Engine Balance Help! 3

[skyline69]

I have read Heisler's book on AE but I'm finding it hard to fully understand engine balance due to differing views on it i.e. Heisler's view and these

http://www.epi-eng.com/ET-PistnVelAccel.htm

and

http://yarchive.net/metal/piston_acceleration.html

Can someone who is an expert explain the balancing of an in-line 4 and explain exactly what is preimary and secondary forces - also if possible is what is stated at the above

http://www correct

and therefore that forces are out of balance between TDC (more acceleration) and BDC?

Brian
Software Engineer - so be gentle on my lack of knowledge on the AE front please.


Do it right or don't do it at all.

 

[ivymike]

The links seem to have it right (roughly). TDC acceleration is typically not equal to the negative of BDC acceleration.

I dislike the terms "primary" and "secondary" as they are used in regard to piston motion, as they seem to imply that there are no other axial components to piston motion. If you write out closed-form solutions for piston position, velocity, and acceleration, you'll see that there are higher-order components of piston acceleration as well. In many cases, they are not of great significance compared to the first- and second-order components, but many people are completely unaware that they even exist.

Also see this site for piston motion info:

http://www.wfu.edu/~rollins/piston/

Note the closed-form solution:

 

[patprimmer]

Thanks "I"

This info and the spread sheet from the site is fantastic.

I havn't done this sort of math for about 35 years, so I will just trust the spreadsheet.

My application for this is hobby only (1912 model engine rebuild), but as I am making the pistons, rods and crank, and increasing the stroke from 4 to 5' and rpm from 1800 to maybe 2500 to 3000, I now will analyse the speeds and acceleration vs modern engines. It should help a lot vs my GUESS at rod bolt size.

Regards
pat

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[Tmoose]

Amplitude VS frequency Vibration measurements made on many engines or reciprocating compressors have a "spike" at 2X rotating speed. That's the "secondary" vibration.

Of all the common engine configurations the inline 4 has the worst secondary vibrations by far. That has limited inline 4 cylinder use in "nice" cars to engines under 2 liters, unless the complexity of twin counter-rotating balance shafts spinning at 2X rpm are added (Mitubishi,Porsche, 1953 Diesel FORD tractor). Honda added some rubber mounts when their inline 4 motocycles went up to 900 cc.

As you mentioned the cause is the difference in piston motion between TDC and BDC. The rod length to stroke ratio is the cause. If the con rods were infinitely long the pistons' motion would purely sinusoidal, and their motion at TDC would be the same as BDC, and there would be no secondary vibration. (Opposed twins and 4s are much smoother because they match pairs up so they cancel each other). I visualize when a piston move a few degrees off TDC the crank and rod both get for-shortened, so the piston must "hurry" away from TDC. Conversely at BDC the rod and throw follow a similar arc, so the piston barely moves at all. For a pair of pistons This repeats twice per revolution.

The amplitude of the force is relative to the "reciprocating" weight, so light pistons help reduce it too. When VW went to 1800 cc in the GTI they lengthened the rods and lightened the pistons.

I used to work at a shop that did engine balancing. When I had just begun One of our customers was a speed shop of good local reputation. They built a Hot Rod Opel 4 cylinder engine for one of their customers. We had balanced the rods/pistons/crank/flywheel/clutch. It had been Bored and stroked. Bigger (heavier?) pistons. And if the original rods were used the rod/stroke ratio got shifted in the "wrong" direction. Years later I heard the engine was unusable because the vibration was so severe the intake manifold "shook right off it." Multiple (primary) balance re-checks. A law suit with damages paid. Only years Later did we get familiar with vibration analysis. Now I strongly suspect that the culprit was strong secondary vibration, maybe combined with a manifold whose resonant frequency was inrange. Volvo B-20E engines have a stout triangulated brace between intake and block.

After we had vibration analysis gear a fork lift truck distributor brought in a new fork lift to analyze. It was a new model with a new powerful 2 liter inline 4 engine from a Datsun car. Rigidly mounted in the frame. Operators said the power was nice, but were complaining the seat and controls buzzed un-bearably. Our vibation spectra showed strong 2X vibration. We rigged up some isolation mounts for the seat and floor board. We heard Eventually the factory re-did the truck design.

V-8s sometimes us "flat" cranks that effectively turn the engine into 2 inline 4s. The complaint is vibration. I suspect its at 2X.