Firing order and traction

[antti]

Hi All,

This is a question I have heard a bit about but have never read a decent believable technical explaination. It relates to changes in firing order has on traction and engine characteristics, especially concerning motorcycle engines.

The basic belief is that a 1000cc V-twin powered bike can effectively find more traction off a corner than a, say, a 1000cc in-line 4, purely because of the wider spacing of firing pulses. Also, back in days of 500cc GP motorcycles (pre-2002), the last decade was dominated by so called "big-bang" engines. Here the firing order of the 500cc V-4 was changed so the cylinders would fire more closely than the usual, evenly spread firing orders. Again, it was supposed to improve the traction of the bike.

The explainations I've heard is that having longer periods between firing allows rear traction to be regainged if the tyre was pushed into slipping during the pulse (basically a static vs kinetic friction thing). Personally I would expect larger widely spaced pulses to more readily drive a tyre into slippage than smaller even pulses.

Some also claim the close firing order makes egines more "torquey" or more suited to running at lower rpms, but whether that is due to the additional flywheel weight, I don't know.

I was wondering if anybody had some insight into the situation because it does seem to be effective, eg, in the "big bang" 500 cc GP bikes?

 

[Azmio]

Antii
This is an interesting question. I also wonder whether the V twin gives more traction to the tire. I am also not sure who is right or who is wrong.
One thing that i know is that the V twin power pulse is bigger than the 4 cylinder power pulse. This comes from the bigger displacement for each of the twin cylinders. This is the reason why I keep on wondering whether the theory of bigger interval of the twin gives more traction to the tire.
Anyway, one good reason that I can think of is that the twin cylinder has better low and midrange torque. The torque spread is much better than the 4 cylinders. The twin also has less peaky torque if compared to the 4 cylinders. Better low and midrange plus the less peaky torque will translate to better traction.
We also have to understand that the current world superbike championship decision makers are dominated by Italians. I heard that the technical group of the comittee made the calculations for 2,3 and 4 cylinders engine. They made it extremely difficult for bikes with more than 2 cylinders to have advantages by restricting the intake flow through the use of restrictors. Before this, the 4 cylinders can have only 750cc compared to 1000cc for the twins.
That is the reason why WSB is dominated by the Italian Ducati two cylinders bikes. Japanese OEMs were very unhappy over the unfair rules and made a right move by staying away from the championship.
Perhaps, the only reason that the twins to be winning a lot in the WSB is that they have the unfair advantage over other bikes.

AO

 

[Metalguy]

HP/cu in vs fuel use/HP--that's the difference. With everything else equal, more cyls. favor the former, less cyls. the latter. Many cyl. engines can be built to have very high amounts of low speed torque, but they have a huge advantage at high RPM. This is all very elementary.

 

[Azmio]

Metalguy,
Back in the 90s, F1 engines can have as many as 12 cylinders. Yet the most successful engines have only 10 cylinders and not 10. The 12 cylinders benefit the most at Monza and Hockenheim. The rest of the tracks favor the V8 or V10. They ended up having only 10 cylinders and not more and not less. To win the races, it's not about having the most cylinders to get the most power.

 

[antti]

OK, I think I might not have made my question clear enough. I wasn't asking regardling different numbers of cylinders, just different numbers of firing impulses. Will firing order ALONE, make any difference to an engine's ability to get traction or any other characteristic.

The 500cc GP engine is the best example I can thing of. Take 2 V4 engines otherwise identical - one with many cylinders firing at once, one with the pulses evenly spread. The former is supposed to provide better traction. Why?

 

[Metalguy]

Azmio,

Probably a HP vs fuel use issue. *Never* bet against more cylinders if you need HP/cu in. Yes, HP alone doesn't win races.

 

[patprimmer]

I think in the 90's F1 had fuel use limits, which would completely distort the picture, as both maximum power and good fuel economy were both important. As the requirements for each are often conflicting, therefore, the trick was the best compromise.

Regards
pat

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[Chumley]

Interesting question. I'm not sure if braking generated traction and power generated traction would be similar but, intuitively they seem similar.

Engineers who design braking systems seem to think that ABS when properly applied is more effective than standard brakes in that the ABS systems lock and unlock (pulse) the brakes as many as 20 times per second depending on the system providing shorter stopping distances as opposed to being in a partial to complete skid.

The coefficient of static friction is usually significantly higher than the coefficient of sliding friction for most materials.

So, if leaving a corner with a smooth power application causes the tire(s) to break loose or be right on the edge of it as opposed to lots of power pulses which continuously break the tire(s) loose then allow it to regain traction in a similar way to an ABS setup, that could cause the tire to continuously move back and forth from static to sliding friction. Since the coefficient of static friction is higher, "in theory" it might just make sense. In practice, well, the stopwatch tells all! I'm sure rider/driver experience, other engine torque characteristics, weight and balance, and track design/conditions are just as significant, if not more.

Chumley

 

[76GMC1500]

Ok, I figrued out why big twins get better traction than the 4 cylinder engines. The engines with 2 large cylinders require more of energy be stored in a flywheel than a similarly displaced 4 cylinder in order to compress that large volume of air. A large flywheel means the rotating assembly has a lot of inertia and cannot accelerate fast enough to break traction. It also means that if you lose traction, the rpms wont build as quickly so you'll have time to react.

 

[antti]

Thanks for your reply. I figured the flywheel weight would be a big part of it, but I wonder if that is all. People do make 4 cylinders with differing firing orders, hence, maybe require different flywheels. I think if it was purely a flywheel weight issue, why not just use a heavier flywheel on a normal 4 cylinder.

I wonder also if people working on abs find significantly different performance from different system cycle rates.

 

[76GMC1500]

Everything in automotive design is a compromise. A lighter flywheel gives better throttle response because there is less mass to accelerate, and shifts are shorter because it takes less time for the engine to decelerate from high rpms. Heavier flywheels improve rideability by reducing the rate at which the engine can accelerate during wheel spin, and it effectivly gives the bike a little more low end torque to get it moving. One of the major differences between a motocross bike and a trail bike (dirtbikes) is the flywheel. Motocross bikes are fast reving, high winding engines while woods bikes need to be nimble and torquey. They only difference internally is the flywheel, compression and camshafts remain the same.

 

[patprimmer]

I thought flywheels also had a pronounced gyroscopic effect on motor cycles

Regards
pat pprimmer@acay.com.au
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[76GMC1500]

Could be, I ride dirtbikes and they are tall so it is very easy to upset the balance and initiate a turn. They wheels have a very pronounced gyroscopic effect, and you can use this to initiate a turn. In order to make a left-hand turn, you push the left handle bar (turn them to the right) and the gyroscopics will cause the bike to lean to the left. It takes so much less energy than trying to make the bike lean by yourself. I don't notice any gyroscopic effects due to the engine. The rotating parts don't weigh very much and are of very small diameter so their gyroscopic forces aren't very pronounced.

 

[Azmio]

Guys,
Are you guys sure that the race bikes competing in the WSB championship have flywheels? I may be wrong but I dont recall flywheel being used in sportbikes. Same with F1 race cars, I guess.
However, I do know that touring bikes like the ones from BMW have flywheel. The touring bikes are different from the sportbikes in which the sportbikes rev rapidly up to 15,000rpm. Flywheel will really slow it down but the touring bikes will benefit in term of better ridability.
There is also not much packaging space for flywheel to fit in. Otherwise the riders wont be able to lean the bikes very close to the tarmac.

AO

 

[76GMC1500]

You're right, motorcycles don't actually have a flywheel, they use the alternator as a flywheel. Formula 1 uses a multiple disk clutch very similar to what is found on a motorcycle and probably uses the basket as a flywheel because there is a lot of weight there.

 

[mburgess]

Antti,
To have a different firing order in a 4cyl you would need a different counterweighted crank and possibly a counter weight shaft to control NVH. The combination of these may have a flywheel effect but a flywheel at the end of the crank will not dampen out inherent unbalances in the crank. Also remember that a lighter crank/flywheel can decelerate at a greater rate as well as accelerate (ie spin up faster) and this itself can have a major effect in the stability of a motorcycle, esepecially under braking. A greater flywheel effect will not make an engine more "torquey" but maintain a greater energy for a given rpm, this may feel like torque in some instances like when releasing the clutch. It will also carry you into a corner more under a closed throttle as there is more crank inertia to dissapate. Also from what I have read GP engines prior to the big bang firing order had a very savage power delivery (ie a big step in the torque curve) and the big bang firing order an effect of softening the throttle response making the bikes easier (relaitvely) to ride. In the last years of 500's many riders switched back to the even firing order engines as electronics & engine design were able to deliver a more manageble power delivery.

76GMC1500,
The major effect on stability of a motorcycle is the chassis geometry and also the geometry of the tyres. Mainly the rake and trail of the front fork. The inertia of crank and wheels is minimal by comparison.

Regards,
MB

 

[antti]

OK, thanks for your thoughts folks.

Antti.

 

[seniorbigs]

I remember a teacher telling us about motorcycle hill climbs. Some people were cutting and welding the camshaft so the big four cylinders had #'s 1&4 firing at the same time, then 180 degrees later #'s 3&4 would fire followed by 1&4 540 degrees later. The idea was to let the tire hook up. Now it seems like the big Harleys dominate the unlimited classes. Great question, Mike

 

[rd400guy]

I'm a little late in responding, but are you referring to Ducati's "Desmosidici" GP bike?

http://www.ducati.com

Check out the MotoGP part under Racing links. It details the Desmosedici, AKA "Twin Pulse" GP bike.

I think this is exactly what antti had in mind with the question. Ducati designed a 989cc, 90 degree V4, but the pistons in the forward bank travel side-by-side and fire simultaneously, as do the pistons in the vertical bank. Claimed horsepower is 220+ @ 16,500 RPM.

In one of the articles they claim that having two smaller pistons fire simultaneously gives the "feel" of one big piston. Or in this case, a V4 that rides like a big twin. BUT, there is a blurb about testing a "conventional" firing order where each piston goes through its cycle independently, however there were concerns about the rear tire losing traction.

 

[mburgess]

Prob with the theory was ducati found in testing that a more conventional V4 firing order was a faster setup. Didn't destroy the cases as much & NVH was far superior.
Cheers,
MB