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Old 08-25-2007, 07:39 AM   #1
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Passenger car aerodynamics

I've found some interesting info about car aerodynamics your mileage

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Old 08-25-2007, 11:24 AM   #2
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Some good stuff. Like most of us shade tree engineers, some of his suggestions are questionable. One is the area of engine modifications. Unless I'm changing effective gear ratios, making the engine consume more air and fuel seem counterproductive when after greater FE. I've spent nearly fifty years tinkering with cars and I'm used to immediately jumping to cold air intakes, better exhaust, ported heads, etc. But unless I use that greater horsepower (gallons of gas) to drive a numerically lower effective gear ratio, I'm just burning more gas per mile--I think. It gives me a headache.

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Old 08-28-2007, 03:07 AM   #3
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Graeme, I always thought the hot air increased FE by putting a natural acceleration limiter on the driver, i.e. the accelerator pedal is less sensitive so you can't "stomp on it".

Add this to your headache: Doesn't a piston changing directions in the stroke "use up" power because the momentum changes? Also, for every inch the piston rubs against the wall there is friction, so you would in theory want the MOST power for the FEWEST revolutions per minute possible, for the engine to run efficiently and not waste power.

Installing an HAI decreases the power per stroke, right, because less combustion reactants can enter the chamber? So in my opinion, all the physics & chemistry point to the only benefit of Hot Air as being control over the driver. I experienced this firsthand because when I had my custom CAI pointing thru my fog light hole, I could touch the gas pedal and the car jumped.

Then again, there are those who point to "pumping losses", i.e. the engine wastes power in sucking cold [dense] air into the chambers, so hot [less dense] air contributes to efficiency.

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Old 08-28-2007, 06:12 AM   #4
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Yes the friction losses in the ICE are significant - especially when running at low throttle (high vacuum). Look in "experients", there was a post or thread recently where someone measured gallons per hour while idleing at various rpms. gph increase was pretty consistent with rpms increase. So there's a "tax" just to keep the ICE spinning that's related pretty directly to the engine speed.

I confess to reading that site quickly.
Mostly good stuff though!

Anyway, my understanding is, warm air intake = good.
Somewhere I read that optimum combustion air is 60-80 deg. F.
Hotter than that probably gets into the range where air density decreases too much. Colder inhibits combustion to some degree.

Only a calibrated air mixer of some kind can really get an optimum intake air temp. Some guys here had a thread re. building one but the electronics involved got beyond what I could follow.

The rear drive Volvos that my family drives had such an air mixer, but mechanical. One tube pulled from in front of radiator, the other pulled from a shell baffle positioned over the exhaust manifold. A wax-based thermostat controls a damper regulating the hot/cold mixture. Trouble is, the thermostat fails after x number of years with no warning, in the "pull in hot air" position. Result is that the air mass meter fails from being inundated with hot air from the exhaust manifold. So most of these cars now have had the damper removed and run on straight cold air.

My FE has been good this summer, mostly around 31-32. I fear a serious drop when temps get below 60. I could reinstate my original airbox with a new thermostat but I just don't trust it - could fail and kill the amm which is very expensive.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 08-28-2007, 07:23 AM   #5
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my car has this too though i think it still works proppertl after 20 years... at least it does something...on hot days the valve thends to produce flapping noises... i think it's because when it's fully opened the mechanism allows it to vibrate in the airstream. althoug winter vs summer economy is vastly different.

i also notice that when my car starts to warm up there's a sudden increase in power... just when the temperature needle starts to move up... maybe it's the cold air valve opening?

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