Mailing List flyrotary@lancaironline.net Message #8020
From: Ed Anderson <eanderson@carolina.rr.com>
Subject: Re: [FlyRotary] Re: Runner Length Was Ref: injector relocation predictions?
Date: Tue, 11 May 2004 12:17:28 -0400
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Message
Al, in many of the induction articles I have read, I frequently come across velocity flow numbers of 300-400 fps.  Some caution not to exceed airflow of mach 0.6 in the inlets.  I'll find some of the articles for you to read, it seemed high to me as well, but I ran across it in a number of different articles..
 
Ed
 
 
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
From: Al Gietzen
Sent: Tuesday, May 11, 2004 11:37 AM
Subject: [FlyRotary] Re: Runner Length Was Ref: injector relocation predictions?

 

Now taking the fact that at 6000 rpm and 100% VE a 80 CID rotary will suck approx 277 CFM of air, so each set of runners flow approx `1/2 *277 or 138 CFM.  Assuming that the air divides between the two runners proportional to their areas 40% for the primary and 60% for the secondary.  We have .4*138 = 52 CFM and 0.6* 138 = 82 CFM

 

To find the air velocity in each runner we divide the flow in each runner by the area of that runner we find that the average velocity of air in the intake is only on the order of  110 fps or approx 75 mph.  Now most induction theory says you want an intake velocity on the order of 300 fps,

Ed;

 

Where does that number come from, and what does it mean?  Or maybe the better question - Where should the velocity be 300 fps, and over what length?  That is a very high velocity, and if it occurs over a very long length of runner, there will be pressure loss.  As I understand it, in a piston engine with valves, it is good to have that kind of velocity entering the combustion chamber for good mixing and some interia, and maybe at the injectors (usually at the ports).  In the rotary, as I understand from talking to Paul Yaw, relatively high velocity (I don’t know how high) is good as it enters the port for the same reasons, but the port area is rather large giving velocities much less than 300 fps.

 

Velocity in the runner contributes to the momentum as the air is flowing into the chamber, which can be helpful, but the offset is that it stops when the port is closed and has to be accelerated again. Between that, and frictional and turbulence losses in the runner, I’d just guess that 100 fps in the runner is plenty high enough.  Maybe keep the average velocity relatively lower, and use the dynamic wave to add some energy into the port just before it closes.

 

I have a 5” long, 44mm (~1.73”) dia 3-barrel TB on my 3-rotor; one barrel/rotor. At 6000 rpm that gives me a bit over 140 fps in the TB.  Based on my limited dyno experience, I think there would be a bit more high-end (5000 up) power if the TB diameter were larger.

 

Or maybe the issue is – the TB barrel area is 2.34 in.2; from there it is into a short manifold where it divides into primary and secondary runners with a total area of 3.88 in.2 which matches the port area on the housing fairly closely.  So the velocity is relatively high in the TB (where the secondary injectors are), which is good for vaporization and throttle control, but then slows to about 85 fps (at 6000) in the runners and entering the ports. 

 

Let’s see; does that all go into the category of rumination, of little interest to anyone? Well; whatever, it is built and it works fine.  I suspect I will never be as ambitious as you, ED, and rebuild the manifold 2-3 times.

 

Al

 

 

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