Runner Lengths Formula

[smokebreak]

Hello everyone,

My name is John Strudel and I am the 2015 Powertrain team leader for Florida Atlantic University's Owls Racing and we will be competing in FSAE Michigan this year. I am a junior and this is my first year with the team. My experience is 15 years as a automotive technician and I returned to school to learn the math/science behind the functions of automotive systems. That being said, I am working on designing the intake and exhaust manifolds for the 2016 engine and a former member created a spreadsheet for calculating the runner lengths and I have been working to decipher it in hopes of using it as a design tool and as an engineering document for future members. Unfortunately the member that created it is not available for reference so the question I have is if the formula I derived from the spreadsheet looks correct:

<Runner length (mm)=((Speed of Sound in air @ IAT (m/s)*1000)/(2*Harmonic wave))*((60/RPM)/360)*Cam Duration @ .050>.

The current design correlates with this equation and performs well. I have ordered Scientific Design of Exhaust and Intake Systems to read. Any other documents or books you can suggest would be great.

Thanks,
John Strudel

[MCoach]

John, welcome back to the academic world!

There is a sizable list of books and references located here on the forum:

http://www.fsae.com/forums/showthread.php?1834-BOOK-LIST-to-reference-for-car-design

A very good one is Gordan P. Blair's "Design and Simulation of Four-stroke Engines":
http://www.amazon.com/Design-Simulation-Stroke-Engines-R-186/dp/0768004403

Gordan is very comprehensive.

Good luck.

[Z]

John,

I recalled that there was a good short paper about this stuff by Prof. Gordon Blair, so went searching the Forum for the link. I found one of my posts that gives an "Explosion Engine Summary" (http://www.fsae.com/forums/showthread.php?7954-Bore-x-Stroke&p=16934&viewfull=1#post16934). Blair's "Back to Basics" paper is linked by Adam in the post directly above mine.

There is quite a lot more good advice on this Forum. There is no magic. Don't expect miracles. Just keep it simple. The quote in this post about variable length runners (http://www.fsae.com/forums/showthread.php?6766-Plenum-Volume-Intake-Runner-Calculations&p=49173&viewfull=1#post49173) is a cracker! :) Owen's post at the end of that thread should also help directly answer your question.

Z

[smokebreak]

Thanks guys!

This is exactly what I was looking for, I ordered Blair's book and am looking forward to reading it.

John

[smokebreak]

I had a message from a FSAE competitor asking about this post so I figured I would share it here.

The basic theory, as I have come to understand it, is that there will be a wave generated by the intake valve closing. The wave generated travels from the intake valve to the inlet of the air filter, when the wave exits the air filter another wave is generated which travels back toward the intake valve. These waves are traveling at the speed of sound and the goal is to get the wave back to the valve at ~90° AFTER TDC (when the intake valve is almost at full lift) at your target rpm. This wave keeps bouncing back and forth until it either hits an open intake valve or an opposite wave canceling it out. This is a simple 1D method of determining an approximate range of lengths which work at a single rpm value. However, there are complex flow dynamics going on inside the system which requires computer modeling software develop precision in this aspect.

Hope this gets you on the right path.

Strudel
UNCC FSAE 2018