The crossplane I4

Thursday, November 10, 2011

Tags :  crankshafts

crankshaftsYamaha is the only company running an I4 in the MotoGP series, a layout to which it has been loyal since the inception of the current four-stroke formula; all the other engines are now V4s. Having the cylinders in a vee, rather than in a line, allows the crankshaft to be shorter and stiffer, with fewer main bearings, and the engine to be narrower. This is, however, perhaps not such a great advantage.

The reasoning can be found in the width of the human knee. Complete with protective clothing, a biker is at least 350 mm across the knees, even with them clamped tightly together. With a chassis in between them, the rider's knees are a lot wider than most engines. Using similar reasoning, ankles are about 125 mm wide each, and they have to go each side of the swinging arm (which, in turn, must be wider than the rear tyre) so there's no point in trying to make an engine much narrower than this. (In the previous Grand Prix motorcycle racing formula, for 500 cc two-stroke engines, Honda made both V2- and V4-engined bikes, and the difference between them in overall width amounted to only a few millimeters.)

Perhaps surprisingly, having a light crankshaft of low inertia seems to give no particular advantage in bike racing. Riders are usually happier with the bigger flywheel effect of a heavy crankshaft. In other forms of bike racing, like motocross or flat-tracking, it is common for riders to optimise the crankshaft inertia with bolt-on flywheel weights. Bizarrely, the motorcycle crankshaft is thus now a piece of sports equipment which, like a shotgun, cricket bat, or golf club, has an optimum inertia that must be tuned to individual competitors.

A current MotoGP bike spends a surprisingly small proportion of a race developing full power. Wide-open throttle (WOT) is commonly used for less than 20% of a lap, even on high-speed circuits. Critical to improved lap times is best possible use of available grip when accelerating away from the apex of a corner. To this end, MotoGP bikes rely on a combination of advanced traction control systems and measures to improve the riders' 'feel' of the tyre grip.

A racing motorcycle can bank to more than 50º from the vertical, and its movements in yaw and pitch are much more significant than those which a racecar experiences. The inertial and gyroscopic effects of the crankshaft are certainly large enough for riders to notice, and they prefer the feel of a heavy crankshaft because the flywheel effect of a high-inertia crank slows down its rate of response and makes it easier to control.

In trying to improve the performance of their I4, Yamaha engineers speculated that an inherent (and hitherto unnoticed) advantage of the vee-configuration engine was the constant crankshaft speed, due to one piston being somewhere near to maximum velocity as its partner (which shares the same crankpin) is at standstill. By contrast, the cyclic speed variation of the crankshaft of a conventional I4 engine is relatively large, because all the pistons change direction - that is, come to a halt - at the same time, and all of them reach their maximum speeds at more or less the same time.


Yamaha's solution, the 'crossplane' layout of the latest I4 MotoGP crank designed to mimic the V4's low 'inertia torque', gave rise to a more constant crankshaft speed, and also to a lop-sided exhaust note due to the irregular firing intervals.

As is often the case in race engine design, an elegant theory has failed to make much impact in the real world. The Yamaha MotoGP engine is down on power compared to its V4 rivals. Yamaha released a production sports bike with a crossplane crank, which was hailed with great fanfare by journalists for its uncanny grip out of corners, and yet in racing for modified production bikes - where the crossplane crank competes with conventional 'two up, two down' I4s - the Yamaha is only tolerably competitive in the World and British SuperBike series. It also made no impression on the leader board at the 2011 Isle of Man TT races, a 'real roads' event where one might expect the Yamaha's supposed improved traction to be most significant. All the TT races this year were won by conventional flat-crank I4s.

Fig. 1 - Yamaha's crossplane I4 crankshaft

Written by Ian Cramp

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Simply considering crank/conrod/piston inertias with regards to throttle response is a bit myopic.  Since the engine is routinely coupled to the clutch, transmission and rear wheel, these inertias/loads must also be considered, and they are typically much greater than the internal engine inertias.

As for a V4 versus an I4, there is not much difference with regards to inertias.  If other bikes are faster than the Yamaha, it likely has more to do with the rider than the engine configuration.
Why would it be down on power? Is that a consequence of the crossplane?

If each bore is giving a certain amount of torque, it then just a case of assembling the 4 bores to fire in some order, at speed and thus make power.

Point being, if the same bores were aligned in a V4, like Honda RC212V or Suzuki GSV-R, it should give the same power.

The I4 would not produce less horsepower than the V4, all other things being equal.  Where I4 and V4 engines would differ is in their instantaneous torque output profiles.  If you were to plot the net torque produced at the crank end versus crank rotation over a full 720 degrees of crank rotation, you would note that the torque varies greatly over that period.  

If you then made a more detailed plot showing 4 overlaid curves of the net torque produced by each individual cylinder over the same 720 degrees of crank rotation, you would see how the total resulting torque at the crank output differs between the two engine configurations.  If you then also made plots of the crank instantaneous angular velocity over that 720 degree interval, you'd note that they have similar differences.

What the author tries to point out is that with two engines that have different cylinder configurations but similar rotational inertias, the configuration that gives more evenly spaced firing intervals (ie. the I4) will have a more uniform crank angular velocity over a given interval than another (ie. the V4).
Sorry to come to the party late on this.

No Terry, that’s not what the author says at all.

He states that (amongst many reasons) the I4 has a less constant crankshaft velocity than (all things being equal) V shaped engines.

This, in the article, was attributed to the fact that most I4 engines' pistons' pairs come to a standstill at TDC & BDC simultaneously. Whereas with the V2 example given, whilst one piston was coming to a standstill the other was approaching V max - effectively offsetting the otherwise minimal instantaneous torque (of the cylinder whose piston it was that coming to a standstill) contribution to the crankshaft's rotational speed, that would have otherwise (in the I4 configuration) resulted in the crankshaft speed slowing down or fluctuating.

It all comes down to when you sum the instantaneous torque and/or combustion product/torque output from each cylinder. When this is done in a way where the crankshaft rotation speed is either favourably fluctuating or as minimised (constant crankshaft speed) as possible; the conditions can suit rider performance. Particularly when the bike is leant over and the coiled spring and typical suspension mechanisms are not necessarily providing the bulk of the entire motorcycle's operating suspension and therefore other systems are in place to control tyre contact patch and rider feedback.

This characteristic (constant crankshaft speed) was said by the author to be attributed to the V configuration engines sharing the same crankpin - which is (a bit like) like Siamese (V) twins - where one goes to stand up whilst the other sits down. The net effect of Siamese (V) twins is that something in between happens; that is usually not described by stationary. If they were not co-joined twins, neither would have a direct input on the other's intentions and one may be stationary; therefore imposing one of the most significant and undesirable crankshaft moments on the composite torque relationship and crankshaft speed. (I hope that makes sense).

In short the Yamaha engineers (according to the author) noted the inherent advantage of the V configuration was the constant crankshaft speed. Because most given piston's V min expressed less influence upon crankshaft rotational speeds when co-joined at the crankpin (like Siamese twins) with another piston conrod assembly that was not stationary.

By contrast, therefore crankshaft fluctuations associated a conventional I4 engine are relatively large (even though their firing order makes them sound smoother than, say V configuration motors or even X plane I4 motors), because whenever any piston comes to a momentary halt within a traditional I4 motor there is no other piston connected to its same crankpin that is moving, let alone approaching V max.

The Yamaha crankshaft design goes a little further than this though, as the way it superimposes instantaneous or combustion torque across the derived composite torque - there are several variations of how to do this with firing order and other things - is said to offer the rider greater feel and assist to deal with chatter. Another story.