I wrote most of the following 5-years ago in another forum:
An in-line four (or I-4) four-cycle with cross-plane crankshaft means uneven firing intervals.
Instead of the standard 180 crankshaft degrees interval between successive combustions of the conventional “plane-crankshaft” four in-line, like:
in the cross-plane the firing intervals are 270 crankshaft degrees, then 180 crankshaft degrees, then 90 crankshaft degrees and then 180 crankshaft degrees, like:
and the sound from the exhaust reminds something like the “gallop pace”.
Regarding the unbalance of the cross-plane I-4 (by the way, with the balance program at http://www.pattakon....duc/balance.exe
you can see graphically and by numbers all these), it is different than the unbalance of the conventional I-4.
Besides the heavy unbalanced 2nd order inertia force, the conventional I-4 has also a heavy 2nd order inertia torque:
(the inertia torque on the block and the inertia torque on the crankshaft are the same).
The short curve is for the cross-plane I-4, for equal stroke, rpm, piston mass etc.
2nd order means that it maximizes and minimizes two times per crankshaft rotation.
An idea for this “heavy inertia force”?
In a typical 1600cc I-4, at the red line the unbalanced inertia force is more than 1 ton, i.e. more than 7 times the total weight of the engine.
In the reasonable question: “and why the engine, under the action of such a strong force, doesn’t leave its place/position?” the answer is simple: “it does” and the engine vibrates, i.e. it moves up and down in the rhythm of the acting force, i.e. if you take a “fast” video of your engine running at 6000 rpm (i.e. at 100 crankshaft rotations per second) you will see the engine going up and down 200 times per second .
The elastic engine mounts isolate these vibrations from the car – or motorcycle- frame.
In the expensive cars they put a pair of counter-rotating 2nd order balance shafts (i.e. they rotate at double crankshaft speed) to create an equal and opposite force and so to cancel the 2nd order unbalance force of the conventional I-4.
With the proper offset (along the cylinder axis) of the two balance shafts, besides the inertia force, it is also counterbalanced the inertia torque on the block of the engine (however, the heavy inertia torque on the crankshaft and on the flywheel cannot be reduced).
What the “heavy inertia torque” means?
At high revs the inertia torque is stronger than the instant torque generated by the expansion of the gas into the cylinder. This actually means that if you take the actual torque that arrives to the flywheel from the crankshaft, it comprises a strong “inertia” part that offers nothing but stress and friction and a useful torque that finally moves the vehicle.
To really understand the meaning of the inertia torque, look it from the energy view point: in a conventional four in line (I-4) with plane crankshaft (i.e. with crankpins at 0, 180, 180 and 0 crankshaft degrees) when the first piston is at the TDC (top dead center) all the four pistons are immovable (their velocity is zero), i.e. their total kinetic energy is zero (kinetic energy: 0.5*m*V*V, where V is the speed of the piston and m is the mass of the piston with the piston rings and the piston pin and the upper part of the connecting rod). After 90 crankshaft degrees, all the four pistons move quickly and have concentrate a strong amount of kinetic energy. At the BDC (i.e. after another 90 crankshaft degrees) the total kinetic energy of the pistons is zero again. I.e. there is a strong “take and return” of mechanical energy between the flywheel and the pistons two times per crank rotation. And this strong inertia torque does not contribute to the vehicle motion. It just stresses the moving parts of the engine and increases the mechanical friction.
The conventional four in line is not so perfect as you used to think.
And what about the cross-plane I-4?
Here the total unbalanced inertia force is zero.
Also the total unbalanced inertia torque is actually zero (i.e. there is no reciprocation of inertia torque between the flywheel and the pistons). I.e. in the rear tire of the Yamaha R1 arrives only useful energy that makes easier the life of the rider (better feeling, drive closer to the limit etc). The clutch can operate without springs.
Just like in the V-4 with 180 degrees crankshaft.
But there is a pure 1st order strong plane inertia moment. By using a 1st order counter-rotating balance shaft (i.e. it rotates with the crankshaft speed at opposite direction) the total unbalanced inertia moment of the engine becomes zero.
That is, with one balance shaft the cross-plane straight four is better, as regards its inertia balancing, than the best V-8 ! and way, way better than the conventional I-4
The only asymmetry that remains is the uneven firing.
For someone who has a V-8 in his car, he can simulate the cross-plane I-4 by removing the ignition and the injection from the one bank of cylinders. That simple.
As for the turbo applications, one can start by seeing how the V-8 deals with this problem. If a turbo can serve successfully one bank of cylinders, like in the picture below (chevy), the same is true for the turbo-charging of the cross-plane four in line.
The drawback of the cross-plane I-4 is the uneven firing and the need for a 1st order balance shaft (which is way easier and involves way less friction than the two 2nd order balance shafts necessary for the balancing of the conventional I-4).
Another drawback is the lower peak power as compared to the flat-crankshaft straight four (with equal intervals between the exhaust pulses, the tuning / breathing is better).
Back to the conventional flat crankshaft I-4:
The clutch passes to the gearbox both, the “working” torque (i.e. that from the combustion-expansion-compression) and the inertia torque (which, at high revs, is several times stronger).
From another viewpoint, the main torque the clutch and the gearbox passes to the wheel is the inertia torque, with the “working” torque passing like “noise” onto the inertia torque! This is the case.
These two torques have the same “main frequency”.
Any attempt to damp the inertia torque, kills a part of the “working torque” too.
A solution is to allow the crankshaft to rotate at slightly variable angular speed (slightly faster at TDC and BDC wherein the pistons stop moving, and slightly slower at middle stroke wherein the four pistons move quickly) while the primary axle of the gearbox rotates at constant angular speed.
If in the primary transmission (from the crankshaft to the clutch) the gear-wheel on the crankshaft is of special design like:
then the flat crankshaft I-4 no longer passes inertia torque to the gearbox, keeping all its rest advantages.
Edited by manolis, 20 April 2012 - 07:37.