wesaysoracing

i was doing a bit of research and ran across something that got me to thinking, it said that boxer engines have a natural balance to them and dont need counter weights on the crank,
all of the boxer twin model engines i have seen has counter weights on the crank ,would this reduce the rocking couple because of the cylinder offset? would this be a question for antique?

TimBle

twins are very hard to balance. I think the comment of boxers being in balance is limited to 6 cyclinder and 12 cyclinder boxer engines, where its simpler o have mass moving in opposing directions to create dynamic balnce.
 Yes the balance weight would reduce the rocking couple in a twin ( Iassume you refer to dynamic balance?)

Tired Old Man

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Some Brison twins do not have a counter balance.

BadAzzMaxx

I think the counter balance on twins are there more for stuffing than anything.

Ralph told me he cut the counter weights off a GT80 and it still ran fine.

Milton

spaceworm

It may depend on whether the engines are 2 stroke or 4 stroke, and if they are simultaneous firing or alternate firing?

Tired Old Man

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The smoothest running 2 stroke opposed boxer twin I've ever seen had a smaller displacement piston (30-50% of primary) driven in a cylinder about half the stroke of the main cylinders, oriented 90* to the primary cylinders. It was used to drive an accessory pump. You could place a glass of water on either of the primary cylinders and see only a little vibration at idle RPM.

TimBle

<Slaps Forehead> Sorry lads clearly it was late  when I responded and not thinking straight.

Dynamic rotational balance is good on a boxer yet somehow I managed to think about a 180 degree V engine..... :/
Engine where the two pistons are on the same journal as per conventional V engine. Mercedes Benz built a 180degree V12 in the early nineties for sportscar racing and the Zundapp motorscycle has a 170 degree V

The poster asked about rocking couple which is a dynamic imbalance due to the one piston being staggered slightly ahead of the other. Yes that's what the balance mass is for. The two pistons and their rotational mass plus combustion forces want to precesse the crankshaft about a point somewhere around half its length. Designers need to balance the shaft to counteract that precession. Often some engine will have the balance mass incorporated into the casting of the crank either as an eccentric mass. I imagine the Brisson's crank has this. Never seen one so Final balance is usually not necessary if parts are matched to account for any imbalance cast into the crank.

apalsson

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Outch ..... it must be very late there still (or very early)
180degree V????????????
What degrees is a boxer?

Antique

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Cut a 289 Herbrandson boxer and a GT80..both ran/run well, can't tell much difference...some BME boxers also have no counterweights...

TimBle

It is accepted that a horizontally opposed engine is called a boxer and an engine with <180degrees between banks is called a "V" engine. But to people who design multi cylinder engines the definition between a Boxer (flat) and 180 degree V is that for an engine to be calleda boxer the conrods each have their own crank journal, anda vee engine typically the pistons of each bank share a journal.
Using a 180degee V the sharing of crank journals allows for a shorter crankshaft and therfore a shorter engine.

So e.g. the Zundap, which was a 170degree V twin engine, the pistons shared one common crank journal. They built a 180 degree V as well but it was a little too long so reduced the angle to 170 to reduce the length of the engine. I believe it was mounted transvers in the bikes frame i.e. with the flywheel in the same axis as the wheels.
Seems a game of semantics but a 180 degree Vee engine has been built by Mercedes Benz in 1991 or 92 when they raced the World Sportscar championship with such an engine; 180 degrees layout but conrods from opposing cyclinder bankssharing crank journal's.

When the VWBeetle was designed the engine was called a "boxer" because it was designed as 4 separate single cyclinder engines in one crankcase. Each cyclinder was designed to produce a different power output. Usually an engine designer wants to ensure equal output per cyclinder.

With a two stroke it is not so critical if the journals are shared or not. You either get two bangs one after the other or two together. The two together with a separate journals resuklts in a longer crank with larger rocking couple. If sharing a journal typicall you have a shorter crank with less rocking couple. But on small two strokes is it important? Depand on the design

But it is the Indian GPthis weekend and friend and Iwere talking late last night about engine rules changes for 2014 and he mentioned the Mercedes 180degree V and how compromised the layout was. So anyway we were stil debating on the merrits of that definition but he havingworked forthat manufacturer in engine testing at the time of its development, was very clear onwhatthe differenceis.

pe reivers

How then is the scavenge pumping done in a 180° "V" engine
The V-engine typical has 90° bank angles or less. Nearing 180°, it's called opposed cylinders. When cylinder pairs have the pistons moving out and in simultaneously, it is called a boxer. The two stroke principle needs to be a boxer when cylinders are at 180°. A 70° or less angled bank also can work as a two stroke. All opposed cylinder engines were 4-stroke engines.

jedijody

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In TimBle's example of a "180 degree V" engine there is no way it could be made to scavenge as a two cycle engine. Been involved with many opposed cylinder engines, if two rods share a journal on the crankshaft it must scavenge four cycle, must have four or more cylinders, and it's termed as an opposed cylinder engine. A boxer design can be made with only two cylinders and can be made to scavenge two or four cycle no matter how many cylinders are used.

Two cycle engines can be made with shared crankshaft journals but they are more of a four cycle design forced to operate in a two cycle mode. They require a separate pressurized oil system, oil sump, and pressurized air intake. I don't know of any that don't also use overhead valving for exhaust at least and some form of fuel injection, none use gasoline for fuel.

A true boxer engine uses one crankshaft journal per cylinder, many are made without counter weights being necessary for balance as are many opposed cylinder engines. There should be very little rocking couple in a well made boxer engine as all forces should counter act each other. I have several model airplane boxer twins some have counter weighted crankshafts and some do not. The ones with counter weights do run a bit smoother throughout all RPM ranges but the ones without counter weights are acceptable.

TimBle

Well thats what I realised after some much needed sleep
All that talk of 180 V, 15 degree 5Cylinder V engines and "W" 12's at 1am in the morning gets a head spinning. All 4 strokes of course.

Tired Old Man

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That's what happens when one attempts to become overly techinical. Easier to just generalize until you are putting thoings to paper for design and production.

The Brison cranks were clean, without intentional imbalance. 6.4 in particular.

TimBle

not familiar with them at all but will research it.

wesaysoracing

this has proved interesting and clear up the issue some i was refering to two stroke boxer twins like what DLE ,BME,DA,3W,ZENOAH, ECT is offering