AWESOME engine dyno information thank you! Even I can admit when I learn something new.

Now it's your turn

Defroster used in rear windows of automobiles is a heating element applied directly to the window. However, the defroster used for the front window turns on the air conditioning. A/C consistes of 4 major components of which I'm sure you are familiar with since you've probably removed 3 of them. Compressor, condensor, expansion valve and evaporator...

The front defroster actually utilizes the air conditioning to remove water vapor from the air. The result is very dry air that likes to absorb moisture. When you blow dry air against a surface that has condensed water on it (your window) the water evaporates, clearing the surface. You can blow warm air at it too, but the real key for defogging windows quickly is DRY air. You'll notice, whenever you've run the air conditioner for a while, your car will appear to be leaking water. That is the condensed water collected from the air being blown into the passenger compartment.

BTW, have you ever used an eddy current dyno? I would imagine those would be the most ideal... no head loss from water flow. I was reading about one the other day. Looked pretty sweet... It's essentially a generator that you can apply a variable resistive load to, mounted on a force transducer... voila! torque!

 

Unless the air conditioning is turned on seperately, the compressor is never turned when you hit the defrost/defog button [at least that is true for abou 80% of 4-cylinder vehicles on the road today, V-6 and V-8 cars are a little different], although you are right, dry air does the job quicker, in which case you can turn the air temperature up in winter click on the A/C and clear your windows. The absolute simplest trick, however, is directed airflow, when you hit the defog button, the vent positions change under the dash to direct more air on the window than into the passenger compartment, and in the winter time when you usually want to defog the windows the most, NOTHING happens until the engine is warmed up, the thermostat opens and the heater core is at the proper temperature. Now, if you're driving along and hit a rainstorm with the A/C on, you'll never notice the difference, as you said, the air is dry; in the same conditions if your heater is on, you need to re-direct the airflow from your feet to the glass to correct for the sudden change in temperature between the outside and inside of the glass. The bottom line is that as long as air is being directed over the windows in the necessary volume and at a consistant temperature, the windows are going to clear, otherwise how would automakers deal with the liability resulting from allowing cars to be sold without air conditioning in northern areas where A/C is pointless even during the 2-day summer?

BTW, I'm glad you mentioned the eddy current dynos. I haven't had the chance to use one, they're still very much high $$ equipment, but I do understand that they also have a few problems. Namely, since you're measureing the current draw of [effectively] and electric engine, you run the risk of having a bush bounce on the commutator, causing a carbon arc, which will cause irregular current draw, and skew the results :a: Oh well, I guess. No perfect dyno's yet, the search continues, even the inertial dynos have to deal with the error factor of warn and dragging bearings on the support shaft, but so far, it is still the most fool-proof way to dyno the output of a car and/or engine.

FYI: It's nice to talk with someone that can go back and forth without throwing around credentials and getting all bent outta shape

 

Actually, I challenge you to try it. When you push the defrost button to redirect air flow to the front windshield, the A/C is on. My buddy's 90 Accord does it, so does my Integra. That's how defrosters are designed to work.

Are you sure about the commutator. I couldn't imagine using a brush generator in there. If carbon arcing is truly a problem why wouldn't the manufacturer be using an AC "alternator"?? Eddy currents result from simply moving a magnet near a conductor. Sooo, it could be a completely mechanical system, I'm not familiar with automobile ones, but small table top eddy current dynos measure torque based on the physical proximity of permanent magnets to a rotating conductor.

 

Hmm... maybe I need to check some wiring diagrams a little closer... Alternators aren't the same as generators, you can't reverse flow power into an alternator and force it to turn. Regardless though, anytime you apply counter-current to the rotation of an armature, AC or DC powered, you get a high current draw which causes the contacts [brushes, loops, whatever it uses] to try to fuse with the comm and arc as it's seperated. Unless you're talking about a different style or one designed for something other than high-output car engines, carbon arcing is a problem, as well as being able to supply enough current to accurately measure the results. I've seen similar dynos on a much smaller scale, where the arcing problem is much smaller since you're dealing with less nessary amperage for the testing, but for cars, you're talking about some large and high-draw armatures to counter an engine's crankshaft.

 

Dude, not to refute you or anything, but the A/C comes on automatically with the defrost in my 98 civic coupe, my moms 99 Ex Accord, and my brother 97 Lx Accord. as soon as you hit the defrost button, the Ac light pops on.

 

Yes, I've been corrected, now I know

 

My 94 Del Sol without A/C has 7 hp and -30/40ish lbs over an exactly identical Del Sol with the complete A/C system... just my 2 cents...