Joshy

as above, found this while browsing and havent come across it before.
not terribly in depth but interesing nether the less;

"For the uninitiated a screamer style engine is one that:
For a four cylinder engine, a screamer configuration would see one cylinder firing every 180 degrees of rotation on the crank.

A big bang styled engine is one that:
It takes 720 degrees of crankshaft rotation for a four stroke engine to complete the well known cycle of intake, compression, ignition and exhaust. In a four cylinder, motogp big bang engine, all the cylinders fire within a very small angular range of each other, my guess would be within 90 degrees.

The guiding philosophy being that in a big bang engine, the "relaxation time" is long enough between the time when the engine is applying force to the road (the first 90 or so degrees of crankshaft rotation, or ignition) and the time when it's going through the non-force applying functions of a four stroke engine (exhaust intake and compression), that the remaining 630 degrees allow the tyre to re-grip, such that if too much force is applied during that 90 degrees of ignition and the tyre begins to slide, there is still 630 degrees of non force application. This provides a buffer for the rider to not be sent over the handlebars.

Changing up through the gears means that force is applied to the wheel with decreasing angular frequency. For example, if there is a one to one relationship (we'll call this first gear) between the application of force to the wheel and the engine, then in the big bang style engine cited above, force will be applied to the wheel through the angles 0 to 45 degrees, then no force between 45 to 180 degrees, then force from 180 to 225 degrees then no force from 225 to 360 degrees, thus completing one full rotation of the wheel. As the gearing increases, the angular frequency decreases, if there is a 2 to one relationship (we'll call this 2nd gear) then force is applied between 0 and 22.5 degrees, no force from 22.5 to 90 degrees and so on.

So we have seen how the angular frequency decreases with gearing, shortening the time the tyre has to re-grip the road if too much force is applied during ignition. As wheel speed increases, the time it takes to complete one full rotation naturally decreases. So there are two things that shorten the time a wheel has to re-grip the road if too much force is applied during ignition, wheel speed and gearing. The higher the speed and the higher the gear, the less "buffer" a rider has during a slide.

Until now we have only focussed on a big bang engine, for a screamer engine, the situation is certainly far worse. A big bang engine can provide far more "buffer" as speed and gear increases than can a screamer. The screamer engine effectively is applying force constantly to the wheel, there is never any (useable) relaxation time. When Honda switched the NSR500 to a screamer in 1997, riders we're throwing themselves off all the time, only the skill of the rider could compensate for such a nasty peice of machinery, one of the reasons Mick Doohan won an incredible 12 of 15 races that year.

But there is a way for a screamer to compete with the big bang in this battle to squeeze in some sort of relaxation time for the tyre, should there be a sudden loss of grip. The solution comes from, as indeed the entire development of motorcycles, the bicycle. In time trials, it is not uncommon for cyclists to employ an elliptical front sprocket (a bicycle mechanic would call it a chainring). The reason being that an elliptical sprocket can smooth out the application of force to the tyre by changing the torque profile during the rotation of the crankshaft. On a bicycle, the main application of force is to push down on the pedal, the rest of the rotation of the crankshaft, you are simply waiting until your leg is in position to re-apply that downward force. The elliptical front sprocket drastically shortens this waiting time between the application of force. The torque profile changes as your leg reaches the bottom of the stroke, the gearing decreases at the bottom of the stroke, pedaling becomes easier, and your leg moves quicker through this period of non-force application, significantly extending the time that force is applied to the tyre.

For a motogp machine, the philosophy is the same, but applied in reverse. Where an elliptical sprocket is used to smooth out the force application on a bicycle, it can be used to roughen it up on a motorcycle. What I'm suggesting is that an elliptical rear sprocket would change the torque profile as the wheel rotates, this is exactly what a big bang engine does, except now a screamer can play the same game. At zero degrees of wheel rotation, with an elliptical gear, whose semi major axis is vertical, the engine would "see" a large sprocket, and consequently the wheel would be easier to turn. At 90 degrees of wheel rotation, the semi minor axis would be vertical and the engine would "see" a small sprocket and the wheel would be harder to turn. What this means is that when the semi minor axis is at the vertical, this is the point at which it is most difficult for the engine to continue to over-apply force, and cause the wheel to continue sliding, allowing the rider a buffer that he doesn't get with a circular sprocket.

The added advantage of an elliptical gear is that it also eliminates the problem talked about earlier of decreasing the time between the application of force with increasing gear. Because it is the final drive sprocket, the application of force is greatest at 0 and 180 degrees (semi major axis at the vertical) and least at 90 and 270 degrees (semi minor axis at vertical) no matter what gear the bike is in. So in this case, the only thing acting to shorten the time between the application of force is naturally the increasing wheel speed, as described above.

Finally, there is the other argument for using the big bang engine, and it's an argument I can totally agree with. You can read about it here. It is the idea that a big bang configuration is able to somehow improve the quality of the "signal" that a rider receives from the rear tyre, so that they can better understand what the rear tyre is saying to them. Furusawa talks about a connection between throttle and tyre in terms of harmonics, and for me the explanation is simple. The philosophy is that with a big bang, and as I speculate an elliptical sprocket, you can apply some sort of input, such as the application of throttle to a tyre, and be given in return a useable amount of time to determine what that input has done to affect the motorcycle. It's like a feedback system, but with a screamer engine, the input is constant and permanent, there is no time to "hear" anything other than the input signal, you never get the chance to "hear" the feedback signal.

One potential problem for an elliptical rear sprocket is that it would cause the chain to flop around a bit more than usual, although I'm sure a simple mechanism could be devised to take the slack out of the chain, similar to a bicycle derailleur. In saying that however, the eccentricity of the sprocket would not need to be terribly much to achieve the desired effect, and a "derailleur" of sorts mightn't be necessary, and besides that, if you've ever been to a race and watched a chain driven motorcycle, you'd know how much the chain flops about anyway. It is not clear to me why none of the teams that have ever used a screamer style engine did not or have not tried this, as from an engineering perspective, it is the simplest solution to the problem. And according to Occam's razor, the simplest solution is usually the best"

Joshy

any thoughts?

Chaz888

this is about control right i mean you got 200+ bhp with a cotact patch the size of a pringle .also wot about TC do you get better control for this also ? with big bang layout ?

Joshy

if you mean TC as in traction control then i think yes.

interesting firing order as well seeing as to have all pistons firing within 45 degrees of each other (presuming its a 2 stroke) would mean having a very odd crank pin layout

Joshy

Chaz888

screamer is basic 180 pulses, big bang i think is like 180 but fired like a set of twins or one twin two singles and the last is set like a v8 crank 90 lobs but only four pots but you need to think of power duration how much of a stroke is under power so from TDC to BDC how many degs is power ? this is how v engines gain , more degs under power per rotation .

Joshy

oh i get you now, makes a lot more sense lol.

obviously firing them as a pair means that this firing order would only work on a 2 stroke where the cycle repeats every time the piston rises

JonathanJ

Some years ago the DTM banned traction control , so in an attempt to give a traction control type pulse to the drivetrain the main manufacturers produced "Big Bang" engines , one manufacturer decided it was against the spirit of the rules because it detracted from their marketing stance of producing smooth engines in their road cars , so they pulled the plug on the DTM and concentrate on other forms of motorsport ........the manufacturer was BMW .

Big Bang engines have a purpose , but I believe that their use kind of detracts from good engineering , we as consumers strive for power and smoothness from our engines , and the Big bang engines go against that .

In Motor sport we should either allow traction control or rely on the skill of the driver/rider not to just open her up and rely on the tyres to cope with the sudden increase in torque applied , my thought is that in sport we should develop road going engineering practices , not produce something that is not practical in the "real world" .

Just my two penneth