[71DubBugBug]

Maybe this is a question for the supid question thread, but I am confused about the difference between a boxer engine like subarus have, and a flat engine, like the old air-cooled VWs and ferrari 512s had.

Lets discuss!

courtesy of wikipedia:

 

[@McMike]

Maybe this is a question for the supid question thread, but I am confused about the difference between a boxer engine like subarus have, and a flat engine, like the old air-cooled VWs and ferrari 512s had.

First of all, the only car you listed that does NOT have a boxer is the Ferrari.

In the photo you used, the flat engine is on the left, boxer is on the right

The answer is in the crankpins: (shown in green, below)

On the flat engine (180* V) the pistons share a crank pin (crankshaft journal) and move back and forth at the same time - when one is TDC, the other is BDC
On the boxer engine, the pistons have their own pin, and are set at 180* opposite of the opposing piston. They move back and forth opposite of each other - both reaching TDC at the same time.

The Ferrari "boxer" is not a boxer, but a 180* V12
Subarus, VWs, BMW and Honda motorbikes, Porsches and Corvairs are all boxers.

IF you were a two cylinder engine and your arms were pistons, sit in your chair and punch your fists left, right, left right.. with both of your fists going to the right at once, then both to the left at once, you'll notice that you are moving in your chair a bit. Now do the same but punch out with both, then in with both, out, in, etc... you'll see that you're torso is moving moving around in your chair a lot less.

You can run a 2, 4, 6 cylinder engine and they will be much more balanced as a boxer (the 6 especially, since it's inherently balanced on its own). You can make a boxer 12, but it would be a waste of time, since a 12 cylinder engine is already inherently balanced in the first place.

Edit - here is a gif of the two types of engines. Top is boxer, bottom is flat (or 180* V)

 

[Shmi]

First of all, the only car you listed that does NOT have a boxer is the Ferrari.

In the photo you used, the flat engine is on the left, boxer is on the right

The answer is in the crankpins: (shown in green, below)

On the flat engine (180* V) the pistons share a crank pin (crankshaft journal) and move back and forth at the same time - when one is TDC, the other is BDC
On the boxer engine, the pistons have their own pin, and are set at 180* opposite of the opposing piston. They move back and forth opposite of each other - both reaching TDC at the same time.

The Ferrari "boxer" is not a boxer, but a 180* V12
Subarus, VWs, BMW and Honda motorbikes, Porsches and Corvairs are all boxers.

IF you were a two cylinder engine and your arms were pistons, sit in your chair and punch your fists left, right, left right.. with both of your fists going to the right at once, then both to the left at once, you'll notice that you are moving in your chair a bit. Now do the same but punch out with both, then in with both, out, in, etc... you'll see that you're torso is moving moving around in your chair a lot less.

You can run a 2, 4, 6 cylinder engine and they will be much more balanced as a boxer (the 6 especially, since it's inherently balanced on its own). You can make a boxer 12, but it would be a waste of time, since a 12 cylinder engine is already inherently balanced in the first place.

Edit - here is a gif of the two types of engines. Top is boxer, bottom is flat (or 180* V)

Great post, thank you for explaining all of that. Other than balance, are there any other inherent benefits to the boxer? I'd imagine that if all else (displacement, bore, stroke) were the same, these two would make similar power/torque?

 

[bzcat]

Great post, thank you for explaining all of that. Other than balance, are there any other inherent benefits to the boxer? I'd imagine that if all else (displacement, bore, stroke) were the same, these two would make similar power/torque?

The inherent benefit is the low deck height, which allows the engine to sit lower, thus lowering the center of gravity (generally better for handling), and/or leave room on top of engine for other uses (e.g. storage compartment in Boxster).

 

[Jimmy Russells]

First of all, the only car you listed that does NOT have a boxer is the Ferrari.

In the photo you used, the flat engine is on the left, boxer is on the right

The answer is in the crankpins: (shown in green, below)

On the flat engine (180* V) the pistons share a crank pin (crankshaft journal) and move back and forth at the same time - when one is TDC, the other is BDC
On the boxer engine, the pistons have their own pin, and are set at 180* opposite of the opposing piston. They move back and forth opposite of each other - both reaching TDC at the same time.

The Ferrari "boxer" is not a boxer, but a 180* V12
Subarus, VWs, BMW and Honda motorbikes, Porsches and Corvairs are all boxers.

IF you were a two cylinder engine and your arms were pistons, sit in your chair and punch your fists left, right, left right.. with both of your fists going to the right at once, then both to the left at once, you'll notice that you are moving in your chair a bit. Now do the same but punch out with both, then in with both, out, in, etc... you'll see that you're torso is moving moving around in your chair a lot less.

You can run a 2, 4, 6 cylinder engine and they will be much more balanced as a boxer (the 6 especially, since it's inherently balanced on its own). You can make a boxer 12, but it would be a waste of time, since a 12 cylinder engine is already inherently balanced in the first place.

Edit - here is a gif of the two types of engines. Top is boxer, bottom is flat (or 180* V)

sweet gif

This is seriously a great post. Many internet rewards for you sir.

 

[Grinder]

Do these work if only 2 cylinders? I looks like they are common on older motorcycles. When the crankpin is horizontal it can turn either backwards or forwards. maybe this issue is moot once the engine is turning and the crankshaft has momentum.

I can't remember the terminology for this issue. I did see a representation of it at a kids train museum regarding steam locomotives.

 

[Slipstream]

IF you were a two cylinder engine and your arms were pistons, sit in your chair and punch your fists left, right, left right.. with both of your fists going to the right at once, then both to the left at once, you'll notice that you are moving in your chair a bit. Now do the same but punch out with both, then in with both, out, in, etc... you'll see that you're torso is moving moving around in your chair a lot less.

I'm getting some weird looks from people passing by my office. :laugh:

Great info! I had no idea there WAS a difference between flat / boxer configurations.

 

[Aonarch]

I just punched my fists to the side and my co-worker thought I was going crazy.

:beer:

 

[Shmi]

The inherent benefit is the low deck height, which allows the engine to sit lower, thus lowering the center of gravity (generally better for handling), and/or leave room on top of engine for other uses (e.g. storage compartment in Boxster).

Correct me if I'm wrong but wouldn't a flat engine also have this benefit?

 

[Shmi]

I'm getting some weird looks from people passing by my office. :laugh:

Great info! I had no idea there WAS a difference between flat / boxer configurations.

I just punched my fists to the side and my co-worker thought I was going crazy.

:beer:

I did the same. Got the same looks :laugh:

 

[Ghostrider III]

If the venerable six in a 911, is a boxer then why is it called a flat six?

 

[l88m22vette]

I've always thought they were used interchangeably to denote an engine with pistons 180* to each other - to me, wouldn't it be the same as a normal crank vs a flat plane? Its still a V8 (or whatever) but the internals' organization is a bit different. How is a flat-12 a V12? Not trying to be obtuse, but very curious because I've never seen this distinction...

 

[two.twoliter]

If the venerable six in a 911, is a boxer then why is it called a flat six?

All boxers are flat but not all flat engines are boxers.

 

[Jimmy Russells]

I've always thought they were used interchangeably to denote an engine with pistons 180* to each other - to me, wouldn't it be the same as a normal crank vs a flat plane? Its still a V8 (or whatever) but the internals' organization is a bit different. How is a flat-12 a V12? Not trying to be obtuse, but very curious because I've never seen this distinction...

All he's saying is the way the rods connect to the crank (and therefore the way the pistons move in relation to each other) is the same as in a V. In a typical engine you never have more than one cylinder at TDC at a time. In a boxer, you do.

 

[@McMike]

Correct me if I'm wrong but wouldn't a flat engine also have this benefit?

Yes, but it would shake itself apart.

 

[crannky]

Stuff

:beer:

 

[@McMike]

I've always thought they were used interchangeably to denote an engine with pistons 180* to each other - to me, wouldn't it be the same as a normal crank vs a flat plane? Its still a V8 (or whatever) but the internals' organization is a bit different. How is a flat-12 a V12? Not trying to be obtuse, but very curious because I've never seen this distinction...

This is where it starts to get complicated. For every 720 degrees of crankshaft revolution, each cylinder has to fire once - spaced out evenly.

For a one cylinder engine, its pretty simple Once, then 720 degrees later it fires again. (720/1 = 720)
For a two cylinder engine, you fire twice. Once at 0 and once at 360 (720/2 = 360)
For a four, it's once every 180*
For a 12, it's once every 60 degrees of crankshaft revolution. For this reason I believe you can put a V12 in any damn V angle you want, and it'll run smoothly.

in order to get have the crank configured to get these pistons to all reach TDC at the appropriate time, there's more math involved. If it's an inline engine, it's pretty easy to arrange the crank pins. (I think it's the same formula as above... an inline four has two pins at 0* and two at 180*... an inline 2 has them both at 0* an inline 6 has two @ 0, two @ 120, and two @ 240*.

It gets interesting when you start arranging them in a V. you need to stagger the pins just enough to have the pistons reach TDC at the right time (V6 engine below). I don't think 90* V8s have this problem, and

There are preferred angles for V engines, too. I think it's 90* for an 8, 60* for a 12, and I can't remember for a 6. I'm not an expert with this at all, and just have the basic understandings of how it all is supposed to work.

There are some drawings of crankshafts and some of this math nonsense here. There are other sites I have found in the past that I can't seem to find right now.
http://www.e31.net/engines_e.html

And a drawing that might help explain something.

 

[71DubBugBug]

All boxers are flat but not all flat engines are boxers.

hmmm, all boxers and flat engines are horizontally opposed

so for the 512 and the testarossa need a balance shaft? while boxer engines do not

 

[16volt]

SCIENCE!

Thanks Banagon.

 

[Viking]

This is seriously a great post. Many internet rewards for you sir.

It really is a great post. Thank you.

Honestly, I just thought they were two terms for the same thing. Glad to learn how dumb I was to assume that all these years.