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Old 03-20-2009, 12:12 PM   #11
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Originally Posted by bobc455 View Post
Don't forget, though, that warm air generally helps MPG so it is beneficial to pre-heat the air going into the engine compartment...
He's talking about air used to cool the engine, not air for the engine to intake.

Also, warm air is not more efficient. You want a warm engine (coolant and oil) but not warm intake air. Cool air is more dense and makes more power.

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I think a better option would be to better route the air out of the engine compartment. The most efficient way to cool the engine would be to use the wind and not the fan. Blocking the grille area would just cause the fan to come on sooner. I think if the air was able to freely excape the engine compartment you would lower the drag on the car.

The idea is used on racecars, the Ford GT and even the Mitsu EVO.

Below is the Ford GT. Air enters the radiator below the bumper and then exits through the hood and over the car.


Below is a similar (less effective) design used on the Mitsu EVO. Air is allowed to excape the engine compartment just behind the radiator.
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Old 03-20-2009, 12:15 PM   #12
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That makes a lot of sense. Cutting the back out of the box. The weather going right in there doesn't matter?
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Old 03-20-2009, 12:20 PM   #13
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That makes a lot of sense. Cutting the back out of the box. The weather going right in there doesn't matter?
Exactly.
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Old 03-20-2009, 01:37 PM   #14
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Quote:
Originally Posted by NovaResource View Post
Also, warm air is not more efficient. You want a warm engine (coolant and oil) but not warm intake air. Cool air is more dense and makes more power.
Cooler, more dense air makes more power by allowing you to move more air and inject more fuel.

A warm-air intake is commonly observed to be effective for increasing fuel economy. On some vehicles it works, and on others it doesn't have any effect, but I don't think I've heard of anyone whose fuel economy went down when their intake air temperature went up.

Quote:
Blocking the grille area would just cause the fan to come on sooner. I think if the air was able to freely excape the engine compartment you would lower the drag on the car.
The grille block is another modification that has proven effective. It does seem like it would cause the fan to run more (and it surely does), but the end result is an improvement. It seems to work by reducing aerodynamic drag from air entering the engine bay as well as keeping the engine bay temperatures warmer.

Allowing air to freely escape may provide a similar aerodynamic benefit, but it will cool the engine bay.

There are some things that are done for speed that help fuel economy, and there are some that hurt it.
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Old 03-20-2009, 01:40 PM   #15
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I did notice all the cars with that hood vent were pretty powerful. This is turning out to be a very informative topic.
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Old 03-20-2009, 01:56 PM   #16
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Thinking about the shutter grille block some more and a more sophisticated control system than just opening it all the way when the fan comes on wouldn't be too hard. It being partially open in response to temperature. The microcontroller work needed for that wouldn't be terrible. Still need a good design for the shutters, though. I will think some more.
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Old 03-20-2009, 02:09 PM   #17
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I find a lot of early car developments fascinating. Such a variety of different techniques. Microcars had a lot of variety like that too.
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Old 03-20-2009, 03:11 PM   #18
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Originally Posted by theholycow View Post
Cooler, more dense air makes more power by allowing you to move more air and inject more fuel.

A warm-air intake is commonly observed to be effective for increasing fuel economy. On some vehicles it works, and on others it doesn't have any effect, but I don't think I've heard of anyone whose fuel economy went down when their intake air temperature went up.
The WAI helping fuel economy is a myth. While it doesn't hurt economy, it can hurt performance and if too warm, cause detonation.
http://www.metrompg.com/posts/wai-test.htm
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Old 03-20-2009, 03:16 PM   #19
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I made this statement before and the thread went south. The most important part of better mileage is the temperature of the coolant entering the engine.
In colder weather the cooling system has too much heat removing capacity.
This could be addressed by an additional thermostat that worked with a bypass hose. The thermostat would only allow coolant to enter the engine at 120 degrees, which is about the temperature of inlet coolant in hot summer weather.

When the coolant enters the engine in cold weather, it is so cold it causes the combustion chambers to loose heat energy. It also causes the thermostat to close some which increases the resistance of the coolant flow through the water pump.

By specifically controlling the inlet coolant temperature, the regular thermostat would be open more, reducing the restriction at the water pump and increasing the flow through the radiator and bypass hose.

The conceptual part of this is to allow a certain percentage of the coolant to bypass the radiator, with a 120 degree thermostat in the connection point where the bypass and radiator flow inlet meet.

This would allow the inlet coolant temperature to be the same regardless of the outside temperature. The colder the outside temp the greater the bypass volume. The greater load placed on the engine, such as sustained hill climbing the more coolant flows through the radiator.

Remember two crucial points;
67% of the energy loss in auto engines is heat
Cooling systems are designed to provide sufficient capacity for the worst case scenario.

There is no control whatsoever of the inlet coolant temperature which can vary by close to 80 degrees depending on outside temperatures. It also will vary depending on the condition of the radiator, with a old almost plugged radiator giving you better mileage in the winter when it does not have the cooling capacity for summer heat.

Maintining inlet air temperature at summer levels as well as coolant inlet temperatures would virtually eliminate winter fuel economy losses.

Anyone who thinks cold engines with cold intake air are more economical does not understand the correlation between winter and summer fuel economy differences which can easily exceed 10 %. Warmer intake air allows greater throttle opening and higher compression with less fuel. Most modern fuel delivery systems regulate fuel delivery based on mass and temperature of the incoming air. Cold air allows more peak horsepower but also requires more fuel than hot air. Since all modern engines are way overpowered for their application they run at small throttle openings with relatively high manifold vacuum. This basically means your effective compression is much lower than it would be if the throttle position wa high enough to have atmospheric pressure available to the engine. So your super high performance engine is cruising at normal speeds with effective compression ratios of less than 5 to 1.

Take a manifold vacuum reading while cruising. If it is 15 inches you are only working with half of your atmospheric pressure. Your effective compression is half of your engines capability. This is much more inefficient than any "pumping losses".


Engines produce power by expansion of the mix of air and fuel during combustion. That expansion capability is reduced by 50 % when you only allow half of the available atmospheric pressure to enter the combustion chamber.

A typical 4 cylinder engine on a dyno producing 20 HP, will produce 50 HP with a 50% increase in fuel with the throttle wide open. That is the reason why real hypermilers can get so much better mileage. They pulse with 50 no vacuum HP and glide with the engine off. They extract 50 HP with 1.5 units of fuel instead of 20 with 1 unit of fuel. This means they get the 30 extra pulse horsepower with only half again as much fuel as the 20 sustained low effective compression horsepower of the person who is not hypermiling.

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Old 03-20-2009, 03:29 PM   #20
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Originally Posted by NovaResource View Post
The WAI helping fuel economy is a myth. While it doesn't hurt economy, it can hurt performance and if too warm, cause detonation.
http://www.metrompg.com/posts/wai-test.htm
No myth, I managed 68.5 MPG in my Civic VX with WAI, and have averaged 60.3 in my Insight beginning December 5th 08 in the coldest part of the year in Virginia.

More air and fuel=more power

Warm air and less fuel + higher effective compression, due to WAI give you better mileage. Proven over and over.

I have never seen any proof that colder intake air and or colder engine operating temps produce better mileage.

NEVER IN 42 YEARS DRIVING.

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