MotoGp engine theory

[DanJR1]

with all these teams rephasing engines at the moment, is it possible that the bigger the cylinder size (and the less amount of cylinders it has) the amount of power from each cylinder firing is still great enough to help destroy the tyre ie Aprilia RS3 cube. The honda RCV has the smallest cylinder size (and the most amount of cylinders)so the amount of energy from each piston firing is less, so it doesnt completely destroy the tyre.

Also, it isnt possible to rephase a 3 or 5 cylinder engine is it because you have an odd number of cylinders :confused:

[Chucky]

Rephase? Do you mean playing with firing orders?

[DanJR1]

yep

[halecmini]

you still have the problem with getting the thing ballanced.. and are you sure about your theory? smaller cc of each bore less tyre wear? i can understand it to a certain degree but surely the riders have got the most to do with it?!?

[shiny side up!]

I would think the rider, weight distribution, and gearing have far more to do with it than the amount of combustion force per cylinder...

[bartman71]

i have read that the Honda V5 fires 2cylinders at the same time and 1 difrent. this way they get one with less power and helps the rear tyre life
EG: 2,1,2,1,2,1 ect

[Carse]

does the 2,1,2,1 etc firing order mean the single firing cylinder has 2 the amount of ignitions/combustions of the pairs of cylinders firing simultaneously?

[Carse]

sorry, twice the amount of combustions/ignitions.

[Paul Ranson]

No, each cylinder fires every other revolution (give it's 4 stroke).

So it must be 2, 2, 1, 2, 2, 1, 2, 2, 1...

But what the angles are between the firings is anybody's guess. I'd guess 288, 288, 144 but only because it seems reasonable.

Paul

[Carse]

Angles between firings? as in the angle of the conrods to the crankshft?

[Paul Ranson]

The angle the crank turns through between firing one cylinder and the next. A four stroke engine repeats itself every two turns of the crank, 720 degrees, so every cylinder must have fired every two turns.

Paul

[RWC]

Please note that the following is not ironclad fact,just bits and peices i've learn over the years.It may not be completely correct

This has been a very important design feature for the engine designers/tuners for many years now
As far as i know it started back in the late eighties when they got serious with 'big bang' 2 stroke 500 GP engines
The idea then was that if they fired two or more cylinders together (or close together),that gave the tyres much more "feel" when sliding.
The average rider could then slide a bike much more consistantly which was VERY important for fast lap times in that era.
Basically with the tyres they had and the riding styles they used -the tyres worked best if given less regular but bigger 'lumps' of torque rather than many smaller ones.

Since then technology has moved on and tyres are .....just different.They tend to lean the big bikes much more now,like they do with the smaller class bikes.

I seem to remember that the transition period was around the mid nineties (?).Even mick doohan gave up the 'big bang' engine for the 'screamer'.The reasoning was that-with the improvements in tyres,chassis etc ,the more difficult to ride but faster screamer bike wasn't all that hard to get along with but gave a significant extra edge

The modern bikes tend to use clustered firing orders for slightly different reasons.Now-as i understand it from articles i've read-they still group the firings together but never two at exactly the same time
The yamaha for instance is supposed to have the four cylinders firing in two closely grouped pairs.
The idea being that as the first one of the pair is providing it's maximum firing force ,it's partner is nearing it's maximum compression.(maybe phased only 100 deg or less apart?)
That way the engine has two very smooth power pulses from four cylinders instead of the many power/drag phases that an equal firing order engine has
While it sounds abit odd that two big pulses could feel smoother that four little ones aparently that's the general idea for why they are doing it

As in the earlier 'big bang' era they are customizing the engine output to exactly what the tyres work best with

The big bang 500 two strokes needed the larger power pulses to control the brutal slides effectively

The current 1000 four strokes need super smooth feeling power to slide more subtly -and more efficiently

[ysofast]

Certainly sounds like a warrenty would be out of the question with this uneven firing. Would it also affect engine reliability in the race, or would the advantages mention out way that.

[Hazza]

Well, for what its worth. I do remember Jeremy Burgess saying that he didn't have to replace the engine in the fourstroke after qualifying like he did in the 2-strokes. So obviously they last longer.

[RWC]

As long as the drive parts of the engine are properly designed for the extra forces ,everything will be the same (if these do increase-it depends on the design)

The point that Hazza makes is not really related to this but is interesting just the same.
four strokes tend to last longer but cost more to rebuild
two strokes need more regular rebuilds but it's cheaper to do so

This tends to be a rule that applies wether for race engines or production engines