[Im]balance of inline four cylinder engines
[Im]balance of inline four cylinder engines
(OP)
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?
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?





RE: [Im]balance of inline four cylinder engines
- Steve
RE: [Im]balance of inline four cylinder engines
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!
RE: [Im]balance of inline four cylinder engines
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?
RE: [Im]balance of inline four cylinder engines
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
RE: [Im]balance of inline four cylinder engines
A "sixpence", indeed. Frupnee bits to you.
Bill
RE: [Im]balance of inline four cylinder engines
- Steve
RE: [Im]balance of inline four cylinder engines
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.
RE: [Im]balance of inline four cylinder engines
RE: [Im]balance of inline four cylinder engines
RE: [Im]balance of inline four cylinder engines
L/R is key for I4 engines.
- Steve
RE: [Im]balance of inline four cylinder engines
Is "2EO" the secondary vibration created by less than infinite rod length"
RE: [Im]balance of inline four cylinder engines
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.
RE: [Im]balance of inline four cylinder engines
- Steve
RE: [Im]balance of inline four cylinder engines
Yes but a balance shaft module was designed and could appear.