Engine Desing

wrote:
Bertie the Bunyip wrote:

...
What th ewirter seems to be describing is an arrangement where two

opposing

pistons share a crankpin. I've seen twins like this and they vibrate

like

hell. A boxer (whihc most all flat opposed engines are) is a much

smoother

arrangement.



What is the distinguishing characteristic of a boxer, that opposing
cylinders fire simultaneously?


I think the main feature is that the cylinders go in and out in
opposition thus giving very good primary balance. You always have the
torsional vibration from firing and there is also secondary imbalance.

Matt

Matt Whiting
:

wrote:
Bertie the Bunyip wrote:

...
What th ewirter seems to be describing is an arrangement where two

opposing

pistons share a crankpin. I've seen twins like this and they vibrate

like

hell. A boxer (whihc most all flat opposed engines are) is a much

smoother

arrangement.



What is the distinguishing characteristic of a boxer, that opposing
cylinders fire simultaneously?


I think the main feature is that the cylinders go in and out in
opposition thus giving very good primary balance. You always have the
torsional vibration from firing and there is also secondary imbalance.

that's right. I have a BMW bike that's a twin boxer. there's two throws on
the crank and the pistons go in and out in oppostion to each other. AFAIK
all flat four airplane engines are the same, (though I'm sure there's some
ddball one out there somewhere) The ignition is alternating, though..
There have been some twins that have a single throw crank. It's as if
someone said "hey, that old single cylinder engine vibrates like hell, but
you know what'd make it vibrate even more? If we added another cylinder on
the other side!" These engines, while lighter than a boxer, have the added
disadvantage of supplying their power strokes at uneven points in the
rotation cycle. So you get two quick thumps and then a space in between..


Bertie

Most flat fours are not a boxer, and many twins are not. A boxer
engine will weigh and cost more. It's not really needed on a flat four
anyway, as on a non boxer the pistons on the front and rear pair will
be going opposite directions

Sport Pilot wrote:
Most flat fours are not a boxer, and many twins are not. A boxer
engine will weigh and cost more. It's not really needed on a flat four
anyway, as on a non boxer the pistons on the front and rear pair will
be going opposite directions


True, but this doesn introduce a "rocking couple" vibration mode.
There's no free lunch WRT to engine design. :-)

Although some configurations come close (I-6, V-8, etc.)


Matt

"Sport Pilot"
groups.com:

Most flat fours are not a boxer, and many twins are not. A boxer
engine will weigh and cost more. It's not really needed on a flat four
anyway, as on a non boxer the pistons on the front and rear pair will
be going opposite directions


Well, i can't thnk of an aviation flat four that isn't, and even the humble
VW is a boxer. So are all Subarus. In fact, I'm hard pressed to think of a
flat four that isn't. maybe the Mucculogh...
As for twins, there were a lot of non boxer opposed twins in the early
days, and they were diabolically vibratory. Much worse even than the
paralell twins that came in the thirties. ( I have a an old Triumph and
will knock your teeth out on a long trip, and it was meticulously balanced
when I rebuilt it)
what you say isn't entirely untrue, but the torsional loads on an engine
with pistons paired on crankpins front and aft would be very high and need
to be stiffened to the point it would be just as heavy if not heavier than
it's boxer counterpart.


Bertie

"Matt Whiting" wrote in message
...
Sport Pilot wrote:
Most flat fours are not a boxer, and many twins are not. A boxer
engine will weigh and cost more. It's not really needed on a flat four
anyway, as on a non boxer the pistons on the front and rear pair will
be going opposite directions


True, but this doesn introduce a "rocking couple" vibration mode. There's
no free lunch WRT to engine design. :-)

Although some configurations come close (I-6, V-8, etc.)


Matt

A very interesting design is the Diesel Air engine (www.dair.co.uk). They
have two pistons per cylinder, moving in opposite directions. The ignition
happens in the center of the cylinder, which is also the center of the
engine. All forces that can cause vibration are supposed to cancel eachother
out, so there's a minimum of vibration. The engine has two cranks which are
mechanically coupled on the outside to bundle the power to the prop.
Pictures are on their site.

Rob

"Rob Turk"
sednews:xmXie.4098$184.3491@amstwist00:

"Matt Whiting" wrote in message
...
Sport Pilot wrote:
Most flat fours are not a boxer, and many twins are not. A boxer
engine will weigh and cost more. It's not really needed on a flat
four anyway, as on a non boxer the pistons on the front and rear
pair will be going opposite directions


True, but this doesn introduce a "rocking couple" vibration mode.
There's no free lunch WRT to engine design. :-)

Although some configurations come close (I-6, V-8, etc.)


Matt

A very interesting design is the Diesel Air engine (www.dair.co.uk).
They have two pistons per cylinder, moving in opposite directions. The
ignition happens in the center of the cylinder, which is also the
center of the engine. All forces that can cause vibration are supposed
to cancel eachother out, so there's a minimum of vibration. The engine
has two cranks which are mechanically coupled on the outside to bundle
the power to the prop. Pictures are on their site.

A fairly old idea, but it was quite successful for the Germans in WW2, the
engine giving a good SFC and allowing long range maritime patrols.
I've seen a sectioned example of this engine up close and it's fairly
complicated, not to mention absolutely huge!
http://www.billzilla.org/ideas4.htm
But it goes back even further than that, at least to 1898 when the Arrol
Johnston company made that same design into this contraption.
http://www.britishmm.co.uk/history.asp?id=65

Earlier, Sport Pilot wrote:

Most flat fours are not a boxer, and
many twins are not...

...on a non boxer
the pistons on the front and rear pair
will be going opposite directions

First, let me define the terms as I understand them:

The way I understand it, boxer motors are flat opposed engines in which
the connecting rods of opposing cylinder pairs do not share share a rod
journal on the crankshaft. Instead, they connect to the crankshaft at
journals that are spaced 180 degrees from each other. So arranged, each
opposing pair of pistons are both either on the down (power or intake)
stroke or on the up (exhaust or compression) stroke.

Conversely, flat opposed engines in which the connecting rods of
opposing cylinder pairs _do_ share share a rod journal on the
crankshaft are _not_ boxers. And again, that's just the way I
understand it, but a Dogpile or Google search pulls up lots of Web
pages that bear out that understanding.

And by that measure, most flat four motors _are_ boxers. VWs are that
way, and so are Soobs and Lycomings and Continentals. And certainly,
the Ferarri flat 12s are that way, or else the factory probably
wouldn't be calling them "Boxers."

As far as the relative motions of the various pairs of opposing
cylinders goes, I have never heard of that entering into the definition
of "Boxer." I won't say that it doesn't, but I will say that I won't
believe it until I see a credible cite to that effect.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com

"Bertie the Bunyip" wrote in message
...
"Rob Turk"
sednews:xmXie.4098$184.3491@amstwist00:

"Matt Whiting" wrote in message
...
Sport Pilot wrote:
Most flat fours are not a boxer, and many twins are not. A boxer
engine will weigh and cost more. It's not really needed on a flat
four anyway, as on a non boxer the pistons on the front and rear
pair will be going opposite directions


True, but this doesn introduce a "rocking couple" vibration mode.
There's no free lunch WRT to engine design. :-)

Although some configurations come close (I-6, V-8, etc.)


Matt

A very interesting design is the Diesel Air engine (www.dair.co.uk).
They have two pistons per cylinder, moving in opposite directions. The
ignition happens in the center of the cylinder, which is also the
center of the engine. All forces that can cause vibration are supposed
to cancel eachother out, so there's a minimum of vibration. The engine
has two cranks which are mechanically coupled on the outside to bundle
the power to the prop. Pictures are on their site.

A fairly old idea, but it was quite successful for the Germans in WW2, the
engine giving a good SFC and allowing long range maritime patrols.
I've seen a sectioned example of this engine up close and it's fairly
complicated, not to mention absolutely huge!
http://www.billzilla.org/ideas4.htm
But it goes back even further than that, at least to 1898 when the Arrol
Johnston company made that same design into this contraption.
http://www.britishmm.co.uk/history.asp?id=65

Opposed piston (not opposed cylinder) engines of the diesel persuasion are
very close to an ideal concept. Junkers engine division JUMO used the
concept in their pre WWII 200 series. Fairbanks Morse in the US used the
concept for diesel submarines and later diesel locomotives. The Russians
used them in their tanks and the British developed the incredible three
crankshaft, 18 cylinder, 36 piston Deltec. Diesel Air Ltd. has updated the
concept in their very cool light aircraft engine.

These are all two-stroke direct injection diesels with intake and exhaust
ports at the ends of the pistons stroke. One piston uncovers the intake
ports and the opposing piston uncovers the exhaust ports so there is a
'uni-flow' scavenging effect. The cranks for the opposing pistons are out
of phase by about 20 degrees so the exhaust port opens first but closes
before the intake port does. This allows true supercharging of the
cylinder. There are neither cylinder heads nor valves to leak or cool. The
whole cylinder is bathed in coolant.

I wish all the diesel aircraft engine developers the best of luck. Their
day has come.

Bill Daniels

The Harley Davidson engine is a copy of the 1898 or so De Dion-Bouton.
HD didn't originate it, and there is no way in hell they can keep
anyone else from building a v-twin engine. They lost their essential
point.

Offsetting the cylinders gives you a "rocking couple". But since the
Japanese imitation Harleys are for people who want approximate Harley
appearance and are mechanical idiots....