Firing order - position
 

I've seen the animated GIF on the front page of the v-twin doing it's thing.
It's obvious the pistons run in sync to each other and not opposed.

Question is does anyone know if all twins run like this? (Not just SV's) Or do some run opposed to each other, IE while one piston is on BDC the other is TDC.

I have a friend who thinks they're opposed but I think they're in sync. :?:

Any engine bods who know? :wink:

Uneducated guestimation (my second of the day :O)

But would pistons that oppose each other both be firing at the same time? That'd mean riding along to a BANG BANG BANG type noise, and not caused by the can!

The rev's wouldn't sound half as civilised as they are. I suspect it would also cause a very lumpy acceleration.

Yours sincerely,
Mechanical numpty.

No, because opposed pistons can't fire at the same time if one is TDC and the other is BDC!..

I wondered if there was a configuration other than..
http://www.sv650.org/images/90-Vtwin-sm.gif

My mistake, as I said...

V4 in a sequential firing order or possably a 2 stroke twin could do that i think.

that seems to be a 90degree [spacing between firing strokes] crank

I'm sure there's lots of options - not least "big bang" - although whether it's easy to balance those other options I don't know

I don't know of any 90 degree V-twins that are arranged other than one at TDC and the other midstroke - there's a good reason for this:
The pistons whilst light are moving quickly and therefore have significant momentum, at midstroke they are moving the quickest for any given RPM and thus their momentum is at it's greatest, instead of having a larger flywheel to store energy in and turn the motor when not under gas pressure, (that is; the firing strokes), each piston's momentum carries the motor when the other piston has none - at TDC and BDC the piston comes to rest and reverses it's motion at this time it provides no turning force to the engine.
In the case of V-twins that aren't at 90 degrees they may share a crankpin and just go with whatever timing that gives or sometimes the exact postion of each piston is arranged to maximise the momentum effect.
In the case of parallel twins the pistons may rise and fall together but not on the same stroke, (aircool Triumph twins, lots of Hondas), or they may be at complete odds - ie one at TDC and one at BDC, or sometimes, (TDM Yamaha), they may be arranged like a 90 degree V-twin one at TDC and one midstroke.
BMWs have both at TDC at the sime time, (but not on the same stroke), otherwise the motor would shake from side to side unacceptably.

Just to add a little to the above, the normal balance philosophy is to add weight to the crank to balance out 100% of the "Rotating" mass, which is basically the crankpin and the weight of the big end of the con-rod, and also 50% of the "Reciprocating" mass, which is the piston asssembly (rings+gudgeon pin) and the weight of the small (or little) end of the con-rod.

This 100%+50% applies more or less for each cylinder in most engines, however what you end up with in a single cylinder engine is the 50% reciprocating mass being "out of balance" at mid stroke, i.e. you get a fore and aft force shaking the engine. At TDC and BDC it is half way cancelling the effect of the piston etc, so you also get some up and down forces. 50% represents a good compromise between fore/aft and up/down.

However if you then add another cylinder at 90deg, what is fore/aft for one becomes up/down for the other, so the balacing reciprocating mass ends up working on both cylinders more or less ideally, so it is close to being properly balanced.

Strictly it's never quite right because the piston accelerations at TDC and BDC are different due to the con-rod rotating in opposite directions relative to the crank (think about it), and if the con-rods are side-by-side (usually) instead of forked (uncommon nowadays) then there is also an offset between the sets of forces, hence a small rotating couple. Opposed twins (BMW etc) suffer this offset effect significantly because there's also a centre web between the crank throws so that they can both be at TDC together.

The Honda NTV series (Bros/Deauville etc) have a split or staggered crankpin with a thin centre web, the cylinders are at 52deg but the firing intervals are
Front-232deg-Rear-488deg-Front, which I make to be a 76deg offset. I don't pretend to understand the exact philosophy behind this, I'll just trust Honda-san to know what he was doing. :wink:

Have you heard an Aprilia Mille? Sound very distinctive, 60 degree twin though and I think it has an altered firing order and a balancer shaft. I thought 90 degree twins fired the way they do to give perfect primary balance so no need for a balancer shaft.