<!--quoteo(post=80704:date=Sep 26 2009, 09:35 AM:name=durtkillon)-->QUOTE (durtkillon @ Sep 26 2009, 09:35 AM)
<{POST_SNAPBACK}><!--quotec-->Having only worked on boxer engines in the past, I actually had to google why balance shafts were so critical for the 3.0 inline 4. I found this great little article that explains it well
http://www.autozine.org/technical_school...mooth2.htm . So I now understand the concept of why an inline 4, particularly a large inline 4, can never be properly balanced without one. My question is why are the shafts mounted asymmetrically around the crank? In other words, why is one high and one low? It seems like putting them both low would be simpler. Better yet, why not mount them both below the crank and use a timing gear instead of a belt?<!--QuoteEnd--><!--QuoteEEnd-->
I went through the tread and it looks like no one knows the answer. I remember the principle from University so I did a little search in my Library and re-typed an article regarding the balancer shaft configuration of our engines. It is very close in comparison, only Porsche design differs in using the double sided belt so doesn't need the spur gears. It is rather long so if you are not interested feel free to skip it. [img]style_emoticons/<#EMO_DIR#>/smile.gif[/img] I think the article explains it all so enjoy the reading.
<!--sizeo:3--><!--/sizeo--><b>Front-toothed belt-drive parallel twin countershaft harmonic secondary force balancer</b><!--sizec--><!--/sizec-->
A belt-drive twin-countershaft secondary force balancer (Fig. 3.63) incorporates long twin countershafts with the right-hand countershaft being directly driven by the belt via a pulley, whereas the left-hand countershaft is indirectly driven by a pair of similar-sized spur gears which drive this shaft in the opposite rotational direction to the directly driven shaft. The belt pulleys are chosen to give a 2:1 speed step-up and a jockey pulley is included to provide the correct tension to the belt drive.
Thus, the balance countershafts rotate counter to each other at twice the crankshaft speed and are so phased that they counteract the positive secondary forces at either dead centre, and the negative secondary forces at mid-stroke. This is achieved by the weights facing in the opposite direction to those secondary forces when in the vertical plane, and then for both balance-weights to face either inward or outward and thereby oppose each other when they have moved to one of the two horizontal positions.
An important feature of this secondary force balancer is that the countershafts are both mounted the same distance out from the cylinder's line-of-stroke but the left-hand countershaft is mounted considerably higher than the right-hand countershaft, which is itself positioned just above the crankshaft centre (Fig. 3.63).
The object of positioning the balance counter-shafts at different heights is so that when the countershaft weights are in horizontal plane, the height offset produces a balance-countershaft torque which opposes the fluctuating torque generated by all the piston secondary forces which occur at both dead centers and mid-stroke, otherwise this secondary inertia torque will promote engine roll (Fig. 3.64).
In addition to neutralizing the secondary torque fluctuations the offset balance-countershaft torque partially counteracts the explosion impulse torque produced over a wide speed and load range.
Measurements of secondary force vibration (Fig. 3.65) at the gearbox mounting rise at an increasing rate with engine speed, but when twin countershafts are incorporated these secondary force vibrations are considerably reduced, thus producing a much smoother running engine, particularly in the upper speed range.
Best regards
Jan
