edredas

The idea of the balance shafts are to make the engine feel as smooth as a straight six, which also isn't perfectly balanced. The shafts on their own do not accomplish this and need hydraulic mounts.

refresh951

Again, this is not accurate. The balance shafts are ONLY to counteract the vibration caused by differences in piston acceleration in our inline 4 cylinder engine.

http://www.epi-eng.com/piston_engine...ion_basics.htm

http://www.autozine.org/technical_sc...ne/smooth2.htm

edredas

They only use them in four cylinders because the second order forces are considered to be tolerable for a straight six... but still very much exist.

Voith

Wrong.


Wrong again.


http://en.wikipedia.org/wiki/Straight-six_engine

edredas

I like how that was cherry picked... in the very first paragraph of wikipedia it says; "resulting in much less vibration than engines with fewer cylinders".

That in no way says that straight six engines have no vibrations.

Having no "primary or secondary imbalances" does not mean that those engines are perfectly smooth... they are well balanced, and perhaps the smoothest engines, but they aren't perfect. Besides, the point I was trying to make was, why didn't Porsche shoot for "Smoother than a straight-six".

Also, straight six engines are balanced with pistons that put a lot of force on the crankshaft. So how exactly are two big cell phone vibrators exactly supposed to take the place of those two pistons and the force they put on the crankshaft? They don't, and the engine has no clue they exist.

Voith

Balancers.

They balance secondary imbalances that straight 4 bangers have, and straight 6 do not.

Straight 6 has reciprocating mass perfectly balanced, straight 4 does not.

Two opposing phone vibrators rotating at same speed, will cancel each other, so there will be no vibration at all.



That is not true. Force that piston puts on crankshaft is converted to torque and is used via the shaft. The vibration comes from 2 pistons accelerating faster arround TDC, than the other two. Those two pistons and percentage of rod that affect this, is cca 2kg. 2kg vibrator can be canceled by another 2kg vibrator.

Raceboy

Don't tell that to the ladies

blade7

Didn't the 968 RS retain the balancer shafts.

Chris White

General statement....if you want to offer an opinion on a technical subject please try and understand the basic physics behind the issue (kudos to Voith for posting good info).

The FACTS behind the balance shaft design are pretty simple -

The Balance Shafts have nothing to do with static engine balance. I balance my engines to a tenth of a gram and it has no effect on the harmonics - they are a fact of life with in line 4s.

Race engines have the same issue. Making the reciprocating parts lighter helps but does not cure the issue. Making the rotating parts lighter does nothing to help the vibrations.

As one poster incorrectly mentioned - the balance shafts lower the vibrations of the engine assembly. This is what keeps your oil pump pick up from fatiguing, bolts from loosening and other catastrophic events.

As another point of fact I have disassembled more than half a dozen engines that have failed due to oil pickup failure....all balance shaft delete engines.

One more point of fact - Porsche kept the balance shafts in their LeMans engines...they found that the high frequency vibrations caused various metal fatigue failures and it was difficult to keep all the engine mounted parts from cracking or vibrating loose.

There is only one application where I would consider deleting the balance shafts - a 16v dry sump race engine with light pistons. The only reason that I consider this is that the balance shafts are spinning at twice the crank speed and the though of spinning a balance shaft at 16,000 rpm with the non pressurized front bearing does not give me a lot of confidence!

BTW - that's a good thing to think about when you set the balance shaft belt tension - it puts side load on a non pressurized journal bearing....

edredas

https://rennlist.com/forums/944-turb...-shafts-2.html

Evidently, my previous post missed the mark. The guy I quoted above is 100% correct... balance shafts merely mask the vibration. This was what I was eluding to in my previous post. The only reason to even mask the vibrations, (since they are still internally present) is to prevent that frequency from resonating through the cabin.

http://en.wikipedia.org/wiki/Noise,_..._and_harshness

Oh, and thanks for flaming me on my first post in this section... apparently I didn't get the memo where I was supposed to repeat the myth that your car will break into pieces if you remove the shafts...

Can we have a conversation and discuss this topic like gentlemen, or should I put my flame suit back on?

Voith

So, car moving forward uphill is in reality engine crankshaft masking a car that moves in reverse?

Without balancers vibration is present all the time since it is built in design. Only the frequency is more obvious at certain speeds, but since the balancers are directly connected to crank, and are rotating twice the speed (to catch both pairs of pistons at TDC), they are acting as piston counterweights when needed(at TDC), and as counterweight to each other when pistons are not at TDC, and work at any rpm.

What is present internally is force, that is applied in both opposing directions at the same time and is canceling each other. This causes stress to materials involved, but everything that is going on inside an engine causes stress. That is why high tensile strength materials with good fatigue properties are used to build engines.

http://en.wikipedia.org/wiki/Balance_shaft








Chris: thanks

edredas

That's the thing, they aren't "directly" connected to the crankshaft, as the belt doesn't count... if you wanted to cancel the internal vibrations, about the only thing that would work is to add two more pistons. That would be kinda like putting your hand over a plucked bass string... instead the shafts merely cancel out the sound of that plucked string, and even though you can't hear it, it's still very much vibrating.

Voith

Same would then apply to valves, if the belt wouldn't count, valves would just open at random.

Both forces are connected directly with solid aluminum engine block. Driving force doesn't matter as long as its timing is right.

http://www.sciencedirect.com/science...94114X11002229

Nice korean in depth article.

They even go so far to call it " a smart device ".

Porsche with a smart device.

Probably the best key to understand importance of this element, is to imagine how Porsche (one of the best if not the best car engineering and development company in the world) felt when they had to use this technology and pay Mitsu a fee on every car that was built with these shafts.

Ego killer for sure.

Porsche ego < Mitsubishi shafts.

edredas

Well, I didn't mean they weren't connected in some way... they are connected via belt and kept in sync with each other, but very little vibration is transferred. The shafts wouldn't be large enough to keep the crank from vibrating anyway. To use the guitar analogy again, you could shake a building to vibrate at a certain frequency, and then pluck a bass string to cancel out the noise, but the tiny bass string won't stop the building from shaking, even if it it's vibrations are transferred...

I realize that it is a flawed analogy, but my point is that it takes a lot of force to make the crank vibrate, and it would require equal force for it to be cancelled. Balance shafts might emit a similar frequency, but requires much less force to make them do so.

It is my understanding that Porsche was trying to design their own balance-shafts, but found the two bearing design that Mitsubishi held the patent for worked best.

gruhsy

lol