By Adam Arnold
Following the announcement that the Virgin Yamaha British Superbike team will run a 'Big-Bang' engine configuration in this season's British Championship, there has been a great deal of head-scratching as to how exactly they have managed to pull it off.
However, an independent investigation, conducted by
Crash.net's Adam Arnold, has come up with the answers:
The 'Big-Bang' Superbike engine is nothing new but, although extensively tested, it has never previously been raced due to the problems caused by the two big power pulses produced. These two 'Big-Bang' pulses, within a four-cylinder engine, create a tremendous amount of stress, often leading to failure within 30 minutes.
The problems start with the sheer force of two-cylinders being firing together closely in an engine that was built for one cylinder to fire at a time. This tends to put enormous stress on the crank-shaft - the part of the engine that first comes into contact with the force of the 'bangs' - and the gearbox, which is accelerated tremendously after each massive pulse.
In
MotoGP the rules allow the teams to use bespoke crank-shafts, which they use to offset the firing of each of the cylinders in the 'Big-Bang' arrangement by 40-50 degrees of crank rotation, thus lowering the stresses put on the engine. However, Superbike rules dictate that the 1000cc four-cylinder machines must use a standard crank-shaft.
There is also a massive increase in stress on the camshafts, as Rizla Suzuki's Scott Smart - well versed in the technical aspects of race engineering - describes:
"When we tried it ('Big-Bang') with a GSX-R600 K1 years ago, the force on the cams of opening two sets of inlet valves at the same time, with no help from the closing of another set, was simply too much strain on the cam chain," he said.
The failure in the cam chain arises when the cam-shaft tries to open two-cylinders worth of inlet valves simultaneously, the force from the valve springs being simply too great to overcome for long periods.
In a conventional engine, whether it be a four-cylinder or a twin, the opening of one set of valves is helped by the force of the springs closing on another set of valves. This effectively reduces the amount of strain the cam arrangement has to go through in an engine cycle.
Crash.net believe Virgin Yamaha have overcome this problem by not firing their 'Big-Bang' engine in the traditional two-by-two firing order expected from a four-cylinder.
Instead of firing the cylinders in two groups of two,
Crash.net believes the Virgin Yamaha team fire one cylinder on its own first, then two cylinders together, and then the fourth and final cylinder on its own. This effectively reduces the amount of stress on the engine by half.
But don't just take our word for it; here is our evidence to back it up:
Analysis of audio recordings of the Virgin Yamaha 'Big-Bang' engine showed three distinctive 'bangs': One bang, followed by a second - even bigger - bang, and then another smaller bang. The audio waves the engine pulses produce can be seen in the lower picture.
The time between each of the pulses was worked out to be 180 degrees of crank rotation with 360 degrees of rotation rest in-between each cycle, as would be expected.
To back up these claims, the same experiment was used on two other engines - where the firing order is already known: A 90 degree V-Twin Honda and a Triumph 855cc triple. Because the firing order is known it is possible to see if the method is correct, which it was found to be.
"My engineering head tells me that this makes perfect sense," commented Smart on the evidence presented to him. "The firing of the single cylinder before the two together sets up the stressed members so when it (the two cylinders together) does come, it is not such a big shock."
This 'preparation' of the engine, firing a single cylinder first, is believed to be the key to this design working and being reliable over race distance.
