Secondary balance: • Deep Dive: Inline 4 vs...
The inline three is the new inline four. In the last few years we have seen this humble engine configuration become ever more widespread. A 1.0 liter inline three together with a turbocharger has replaced 1.6 or even 2.0 liter inline fours on many cars. An inline three has less cylinders therefore less friction and more efficiency. It’s also of course cheaper to manufacture and easier to package due to the reduced overall length. But when it comes to motorcycles an inline three of course isn’t considered downsizing, seeing that motorcycles on average have less cylinders than cars. Instead inline threes are a mix of luxury and oddity in the motorcycle word, and while there have been some very impressive and iconic motorcycles over the years, the configuration remains uncommon.
Now, one of the companies that has definitely done their part when it comes to contributing to increasing the percentage of inline threes is Triumph and in today’s video we will see how their T-plane inline 3 engine (Triumph Tiger 900 and Tiger 1200) abandons decades of established design logic to try and create an engine with a split personality, and we will also see how the T-plane compares to other inline three engines including Yamaha's “crossplane” inline three (aka CP3) engine (MT-09, Tracer, XSR900)
If we take a circle which is 360 degrees and divide it by three we will of course get 120. And this the crankshaft configuration that pretty much all inline three cylinder engines employ. We have the crank pins 120 degrees apart from each other which means that we have a piston reaching top dead center every 120 degrees of engine rotation. The result is of course an even firing interval. Now in a four stroke engine we need 720 degrees to complete a full combustion cycle. 180 for intake, 180 for compression, 180 for combustion and 180 for exhaust. To get the firing interval we simply divide 720 by the number of cylinders. The result is 240 and this tells us that the inline three engine fires every 240 degrees of engine rotation.
When we sum everything up the traditional inline three is a humble engine but it’s a good deal. It’s overall less smooth than an inline four thanks to a gap between power pulses and worse primary balance but it makes up for it by being more cost effective, more compact and more efficient.
But despite this Triumph rejected the traditional good deal offered by the inline three and chose to up-end the logic of this engine.
Instead of having of all the crank pins evenly spaced out and separated by 120 degrees they separated them by 90 degrees creating a configuration which looks like the letter T when viewed from the nose of the crankshaft - hence the name T-plane crankshaft.
Of course having the crank pins 90 degrees apart means that our even firing interval goes out the window. Instead of firing every 240 degrees of engine rotation the t-plane has an uneven firing interval where we fire cylinder 1 rotate 180 degrees and then fire cylinder 3 after which we rotate 270 degrees to fire cylinder 2 and then again 270 degrees to fire cylinder 1 again. So our firing interval is 180 270 270 and our firing order is 1 - 3 - 2.
So why would Triumph chose to take an engine that is barely smooth enough and make it less smooth by employing an uneven firing interval?
Well, there are two main reasons behind this. The first one is that an uneven firing interval creates a very distinguished sound character which sets the motorcycle apart from competitors.
A specific sound gives the engine a unique character and definitely helps sales. It definitely worked for Yamaha and their crossplane inline four which sounds completely different from any other mass produced inline four.
In fact the marketing worked so well for Yamaha that they tried to forcefully trickle down the word crossplane into their engine offerings with fewer cylinders, which is why their inline two and inline three cylinders are called CP2 and CP3….the cp being crossplane.
Now I completely understand the need for marketing, brand identity and so on but Yamaha’s CP3 engine is just a conventional inline 3 cylinder engine and it has the same crankshaft configuration as any other inline three. 120 degrees apart for the crank pins and an even firing interval.
Calling an inline four crossplane definitely makes sense because all the other inline fours are flatplane. All the crank pins lie in one single plane. But the inline three configuration is naturally crossplane, that’s at the core of the engine’s design. All the other inline 3 on the market are crossplane just like the CP3.
A special thank you to my patrons:
Daniel
Pepe
Brian Alvarez
Peter Della Flora
Dave Westwood
Joe C
Zwoa Meda Beda
Toma Marini
Cole Philips
#d4a #triumph #tplane
00:00 Conventional i3 firing interval
03:15 i3 Primary balance
07:13 i3 Secondary balance
09:09 T-plane interval and sound
17:26 T-plane primary and secondary