4-stroke triples - and their firing sequence

[firg]

Hi,

Something occured to me a few days ago. I know how a 4 stroke 4 fires (1 piston fire every stroke), and how a 4 stoke 2 fires (fire, blank, fire, blank), however I don't know how a triple fires. It seems odd to me to have 'fire, fire, fire, blank', as this would give loose power every 4th stroke. And if it was 'doublefire, blank, fire, blank' this would give you a very jerky engine movement, with the 2nd half being weaker than the first.

So, what is the firing sequence of a 4 stroke tripple? (like the daytona 995), and what is the advantage of a triple over a 4, or a 2?, 'cause I just can't see it.

Any help appreciated,

Teh Cabbage Man
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Ex-'96 Bandit 600S -> '99 Bandit 600 -> K7 V-Strom 650

[MarJay]

You haven't quite got the concept of four stroke engines right in your head I'm afraid...

Basically, you are not factoring in the shape of the crankshaft. The way you are figuring, the pistons are always an equal distance apart from one another IE, 2 pistons 180 degrees apart, 4 pistons ninety degrees apart. It doesn't (always) work like that. Also you are considering that when there is an exhaust stroke, there has to be a power stroke to keep the engine running. Not so.

For example a 90 degree V twin (or more accurately an L twin) has its pistons on the same part of the crankshaft, IE the crank is U shaped rather than S shaped. This means that the horizontal piston moves toward the crank just after the vertical one does.

In an 'L' twin as detailed above, both of these strokes are the same in the four stroke cycle, so instead of the firing order being (Bang - Exhaust) then 270 degrees of movement and (Exhaust - Bang) - it is in fact (Bang - Bang) then 270 degrees of movement then (exhaust - exhaust). This produces what is known as uneven power pulses.

Now you might think that these uneven power pulses might produce bad vibration similar to a single cylinder? again not so. L twins are known to have very little primary vibration because the power strokes even each other out. I guess there is some sort of damping effect to having the pistons pointiong toward the crank at an angle of ninety degrees.

In the case of four cylinder engines they often work as you describe with a ninety degree crank and evenly spread power pulses. In actual fact if this was the case and the firing order was 1234 then the engine would shake itself apart!
Generally on an inline four the firing order is 1324 or something like that to help to balance the vibration. Even so there still doesn not necessarily have to be one power pulse per stroke. Valentino Rossi has recently asked that all of the power strokes on the Yamaha M1 GP bike be made closer together to create what is known as a "Big Bang" engine. This is in order that the tyre has time to grip again after a large power pulse.

An example of this was shown when the Ducati's were dominating the world superbikes and the inline four 750's were nowhere. This was because the Ducati V twin had large power pulses to allow the tyre to grip again after being dislodged from the ground, and therefore coud get more power down earlier. If the tyre on the 4 broke away then the flurry of smaller power pulses (in one smooth motion rather than huge gobs of power) would not allow the tyre to grip again. This is mostly interesting because the 750cc fours were in fact more powerful than the 1000cc twins!

Now finally we come to Triples. The firing order from left to right on my bike is in fact 123. This is good for a similar reason to that of the Ducati mentioned above. It allows more power to be laid down onto the road because the pause where the 4th cylinder might be and where the exhaust strokes are, means that it can lay better power down in theory, but still be more powerful than the equvalent twin.

The downside of this is that a power sapping balance shaft is required to stop primary vibration.

So Triples are in fact a superior compromise between the liquid power of a 4 with the laying power down ability of a twin.

And before anyone says that the balance shaft is inherently ineficient, then I ask them to look at the 60 degree V twin of the Mille. That has TWO balance shafts!
The advantage of the 60 degree V twin over the 90 degree V or L twin is that it is shorter and can fit into the chassis without making the wheelbase too short.

Also Triples are widely considered (along with expensive and complex V-fours) to be the best overall configuration for a motorcycle engine, because it is easy to position in the frame and lays down a lot of good power.

In conclusion (for those of you who hate my long posts) there are advantages and disadvantages to any engine configuration, and there is no one that is better than another (otherwise all bikes would have the same engine! )

Next lesson, I shal go into the differences between the three possible crank types of a parallel twin!
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Kickstart]

[firg]

Thank you for your very in depth replies - they have enlightened me - , however, I have one more question:

What is a balance shaft? I get that it dampens vibration, but how? Where is it placed? Is it just a wierdly shaped [/me doesn't know the name for this bit, but its the long(ish) kinked rod that all the sticks that come out piston fix to, and it rotates (is it called the comrod? camshaft?)] that minimises vibration, but doesn't transmit power very well? Is it two shafts that attch to the engine, cusioning it between the engine and the frame (like suspention between wheels and frame)?

Any help appreciated,

Teh Cabbage Man

PS, Don't worry about being very technical in your replies, I get all the thoery, I just dont know the names for all the fancy stuff. (Top end, bottom end? wtf? Just stick to piston head, stick that comes out of the bottom, and kinked rod that rotates. Meh. Plus loooooooots of bearings. And oil. And rubber rings. And shims (/me knows a proper name!). Yay! /me could be a mechanic!)
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Ex-'96 Bandit 600S -> '99 Bandit 600 -> K7 V-Strom 650

[Kickstart]

Hi

A balance shaft is a shaft with some weights on it, with the weights mounted off central. The shaft is rotated at a certain speed, and the idea is that the vibration it causes from the off central mounted weight will cancel out the vibration from other parts of the bike.

The exact design of the balance shaft will depend on how much vibration you need to get rid of. Think in theory a 4 stroke single needs 16 balance shafts to cancel out all the vibration, some spinning at twice engine speed.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[firg]

And are these balance shafts spun by the turning power of the engine, and thus sapping its power?
____________________
Ex-'96 Bandit 600S -> '99 Bandit 600 -> K7 V-Strom 650

[Kickstart]

Hi

Yes, they are powered by the engine. Yes they do use a small amount of power, and add a bit of weight. However as they smooth the engine out they allow various components to be lightened.

All swings and roundabouts.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[firg]

Okelydokely.

Cheers for replying.
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Ex-'96 Bandit 600S -> '99 Bandit 600 -> K7 V-Strom 650