Ok this is a little something I am trying to prove. A guy from work just got a used miata. He put an intake on it that is an open element( like a K&N ) and it is sucking pretty much only hot air. I tried explaining the problems with sucking hot air. He doesn't think it is an issue, does any one have an document or proof of this so I can enlighten him. Or am I total wrong about this.

 

Hot air contains less oxygen by volume than cooler air so it will burn less efficiently. ( i think )

 

Thats what I am trying to get at, it seems to be a lot of the trouble with the K&N units old and new. I only had trouble with my intake when the car was warm. but he won't seem to get it

 

Less O2 in hot air because of larger volume for gases, but the ECU adjusts. Air temp doesn't really make a difference, but air hudmity levels and pressure can affect performance. Don't know all the thermodynamics behind it all... someone with mech or chem eng can chime in please and explain...

 

Air temp does make a difference.

 

Ok, here goes.

1. Automotive engines are volumetric displacement machines. Each rotation of the rotor or movement of the piston displaces a certain volume of air. This volume of air +- stays the same except for changes in volumetric efficiency which is primarily a function of intake and exhaust system design/tuning. So, for the time being, let's assume that the volume of air entering the combustion chamber is a constant for each intake process (V).

2. The idea gas equation of state can be written a number of ways but for the time being I would like to use the following form: P=(rho)*R*T. Where P= the atmospheric pressure in absolute units (for this example psia), (rho) is the density of the air (lbm/in^3), R is the specific gas constant (53.35 sorry didn't look up the specific units), and T is the air temperature in absolute units (R). Yes... I know that we could use other unit systems.

Now, if the atmospheric pressure of the intake charge stays the same but the temperature of the intake charge increases... then something else in the idea gas equation must change. Of course the variable that changes is the air density and if the temperature goes up then the density must go down.

As we can see, the units of density are mass per volume so if the volume entering the combustion chamber stays the same and the density of the air charge goes down then the mass charge entering the combustion chamber must drop as the charge temperature increases.

Because of the lower amount of mass entering the combustion chamber the fuel flow to the chamber is reduced to maintain the same fuel air ratio. The smaller amount of fuel and air result in lower available power.

All of these details are why superchargers and turbo chargers work. Effectively, they increase the density of the charge entering the combustion chamber resulting in a larger mass of fuel and air and larger power levels.

Of course what we really want from the supercharger/turbocharger is the increase in pressure. Unfortunately we get a temperature increase that comes along for the ride. This brought about the motivation for the intercooler to drop the temperature of the charge after the super/turbocharger. As we can see from the discussion above... the whole reason for the intercooler is to drop the temperature of the incoming charge. By doing so we increase the charge density thus increasing the amount of mass entering the combustion chamber. Of course we get the added benefit of reduced charge temperature which should reduce hot spots in the combustion chamber (slightly) and thereby reduce the likelihood of predetonation (pinging).

Hope this addresses the question. Regards.

 

^^air temp makes a huge difference. with almost all aftermarket intakes, the filter is an open element (filter is exposed directly to air source) unlike a stock air box where the air travels through tubing and ducting before it gets to the filter. a short ram, or warm air intake, should still see some gains in performance as there is much less restriction in the intake tubing, but will not make as much power as a true cold air intake, where the air temp is closer to ambient than what is being ingestested from a short ram where the air source is the already heated air from the engine bay.

from what i remember, power output is affected to the tune of about 1% per 11 degrees of air temp, meaning that if the air your car is taking in is 70 degrees, and you are making 200 hp, then the air intake temp goes down to 59, you should now be making 202 hp, but if it goes up to 81, then you should be about 198 hp.

this is a rough approximation, so please dont refer to this as gospel, but it should give you an idea as to what happens due to temp changes.

 

Sucking hot air is more an issue with forced induction. Combined they can lead to detonation which is a bad thing.

On naturally aspirated cars hot air alone should not cause detonation but it will result in lower horsepower. Sure the ECU adjusts and delivers the correct amount of fuel for hot air, but the bottom line is you have less air. If a turbo is good because it stuffs more air in the engine, this is the opposite result. You can use Charles and Gay-Lussac's Law to make a quick estimate. 75 degree air is 12% denser than 150 degree air.

Or you can ditch this entire scientific explanation and get him to agree that cold air intakes make more power--after all, that's what all the tuner mags say right? So if CAI makes more power then hot air intake makes less.
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Its basically this:

You need oxygen to burn. The atmospshere is not pure oxygen, it is a mix of gasses. If the temp goes up, then the atmosphere heats up and the molecules spread themselves apart.

So lets say previously at a base temp level of 0 (just an arbitrary value) there were 50 molecules of oxygen in 1 cubic liter of air (arbitrary values). As you raise or lower the temperature value, the molecules are excited more and bounce between each other more, thus increasing the resting volume of the gas. (the resting volume of gas is determined by the ambient pressure, it gets complicated in open systems so lets just talk about temp)

So, if you changed the base value of temp to 10 the molecules would have spread apart and if you counted the number of oxygen molecules in the cubic liter of air, there would only be 20 or so. The problem lies in that it is the same volume of air, but just contains less oxygen.

The engine can only flow air at a certain maximum rate, there is nothing the ECU can do to compensate for the amount of oxygen present in this(you must be thinking of overheating and reducing timing). (I'm talking about maximum power production)

So basically, your engine always flows the same rate, but depending on the density of the air more/less oxygen will get into the engine, and oxygen is what counts.

 

actually, the pressure is a byproduct of more volume. we want more volume of air and fuel pushed into the combustion chambers, and the pressure is created by squeezing all this air into a small space. it all depends on the efficiency of the turbo or supercharger how much this intake charge will be heated up. a small turbo will spool up quickly, but at higher boost levels, will lose efficiency, so it is creating a "higher" boost, but the volume of air that it is moving will actually be less as the temperature will be much higher.

 

lurch519:

What you describe is just a different way of looking at things in the turbo/super charger.

However, it's the engine combustion chamber I am talking about when I refer to volume. The engine controls how much volume of charge it will swallow. So... in the end we are trying to increase the density of the air charge to increase power. Whether you want to think of this as an increased amount(volume) of air stuffed into the same box... or an increase in air density... the result is the same. The rest is just semantics

 

Just have him ditch the intake and get a gently used Jackson Racing CAI from www.miata.net. He will notice the differrence.

 

The ECU can compensate to allow the car to run. However, you are still running with a lesser amount of oxygen by volume so the fuel combustion is going to be less efficient. Basically, you are flowing the same amount of air with a lesser amount of oxygen molecules.

 

This is what I love about this place ask and you shall receive

 

This also good backing for a revi and ram air duct