Tommy Cookers wrote: ↑
Wed Oct 16, 2019 9:48 am
Mudflap wrote: ↑
Tue Oct 15, 2019 10:09 pm
....I agree that there will always be a 2x inertial excitation but this is always small compared to the excitation produced by the firing frequency. ....
I thank you for your post ....
but Furusawa said ....
(of a conventional NA inline 4 at race rpm) that the inertial excitation is greater than the firing excitation
then using a crossplane crank (Yamaha M1) eliminated the inertial excitation and helped the rider to find the traction limit
imo the crossplane crank makes the M1 excitation the same as the Moto GP V4's
of course the conventional inline 4 inertial excitation is no different to the traditional eg an inline twin's or a single's
in the 1970s-80s etc millions of (GM. Honda and PRV) road cars had 90 deg 'true' V6 engines ie 3 crank throws & uneven firing
What Furusawa said:
‘What is Big Bang?’ Furusawa’s area of expertise is harmonics, so perhaps he chose to use the analogy of signal-to-noise ratio to explain his theory:
Noise is always present, what you want is a strong enough signal to render it irrelevant. So what is signal and what is noise in the context of a motorcycle engine? This is best explained by thinking about that word ‘connection’ you keep on hearing riders use in testing. This is shorthand for the connection between the throttle and the rear tire. In an ideal world, opening the throttle by 10% would deliver 10% of available power (actually torque, but never mind) to the rear tire. Life is rarely this convenient or simple, and racing engines certainly aren’t.
Modern electronics should be able to provide the linear throttle response riders crave; a high signal-to-noise ratio. And our research suggests is that that is what you do get—up to a critical rev level where the signal is severely distorted by ‘noise’. The question is, what is this interference? This is ‘inertia torque’, that is the torque due to the motion of the heavy moving parts in the engine—crankshaft, con rods and pistons. This is totally separate from the torque generated by the combustion process. At low revs, the level of interference from the rotating mass is insignificant, but around 12,000rpm it starts to become greater than combustion torque and by around 16,000 is double. This is counter-intuitive because you would assume, with a conventional 180-degree crank, that everything would balance out. Not so, as you discover when you look more deeply at the direction in which torque is exerted at different points of a crank’s rotation.
Combustion torque is easy to understand: it’s produced by ignition of the fuel/air mixture. Inertia torque is much trickier to define and understand. Let’s try. Forget combustion and just consider the piston and con rod travelling up the bore. At BDC the piston, con rod and crank pin are in line and no torque can be applied to the crankshaft (in fact at top and bottom dead centres, the con rod is momentarily stationary and vertical). Now move through 90 degrees. The big end of the con rod together with the piston is moving quickly with lots of energy and is about to decelerate to a halt at TDC. That energy of motion (kinetic energy) has to go somewhere, and the only place it can go is into the crankshaft. So inertia torque is positive in that it is applied in the direction of rotation of the crank. On the down stroke, the converse is true. The lower part of the con rod together with the piston has to be rapidly accelerated from rest at TDC to a high velocity, which requires an input of energy. That removes energy from the crankshaft so here inertia torque acts against the direction of rotation.
Without doing the math, you can see how this variation of torque over each revolution might produce some small variations in the torque seen by the tire contact patch. On your 180-crank, four-cylinder road bike, you won’t notice the effect because you don’t use high enough revs, but as this inertia torque is proportional to rpm squared, you can see how a 17,000rpm MotoGP engine might have problems. At those sort of engine speeds, the ‘noise’ of the inertia torque is ‘louder’ than the ‘signal’ of the combustion torque. The rider’s connection with what’s happening at the rear tire’s contact patch is lost both with the throttle open and with it closed.
Thanks to GPS and Yamaha's electronics package, the M1 not only knew what gear it was in but which corner it was in. Very helpful data in setting up the bike.
The cure is equally counter-intuitive; an irregular firing pattern, 90- degree crankshaft. The conventional 180 crank has its two outer pistons at TDC while the centre pair are at BDC. Leave cylinders number one and three as they are then move two and four through 90 degrees in opposite directions and you have the 90-degree crank with one piston coming to TDC every 90 degrees of crank rotation. Yamaha tried firing all four cylinders in one revolution and compared the result to the more conventional firing order of two cylinder firing at a 270-degree interval in the first revolution of the crank and the other two firing just 90-degrees apart in the middle of the next revolution. The first surprise is that they sounded the same, the second is that there was no difference in traction. That effectively killed off the ‘big sneeze’ theory.
The mathematics say that inertia torque is reduced to almost zero before 10,000rpm and—crucially—to only about 3% of the 180-crank’s value at 15,000rpm. The experimental test to confirm the theory involved measuring rotational fluctuation of the rear wheel, a consequence of uneven torque delivery. With the 180 crank there are big torque spikes at all throttle openings, but with the highest peaks just as the rider gets on or off the throttle. The 90-degree crank shows no such behaviour, suggesting it would make getting into and out of corners a lot easier for the rider. Inertia torque (noise) is still there, it’s just at such a low level it doesn’t have a significant effect. Of course the first law of engineering says you never get something for nothing and an irregular firing order means vibration that may require a balance shaft or heavier components to tame, thus losing you part of what you’ve just gained.
These findings are of course all for in-line four-cylinder motors, but it’s easy to see how the 90-degree crankshaft can effectively mimic a V4—the back tire doesn’t know what direction the cylinders are pointing in! Is this an inherent advantage of the 90-degree V4 engine? Yamaha think not, and will continue with the in-line engine which they regard as enabling them to build a shorter and therefore more nimble machine. But they will have to use an irregular firing order crank.