Motorcycle engines and their cylinders


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

You lose torque as one cylinder needs to push another cylinder around. That is one more cylinder to move yhat wouldn’t be there otherwise.

The fewer cylinders, the larger the borehole. That means you have a bigger explosion causing more force to cycle the motor.

When you add more cylinders, you have a lot more smaller explosions so the engine cycles quicker at high revolutions as the borehole are smaller/shorter.

Big explosions have higher torque while a lot of smaller explosions will create the same amount of power with a quicker turnover for higher speed.

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.

Torque is the twisting force. For a single cylinder engine you have one big cylinder that pushes down *really hard* once every two cycles (assuming 4 stroke) and causes the crankshaft to rotate. For a two cylinder you have two slightly smaller cylinders that push down less hard but you get one each cycle. The smaller cylinders pushing down less hard (because there’s less fuel/air mixture pushing) means that you get a lower peak torque figure.

The trade off is that the single large cylinder has a single large piston, there’s a lot of mass moving up and down and you can’t throw it around too quickly or you’ll start breaking things. Two smaller pistons are each much light and can be revved up higher without over stressing components. Since Power = Torque x RPM (with a scaling factor) the reduction in torque can be made up for by the increase in RPM to get more power in the end

Cylinder count vaguely translates into torque vs power, but it really comes down to torque being closely tied to the size of the individual cylinder and power is going to be determined by the torque and how quickly you can spin the engine with bigger cylinders requiring lower rev limits and small cylinders letting the engine spin stupid fast (See F1 1.6L V6s that run at 15,000 RPM)