Physics of Engine Vibration?

[Mr. Potatohead]

General question here that bugs me:
I know that an unbalanced engine loses efficiency because some of the fuel energy is wasted vibrating the heavy engine back and forth.

I understand that straight six cylinder engines tend to be naturally in balance, for example.
If you have a inline four cylinder that needs a balance shaft to smooth it out to be as smooth as the straight six, does this lose some of its efficiency?

In other words, now that the four isn't wasting energy bouncing around, is it as efficient (all other things being equal) as the six, or is there energy now being used up by the balance shaft?
I know there will be added friction to turn the four's balance shaft, but is it also using energy to swing the counterweights against the pistons' moving mass?
Hope this makes sense.


Rob

[curtis73]

Usually a balance shaft is to make customers happy. Any gains in efficiency are eaten up by the extra energy it takes to drive the balance shaft.

The secret to efficiency from lack of vibration is to engineer things so that the vibes are minimized in the first place. It doesn't matter where the vibes go (balance shaft, motor mounts, frame) there is still energy being wasted in vibration. The secret is to not make the vibes in the first place.

Its all a trade off though. A 3-liter 4cylinder will have more vibes than a 3-liter 6 cylinder, but there will be more frictional surface area in the 6, plus more overall external size, etc. Engineers usually temper market niche with NVH (noise, vibration, harshness) issues. For instance, an inline 6 has long been associated with HD trucks, and 4-cylinders have often been associated with economy cars. Someone who buys a Kia Rio doesn't really care about NVH or power, but if you're buying a full-size work truck, even a big 4-banger wouldn't compete in a 6 and 8 cylinder market regardless of its power and size.

[Mr. Potatohead]

Originally Posted by curtis73

... It doesn't matter where the vibes go (balance shaft, motor mounts, frame) there is still energy being wasted in vibration... The secret is to not make the vibes in the first place.

... A 3-liter 4cylinder will have more vibes than a 3-liter 6 cylinder, but there will be more frictional surface area in the 6, ...

Interesting and sensible. Conservation of momentum and all.
I never considered the extra friction surface in the equal displacement six, either.
Thanks for the reply.


Rob

[GreyGoose006]

although it is true that a straight 6 is one of the more balanced engines, it is also hard to get good head and exhaust flow, expecially in a carburetted engine (yes, we dont use those any more, but hey)
V8 is big and imbalanced, but it makes people happy and fills the niche
small I-4 engines are very efficient, but again, you wouldnt put one in a silverado.

the list goes on...

[KiwiBacon]

Originally Posted by GreyGoose006

although it is true that a straight 6 is one of the more balanced engines, it is also hard to get good head and exhaust flow, expecially in a carburetted engine (yes, we dont use those any more, but hey)
V8 is big and imbalanced, but it makes people happy and fills the niche
small I-4 engines are very efficient, but again, you wouldnt put one in a silverado.

the list goes on...

Actually a 90deg V8 is very well balanced in it's natural form. Nothing like a V12 is though, they're the best but soo uncommon these days.

Balance shafts work by creating imbalance in the opposite direction, the engine block which holds it all together suffers more because of it, not enough to cause a problem, but extra bearings and drive belts (or gears) all create more drag.

Check out www.4btswaps.com, chances are you'll find someone there putting a 4 cylinder into a silverado.

[slideways...]

we should expand the discussion about how straight 6 motors are naturally balanced...tell me how when the dynamics are the same as any other inline engine. oh and if your concerned with conservation of momentum, go buy a rotary.

[Mr. Potatohead]

Originally Posted by slideways...

we should expand the discussion about how straight 6 motors are naturally balanced...tell me how when the dynamics are the same as any other inline engine. oh and if your concerned with conservation of momentum, go buy a rotary.

I knew a link to the balance characteristic of the different types, but I can't seem to find it right now.
I'm not shopping for the most efficient engine, but this relates to a discussion I had maybe five years ago with a guy I worked with. I wondered why the bigger, late model Chevy Trailblazer went to a straight six (from the V-6 I had on my S-10 Blazer at the time).
We were discussing pros and cons on coffee break.
Edit: Found it: http://www.autozine.org/technical_sc...ne/smooth3.htm
But I still didn't understand the efficiency aspect of it until now.

Rob