Three common kinds of piston engine move their piston(s) up and down mostly together. These are (1) singles, (2) 360-degree-firing parallel twins, and their cousins, narrow-angle V-twins (such as Harleys with their 45-degree Vee angle). A natural result of this up-and down starting and stopping of pistons is a vertical shaking force. As the engine turns faster, this shaking force increases in proportion to the square of rpm – double the rpm and shaking force becomes four times greater. This quickly becomes bothersome, and as parts begin to break or vibrate loose, we start looking for ways to reduce vibration.
Before we do anything else to reduce vibration, we take care of the easy part – the rotating imbalance that consists of the crankpin, its bearing, and the big end of the connecting-rod, all of which move in a circular path. Just as we can balance a wheel, we add counterweight to the crankshaft at 180-degrees to the crankpin until this rotating imbalance is zero.
The hard part is vertical shaking force, for our intuition tells us that we can’t balance a shaking force that moves in a straight line by adding counterweight to a crankshaft which rotates.
Here’s what happens when we try. Reciprocating weight (weight that moves back and forth in a straight line) consists of the piston, its rings, wristpin, and the small end of the con-rod. If we now add 10% of that weight to the crankshaft at 180-degrees to the crankpin, we will find that the up-and-down shaking force has been reduced by 10%. Progress! But we also notice that now, when the crankpin is at 90 or 270 degrees (measured from TDC, which is 0 degrees), this new counterweight is creating a brand-new imbalance acting forward and back at right angles to the cylinder axis. As we add counterweight, up-and-down shaking force decreases but the new forward-and-back shaking force increases.
Finally, when we counterweight 50% of the reciprocating weight, the up-and-down shaking force has been cut in half but the new forward-and-back shaking has grown to equal it. If we graph out the force for 360 degrees, we find this takes the form of a constant imbalance force that rotates opposite to the crankshaft (which is why no crank counterweight can cancel it).
In the case of British twins, whose two pistons moved up and down together, chassis flexibility made it easier for the rider to feel up-and-down shaking (putting his/her bum to sleep) than it is forward-and-back shaking. Because of this, such engines were “over-balanced” at 65-85% of reciprocating weight, delaying numbness of the fundament. Because this high balance factor creates so much forward-and-back shaking, at idle British twins and Harley Sportsters set their front wheels to shaking as well.
In the case of automotive engines with lots of cylinders, designers stopped at a 50% balance factor because this cut the peak load on crankshaft main bearings neatly in half, thereby usefully extending their life. Remaining imbalance was canceled by arranging cylinders in such a way that the several shaking forces and directions added up to zero (V8s, in-line sixes, &c).
The above shows why we can improve engine balance by adding crankshaft counterweights, but only by accepting a new forward-and-back shaking that grows in step with each reduction in up-and-down shaking.
One path to perfect balance is obviously to shake some form of counter-mass opposite to the piston. One way to do this is to provide a second crankpin at 180-degrees to the first, and to attach to it a second con-rod and piston. Because the two pistons now move opposite to one another, primary shaking force becomes zero. This is the classic flat twin, brought to BMW by 1920s engineer Max Friz.
Another way to accomplish the same thing is used in the British Maxsym and German BMW parallel twins – to drive some mass other than a piston up-and-down at 180-degrees out-of-phase with each piston. This has the benefit of making the resulting engine more compact than a flat twin.
Now for some logic. If we start with a single and add more and more counterweight until we have zeroed-out its vertical shaking force, creating a forward-and-back shaking force as great as the vertical shaking we started with, the obvious way to save the day is to cancel that new shaking with a second cylinder and piston at 90-degrees to the first. This is what designers of 90-degree Vee engines have done for over 100 years, and it is why Ducati have stayed with a 90-degree cylinder Vee angle since Dr. Taglioni first adopted it.
Ducati engineer Massimo Bordi created a variant of this to balance his elegant ‘Supermono’ single cylinder. In that engine, a second con-rod at 90-degrees to the one in its one almost-horizontal cylinder, but sharing its crankpin, shakes a weight up-and-down, achieving balance just as in a 90-degree single-crankpin Vee Twin.
Now go back to paragraph 5 above, where it is noted that by balancing 50% of the reciprocating mass, the result is a constant, unchanging imbalance force rotating opposite to the crankshaft. Since we are now talking about a rotating imbalance rather than a straight-line shaking force, we can balance this by gearing a second shaft to rotate opposite to the crankshaft and at the same speed, and to put on that second shaft an eccentric weight whose rotating imbalance force is equal and opposite to that of the crankshaft. Ideally, such a balancer should be concentric with the crank, but useful reductions of vibration can be achieved with other locations (Aprilia’s 60-degree Vee Twin had a pair of such balancers and Harley’s VR1000 Superbike had a single one below its crank).
At present it is normal to reduce reciprocating mass as much as possible through use of thin-crowned, short-skirted pistons whose pared-down weight can be moved through shorter strokes by less massive con-rods, whether steel or titanium, and by smaller or narrower rod and main bearings, all exerting reduced stress on engine structure. These benefits cascade through an engine, allowing use of lighter, less bulky counterweights or balancers. Much of the weight of traditional engines was necessary to make them strong enough to survive their own internal thrashing.
A piston, moving up-and-down in a cylinder is only one of many possible ways to carry out the processes of compressing air, adding heat by combustion, then expanding the resulting hot, high-pressure gas against a load. The Wankel ‘rotary piston’ engine generates no back-and-forth shaking yet can faithfully perform the traditional four strokes of intake, compression, power, and exhaust. Gas turbine stages smoothly and vibrationlessly rotate as intake air enters, is compressed, heated by combustion of injected fuel, and allowed to expand, either performing shaft work through a power turbine (turboshaft) or expelling a propulsive jet (turbojet).