The crossplane or cross-plane is a crankshaft design for piston engines with a 90° angle (phase in crank rotation) between the crank throws. The crossplane crankshaft is the most popular configuration used in V8 road cars.
Crossplane crankshaft could be used in many cylinder configurations to have an evenly spaced firing as long as the number of cylinders is multiples of four in two-stroke engines, or multiples of eight in 4 stroke engines. Unless the crank pins have bigend phase offset, the V-angle requirement must be met for an evenly spaced firing on V configurations as listed below.
4 cycle: L8, L16, V8 (V-angle of 90°), V16 (45°,90° or 135°), flat8, flat16, etc.
However, crossplane crankshaft has been used on other 4 stroke configurations like L2, L4, V2 and V4 engines with unevenly spaced firing when its prominent advantage of smaller secondary (non-sinusoidal) vibration, which increases exponentially with crankshaft rotational speed, out-weigh the disadvantages like the imbalance in firing spacing and 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 have 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 double as frequent as in the flatplane design that 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. 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 3rd 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
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.
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 achieve 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 essential for uniform combustion and torque generation.
However, since the cylinders that fire (and thus exhaust) at 360° phase difference reside on the opposite banks on a crossplane V8, long exhaust pipes that merge the pairs are needed to achieve uniform scavenging (the 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, Ford GT40 made the concept on production-based engines famous with an elaborate arrangement of long exhaust pipes nicknamed 'Bundle of Snakes'.
Inline-four crossplane crankshaft
2009+ Yamaha YZF-R1
Inline-4 engines can also use the crossplane concept. The 2009 Yamaha YZF-R1 motorcycle uses the crossplane crankshaft and, in the absence of the 90° bank angle of the V8, must use a separate balance shaft geared off the crankshaft to eliminate the inherent vibration (a primary rocking couple) found in this type of crank.
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 cross-plane 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 accelerating (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.
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°.
Crossplane cranks used in a four-stroke, four-cylinder engine must result in uneven firing, since the natural separation of ignition events is 180° in such an engine (hence the 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.
Twin-cylinder crossplane cranks
Twin-cylinder motorcycle engines can also use the crossplane concept. The Triumph Bonneville T100 and the 1991 Yamaha TDM 850 motorcycle use crossplane crankshafts with specially designed balancing systems. British twin motorcycles of the 1950 to 1980 era, and Yamaha 650 and 750 motorcycles of the late 60's through the early 80's, 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. These modified engines do not use additional balancing systems but produce an acceptably smoother engine with lighter flywheels and more torque than a stock engine.
- See Crankshaft for "crank throw".
- See Engine balance.
- See Engine balance under "*Fluid - *Torque Balance"
- "Making exhaust headers for Ford Quad Cammer Indy engine". Retrieved 2013-11-10.
- Entwicklungsgeschichte des URS-Rennmotor, German language article on the developmental history of the URS engine.
- V8 Engines
- A crossplane-engined URS sidecar outfit being warmed up (YouTube video)