[Corvettaholic]

So I was cruising through rush hour yesterday, and I watched my temp spike to 250 F. Thats bad, I assume the thermostat got stuck, not to mention the stupid thing has terrible airflow to the radiator.

The chemistry part: if you have aluminum cylinder heads, and they're surrounded in 250 F water, can that temp damage aluminum? I think its fine cause the oil isn't chocolate milk, but I'm wondering how quick heat will transfer from coolant to metal parts, and vica-versa. I'm really fuzzy on the math of thermodynamics, so I figured I'd ask. If heat transfers very slowly, everything should be fine cause I killed the engine as soon as I noticed the spike in coolant temp. Gotta love arizona.

[Donaldson Tan]

You don't have to worry about Aluminium. It's coated with an inert oxide layer that will protect it from chemical erosion with water. Moreoever, the temperature of water won't exceed 100 degree celcius, as long it is maintained at liquid state inside the radiator.

I think pp add certain chemical additives to water to make it a more effective coolent, and also anti-freeze. Maybe you can check the water pump system (including the pipes) to see if it's pumping sufficient water to cool down the engine, and also you may need a better radiator to aid to cooling rate of water.

but then again, I'm not a car mechanic  

[Mitch]

similar thing happened to me. Could only go a couple of miles until the car was overheating again. The hose from the coolant tank got disconnected from the radiator.

[Corvettaholic]

Well yesterday I replaced the thermostat with a 160 degree one, now it didn't overheat immediately but sat around 210 degrees (fahrenheight) while cruising. Something was still wrong, blown head gasket? So on impulse, I checked the radiator this morning and didn't see any liquid in it. After adding some dihydrogen oxide, it runs like a champ now. Relatively, anyway.

Another car chemistry question:
The optimal air to fuel ratio is something like 14.7:1, how did people figure that out? Cause air isn't pure oxygen, its only something like 20%, right? Is there a way to set up a reaction equation to figure optimal levels of each? I'm very inexperienced with organic chemistry. For anyone who wants to know whats in gasoline, I looked it up here:

http://www.atsdr.cdc.gov/toxprofiles/tp72-c3.pdf

A lot more complicated that I originally thought. One last thing, I know if I lean out the air/fuel mix, I get a lot more power when I go drag racing, but I have a much higher detonation risk. Why is that?

[gregpawin]

Lean? does that mean more air to fuel?

[Corvettaholic]

Right, if an engine is running lean then there is too much air. You get more power this way, to an extent. If an engine is running rich (like mine), there is too much fuel and some of the uncombusted stuff leaves via the exhaust. I'm wondering what determines the optimal ratio of air/fuel and how adjusting that relates to performace, on a chemistry level as opposed to a gearhead level.

[Donaldson Tan]

I'm just guessing..

Increase the proportion of air, as this aids complete combustion, giving raise to more power, thus would be helpful for racing.

For fuel economy, I think this boils down to the fact that our engine components can't transfer energy efficiently. You need to compromise the air-fuel mixture to produce sufficient energy such that most of it becomes kinetic energy to drive the car.

[gregpawin]

Ideally you'd use the exact stoichiometric amount necessary of oxygen for combustion, but combustion's a very complex event.  Much has to do with the temperature of the air going in and the size of the droplets created by the fuel injectors... etc etc...

But I do know that engineers or at least car fanatics test air/fuel ratios out to get the most aggressive settings and you can probably find them on a car nut site specific to your make and model.

[Corvettaholic]

Oh I know that I want to be running 15:1 for what I plan on doing. I'm a bit of a car nut myself, but I'm still learning. Is explaining how to figure optimal combustion beyond me? I'm thinking if it involves a lot of o-chem, it is. I know that the more atomized the fuel, the better the boom. Same with tossing a bag of flour in the air and lighting a match, if it disperses enough anyway. Why does grain size matter to combustion/explosions? Something to do with speed of reaction or surface area of the fuel?

[jdurg]

It has to do with both.  When you decrease the size of the fuel particle, it means that for a given mass of fuel, it has a much larger surface area.  If there is more surface area it means that there are more atoms exposed to the oxidizer which means that the oxidation of the fuel can occur a lot faster.  (Since there is more surface area to react).  By having the reaction proceed faster, it increases the heat of the reaction which will cause the reaction to proceed even faster.  So the increased surface area and the increased rate of reaction are what make smaller fuel particles combust easier.  (It's the whole reason why aluminum metal next to some rust won't ignite into a thermite reaction.  The surface area of the components just isn't great enough.  Now increase the surface area of that aluminum metal and watch out).  

[Corvettaholic]

Alrighty, that seems perfectly reasonable. So to boost the combustion process, I should find a way to better atomize my fuel out of the fuel injector. Already they spray fairly well (when clean), but I wonder if I could extend the nozzle a bit for a finer spray? Nothing superglue and a bit of plastic can't accomplish! Better flow test it before I install it in an engine though... and all that heat would be bad for my aluminum cylinder heads. Oh, speaking of which. I know aluminum has a higher melting point than what the temp of coolant typically reaches, but why would an alumium head warp (thereby crack gaskets) if coolant temp is only 280 degrees F? Is metal more likely to warp or crack when a lot of heat is added but below its melting point?

[eugenedakin]

Hello Corvettaholic,

Aluminum heads warp due to the improper design of the cooling system or lack of fluid.  Yes it is true that water boils at 100 Celcius.  If your coolant is half full of water, the portion of the head that is cooling by air only spikes to a higher temperature (above 100 Celcius).  Since the coolant generally moves from the front of the engine to the rear of the engine, the rear of the engine is typically warmer than the front of the engine.  When this uneven cooling is combined with a low fluid level (in which water boils to its gaseous state, and is signifacently less efficient when conducting heat) the rear portion of the head signifacently heats up.  Occasionally, coolant is splashed (through the chaos theory) on the heated rear portion of the heat, and is cooled very rapidly.  This sudden contraction of the metal by cooling eventually weakens the metal and it cracks.  This same time of phenomenon can be seen when taking a piece of copper wire and bending it back and forth until it breaks (metal hardening).

I hope this helps,

Eugene