Search This Blog

Wednesday, February 3, 2010

Cross Plane Crankshaft

The crossplane design was first proposed in 1915, and developed by Cadillac and Peerless, both of whom produced flatplane V8s before introducing the crossplane design. Cadillac introduced the first crossplane in 1923, with Peerless following in 1924.
In 4 cylinder engines, the technology is developed by YAMAHA Motor Corporation. They introduced this successful design in their MOTO GP bikes and the latest superbike.
This technology interprets the positioning of the pistons or arrangement in the crankshaft in an inline engine and also the alteration of the firing order. The rapid torque pulse from the engine is diluted in this way to get more traction from the bike when it is exiting a corner.
A crankshaft is a mechanical part in an engine which connects the big end of the connecting rod. Several big ends will be connected to the crank shaft. The crankshaft is also the output part of an engine.
Here, in the case of a four cylinder engine, the pistons 2 & 3 are moved opposite by 90 degrees instead of 180 degrees in a conventional inline four cylinder engine. The cylinders are then given an uneven firing order to create a different wave of power from the engine. This drastically cut short the high torque impulse which will reduce the traction of the tyre.
By doing these minor changes in the common working principal of an engine will alter the power pulse produced, thus allowing the vehicle to exit the corner with more speed without loosing traction and thus improve the overall performance of the vehicle

1.1 CRANKSHAFT


The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation. To convert the reciprocating motion into rotation, the crankshaft has "crank throws" or "crankpins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach.
A Dutch "farmer" Cornelis Corneliszoon van Uitgeest also described a crankshaft in 1592. His wind-powered sawmill used a crankshaft to convert a windmill's circular motion into a back-and-forward motion powering the saw. Corneliszoon was granted a patent for the crankshaft in 1597.



1.2 FUNCTION

The crankshaft should transfer the force from the piston and piston rod, as torque to the flywheel and/or the clutch. Together with its main bearing and connecting rod bearing it is part of the crank mechanism. Other parts belonging to the crank mechanism are:
- The piston and the piston pin,
- The connecting rod with the bearing bush at the small, and the bearing shell at the large connecting rod.

1.3 HOW IT WORKS


The piston rod encloses the connecting rod bearing of the crankshaft. The connecting rod bearings (here 4) are staggered by half a stroke in relation to the main bearings. Thus the crankshaft contributes its capacity, which results from cylinder bore and stroke. Opposite the connecting rod bearings frequently there are counterweights arranged, which adjust their out-of-balances at higher numbers of revolutions. If the crankshaft still exhibits imbalance after the manufacturing process, material is bored-away here. To the right the drive wheel for the camshaft drive via cam belts can be recognized. The lubricating of the connecting rod bearings takes place via (in fig. 4 above red marked) cross holes within the crankshaft from the side of the main bearings. The oil withdrawing at the connecting rod bearings is hurled by the rotating motion of the crankshaft against the cylinder walls and lubricates the piston and possibly also the gudgeon pin.

1.4 SPECIAL DESIGNS

Older v-engines might have two piston rods on a crankpin. These are not only broader, but they also have two drillings for oil lubrication of the connecting rod bearings (2nd picture above). Four-stroke engines usually have two-piece three-layer sliding bearings. The materials of the layers from inside (more soft) to outside (more hard): lead bronze, nickel and white metal. One of the bearings serves as thrust bearing. The crankshaft with a play of max. 0. 2 - 0.3 mm is axially stored. Mixture-lubricated two-stroke engines have antifriction bearings.

1.5 MATERIAL

Crankshafts are made from nodular graphite iron, quenched and tempered or nitriding steel. Heavily used crankshafts can be forged.

2.1 BALANCING OF INERTIAL FORCES IN THE MULTI-CYLINDER ENGINE

In multi-cylinder engines the mutual counteractions of the various components in the crankshaft assembly are one of the essential factors determining the selection of the crankshaft's configuration, and with it the design of the engine itself. The inertial forces are balanced if the common center of gravity for all moving crankshaft-assembly components lies at the crankshaft's midpoint, i.e. if the crankshaft is symmetrical (as viewed from the front).
The crankshaft's symmetry level can be defined using geometrical representations of 1st- and 2nd-order forces (star diagrams). The 2nd order star diagram for the four-cylinder in-line engine is asymmetrical, meaning that this order is characterized by substantial free inertial forces. These forces can be balanced using two countershafts rotating in opposite directions at double the rate of the crankshaft (Lanchester system).

2.2 BALANCING OF INERTIAL AND GAS FORCES

The tangential gas forces produce yet another periodic torque; this can be detected as reaction torque in the engine block. The composite forces generated in a four-cylinder in-line engine include free mass forces of the 2nd order as well as variable torque forces from the 2nd order mass and gas forces. Compensation for 2nd order mass forces, along with a reduction in the intensity of the 2nd order force transitions, is available from two offset balance shafts.



Balancing 2nd order inertial and transitional forces in a four-cylinder, in-line engine with two offset countershafts

1 Inertial torque only;
2 Gas torque only or complete balancing of inertial torque, zI – zII = – 2 B2/A2.r;
3 Gas and inertial torque without force compensation;
4 Gas and inertial torque with half of the inertial torque balanced, zI – zII ≈ 0.5.I.

3.1 V-TWIN

90 degree v-twins are famous for their drive out of the corners, and sure enough, they have zero inertial torque. As one piston is accelerating so the other is slowing down, and when one is stopped the other is at full speed. This is the reason why twin cylinders V-engines are able to accelerate faster from a corner than a conventional 4 cyl engine.

4.1 CROSSPLANE CRANKSHAFT


The crossplane crankshaft has four crankpins, each offset at 90° from the adjacent crankpins. The crankpins are therefore in two planes crossed at 90°, hence the name crossplane. A crossplane crank may have up to five main bearings, and normally does, as well as large balancing weights.
Their second-order balance means no additional balance shaft is necessary to achieve great smoothness.



4.2 WORKING PRINCIPLE


A conventional four-cylinder engine has its crankpins all in the same plane – a flat-plane crank – with the two inner ones 180 degrees from the two outer ones. The inner two pistons move up and down together, and so do the two outer ones, and it’s this particular configuration which generates something called inertial torque. This is independent of the main torque output generated by the combustion and cylinder pressure and happens entirely because of the crank layout.



It 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.

To understand it, first imagine a crankshaft on its own, no pistons or conrods, spinning in friction-free bearings. There’s nothing to slow it down or speed it up so it just keeps spinning at a smooth, constant speed. Now attach the conrods and pistons, and for the sake of this mind experiment, we’ll make them friction-free too, so you can spin the crank again and the pistons bob up and down, and the whole system keeps on rotating and reciprocating.

At this stage there’s no combustion or valve gear or anything to confuse the issue, and crucially, there is no energy being put into our system and none being extracted or lost. This matter because it is a fundamental law of the universe that energy cannot be created or destroyed, only converted into another form – physicists know this as the first law of thermodynamics.

Within this system, the pistons are travelling at high speed when they’re half way along their cylinders, and at this point they have a lot of kinetic energy. Yet 90 degrees of crankshaft rotation later, all four pistons are stationary, two at the top, two at the bottom. Their kinetic energy hasn’t simply vanished because it can’t: instead it’s been transferred to the crankshaft, which was responsible for slowing the pistons down. As a result, the crank itself has increased its speed. Another 90 degrees on and the pistons are back up to maximum speed, accelerated by the crank which has returned some energy to them and in turn, it’s slowed down again.

In a full rotation the crank will have sped up and slowed down twice, generating rapid negative and positive torque pulses completely independent of the torque produced by the combustion. This constant pulsing torque is like a background noise to the main torque output, blurring its edges and taking away a small element of rider control and precision as he tries to hold the back tyre on the very edge of its grip.
Basically what they do is they take the standard 1-2-4-3 firing order, flat plane crankshaft and move both cylinders 2 and 3 by 90 degrees and 4 by 180. This yields a firing interval of 270-180-90-180. It's essentially one bank of a V8. Their claim is that by moving cylinders 2 and 3 out of the plane, the inertial torque is split evenly throughout the cycle, yielding a smoother variation in crankshaft.
Engineers ran their firing order and phasing through the crankshaft program and got the same mean torque throughout the cycle. This makes sense (same amount of fuel and air combusted over the same period of time). Okay, so same mean power is produced over the same cycle. What is different are the peaks and valleys. cross plane crank are smaller (blue line) as opposed to the normal flat plane (pink line).



On Yamaha’s cross-plane crankshaft, these fluctuations are all but reduced. In this layout the crankpins are distributed at 90 degrees to each other around the crankshaft (in two planes which form a cross). So as one piston is slowing down and losing energy to the crank, another is speeding up and taking the same amount back. At no point do all the pistons stop together, as they do on a flat-plane crank. Instead the energy flow is evened out and the rotation of the crank is almost completely smooth and steady. This improves the ability of a cross-plane-crank, in-line-four bike to accelerate out of corners.

5.1 MATH GOING BEHIND

To see why, imagine a bike at the apex of a turn, where it’s fully leaned over and the rider is about to apply the throttle to accelerate. The rear tyre is already close to its limit from the cornering forces so it can’t take much torque without sliding. Let’s suppose it’ll just take 20lb.ft of engine torque before losing grip. On a cross-plane four with no inertial torque the rider can carefully turn the throttle until he feels the tyre just sliding – at that point he’s getting the engine to deliver 20lb.ft and he gets the maximum acceleration possible, with no interference.
With a flat-plane four the inertial torque effect means there’s a background torque pulsing of, say, 2lb.ft: 1lb.ft is added to the total output as the pistons are slowing down and the crank is accelerating, while 1lb.ft is taken away when the pistons are speeding up and slowing the crank again. If the tyre is going to slide at 20lb.ft, then the rider can only turn the throttle enough to deliver 19lb.ft, because twice every crank revolution the inertial torque is going to add another 1lb.ft and take it up to that 20lb.ft limit. Twice more per engine rev, the torque will drop to 18lb.ft, so the average delivered is still 19lb.ft.
But that’s 1lb.ft less than the cross-plane-crank engine before the tyre begins to slide, and if that doesn’t sound like a lot, it’s five per cent, which is a big difference in racing, equivalent to a peak power disadvantage of 12bhp on a MotoGP bike. So the engine with no inertial torque can accelerate harder out of corners, and it also gives the rider finer control as there’s no fuzziness to the output.

6.1 MORE FACTS

Even though Yamaha makes no claims of improved traction because of the uneven firing intervals of the 2009 R1 – the so-called Big Bang effect – there are still many who cite this as the motive behind the cross-plane crankshaft design. It’s to eliminate the high frequency torque fluctuations, so the uneven firing intervals are only a side-effect, not the objective.
So what becomes of the Big Bang theory of improving traction by introducing uneven firing intervals? The principle behind this depends on the difference between static and dynamic friction: a big, heavy wooden box might take two people pushing to start it moving, but once it’s sliding it’s much easier to keep moving, and only one person could do it. This is because at a microscopic level the rough surfaces of the box and the ground interlock when it’s stationary, but when it’s sliding they ride over each other.

Apply that to a bike’s rear tyre being fed pulses of torque by an engine. If there are fewer pulses the tyre has time in between each to recover any lost grip, so its surface can interlock with the road’s again, but when there are more pulses (as with a four compared with a twin), once the tyre is sliding the next pulse of torque comes along more quickly, before the grip can be regained, and the tyre keeps sliding. This means more torque overall can be applied by an engine with fewer, larger power pulses, an idea that came from seeing V-twins (usually Ducati’s) driving out of corners faster than the four-cylinder competition.
The problem with the theory is that its main principles are for static friction, and a rear tyre is clearly not static. The behavior of a rolling tyre is very different to a stationary box, and it is not clear if this static-dynamic situation would be the same. It’s likely to have similarities: we know from heavy braking tests that a skidding tyre results in longer stopping distances than if the wheels don’t lock up, similar to the sliding box situation. But a tyre creeping across a road, as it does under power out of a turn, is in a grey zone between sliding and grip, and we can’t be certain those principles are valid.
No proof has been offered either that the frequency of torque pulses from an engine is anything like that which might be needed to allow a tyre time to recover. Maybe they are, maybe not, but it’s vague enough to make Big Bang no more than a guess, rather than a true theory.

7.1 ADVANTAGES & DISADVANTAGES

ADVANTAGES

Better acceleration due to more even power delivery.
Better corner speeds compared to flat plane crank vehicle.
Better throttle response to the driver.
Better control over the vehicle.
Improved tyre life.
No need of a balancer shaft as its second order rotation is smoothened.
Superior mechanical balance of the crossplane design, do not require the large crankshaft balancing weights.

DISADVANTAGES

Only vehicles with very high engine speed will get advantage.
Uneven sound from the engine.
Uneven nature of the intake and exhaust systems due to the uneven firing order.
Manufacturing cost will be a little higher


This was part of my seminar done for the final year, Engineering.

2 comments:

  1. An excellent article with some good theoretical information, unsure whether the Cross-plane crank is an advancement on the Flat-plane crank for under 12,000rpm machines, no-one else seems to be adopting the lay-out, what about Honda's high revving machines, why have they not adopted this layout, 5% is a good advantage to add in racing terms.

    ReplyDelete