inline six cylinder question..

[ikeepmychevytoo]

Touching the ignition key a bit is an old mechnics trick. I do it to place the distr. rotor screws at my advantage for easier removal. It can stop at any point, and starter may rotate the v6 with any increment

Compression might not be much of an issue with heavier flywheels and rotating mass but I do observe this with small non-car engines as others mention.

 

[Matt_S]

Tracker8, using the same logic as with 4, 6 or 8 cylinder engines, you would expect the engine stopping point (when viewed at the flywheel) to be rougly correlated to the start of a compression cycle. In addition, in a 4-stroke engine, all cylinders take 720 degrees (2 revolutions) to fire. A circle is 360 degrees. In a V8, you get one compression every 720/8 = 90 degrees. From the flywheel, that means you'd see 4 points spaced 90 degrees apart. The engine could stop at any one of those points when either of two specific cylinders hits a compression stroke. In a four, you get one compression every 720/4=180 degrees. You'd see 2 stopping points on the flywheel for the same reasons you'd see 4 points on the V8. A 3 cylinder compresses every 720/3=240 degrees. That would mean you'd get 3 possible stopping points spaced over 720 degrees (divide the spacing of the compression cycle by 2 to get the stopping point spacing of 120 degrees). In a 5 cylinder, you'd get 720/5=144 degrees between compression, which would mean 5 points 72 degrees apart. To get even more specific, those stopping points on the 5 cylinder would be at 144, 288, 432, 576 and 720 degrees. If you subtract 360 from the last 3 numbers (since a circle is only 360 degrees, therefore the last three numbers represent a second revolution of the shaft) you get 144, 288, 72, 216 and 360 degree stopping points or, to put it in order, 72, 144, 216, 288 and 360.

I hope I haven't completely confused you yet.

 

[Shannow]

Quote:

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I've never reasoned it soundly in my head ..but all inline 6 cylinders have the same firing order ..unlike any V or other inline application.

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Gary,
look at a 6 cyl crank. 1 and 6 are TDC, 2 and 5 are the same, as are 3 and 4. The throws give you 1,5,3,6,2,4, to give you the lowest rocking couple (except for a double 3 cyl an example of which could fire two cylinders together 1-6, 5-2, 3-4, or 1-6, 3-4, 5-2)...would then have more torque pulsation than a normal fire 6.

All of the above could be reversed but there is no advantage as the two end cylinders are up at the same time, the two middles, and the two intermediates, so primary balance is assured.

You don't want all of the front cylinders firing, then all the rears, as it increases vibration and rocking.

A 4 cyl needs 1 and 4 at TDC same time, so 1 and 4 end up not next to each other. 1342, and 1243 really mean the same thing.

V-8s can be anywhere, depending on where the designer wants to be. A 90 degree V-twin can have perfect primary balance. A V-6, or a V-8 can be made to take advantage of this. Firing order then tries to stop the end to end vibrations that could make the engine rock front/rear...some firing orders (e.g. SBC) sound great, others (like the Holden) sound second best. Optimisation of firing order can help crankshafts to last, by stopping reversals in torque on specific bearings.

 

[Gary Allan]

Well, yes, I agree ..but, as you sorta said, this gives you a sense of "bilateral symmetry" in that the "mirror" cylinder is two strokes behind in the same position ..but I don't equate that with "inherently balanced" in my head. I'm sure that it is ..but my idiot-idiot (as opposed to idiot savant) mind can't see it (there are certain holes in my thought processes).

That is, I see that at any point you'll have your 2 up ..2down- 2 mid span ...or rather whenever 2 are up ..2 are down and 2 are at mid-stroke ..but I can't quite grip the need for any specific order. Under that simple criteria any 2 can be up ..any two down and any two mid-stroke (see where I'm lost

) If it was a matter of the combustion thrust interacting here ..again ..I see "bilateral symmetry" ..but assuming, in whatever physics sense that bear-load-bearing is the same as bearing-span-bearing-load (stretch to 5 or 7 bearings and having equal loads opposing except for the combustion stroke) ..then it still doesn't clearly define the need for the given sequence.

What I "see" is that any other configuration ..let's say perfectly balanced 60° crank throws firing in sequence ..that although you wouldn't have the directly "apparent" 2 up 2 down 2 mid-stroke relationship at those points of reference ...you should surely have "equivalent" opposing mass.

I need someone to bridge this conceptual gap ..unless the difference is just one of "sensible convenience".

Again, pardon the gaps in my education and general thought processes. If anyone's successful at bridging them .this will be one of those "holy grail" lightbulb moments (cue appropriate revelation music - maybe the theme from 2001 ASO where the primate figured out the animal bone as a striking tool type thing).

 

[ikeepmychevytoo]

I also do have many black holes in my understanding so I just write with the hopes that the black holes don't overlap.

So Gary, if I'm not misunderstood you tend to couple each pistons inertia with the respective counterbalance in the crankshaft. So they supposedly cancel out. This could be only true for the vibrations in the stroke axis (axes for a V block) and in a perfect world. Vibrations may be in the x y z planes as displacements or as rotations in the x y z axes. Different balancing strategy needed for each for very different causes. So equal mass on each side of the mid stroke plane is the first phase in the design process. Second would be the equal inertias, and here we have started with problems: at which rpm? Also; since every shock is not perfectly insulated just in time, remaining vibrations in different locatins may start a bigger perceivable vibration. Called second, third of fourth phase vibrations. (And to foresee these, imperceivable minute vibrations are measured at the lower rpms.)

Another consideration for firing order is; Each force imposed on the crankshaft creates a wave in the shaft. If we slow motion this so it takes about 10 secs between fires, crankshaft would have a motion like a bass guitar string. The second (or even third?) impact on the crankshaft is not preferred at the moment somewhere at the apex of the still prevailing wave. For the same reason army don't march on through the bridges. This consideration for the firing orders have great effect on the bearing and crankshaft life. In the end, natural frequency characteristics of the rotating/working masses are also influential on the position/direction and amphitude charactersitics of the vibrations of the previous pharagraph. At certain rpm, chances are you may start a rotational vibration and for this designer may give up a little bit in the initial static balance.

 

[Gary Allan]

I'll go with the mechanical harmonics thingy. That is, I understand these forces. I still don't get any glow from the lightbulb, though. Thanks for the effort.

 

[kotzy]

Don't foget that it is not unusual to have one or more cylinders with higher compression then the others adding to the stopping point. Engines also tend to back off (reverse rotate) when hitting the top of a stoke. He may need a few less to drink but I have seen many engines with one spot chewed far worse then the rest. The chewing is the result of starter timing, not where the engine stopped. GM had quite a problem with their Delco staters in this area, motor running before engagement.

 

[Shannow]

Gary,
another thing on a 6 is that you don't want one end of the manifold to stagnate (well at least with a carb).
153724 makes every next stroke to the other end of the manifold, keeping things moving so to speak, and the carb drawing properly.

When I had triple carbs (S.U.s) on a straigh 6, cylinders 3 and 4 were perfect with their carb. One suction event every 380 degrees. 1 and 2 and 5 and 6 were bad combinations, which makes tuning difficult...two single barrels sometimes work better, with an even 240 degrees between suction events.

 

[Kestas]

153724?

 

[Shannow]

6 and 7 are so close together

 

[occupant]

Good question. Last straight six I owned was the only one. A 230cid Chevy in a 1965 Biscayne. I hardly ever used the starter. It was much more fun to coast down the hill in front of my house and clutch start it at 10-15mph in third, then downshift to second and go. No synchro in first in that car, but it kept me from rolling stop signs since I did have to come to a complete stop to get into first easily.

 

[Gary Allan]

Okay ..now here's where I think I'll make an observed distinction. I say, until proven otherwise, that there is absolutely nothing wrong with a crank configured in 60° (radial distribution) and camming it anyway you want and fire in any sequence that you want.

What I have figured out, or rather reasoned, is that IF you're going to have the "two up, two down, two mid-stroke" crank configuration that you MUST have that firing order.

First pistons in the "pairs" separated by 1 stroke: 1-5-3
Second pistons in the "pairs" separated by 1 stroke 6-4-2

 

[Shannow]

Gary,
close, but only if the cranks are set "normally", say the throws at 120 and 240 are transposed, you can get 135624.

Your idea of a 60 degree throw would result in quite a rough engine, as there'd be a greater or lesser "rocking couple", when you have one piston at one end up, and one at the other down. The whole engine would sort of rock on it's mounts.