[Im]balance of inline four cylinder engines

[pontiacjack]

Ground rule: restrict this discussion to pure 'balance' (F=ma) forces; assume no combustion occurs. Assume a "normal" crank with center two throws aligned and end throws aligned with each other, but 180 degrees from center throws.

From reading various explanations of the "inherent imbalance" of this crankshaft/rod/piston configuration, I conclude that infinitely long rods (R/S ratio = infinity) would eliminate the imbalance. True/false?

 

[SomptingGuy]

True.

- Steve

 

[BrianPetersen]

Doing what was described would eliminate the secondary imbalance (recognizing, of course, that infinitely long rods are not possible, but decently long rods can approach elimination of secondary imbalance).

Conventional inline fours also have a crankshaft speed fluctuation because (neglecting combustion/compression forces) the total kinetic energy of the pistons (up-and-down) and the crankshaft and big ends of the rods (round and round) is a constant at a given average rotation speed. That means the instantaneous rotation speed at TDC/BDC (when the pistons are instantaneously stationary) is higher than it is when the pistons are all level with each other halfway between TDC and BDC. This of course also shows up as a vibratory rotation of the block itself.

The long rods needed to address the secondary imbalance are heavier and make the rotational vibration mode worse. Of course, the trick involves finding the right balance ... and/or using the right design of engine mounts!

 

[pontiacjack]

Thanks for the help. I'm not well-versed on this subject, so I hope you'll be patient with my questions.
Brian- for the block to experience "vibratory rotation", a vibratory torque would need to be applied to it. Where is this torque applied, and to what is it reacted?

 

[SomptingGuy]

Adding to my somewhat "terse" first answer...

2EO is a massive issue for all I4 engine people. It's actually interesting as an engineer, because the interia forces and the gas forces are out of phase, so there is a "sweet spot" where they cancel each other out (~2000-2500 rpm for most small production gasoline engines). The do we or don't we fit a 2E balancer shaft rattles around many a design team meeting. NVH vs economy. Economy normally wins. I3 engines ... now that is a whole new subject.

Interestingly, the equilibrium position of an I4 engine is not neutral, so you can bet there will be forces created when it starts to spin.

V12. Stand a sixpence on the cam cover.

- Steve

 

[WGJ]

SG - giving you age away?

A "sixpence", indeed. Frupnee bits to you.

Bill

 

[SomptingGuy]

My bad. A sixpence would roll off, of course. I vaguely recall them still being legal tender, but my Dad couldn't afford a Jag at the time.

- Steve

 

[BrianPetersen]

PontiacJack, the torque gets applied through the cylinder walls. When a piston is coming up to TDC (and therefore, pulling on its con-rod as it decelerates), that con-rod is at an angle, so the piston applies a little bit of side load to the cylinder walls. Meanwhile, an adjacent piston is coming down to BDC and therefore pushing on its con-rod as it decelerates, but because that con-rod is angled to the other side, the side load to the cylinder walls is in the same direction.

Of course, the opposite side load is being transmitted through the crank journals. After the crank passes through TDC/BDC the rods are angled to the other side, so it all flips around.

On the topic of three-bangers ... I have a car with a three-banger and no balance shaft, and it's pretty smooth inside the car. Careful design of the crank weights and careful design of the engine mounts seems to have gotten the job done. The Ford Ecoboost 1.0 three-banger has no balance shafts, either, and that's a new engine design.

 

[pontiacjack]

So, Brian... if(!) R/S ratio were infinite, could the block experience any torsional vibration? (Again, leaving combustion events out of the current discussion).

 

[pontiacjack]

SometingGuy- I don't know what "equilibrium position is not neutral" means.

 

[SomptingGuy]

"Equilibrium position is not neutral" means that if you had a perfect friction-free I4 engine and put it to an arbitrary angle, it would not stay there. It would move to one of the two equilibrium angles. Based on gravity. I suppose if the con-rods were massless, this would not be the case. Possibly off at a bit of a tangent, I admit.

L/R is key for I4 engines.

- Steve

 

[Tmoose]

"2EO is a massive issue for all I4 engine people. It's actually interesting as an engineer, because the interia forces and the gas forces are out of phase, so there is a "sweet spot" where they cancel each other out."

Is "2EO" the secondary vibration created by less than infinite rod length"

 

[BrianPetersen]

pontiacjack - Conservation of kinetic energy means that regardless of the L/R, if the weight of the pistons is non-zero, the rotation speed of the crankshaft in a traditional-layout inline-four will be irregular. Conservation of momentum means that if the rotation speed of the crankshaft is irregular, the block will vibrate oppositely in the same manner.

As the con-rods approach infinite length, the angle of the rods approaches zero, but the amount of side thrust on the pistons that it takes to generate the relevant amount of torque that makes the block vibrate approaches zero, too.

 

[SomptingGuy]

As in many discussions in this forum, the answer is simple: "Bosch Automotive Handbook". Better than many engineering degrees (if you start with enough maths, physics and chemistry from high school).



- Steve

 

[triumphbob]

"The Ford Ecoboost 1.0 three-banger has no balance shafts, either, and that's a new engine design. "
Yes but a balance shaft module was designed and could appear.