Engine Air Intake Design

[Hardingfv32]

Trying to understand the requirement for designing an engine air scoop design. What should be the criteria for the inlet opening and plenum interior volume.

In regards to converting dynamic pressure to static pressure I have the following quote:

"Below about Mach 0.3 (or about 1/3 the speed of sound, 228 MPH at sea level), air is considered “incompressible”. That is, even if the correct nozzle is selected, and the air is slowed down (the official term is “stagnated”) there will be zero trade. No kinetic energy will be traded in as work capable of compressing the air. The reasons for this are not discussed here; the reader may consult any reputable fluid mechanics textbook for confirmation of this fact. In plain English, a car is just too
slow for ram air to work."

IF true does this mean no ram air effect?

Brian

[RobLav]

What you want to do is trade velocity for pressure. The size of your initial intake into the airbox can theoretically be the same size as one of your individual intake runners (assuming one for each cylinder) in a 4 stroke 4 cylinder engine. The higher the velocity, the lower the pressure. You can use Bernoulli's equation and play around with some numbers...

Basically, Rho V^2 / 2 (at the inlet) must equal Rho V^2 / 2 inside the middle of the airbox. (Rho is static pressure and V is velocity).

You can make an estimate of the velocity of the air entering your airbox given a diameter (and engine RPM). As that diameter increases, you can then calculate the change in pressure (it goes up!)

The airbox on the FC Zetec looks like a halfway decent design to my eyes.

[S Lathrop]

If you consider that all the air surrounding the car is low pressure, high velocity, you will accomplish a lot if you simply get back to static pressure at the intake to the engine. I would not hold the Swift 08-16 as an example of brilliant design but they did do enough wind tunnel work to figure out something about intakes.

[Jnovak]

There is definitely a ram air effect. I have measured it on several race cars and it does work.

Jay Novak

[Rick Ross]

What you want to do is trade velocity for pressure. The size of your initial intake into the airbox can theoretically be the same size as one of your individual intake runners (assuming one for each cylinder) in a 4 stroke 4 cylinder engine. The higher the velocity, the lower the pressure. You can use Bernoulli's equation and play around with some numbers...

Basically, Rho V^2 / 2 (at the inlet) must equal Rho V^2 / 2 inside the middle of the airbox. (Rho is static pressure and V is velocity).

You can make an estimate of the velocity of the air entering your airbox given a diameter (and engine RPM). As that diameter increases, you can then calculate the change in pressure (it goes up!)

The airbox on the FC Zetec looks like a halfway decent design to my eyes.

Rob,

In Bernoulli's equation, the "Rho" term is the air density, not the pressure. For incompressible flow, the density is a constant.

[Rick Ross]

Jay,

May I ask how you measured the ram air effect on your cars? Did you measure static pressure or stagnation pressure inside the air box? If using a differential pressure sensor, did you use the freestream pressures (from a pitot/static tube) as your reference value?

[RobLav]

Rick - Oooops! Rho is density - I wrote that without my reading glasses! Thanks good catch.

[P Burke]

I'm not good with math and am definately not an engineer. However maybe a real world experience with no numbers might help. Started racing in a F/V decades ago. Got an updated chassis and started going faster, thus finding our oil temps higher than they had ever been. This car used the block mounted type 3 oil cooler. The tail section also had an air scoop not sealed to anything. The owner of the car ended up sealing the tail section under the scoop so air could only go out through the cooler or down the carb. After that change only the oil temps did come down but it developed an intermittent miss at speed. We couldn't figure it out. Finally found there was enough upward movement of the tail to half way turn off the master switch, not the key type. Opened the hole only slightly and the intermittent miss went away forever. Assumption made was there was enough pressure under the tail to push it upward slightly at speed and randomely half way shut the switch off. This was long before data acq days and obviously no data to support our theory. Only that the miss went away with the larger hose for the switch. Just thought this might apply.

[BILL LOMENICK]

[FONT=Courier New][SIZE=2]After I made the air box on my DSR I did some test to check the pressure inside at different speeds and with wide open throttle. At first there was zero pressure at low speed so I started reducing the inlet opening and the pressure started to increase. I kept decreasing the inlet opening until the pressure at high speed started to drop off. At that point I went back one step and that is where I run it. I know this is not all that scientific and there may be some more gains to be made but I am pretty sure the air pressure in side the air box never goes negative and does add substantial pressure at higher speeds.[/FONT]
[FONT=Courier New][/FONT]
[FONT=Courier New]I have some data on air box pressure taken at full throttle on the front straight at Cal Speedway. It isn't corrected for wind speed but it was a [/FONT][FONT=Courier New]calm day with little wind.[/FONT]
[FONT=Courier New][/FONT]
[FONT=Courier New]speed PSI[/FONT]
[FONT=Courier New]45 .09[/FONT]
[FONT=Courier New]50 .09[/FONT]
[FONT=Courier New]60 .13[/FONT]
[FONT=Courier New]70 .14[/FONT]
[FONT=Courier New]80 .14[/FONT]
[FONT=Courier New]90 .18 [/FONT]
[FONT=Courier New]100 .20[/FONT]
[FONT=Courier New]110 .23[/FONT]
[FONT=Courier New]120 .27[/FONT]
[FONT=Courier New]130 .31[/FONT]
[FONT=Courier New]140 .38[/FONT]
[FONT=Courier New]147 .40[/FONT]

[FONT=Courier New]Bill Lomenick [/FONT]

[Rick Ross]

Hi Bill,

What exactly were you measuring -- static pressure from a port in the side of the air box? Were you using a pitot/static tube for your reference pressure? Thanks.

[BILL LOMENICK]

The pressure was taken from a port at the base of the air box. It was a port designed to drain water out of the air box on the bike. The reference pressure was taken from a space under the driver seat.

Bill Lomenick

[carnut169]

[FONT=Courier New]At first there was zero pressure at low speed so I started reducing the inlet opening and the pressure started to increase. I kept decreasing the inlet opening until the pressure at high speed started to drop off. [/FONT]

Why would reducing the inlet size increase low speed pressure in your airbox?

[BILL LOMENICK]

[FONT=Arial][FONT=Arial]I am by no means an expert but I think at low speeds the smaller the inlet the more the air slows down therefore the more the pressure increases [/FONT][FONT=Arial]up to the point where there is not enough volume going in the inlet to maintain the pressure at full throttle. At higher speeds there is enough air [/FONT][FONT=Arial]forced into the inlet to keep the pressure up. I don’t remember and haven’t gone back to check the data to see if the high speed pressure was [/FONT][FONT=Arial]changed with the smaller intake but I think it stayed about the same.[/FONT][/FONT]
[FONT=Arial]
Bill Lomenick[/FONT]

[brownslane]

Bill, I have a couple of questions for you.

Can you give us an idea of the size of the inlet? And did you use the factory air box, or was it a larger plenum volume? Myself, I feel that there is a certain benefit to be hjad by increasing the volume in "suspension" available for the intakes. This allows for inlet resonance to take place in a way that equalizes each inlet's intake volume.

Certainly, with my experience with Webers, I have seen the charge standoff visible to at least 6 inches above the inlet.

Thanks, Tom

[S Lathrop]

The pressure was taken from a port at the base of the air box. It was a port designed to drain water out of the air box on the bike. The reference pressure was taken from a space under the driver seat.

Bill Lomenick

Taking the pressure from inside the cockpit will give inaccurate results. The cockpit is a low pressure area and the pressure goes down with speed. In flying you have an analogues situation when you use the alternate static air source which is inside the cockpit.

However, I think you are only overstating the effect by some margin.

The aero testing I did years ago, I used a pitot tube for speed and measured pressure against the static source on the pitot tube.

[BILL LOMENICK]

[FONT=Arial]]Tom, smallest opening in the inlet ended up to be 2.15 inches. I modified an air scoop from a RT-4 and [/FONT][FONT=Arial]grafted it on top of the GSX-R1000 air box so that the inlet was above the roll bar, this left lots of room [/FONT][FONT=Arial]above the stacks for standoff. [/FONT]
[FONT=Arial][/FONT]
[FONT=Arial]I checked the pressure of the reference tube that was placed under the seat and found very little [/FONT][FONT=Arial]change in pressure. This may be because I duct air through the cockpit from the front to the engine [/FONT][FONT=Arial]compartment to help cool the engine. The first data I recorded was on a Race Log system which didn't [/FONT][FONT=Arial]have math channels and the data may not be accurate as far as true values but it should be accurate [/FONT][FONT=Arial]relative to itself so the inlet size I am using should be close to the correct size but may not be the true [/FONT][FONT=Arial]pressure inside the box. The later data I recorded was on a Motec system and should be close to real [/FONT][FONT=Arial]numbers. The numbers I listed before were from the Motec system and using a single input pressure [/FONT][FONT=Arial]sensor mounted in the engine compartment. In order to smooth the data I average 5mph below to 5mph [/FONT][FONT=Arial]above the given speed for lower speeds and 2mph above and below for higher speeds. So for 100 mph [/FONT][FONT=Arial]I average the data for 95mph to 105 mph. In the past I have compared data from single input pressure [/FONT][FONT=Arial]sensors mounted in the engine compartment and differential pressure sensors with the reference pressure [/FONT][FONT=Arial]taken at the pitot tube, in the engine compartment, and under the seat and they all were close to each [/FONT][FONT=Arial]other. There is always a chance the data I have is not accurate and if I get a chance I will do all the test [/FONT][FONT=Arial]again using the Motec system.[/FONT]

BILL LOMENICK