Crossplane

The crossplane or cross-plane is a crankshaft design for piston engines with a 90° angle (phase in crank rotation) between the crank throws.[1] The crossplane crankshaft is the most popular configuration used in V8 road cars.

3d model of a cross-plane crankshaft demonstrating the 90 degree angle between the crank throws.

Crossplane crankshafts could be used in many cylinder configurations that have evenly-spaced firing, as long as the number of cylinders is a multiple of four in two-stroke engines, or a multiple of eight in 4 stroke engines. Unless the crank pins have big-end phase-offset, the V-angle requirement must be met for evenly-spaced firing in V configurations as listed below.

2 cycle: L4, L8, L12, L16, V4 (V-angle of 90°), V8 (45°,90° or 135°), V12 (30°,60°,90°,120° or 150°), V16 (22.5°,45°,67.5°,90°,112.5°,135° or 157.5°), flat4, flat8, flat12, flat16, etc.

4 cycle: L8, L16, V8 (V-angle of 90°), V16 (45°,90° or 135°), flat8, flat16, etc.

However, crossplane crankshafts have been used on other 4 stroke configurations like L2, L4, V2 and V4 engines with unevenly-spaced firing where its prominent advantage of smaller secondary (non-sinusoidal) vibration, which increases exponentially with crankshaft rotational speed, out-weighs the disadvantages like the imbalance in firing spacing and the increase in rocking vibration.

90° V8 crankshaft

The most common crossplane crankshaft for a 90° V8 engine has four crankpins, each serving two cylinders on opposing banks, offset at 90° from the adjacent crankpins. The first and last of the four crank pins are at 180° with respect to each other as are the second and third, with each pair at 90° to the other, so that viewed from the end the crankshaft forms a cross. The crankpins are therefore in two planes crossed at 90°, hence the name crossplane. A crossplane V8 crankshaft may have up to nine main bearings in the case of an eight throw design, and usually has five bearings supporting four throws each with a shared crank pin.

Crossplane V8 engines have unevenly-spaced firing patterns within each cylinder bank, often producing a distinctive burble in the exhaust note, but an even firing pattern overall. Their non-sinusoidal imbalance, owing to the lack of pistons that move together in the reciprocal phase, is half as strong and twice as frequent as in the flatplane design, which does have piston pairs moving together.

In the absence of balancer shafts, the disadvantages include rotating plane imbalances on 1.rotating mass (crankshaft), 2.reciprocating mass (pistons and conrods), 3.torque generation and 4.compression, all of which generate rocking vibrations.[2] These imbalances can be countered to varying degrees with heavy counterweights on each crank throw, so most crossplane V8s have very heavy crankshafts. Early Chrysler Hemi V8 had heavy counterweights, but the middle two positions on both sides of the center main bearing (the third of 5 mains) did not have any counterweight, as these positions are located close to the center of engine and deemed ineffective to counter rocking motions.

Another disadvantage is the aforementioned unevenly spaced firing within a bank of four cylinders, which can be mitigated by what is called a Bundle of Snakes as described below.

The other prominent design for a V8 crankshaft is the flatplane crankshaft, with all crankpins in the same plane and the only offset being 180°. Early V8 engines, modern racing engines and some others have the flatplane crankshaft, which is similar to that used in a straight four or flat-four engine. They lack the V8 burble but have double as strong (and half as frequent) secondary vibration of the crossplane design, and do not require the large crankshaft counterweights. Inherent balance of the reciprocating mass is like a pair of straight fours, and modern designs often incorporate a balance shaft for smoothness. When built without balancer shafts that add to the overall rotating mass, flatplane designs have the least flywheel effect of any V8s, which allows them to be quicker to rev up and down.

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.

Bundle of Snakes

Bundle of Snakes on Ford GT40. This is a modern version for street use. Originals for 7 Liter racing versions had more snake-like appearance.
Click the image to enlarge for details.

The characteristic 'burble' of a crossplane V8 comes from the exhaust manifold design which merge all four exhaust ports on each bank of four cylinders into one exit.
On a 4-stroke engine, each port generates the exhaust pulse once every 720° of crank rotation, and they need to be paired with the pulse generated at 360° phase difference in order to result in an evenly spaced exhaust pulse. This is needed not only for a clean exhaust note, but more importantly for uniform scavenging of residual gas in the cylinders, which is needed to fill the cylinders with the same volume of intake, that is essential for uniform combustion and torque generation.[3]

While the firing of the four-stroke crossplane V8 is evenly spaced overall, the firing order on the 'L'eft and 'R'ight banks are LRLLRLRR or RLRRLRLL, with each 'L' or 'R' ignition being separated by 90° crank rotation for a total of 720° for eight ignitions. As can be seen by counting four characters to the right of each 'L' or 'R' (4 x 90° = 360°), the cylinders that fire (and thus exhaust) at 360° phase difference reside on the opposite banks on a crossplane V8, so long equal-length exhaust pipes that merge the pairs are needed to achieve uniform scavenging. The exhaust design that achieves this is referred to as "tuned exhaust".

One of the earliest examples of a tuned exhaust for crossplane V8 was on 1.5 Liter Coventry Climax FWMV Mk.I and Mk.II in the early 1960s. While many racing crossplane V8 engines (like Ford 4.2L DOHC V8 for Indy racing) had exhaust ports on the inside of the V angle to make these exhaust pipe lengths shorter,[4] Ford GT40 made the concept on production-based V8s famous with an elaborate arrangement of long exhaust pipes nicknamed "Bundle of Snakes".

Inline-four crossplane crankshaft

Crank throw orientation is up-left-right-down in this drawing in contrast to flatplane's up-down-down-up

Unlike in a V8, crossplane arrangement in inline-four engines results in unevenly distributed firing pattern, so the use tends to be limited to extremely high-revving engines, where the advantage of less secondary imbalance outweigh the irregular firing interval disadvantage (in addition to the rocking vibration disadvantages arising from plane imbalances on reciprocating mass and rotating mass, if not countered with a balance shaft). Please see engine balance article for details.

2009+ Yamaha YZF-R1

The 2009 Yamaha YZF-R1 motorcycle uses a crossplane crankshaft and use a balance shaft geared off the crankshaft at crankshaft speed to counter the inherent rocking vibration (primary rocking couples) described above.

A crossplane crank had been used in Yamaha's M1 MotoGP racing models in the past. Yamaha claims advances in metal forging technologies make this a practical consumer product.[5]

URS engine

The so-called Fath-Kuhn straight-four engine, as used to relative success in motorcycle and side-car racing from 1968 by the private URS racing team, was also a crossplane type. It was a different configuration to that normally used in a V8 or indeed in the Yamaha above, with two of the throws being swapped around - i.e. the throws may be described as being at absolute angles of 0, 90, 180, and 270 degrees, versus the more usual 0, 90, 270, 180. This results in a slightly reduced primary rocking couple, but introduces higher order couples of much lower magnitude.

The different layout was primarily chosen to reduce the impact of the inertial torsion inherent with crank throws spaced 90° apart due to the pistons being accelerated (start-stop motion), given this engine was meant to be high revving and inertial forces scale as to the square of engine speed. The reduction in torsion was achieved by splitting the crank into two separate parts, geared together, from their respective midpoints, via a counter-shaft, from which power was delivered to the gearbox.[6]

It is likely this inertial torsion within the crank is the reason for Yamaha citing crank forging improvements as a reason for the cross-plane crank being viable in a road bike. It is less of an issue in the V8 because each throw is shared by two pistons already offset by 90°.

Firing intervals

Crossplane crankshafts used in a four-stroke, four-cylinder engine result in uneven firing, since the natural separation of ignition events is (720°/4 =) 180° in such an engine (hence the popularity of 180° flat-plane crank). The firing intervals (the space between ignition events) for the crossplane R1 and URS engines are 90-180-270-180 (crank degrees), but other intervals are possible including those due to so-called big-bang firing orders. The uneven firing is the cause of the distinctive sound of this configuration, which is superficially a combination of the 270-450 (90° V-Twin), 180-540 (180° straight twin) and 90-630 ("twingled" V-Twin) intervals, the dominant interval perceptually being the 270° one.

The 90° throw separation would make the cross-plane crank a natural choice for a two-stroke straight four, providing the advantages of both evenly spaced firing and less secondary vibration when the increased rocking vibrations are countered with a crank-speed balance shaft.

Twin-cylinder crossplane cranks

For more details, see Straight-twin engine

"Parallel" twin-cylinder motorcycle engines mainly came in two types: 360° cranks (pistons moving in tandem) or 180° cranks (pistons moving in opposite phase). The 180° crank was used in many inline twin cylinder 2 strokes, including quite large capacity such as the 598cc Scott or 498cc Suzuki T/GT500. Most early Honda 4 stroke twins including the 450cc "Black Bomber" and CB500T made before the 1977 Dream/Hawk had 180° cranks. Due to their small displacement, the rocking couple was acceptable without a balance shaft, particularly when compared to a similar sized 360° twin also lacking a balance shaft. As the 400cc Dream/Hawk CB250/400T was to replace the 4 cylinder CB400F, to obtain smooth running closer to that of the four it was a 360° twin with a balance shaft - the even firing of the 360° crank noticeably smoother than the uneven 180° crank.

The 1996 onwards MK2 Yamaha TDM 850 motorcycle used a 270° (90° measured the other way) crankshaft with a specially designed balancing system to counter the resulting combination of free forces and rocking couples. Namely, it has smaller free forces than the 360° crank (but much larger than the 180° crank) and smaller rocking couples than the 180° crank (the 360° crank has no such couple), all else being equal. Whilst it was uneven firing still (exactly like a 90° V-Twin), it was not as uneven as the 180° crank. So the configuration represents a compromise of sorts and has continued to be used more recently, e.g. Honda's NC700 and new Africa Twin, Hinckley Triumph's Scrambler and Thunderbird cruiser, Yamaha's MT-07 / FZ-07 etc.

British twin motorcycles between the 1950s and 1980s, as well as Yamaha 650 and 750 motorcycles of the late '60s through the early '80s, have been modified to use cross-plane crankshafts under the terms offset crankshaft or rephased crankshaft with notable success in reducing the vibration inherent with a stock 360 degree vertical twin.[7] These modified engines do not use additional balancing systems but are smoother with lighter flywheels. The lighter flywheel is possible because the pistons do not start and stop at the same time, so rotational momentum does not need to be stored up as much to compensate, it is simply transferred between the pistons directly (through the crankshaft). This is the same principle Yamaha uses in its crossplane four cylinder engines, where the extra two cylinders account for the non-symmetry of piston motion in the upper and lower halves of their strokes,[8] resulting in greater minimisation of the inertial torque caused by changes in rotational momentum.

See also

References

  1. ↑ See Crankshaft for "crank throw".
  2. ↑ See Engine balance.
  3. ↑ See Engine balance under "*Fluid - *Torque Balance"
  4. ↑ "Making exhaust headers for Ford Quad Cammer Indy engine". Retrieved 2013-11-10.
  5. ↑ http://www.yamaha-motor.com/sport/products/modelfeatures/6/0/features.aspx
  6. ↑ Entwicklungsgeschichte des URS-Rennmotor, German language article on the developmental history of the URS engine.
  7. ↑ http://www.xs650chopper.com/2009/06/rephased-xs650-chopper/
  8. ↑ http://www.epi-eng.com/piston_engine_technology/piston_motion_basics.htm

External links

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