you to try and measure.

 

My more realistic take on it: I wouldn't paint it black unless the body was real straight or I put lots of time into the bodywork, If I paint it white it is to mask the dings. The black car has a lot more effort in it or is in better shape to start with, meaning probably the engine work is done with an equally larger amount of care on the black car.

 

Or maybe the black one just looks faster...

Or if it is a black interior maybe it heats air passing through making more thrust like some piston aircraft engine cowlings of WWII.

[Kevin Shasteen]

This explains whay White Cars are more expensive than Black Cars; blasted dealers-they dont let us get away with anything!

 

Kevin,

(Yea,Still an Inliner)

[Brad-ManQ45]

I believe that the hotter air acts like a recent wax-job on a private plane only not to the degree the wax job does - makes the vehicle "slipperier".

 

The hotter air (from black) radiating off of the car forms a small (I don't want to call it a barrier) buffer for airflow - the airstream instead of hitting the solid coating of the car hits the buffer of air which is less dense, rendering the car slipperier...

 

Yet another tongue-in-cheek response....

 

Brad

[denny411]

I`ve also heard that the dimples in a vynle top will also cause a car to be "slippery" similar to the dimples on a golf ball