3 cylinder engines are very unbalanced. Wouldn’t making the middle cylinder bigger improve the balance of the engine?
No, because you don’t change the fundamental harmonic imbalance, just change it’s location in the cycle.
A large center cylinder would/could solve the rotating mass imbalance of a 3 cylinder. You trade it instead for a huge airflow, combustion, and power output imbalance that would be incredibly difficult to engineer out for a good running engine. The center cylinder will develop much more torque, take longer to burn fuel, get hotter, and result in a lopey torque application that bodes badly for bearing and crankshaft life.
While this is largely impractical for most applications, it does bring to mind the GRX-R1000 which uses an uneven firing order.
Basically the idea is that with an engine more powerful than the motorcycle needs, having two cylinders firing close together can cause you to lose grip for just a fraction of a second, before having almost 180° to recover stability. Using this an experienced rider can feel when they’re about to lose grip when riding through a corner at top speed, and make tiny adjustments to prevent a loss of control. Whereas with an even firing order, the loss of grip is more likely to continue evenly with the firing of each cylinder.
I think this could have some similar effect, but might offer up to 360° between bangs if it somehow managed to not blow itself up.
Noob here- wouldn’t you just have 3 at 120° offset, rather than two offset 180° to a bigger one?
No because in 3 cylinder engines, the pistons are not 180 degrees apart. If they were, then a big middle cylinder could in theory maybe balance it but then it would introduce other problems.
But in reality the 3 pistons are 120 degrees apart each and therefore the only way to balance them is to use balancing shafts.
Or you could do what Triumph did and just YOLO into the situation and say it has character because of the imbalance: https://www.cycleworld.com/story/bikes/triumphs-new-t-plane-firing-order-explained/
