Redi3east

i have a stupid question...when ur engine run cooler does that mean you get more power?

babyslapmafro

colder air=more power

2cents

colder air = higher pressure

shrike

Not true. Temperature and pressure are independent, unless we are talking about a fixed number of molecules of air, but we aren't.

For a given pressure, colder air is denser, i.e, has more molecules per unit volume, than warm air. Atmospheric pressure on the Earth is affected some by weather, but does not vary much by temperature. Pressure at any point in the engine's air path is affected primarily by atmospheric prssure, engine speed, and throttle position. Hence, we can take pressure as being fixed for a given engine speed and throttle position.

Temperature does not greatly change the volume of air entering the cylinder at any given rpm and throttle position, so: cooler --> more molecules of air --> more molecules of oxygen --> more fuel you can burn --> more power.

The oft quoted five degrees = 1% hp increase is not far off, as a theoretical matter: five degrees Fahrenheit is fairly close to a one percent change in absolute temperature (meaning measured from absolute zero) in the relevant range, so, to a reasonable approximation, there is about a 1% increase in oxygen per unit volume, which means, in theory, 1% more power. There are some tweaks that need to be done to get it exactly right, but the theory is sound.

Notice that the temperature is that of the air going into the cylinder (or actually, through the most restricted part of the air path, but that will probably be the intake ports), not of the intake or the manifold; I think this difference is the source of much of the skepticism about the rule of thumb. The engine's running cooler may have little effect on the temperature of air entering the ports, but to the extent it does, the effect will be more power.

There are other issues; in particular, the thermodynamic efficiency (the portion of released chemical energy that gets translated into useful mechanical energy) of a piston engine will go up if the combustion chamber walls are warmer, but I am pretty sure this effect is much smaller than the density effect.

The relevant equation, for anyone who cares, is PV = nRT; P is pressure, V is volume, n is the number of moles of gas (a mole is 6.023 x 10^23 molecules), R is the Boltzman constant, about which we care not at all except that it is constant, and T is temperature, measured from absolute zero (which is about -460 Fahrenheit). This equation is reasonably accurate, though not perfect, for air at the temperatures and pressures we are dealing with.

S2k007

I'm a student at uci and i love physics (althought i'm a bio major) so i can respect the fact that your a physics teacher. Good write up (especially in the weight affecting top speed thread).

Infidel

So, why is it that very high charge temps increase the likelihood of detonation? You would think that the hotter the air got, the less oxygen by mass would get into the engine, thereby making you run rich.

How does temperature affect the compressibility of an air/fuel mixture and what, extactly, from a physics perspective, causes gasoline to detonate?

Ranger

Shrike ... I was going to say the same exact thing. No, really.

Axel6

Shrike is correct. Cold air is denser than warm air and therefore per given volume will contain more molecules than the same volume of air at warmer temp.

shrike

I'll hazard a guess you're a premed. I actually have a great job getting people into medical school -- I am approximately full time for the Princeton Review. MCAT physics, verbal, and bio, plus LSAT, GMAT, and GRE. Basically, it means I teach college- and high school-level stuff in physics, biology, math, composition, reading, logic, grammar, and probably something I'm forgetting. And my students are all desperate for my help, and when I help them succeed I am being part of their achieving their dreams.

Very good for the soul, if not the wallet: I don't really make enough to afford my S, but I couldn't resist.

Be sure to indulge your appreciation of physics when you can -- it is, after all, how the world works.

SpeedxRacer

Wow shrike do you teach at UTA? I attended UTA for 1 yr before I transferred out... I couldnt pass any of the phsyics class =[