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Discussion Starter #1 (Edited)
Function of counterbalancer

Some engines have a counterbalancer to help reduce vibrations. What produces the vibration that a counterbalancer is trying to reduce?
 

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Discussion Starter #3
Sorry Kevlar, I think you got it wrong.

The added weight (Fig. A) on the crankshaft cancels the vibration caused by the piston moving up and down.
 

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A counterbalancer has nothing to do with the pistons. A crank with a big weight on one side will create its own imbalance.The piston's up and down motion is offset by the crank. But the crank has an imbalance that is front to back, so a counterbalancer is used to offset the crank's weight.:p I think I am right, but its hard to describe.....:confused:
 

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Balance shaft A shaft designed so that, as it rotates, it vibrates in a way that reduces or cancels some of the vibration produced by an engine. Not essential to an engine's operation, balance shafts are nonetheless becoming increasingly common as a means of engine refinement. Balance-shafted four-cylinder engines use two shafts turning in opposite directions on either side of the engine's crankshaft. A single balance shaft is used when fitted to three-cylinder and V-6 engines.


The vibration comes from the combustion pulse of the engine. On a 4 cyl its every half crankshaft revolution. On a V8 its every quarter rev.

:)
 

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The added weight to the crankshaft at the opposite side of the piston is there to offset the weight of the piston mainly.
The counterbalancer shaft is there to reduce the Harmonic Vibrations mainly.
Don't confuse the two, every crankshaft has counterbalance weights, but not all engines have a counterbalance shaft!
If you ve ridden a single without a counterbalance shaft, you will understand better ;)
The article makes a very simplistic assumption that the only source of vibration are the reciprocating parts.
Truth is the most important source of vibration is the extremely high pressure at the top of the piston at TDC! This is the cause of Harmonic Vibrations (of many orders!) that are a nightmare to calculate and reduce in order to manufacture an engine that can work! Configuration and firing order are crucial for the design and the vibrations handling.

The inline 6 has an advantage, and the V2-90deg (like Ducati) has another advantage. What are the advantages of these engines?

(tell me if I overdid it here :) , I couldn't help it because my engineering spirit woke up!)

Aris
 

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A 90 degree twin has perfect balance of the pistons because they are moving opposite of eachother. They do not need a counterweight on the crankshaft.
 

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Discussion Starter #10
No Aris... you did not over do it here... :) I'm glad you make us think about physics and engine design!

Here is some information as to why the inline 6 is such a good design regarding balance.

"As I understand it, in simplified but essentially correct terms, due to the mass of the piston moving up and down in the cylinder, the engine block wants to shake up and down (i.e. along the cylinder bore axis); this is the primary or first order force shaking the engine. Due to the mass of the connecting rod swinging from side to side, the engine block wants to shake from side to side; this is the secondary shaking force.

From what I know at this point, it is these two forces which cancel out in a spinning - BUT NOT FIRING - inline-six, flat six, or V-12. By "cancel" we mean that the interrelated motion of the various pistons and connecting rods act in such a way that no net force acts on the engine block to shake it. As one piston moves down, another moves up; as one piston rod swings left, another swings right. Net result: no shaking force in either the axial direction (along cylinder bore) or at right angles to this."

Link source: http://www.zhome.com/ZCMnL/tech/harmonics.htm

Here is a picture of a inline 6 crankshaft...



And here is a front view of the same crankshaft...



From what I gather, if you look at 2 & 5 the rods big end would be coming towards you, while 3 & 4 the rods big end are moving away from you.

Andy
 

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In sizing power transmission equipment, gear drives and the like, it is common to rate a device as suitable for a certain horsepower and then use service factors for the types of driving and driven loads. On the power end, an electric motor and a V-8 engine are usually given a 1.0, straight up. A six will get something like a 1.1, a four something like a 1.25, and so on, the fewer the cylinders, the higher the service factor. Just a point of interest that puts some numbers to this subject.
 

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Discussion Starter #12 (Edited)
Aris,

Your question "why are 90 degree twins perfectly balanced" is a good one.

There are many diagrams showing the 45 degree twin like the Harley. But we know that design is not balanced! :D

The reason a 90 degree twin is balanced, first you need to think about the single cylinder design. When the piston is half way down the cylinder, the counter weight on the crankshaft is at 90 degrees and no longer is counter acting the weight of the piston. At that point vibration is created.

However, on a 90 degree twin, the second piston balances the counter weight even when it is at a 90 degree angle to the first piston.





Link source: http://www.ukcar.com/sframe.htm?/features/tech/Engine/smoothness/smooth 8.htm

Andy
 

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ninjabeater said:
A 90 degree twin has perfect balance of the pistons because they are moving opposite of eachother. They do not need a counterweight on the crankshaft.
Sorry, Ninjabeater but you re wrong here.
The V90 in fact has counterwights on the crankshaft.

Here is a pic showing a Ducati 900SS crankshaft with Carillo rods :drool:

Aris
 

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Andy said:
Aris,

Your question "why are 90 degree twins perfectly balanced" is a good one.


However, on a 90 degree twin, the second piston balances the counter weight even when it is at a 90 degree angle to the first piston.

Andy I'm glad you liked this question :) This subject can easily last for ages you know...

You re right in what you say, the link you posted is really good!
Basically what you mention above can be described in a few words as follows:
The V 90 degrees engine doesn't need a counterbalance shaft because the Harmonic Vibrations of 2nd order are self cancelled.

The 6 inline on the other hand, is free of 1st AND 2nd order harmonic vibrations!

Aris
 

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Has anyone seen the Ducati Supermono engine?
It is a single cylinder engine, but actually a second rod with a weight attached, positioned at 90 degrees like in the V twin Ducatis, is used instead of a counterbalance shaft!
You can see it here:


Aris
 

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Discussion Starter #16
ariszr7 said:


The 6 inline on the other hand, is free of 1st AND 2nd order harmonic vibrations!

Aris
Hi Aris,

The 90 degree twin is commonly quoted as having "Perfect Primary Balance".

My question... is Primary Balance the same as 1st order harmonics? If so, then is it correct to say that a 90 degree twin like an inline 6 are both free of 1st and 2nd order harmonic vibrations?

The cut-away picture of the Ducati 600cc Supermono engine is great! What a fantastic way to use existing technology to create a new engine.

Andy
 

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Hi Andy,

OK let's take thing from the beginning.

First of all I made a mistake when I said the Ducati V90 is free of 2nd order harmonic vibration :hurl:

In fact it's free of 1st order harmonics, but not of 2nd order.

I had to go back to school to calculate it :)
My mistake was that I didn't take into account the fact that the firing order is unique to this engine, and the two conrods are attached on the same crankpin.

When we say 'Order of Harmonic Vibration' we mean the number of times the vibration occurs during a full revolution (360deg) of the crankshaft.
These vibrations increase at certain rpms, and the designer needs to overlay the graphs to see the result of the various orders along the rpm range of the engine.
It is crucial during the design phase of an engine, to avoid having a crankshaft reasonant frequency within the frequency of the harmonics, as this would result in a self disintegrating crankshaft :eek:

Nowadays the calculation is done using special software which can give a graph for orders from 0.5 to 24 or more at once.
Note that we need sometimes to calculate 'half' orders, ie: 0.5 , 1.5 etc.

To answer your question, Primary Balance means the 1st order ONLY.
So the V90 is primarily balanced, and so is the inline 6.

What I said about the 6 inline is correct, it's free of 1st and 2nd, but not of 3rd ;)

Aris
 
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