I have an idea I want to pass by you all. On intake an exhaust systems.

Is it better to have a high pressure and low velocity intake/exhaust system?

Or a low pressure and high velocity intake/exhaust system?

It seems to me you would want some of each. A high pressure high velocity system.

My idea is based on Bernoulli’s Principle. If you are similar with carburetor systems when you know what I’m talking about.

For an intake you have a large say 3 inch pipe, towards the throttle body you change the diameter of the pipe to 1.5 inch, this will in turn drop the pressure of the air but increase its velocity. With this higher velocity you are pushing air into the engine at a higher speed.

Now I’m not sure this would work correctly because of the low pressure created.

Anyone have any thoughts?

 

max velocity with max pressure. go for both, little back pressure as possible with max velocity, now go out and test out all the exhaust systems you can on a dyno!


[Modified by EE4, 1:30 PM 2/6/2003]

 

get on http://www.team-integra.net ck out the article section, surferX posted a pretty phat article that is extremely informative about most of the exhaust systems out there and they're potential. have fun..peace

 

The best way to look at it is to understand that you can't run a marathon while breathing though a straw. An engine is no different.

 

You're correct that Bernoulli's law is hard at work in modern engines, as velocity is a primary concern in both intake and exhaust systems. Balancing velocity with flow potential is the balance you have to strike.

Take a look at basic intake design, and you'll notice three important things. Runner cross sectional area, runner length (include the head ports in that length), and plenum volume. Varying these three will determine your port velocity and flow potential, but obviously now most people know that large ports kill low RPM power. Why? Because they degrade air velocity at any given RPM point vs. smaller runners, which effects how easily the moving air column can stop and reverse flow. You've mention the Ideal Gas Law, here's another one: KE = (m * v squared)/2 (where M is mass and V is velocity). While this doesn't express bulk flow, what it does show is that if you double an air column's velocity, you quadruple it's kinetic energy (this is actually a bit simplified as I'm not taking into account density, just let that slide for now). In other words, as velocity increases, you incur greater pumping losses but the offset is a charge that's harder to stop once it gets going. On the intake side this is good because it limits potential reversion late in the cycle and helps to ram charge the cylinder after BDC. The velocity also contributes to good fuel atomization. The downsides are an increase in pumping losses (easily offset by the increased cylinder filling) and potential for cylinder starvation if velocity goes too high (not so easily offset). In the end you must balance velocity with bulk flow potential, and find the target point at which your particular engine creates the most power.

On the exhaust side you use velocity for the anti-reversion properties I just mentioned AND the pressure drop, as this helps to further evacuate the cylinder and increases intake flow through increased pressure differential during valve overlap. THe pressure drop portion is defined as delta pressure, which is a way of describing the pressure differential between two points in the exhaust tract. One would be right at the exhaust valve face, another might be down towards the end of the header primary pipe for that cylinder. The greater the pressure differential here (meaning the higher the air velocity, again due to Bernoulli), the greater the exhaust can scavenge the cylinder and the less likely flow reversion is to occur. Again the problem you run into here is bulk flow vs. velocity, and the pumping loss problem. The pumping losses here are not directly made up for by increased cylinder filling, so balancing flow potential and ease of exhaust flow (how hard the piston has to work to pump it all out) is a bit more delicate than the intake side.


EDIT: I wrote this pretty quickly, so it might not be all that easy to understand. Apologies in advance.


[Modified by texan, 12:35 PM 2/6/2003]

 

Joe_docc - I don't think you understand what i'm talking about. You aren't decreasing the intake pipe to 1.5 inches. Look into how a venturi in a carb works and then you might understand.

Texan, thanks for the reply.


I'm taking an aerodyanmics class, so it got me to thinking. Its interesting how a smooth golf ball won't travel as far a one with dimples in it.


[Modified by machaf, 1:48 PM 2/6/2003]

 

Yep, boundary layer black magic. A delayed flow separation caused by an increase in flow turbulence around the object causes an overall reduction in drag. The boundary layer is formed closer to the ball's surface and hence it's aerodynamic wake becomes smaller than it would with a smooth surface.

The funny thing is that this same prinicple is present in intake design too. Polishing the intake runners causes a net decrease in flow potential because of the larger boundary layers; flow separation and wall induced turbulence actually takes up a greater area of the runner than with a slightly rough wall. Hence most of the best engine builders leave intake manifolds and port runners slightly rough.


[Modified by texan, 1:01 PM 2/6/2003]

 

How'd did you learn about all this stuff texan?

It seems you know your ****.

 

Time and effort. Read the books, discuss the principles, give it time to all sink in and you'll understand it too. Most of these principles are in some part still theory anyway, the exact physics of airflow through an engine is not completely understood.