Hey guys, I am an Engineering student, and have been assigned a project in my internal combustion engines class. The project I selected to do was to see how cold air intakes increase power and why, as well as what is the best material to use for the devise. I have found everything I need to calculate my Reynold's numbers and from which I can calculate the power needed to bring air into the motor. I plugged my computer up to my car and got an average air temp from the Inlet Air Temp. My problem is this, I can calculate a theoretical inlet flow rate by taking operational rpm's and dividing by 2 then multiplying by the displacement of the engine. The only problem with this is that it is a theoretical value and my teacher recommended finding an experimental value. So my first thought was to look at the MAF sensor and get a reading, but this is displayed in voltage. I was just curious if anyone has an idea of how to convert this to a flow rate of air or if anyone has a value they have seen for this car, say around 4000 rpm's?

 

As I recall I think there was a way to come up with a number that's practical but I learned it in my fluid dynamics class, but I've gone a different way so I didn't retain a lot of that information(I wish to god that I did) let me dig in my brain, nah that would be useless, but I know a psychics teacher that's a major gear head, I'll call him and see what he says if u think it would help u any.

 

The MAF sensor reading is not displayed in voltage. The output of the MAF sensor is a variable frequency. There is a calibration table in the PCM program that converts the signal in Hz to the mass air flow, in grams/second. A scanner will show you the G/S value, or in some cases the #/MIN value.

Your analysis of air flow has to reflect the mass air flow, since the power the engine can make is proportional to the mass of air in the cylinder. The mass is determined by the volume of the cylinder, using the perfect gas law. That requires that you know the absolute pressure (from MAP) and the absolute temperature of the air entering the engine (the intake manifold is as close as you can get to measuring the temp and pressure in the cylinder).

The CAI increases HP by reducing the absolute temperature of the incoming air, and reducing the frictional losses in the duct, increasing the absolute pressure. Lower absolute temp, and higher absolute pressure increases the density, and hence the mass of air in the cylinder volume.

You also can not just use (RPM/2) X displacement to calculate the effective volume of the cylinder. You have to include the volumetric efficiency of the engine in your calculation.

As you can see... the way to solve your problem is not to calculate the power required to move the air. You need to focus on the engine as an air pump - how does the CAI increase the mass of air in the cylinder?

FWF, PE

 

Thanks guys for the comments. brandons94lt1, anything I could get would be great.....


Injuneer......... That's exactly what I was wanting to know about the sensors i need to use. So your saying model the motor and analyze the power it makes rather than model the intake and the power it takes to pull air through it? I'm slightly confused on what you mean by modeling it as a pump? If you are modeling the motor wouldn't you need to use an Otto cycle and see how the colder temps and higher pressures help performance? This wouldn't really take into account friction factor of plastic vs. metals though. So could explain what you mean a little more? And the question you put at the end, I believe you answered it yourself two paragraphs above that.

 

CAI's don't add power just cuz they may be easier to draw air through, perhaps only a small amount.
If through the exact same you pipe drew 70degree ambient air vs 150degree air behind the radiator, you can calculate a significant difference in the density of the air using PVT laws and the ideal engine modle. If anything the colder, more dense air would have more drag in the pipe. The more air (oxygen) you put in the cylinder, the more gas you can burn at the given air/fuel ratio and therefore more power.

 

You don't need to analyze the complete Otto cycle. The only thing the CAI affects is the pressure and temperature of the air charge. Once it gets in the cylinder, we can assume that the A/F ratio control system adds the proper mass of fuel to correctly combust with the air mass. Your analysis stops at the bottom of the intake stroke. Beyond that, the CAI changes nothing.

By using the "pump" reference, I was simply emphasizing that the mass of air in the cylinder is the controlling factor on power. Look at the design of the path to the combustion chamber, and minimize the pressure losses. Lose 1" of Hg that you don't need to lose, and you have lost the ability to make 3.3% more power, at sea level.

 

Mike......Well part of my project was to see what material is best for the CAI. So I definately have to include something about power losses of say painted steel vs smooth plastic. Also have to include something about how factory intakes aren't smooth which causes power losses also. Just a question though.....On my 94Z I put a KandN set up and this mounts to the same location as the stock intake. So temperature shouldn't be that big an issue if it mounts in the same location, but they are advertising 18 to 20 hp increases. So just curious in you opinion is this bullcrap or why do they get so much more power from the stock location if energy loss isn't a big factor? Is it because the aftermarket ones aren't as baffeled as the stock one and don't allow as much heat transfer before air gets into the intake? I would think this would be more negligable than the friction of the air being pulled through, but I have no idea.

injuneer......O I see what your saying I think. Show how pressure loss and temperature increase is detrimental to the mass of air in the cylinder? So what I think you saying is get real world values from MAP and IAT sensor values for CAI's instead of trying to calculate head losses (which was going to take forever after I realized what I was trying to do). Any idea where i can get some good MAP and IAT readings for different CAI systems then if this is true?

 

A K&N filter will have less resistance to flow compared to a stock paper one. And the K&N CAI I've seen for LT1 fbodies takes air in from down in the bumper cavity, I'm not sure where the stock intake is though. Asuming the same shape, I would imagine the material used would be pretty negligible aside from the better heat insulator delivering cooler air.

 

thanks for the info man........ I got a lot of information so far and have started some of the calculations that he wants.