In December of 2019, I gave an educational seminar to a standing room only crowd at the Performance Racing Industry Tradeshow. I was asked to speak on the topic of valvetrain performance specific to offshore powerboats, and this was likely the first time this topic had been discussed publicly ever! The Camfather himself, Ed Iskenderian was among those in attendance. To say the least, I was honored. The following is part 2 of a synopsis of my talk.
Managing Mass in the Valvetrain
In Part 1, we discussed what frequency is and established that it is a major factor in achieving valvetrain stability. In this installment, we will discuss valvetrain mass, its role in frequency, and how to move masses across a rocker arm to calculate the frequency of the system. Let’s dive in!
Moving Masses Across the Rocker
It is common knowledge that each part of the valvetrain has mass. What might not be so widely known is that not all of these masses are created equal. For purposes of calculating valvetrain frequency, the side of the rocker arm the mass is on matters greatly, and it scales as a function of the rocker ratio.
What we find convenient in engineering is to express cam side masses as equivalent valve side masses. To do this, simply divide the mass of the cam-side part by the rocker ratio.
In the example above, you can see how a higher ratio would result in a lower valve-side equivalent mass, but don’t jump too far ahead and order a set of 2.0:1 arms just yet: we’ll see in a later installment that that hurts you in the long run.
Gather Moment of Inertia Data
Repeat the process for the pushrod mass, and you’re done! But what about the rocker arm inertia? Well, there’s a solution for that, but first you have to know what the inertia is. If you can work with CAD (Computer Aided Design) software, you can model an arm and get a reasonable result. If you’re more of a hands-on type of fellow, you can build a trifilar table, which is a disk that oscillates at a known frequency on three strings or cables. The added inertia of the arm allows you to calculate what the inertia of the rocker is. Of course, if you’d rather let someone else do the work, contact Allmond Marine and we can help you.
Calculating Valve Side Mass from MOI
Assuming you have the Moment of Inertia of your arm, calculating the equivalent valve side mass is pretty simple.
Valvesprings
The last part that’s not obvious is the valvespring. Springs have zero motion on the head side and 100% motion on the retainer side. For cylindrical springs, we have found that 1/3 of the total mass can be considered “Active”. For conical or beehive springs, we would use something less, around 25%.
So far, these lessons have been as straightforward as they can be. That stops here. Join us in the next installment where we’ll dive into the murky waters of stiffnesses and how to sum them. Things are going to get exciting!
