Moderator: JimVonBaden
AutoZine Technical School wrote:While single-cylinder engine fires once every 2 revolutions, twin-cylinder engine fires once every revolution, 3-cylinder fires once every 720 / 3 = 240° crank angle, 4-cylinder fires once every 180° (half a revolution) .... 12-cylinder engine fires once every just 60° crank angle. Obviously, the more cylinders the engine has, the smoother the power delivery becomes.
This explain why we prefer V12 engines than in-line 6, although both of them achieve near perfect internal balance.
. . .
Inline 3-cylinder engines
As the engine fires once every 240° crankshaft angle (720° / 3 = 240°), the crankshaft design is as shown in the below picture. (Firing order is: 1-3-2)
It seems that no matter how the crankshaft rotate, the combined center of gravity of all 3 pistons and con-rods will remain at the same location, hence no vibration generated. By mathematical analysis, you can also find there is no forces generated in vertical direction as well as transverse direction. (actually, I really performed such calculations) So why did we hear that 3-cylinder engine need balancer shaft ?
In fact, the calculation is wrong because it assumes the engine is one point, thus the forces of all 3 cylinders act on this single point and result in complete cancellation. In reality, the forces act on 3 different locations on the crankshaft, thus instead of canceling one another, they make the crankshaft vibrating end to end.
Don’t understand? look at the above picture, the side view of the engine. Piston 1 is at the top now and is going downward, thus generates an upward force to the left end of the crankshaft. Piston 2 is also going downward, thus generates an upward force to the middle of the crankshaft. Piston 3 is going upward, thus generate a downward force to the right end of crankshaft. As the engine’s center of gravity locates in cylinder 2, you can see forces from piston 1 push the left end of the engine upward while forces from piston 3 push the right end of the engine downward; After 180° rotation, the situation will be completely reversed - downward force at left and upward force at the right. In other words, this is an end-to-end vibration with respect to the center in cylinder 2.
(Site is here.)
Mike James wrote:Boxers (flat twins) are two cylinders as well, but I don't hear much about vibration being an issue. Or do I hear wrong?
Could a throttle lock help with the vibration?
Could tire balance contribute to your problem?
Also try the short version of Cramp Buster for $10
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