Flat Plane vs Cross Plane Crankshafts, and Firing Order Considerations

[GreenTwin]

I found some interesting information about flat plane vs cross plane crankshafts on engines, and also firing order considerations.

A rather involved topic, but interesting.
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[GreenTwin]

I had a Yamaha 650 two cylinder motorcycle, and noticed that both pistons traveled up and down at the same time, which places the firing impulses evenly around the crankshaft rotation.

I ran across a forum where some folks "re-phase" their Yamahas to either 270 or 277 degrees apart, and they claim less vibration, although I did not see anyone actually test that across the entire motor rpm range.
Some claim to like the gallop sound better than a perfectly even firing sound.

Since the motor is 4 stroke, then that has to be taken into consideration, since the piston fires every other time at TDC.

It is not just a matter of dynamically balancing moving/rotating masses, but involves balancing firing pulses that act upon the crankshaft.
Very tricky problem in my opinion, and no doubt there are compromises that must be made, depending on exactly what you are trying to achieve with the engine.

And as in the motorcycle re-phasing mentioned above, there is more than one way to make an engine run, but no doubt an engine designer would select a firing order that minimized vibration, and maximized power/torque, and while that firing order may not be perfect, it may be the best of many options.

http://sense.net//~blaine/twin/twin.html

It is interesting that all the crank pins on the flat four engine on the previous page are in the same plane.
I am sure there must be a good technical reason for doing this, as opposed to being at 90 degree angles or something.

Edit:
Some interesting articles:

https://www.motortrend.com/how-to/difference-between-cross-plane-and-flat-plane-cranks/

https://www.onallcylinders.com/2015/01/15/cross-plane-vs-flat-plane-crankshafts/

 

[GreenTwin]

Rephasing the crankshaft and camshaft seems to be a big deal in the 650 motorcycle world, with a great deal of discussion about the pro's and con's of such an arrangement.

One person refers to increased internal stresses in the engine, which could affect engine life.

Again, and compromise of many things, including primary and secondary vibration, horsepower, torque, and the sound that the engine creates. Some apparently rephase their yamaha 650's for the sound alone.

https://www.xs650.com/threads/rephasing-what-is-it.103/page-8#post-401791
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[GreenTwin]

More 650 rephrasing info.
Can negatively affect lower piston skirt wear.

http://chris-kelland.squarespace.com/xs650-rephasing/

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[kf2qd]

If I remember right... The old Triumph and BSA twins had different cranks. Triumph had the crank pins aligned, abd thee BSA had them at 180 degrees. The triumphs had more vibration at certain speeds. My Honda 360 had them at 180 degrees and I had to set the points at 5000 RPM to get it to run smoothly.

 

[GreenTwin]

I had a Yamaha 650 twin, a Honda 4-cylinder 500, and later, a single-cylinder Yamaha SR500.
I still have the SR500.

The SR500 remains my favorite bike of the three, and I got rid of all my multi-cylinder bikes.

The SR500 will not win any races, but it has a perfectly flat torque curve across the entire rpm range (I have a printout), and so it is a real joy to ride, since it really matters little about what your speed is, or what gear you are in; if you want to go, just roll in a lot of throttle, and it will go like being on the back of a hairy gorilla.

If I had more time and money, I would find the dirt version of the SR, which is the TT.
You could literally plow fields with the TT.

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[scottyp]

Good info, it took me back the 80’s on my 400 special.

 

[Nerd1000]

When it comes to flat engines (opposed twin and opposed 4) the crankshaft is arranged as a 'boxer' configuration, the pistons in each row move in opposite directions to each other. This balances both primary and secondary forces leaving only a small rocking couple from the cylinder bore offset. It also gives even firing.

For an inline twin, we have the issue of a conflict between balance and even firing. Aligned crankpins give even firing with a power stroke every 360 degrees, but poor balance akin to a large single. 180 degree cranks have very uneven firing, but ok primary balance (the secondary balance is still poor, and there is a rocking couple). The 270 degree cranks that are popular now are more or less a compromise between the two.

The vast majority of inline twins are the 360 degree type. Multiple reasons for this: one is that uneven firing often produces more significant vibration than the 360 twin's imbalance, especially at low RPM. The other is related to airflow, specifically the evenly spaced induction strokes of a 360 twin work well when fed by a single carburettor. The uneven engines are subject to issues with one cylinder 'starving' the other of mixture, leading to a lean condition on the starved cylinder unless a separate carb barrel is provided for each. Finally, the 360 twin lends itself to simple wasted spark ignition with no distributor.

 

[a41capt]

When it comes to flat engines (opposed twin and opposed 4) the crankshaft is arranged as a 'boxer' configuration, the pistons in each row move in opposite directions to each other. This balances both primary and secondary forces leaving only a small rocking couple from the cylinder bore offset. It also gives even firing.

For an inline twin, we have the issue of a conflict between balance and even firing. Aligned crankpins give even firing with a power stroke every 360 degrees, but poor balance akin to a large single. 180 degree cranks have very uneven firing, but ok primary balance (the secondary balance is still poor, and there is a rocking couple). The 270 degree cranks that are popular now are more or less a compromise between the two.

The vast majority of inline twins are the 360 degree type. Multiple reasons for this: one is that uneven firing often produces more significant vibration than the 360 twin's imbalance, especially at low RPM. The other is related to airflow, specifically the evenly spaced induction strokes of a 360 twin work well when fed by a single carburettor. The uneven engines are subject to issues with one cylinder 'starving' the other of mixture, leading to a lean condition on the starved cylinder unless a separate carb barrel is provided for each. Finally, the 360 twin lends itself to simple wasted spark ignition with no distributor.

I agree wholeheartedly, as my BMW R1200 GSA boxer has got to have the smoothest, most vibration free ride of all the (100 or so) bikes I’ve owned since 1967. Back in the 60s, while racing American flat track and TT, the shop BSA had more torque and power than the Triumphs that I was assigned to, and it seemed to have a smoother power band.

As mentioned above, the big singles I rode (BSA 441 Victor and BSA B50MX) pulled ‘like a hairy gorilla”, and when matched with the right carburetion, tires, and track surface, were king on the short track, many times beating out the big twins (Harley, Triumph, and BSA).

The shops I rode for weren’t equipped or bankrolled well enough to change crank timing, but a custom ground camshaft, experimental pistons, a 4 valve Westlake head, and fancy carburetion were the occasional “extras” that found their way onto the track under my young butt.

This is a fun discussion and takes me back to days lived fast and fancy, handlebar to handlebar, each bike against each bike, and seeing how racing engine technology advanced each year.

Now, perhaps I’d better get back to the shop and finish Rudy’s Pioneer!

John W