On or around Sat, 25 Jun 2005 23:27:19 +0100, steve Taylor
<
[email protected]> enlightened us thusly:
>Austin Shackles wrote:
>
>> I dunno how as they'd fight, as such, if bolted rigidly together. They're
>> both turning the same way, after all, and every power stroke adds to the
>> torque. I daresay you don't actually get 2x the power of one, but equally I
>> don't really see where you lose that much either. I don't, in fact, know
>> what angles your typical straight-8 crankshaft uses, but it ought to be 90
>> degrees, in order to get even firing sequences.
>
>..but unless they are phased within a fraction of a degree you'll get
>some losses of power - hence my tying the cams together...#
I'd think that a slight phasing problem would be more likely to make it run
slightly less evenly. Even that's not necessarily a problem: some shortish
time ago, the cunning Japanese decided that making multi-cylinder GP bike
engines fire all their cylinders close together was better than firing them
evenly: the Big Bang engine, it became known as. Granted, it didn't make
more power (but didn't, I don't think, make much less), but it did have an
effect on grip - the pulses in the power delivery had an effect similar to
ABS and gave reduced wheelspin, the latter being the limiting factor at the
time in deploying more power.
Going back to basics... every 4-stroke cylinder in an IC engine absorbs
power to a degree - all the time due to friction and more once per cycle due
to compression. The combustion process, allied to a heavy flywheel on a
single, gives more energy than is absorbed and the motor can do useful work.
Adding more cylinders means you can reduce the flywheel mass - I suspect an
8 (or more)-cylinder doesn't technically need a flywheel at all as whichever
cylinder is currently compressing is matched by another on its power stroke,
for an even-firing engine (i.e. one with evenly spaced crank pins). It
might be that the flywheel does other things, like smoothing the power
delivery, and in the case of automotive applications, the flywheel is an
ideal large flat surface to run your clutch on; if you didn't have such a
flywheel, you'd need some other form of clutch.
What I don't see is how adding another set of 4 cylinders, not perfectly
synchronised, is going to lose a lot of power. I can see how having 2
complete systems could go out of sync, such as for example having 2
generator sets driving an electric transmission - one could be doing much
more work than the other; in the same fashion, 2 complete transmissions (a
la 4cv citroen sahara[1]) could be imbalanced - mind, in some conditions,
being able to mix the power levels non-linearly would be an advantage,
especially off-road.
I don't see the problem of balancing power from 2 linked engines as being
that much worse than balancing power from 2 or more carbs. Probably, you'd
set it up on a dyno, and tweak the pump linkages to get maximum output.
[1] twin engine 4x4 version of the 2cv. For them as didn't know
--
Austin Shackles.
www.ddol-las.fsnet.co.uk my opinions are just that
"Quos deus vult perdere, prius dementat" Euripedes, quoted in
Boswell's "Johnson".
