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I'm trying to learn more about inline 3 engine balance and engine balance in general (motorcycles)... I3 interests me for a few reasons but i want to further understand how they work.

L4 and crossplane I4 engines have evenly spaced 90 degree cranks i believe, so when one piston is stopped another is at max speed, meaning the crankshaft speed (and thus the engine) is smoother... That's what i understand anyway.

I'v heard that this same smoothness applies to 120 degree crank I3 engines but i'm a bit confused. I get that the cranks are evenly spaced like the examples above, but the piston movement (and thus speed) doesn't look so simple to me, even after watching it move online. Do the pistons speeds really cancel each other out perfectly like the 4 cylinder examples above (creating even crankshaft speed), or does it do worse compared to them?

Also i believe (could be wrong) these I3 engines can use a single balance shaft to deal with the rocking motion caused by the outer 2 cylinders... In this scenario (using the balance shaft), are there any other imbalances remaining?

I find I3 and L4 engines to be really cool. They're even firing/high revving screamers (my favorite kind), narrow, have great inherent balance/power delivery etc. L4 seems great for litrebikes and would love to see some lightweight I3 screamers (maybe 300CCish) as well since i also find small sport bikes to be really cool. Sorry if i'm rambling a bit haha.

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  • I believe a balancer shaft is used to offset vibrations in car 3cyl engines. Not sure how motorcycles deal with that.
    – I have no idea what I'm doing
    Jul 28, 2016 at 8:59

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From http://www.autozine.org/technical_school/engine/smooth1.htm:

It seems that no matter how the crankshaft rotate, the combined center of gravity of all 3 pistons and con-rods will remain at the same location, hence no vibration generated. By mathematical analysis, you can also find there is no forces generated in vertical direction as well as transverse direction. (actually, I really performed such calculations) So why did we hear that 3-cylinder engine need balancer shaft ?

In fact, the calculation is wrong because it assumes the engine is one point, thus the forces of all 3 cylinders act on this single point and result in complete cancellation. In reality, the forces act on 3 different locations on the crankshaft, thus instead of canceling one another, they make the crankshaft vibrating end to end.

Also here's a pretty clear animation (just imagine more red dots for a two-stroke): https://www.youtube.com/watch?v=E2u-zXsioRM

So:

Do the pistons speeds really cancel each other out perfectly like the 4 cylinder examples above (creating even crankshaft speed), or does it do worse compared to them?

The power strokes are evenly spaced. There is one every 240 degrees (four-stroke) or 120 degrees (two-stroke).

The engine's center of gravity stays the same so it's balanced in that sense. The crankshaft rocks because the forces aren't applied to it in symmetric locations along the shaft (See the red dots for the power strokes in that YouTube video, how they constantly travel from one end to the other? This is what makes the crankshaft rock.)

The piston movement isn't that complex. Think of it like a "wave" moving down the crankshaft.

In this scenario (using the balance shaft), are there any other imbalances remaining?

No. The only movement that needs to be countered is the crankshaft movement because of where the pistons apply their force on the shaft. The center of gravity of the engine remains the same throughout the entire rotation though, there are no other imbalances.

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  • Thanks for the info. I guess i'm wondering about the piston speeds compared to each other. For example a crosscrank I4 has a piston at max speed for every piston that is stopped. It's all perfectly offset speed wise meaning the crankshaft speed stays very even. That's why i'm asking about the I3 because when 1 cylinder is stopped the other 2 are neither stopped nor max speed, so it's harder to tell how even the crankshaft speed is compared to the 4 cylinder examples.
    – Guest
    Jul 29, 2016 at 15:34
  • @Guest The pistons are 120 degrees off from eachother. So power is at regular 120 degree intervals in the crankshaft rotation in a two-stroke. That's 3 evenly spaced power strokes per crankshaft rotation. Stop thinking about piston speeds and think about their crank angles and firing order instead. Their actual vertical paths are sort of (not exactly) the sine of the current crankshaft angle, at various phases. The only way piston A would be at max speed when piston B is stopped is if A and B had a +/- 90 degree difference. That doesn't happen in your I3 but it doesn't matter.
    – Jason C
    Jul 29, 2016 at 16:00
  • I'm not very knowledgeable on this but the reason i'm asking is because i use to assume the 180 degree crank I4 was smooth, but since the pistons are all stopped together, the 90 degree crank is better because it prevents this. The I3 is the same in this sense, but i'm wondering if the I4 has a smoothness advantage in that when one is stopped another is at max speed.
    – Guest
    Jul 29, 2016 at 16:11
  • @Guest Smoothness comes from the angle intervals. Relative piston speeds are a consequence of that. The 120 degree I3 has 3 power strokes per rev. A 180 degree I4 has only 2. So the I3 is "smoother" from that pov, I suppose. But again think of it as being because the crank angles are farther apart, not because some pistons are stopped at the same time. Even though those go hand in hand the angles are the reason, the speeds are just a geometric consequence. Piston speeds are effect, not cause. Also keep in mind that a flywheel smooths the whole thing out.
    – Jason C
    Jul 29, 2016 at 18:41

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