Air Flow 101 - applicable to engines
Charter Member
03-22-2002, 07:23 PM
#12
Registered
Join Date: Feb 2001
Location: LANCASTER, NEW YORK 14086
Posts: 422
Likes: 0
Received 4 Likes
on
2 Posts
TOMCAT, Good post , but static pressure is the force being exerted outward against the "port" walls or "pipe walls", it is not why there is flow but it is a result of the flow.
03-23-2002, 03:14 AM
#13
David - You caught me breaking one of my own rules, over simplifying. I didn't explain total pressure (TP). The energy expended in drawing the piston down can appear as either static pressure or velocity pressure. Static pressure can be either positive or negative with respect to the atmosphere; velocity pressure is always positive.
TP = SP + VP
So you can think of total pressure as the total energy put into the air in the pipe by the fan, pump or engine, and whether this total pressure appears as static pressure or velocity pressure depends on how much flow is allowed by the pipe. Ignoring other losses, the amount of flow is controlled by whether or not the pipe is open at the inlet.
In the case where the fan, pump or engine are drawing through the pipe, both total pressure and static pressure are negative, less than atmospheric pressure. Just for illustration purposes, assume that the piston has drawn down and the throttle is closed; there is no flow and therefore no velocity pressure, so total pressure and static pressure are the same, both negative, and VP is zero.
TP - SP = VP = 0
It is this static pressure that will cause flow when we open the throttle. Now open the throttle and flow begins. This means there is now some velocity pressure, but static pressure and velocity pressure still add up to total pressure (in this case total pressure becomes less negative).
This is the actual relationship between static pressure, velocity pressure and the total pressure (difference) created by the descending piston.
It's actually easier to picture this if the pipe is under positive pressure with respect to the atmosphere. I will try to post some diagrams that illustrate this.
TP = SP + VP
So you can think of total pressure as the total energy put into the air in the pipe by the fan, pump or engine, and whether this total pressure appears as static pressure or velocity pressure depends on how much flow is allowed by the pipe. Ignoring other losses, the amount of flow is controlled by whether or not the pipe is open at the inlet.
In the case where the fan, pump or engine are drawing through the pipe, both total pressure and static pressure are negative, less than atmospheric pressure. Just for illustration purposes, assume that the piston has drawn down and the throttle is closed; there is no flow and therefore no velocity pressure, so total pressure and static pressure are the same, both negative, and VP is zero.
TP - SP = VP = 0
It is this static pressure that will cause flow when we open the throttle. Now open the throttle and flow begins. This means there is now some velocity pressure, but static pressure and velocity pressure still add up to total pressure (in this case total pressure becomes less negative).
This is the actual relationship between static pressure, velocity pressure and the total pressure (difference) created by the descending piston.
It's actually easier to picture this if the pipe is under positive pressure with respect to the atmosphere. I will try to post some diagrams that illustrate this.
Last edited by tomcat; 03-23-2002 at 12:52 PM.
03-23-2002, 10:00 AM
#14
Registered
Ok Tom,I think its time you took a holiday down south.The cabin fever has definately got ya.Great info but man do you need a boat ride
Last edited by Airpacker; 03-23-2002 at 10:09 AM.
03-23-2002, 01:57 PM
#17
Diagram #2 - The fluid in the pressure measuring tubes is water. This is where the pressure units "inches of water" comes from. Question: "Is it possible to have a pipe configured such that the SP measures zero and all the total pressure is converted into velocity?"
03-23-2002, 02:01 PM
#18
Registered
Join Date: Dec 2000
Location: Cumming,GA,USA
Posts: 482
Likes: 0
Received 0 Likes
on
0 Posts
Tomcat......now this question does relate to air pressure (kinda), and oughta be a wiz for ya.
If I want to take a vac reading and coorelate to what powervalve I might want to run, what is the difference in vacuum between taking it from a runner vs the carb base. Or better yet, where is the proper place to take the reading.
...I'll give you about 10minutes max, that includes an answer and a chart. ..................... GO
Bob
If I want to take a vac reading and coorelate to what powervalve I might want to run, what is the difference in vacuum between taking it from a runner vs the carb base. Or better yet, where is the proper place to take the reading.
...I'll give you about 10minutes max, that includes an answer and a chart. ..................... GO
Bob
