I’ve seen quite a few videos talking about different motorcycle engine architectures (?) and their pros/cons and comparissons between them.
The general idea I’ve gathered so far is that moving from 1 to more cylinders, you lose low-end torque but gain top speed, and I assume high-end torque as well.
I haven’t come across any video explaining how the cylinders play their part in this. Why wouldn’t more cylinders equate to more torque across the whole range? Is my main premise wrong?
Note, I’m talking mainly about the “typical” engines, without V or any other weird angles, but feel free to educate me about that as well if you think it’s easily digestable.
In: Engineering
Torque is cylinder gas pressure divided by cylinder surface area and multiplied by the crank offset (rod journal to main journal distance), with some scaling for angular misalignment in time (liner reciprocating piston versus axially rotating crankshaft means force alignment changes constantly).
More cylinders means more weight on the rotating assembly and the torque force is identical (assuming same piston as before, just adding more of them). However, at higher RPM you have twice as many combustion strokes per rotation, which means more power.
At low RPM you have a single piston now turning a larger crank with the added weight of another piston’s compression stroke added to it. It means less torque per piston.
You can tweak things like V angle, cylinder count, firing order, etc. to tailor your engine to the application, and automotive engineers have done a great job of that. But the answer to your question is that typically only one piston is firing at a time in a motorcycle engine and that means the pistons that aren’t firing are either exhausting combustion gas, or compressing inlet fuel/air charge. That means some of the torque from the piston currently firing is being used to perform work that *isn’t* simply rotating the output.
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