Breaking the excellent balance of the parallel 3-cylinder?
The parallel 3-cylinder engine in the new Tiger 900 has unequal ignition intervals. When I learned of this, I didn't think it was possible.
I didn't think it was possible, because a 3-cylinder with 120-degree crankshaft and 240-degree equally spaced ignition is very well balanced. The primary and secondary vibrations are perfectly balanced, and even-force vibrations that shake the engine due to the primary inertia force are generated, but can be handled by a balancer.
Moreover, the inertia force caused by the reciprocating motion of the piston, which is added to the rotational force of the crank, is cancelled out. This inertia is much greater than the rotational force from combustion pressure at high RPM, and it hinders the ability to grab traction. The 270-degree twin and the crossplane R1 are the result of YAMAHA's crossplane concept of eliminating this inertia.
The 120-degree crank 3-cylinder is a natural fit for this concept. In fact, the parallel 3-cylinder has a smooth, yet appealing throbbing that the 4-cylinder doesn't have, and that throbbing reaches through and through.
Aiming to be wild like an adventure
The Tiger 900 has been bored up to 4mm from the Tiger 800 and the crank phase angle has been changed.
While the Tiger 800 is a 120 degree crank, the Tiger 900 will be angled at 90 degrees for #2 and 180 degrees for #3 compared to #1. TRIUMPH calls it the T-plane, as if the central one had been removed from the YAMAHA 4-cylinder cross-plane. The ignition sequence is #1-3-2 (800 is #1-2-3) and the ignition interval is 180-270-270 degree.
This has a drawback. The primary inertial force of the central cylinder remains in addition to the primary inertial couple of both side cylinders. However, since a single axis balancer is installed, this is not a problem if it is used to deal with this problem. Single cylinder remains for the secondary vibration, which is twice the engine frequency.
There is a risk of minor vibrations on the high rev range, but it's not serious, considering that a typical parallel 4-cylinder, 2-cylinder, 180-degree and 360-degree cranks, will produce secondary inertia forces several times that of the cylinders (that's why 4-cylinders are often fitted with secondary balancers).
Even so, why did I choose the T-plane?
The aim is certainly to give rhythm and inflection to the throbbing with unequal spacing, for fun and to make traction easier to grasp. Perhaps Triumph wanted to make their personality clearer against the MV Agusta and Yamaha's 3 cylinders. However, it wasn't a significant change from the 240-degree equidistant, and it seemed unlikely that we could hope for a dramatic improvement in traction performance.
The T-Plane made good use of inertial torque.
But I had a gut feeling that this was something to look forward to.
So I examined the effect of piston inertia on rotational force. Take a look at my handwritten notes. This inertial force also has a primary and a secondary force, but the secondary force is about a quarter of the size of the primary force, so for simplicity's sake, I am considering only the primary force.
With 120-degree crankshaft, 3-cylinder cancel each other out and the result is conveniently zero. In the case of a standard 4-cylinder or 2-cylinder, the inertia force is several times greater than that of the cylinders.
However, I wrote the torque variation on a graph and was surprised. Amazingly, the combustion pressure and inertial torque pile up in the central cylinder coincide, and the inflection of the synthetic torque fluctuations before and after the pile-up is dramatically increased. Moreover, the higher the throttle opening and the higher the rev, the more intense this inflection becomes.
For many engines, inertia torque is a hindrance because it is expressed in a disordered manner in response to combustion pressure fluctuations. However, the T-Plane makes you expect that the inertia torque makes the changes in combustion pressure clear and desirable for the grasping sense of pulse and torque.
I haven't had a chance to test ride it yet, so when I do, I might say "what is this thing"...