Why are V Engines Less Smooth When Compared To Inline Engines ?

There is an old saying about Harley Davidson Motorcycles. They call them the "Milwaukee Vibrator" because of they way they run. These engines are a 45 deg V-Twin engine (45 degrees is the angle between the two cylinders). In the HD engine, the two connecting rods attached to the pistons share a common journal or "pin" on the crankshaft. It looks like this drawing:

The deal with this design is when combined with the 45 degree offset of the cylinders, it results in a minimum power pulse interval of 315 degrees. A cylinder fires, the crankshaft rotates 315 degrees while not under power, then the next cylinder fires, the crankshaft rotates another 405 degrees, then the first cylinder fires again, the crank has to rotate 315 degrees, etc. This leads to uneven power production and limits the amount of power it can produce. Energy is lost between the second cylinder firing and the first firing again. 

This is why you get the pop-pop-pause, pop-pop-pause characteristic Harley noise. Since the power pulses are non-symmetrical, power/torque is produced at different times as the crank goes around. This causes power to feel off balanced during acceleration. It will seem to grab more at some places and seem to be slacker at other. This will be a lot more noticeable at lower engine speeds as when the engine speeds up, there is less time between the power pulses and is therefor less noticeable.
You'll notice I said at the beginning that your instructor is "mostly right" ... that's because there are other engine setups where V-twins may NOT be on same pin (or crank journal) and therefor can fire symmetrically and produce power at every 360 degrees of crankshaft rotation. Inline 2-cylinder engines can be setup like this very easily as well as inline and 90 degree V-4 engines. The inline engines can be built using a lot less mass on the crank shaft, which allows them to rotate more easily at a higher rate of speed (higher RPM) as well produce cleaner power, but they take up more space in the bike for the displacement they give. Since these types of engines fire symmetrically, you won't feel the power surge because the engine fires more consistently on each revolution of the crankshaft.

But, everything said and done, you cannot rule out the size benefit V engines have. That is the same reason that has allowed motorcycle manufacturers across the world to have V-twin engines that displace 1800cc but look as compact as an inline-four 1,000cc engine. So each have their own benefits.... 

Major Advantages of V-Type Engine are  :-
  • The V-type of engine has two rows of cylinders at (usually) a ninety degree angle to each other.
  • A V engine also produces more torque at lower rpms because of the power stroke coming from two sides of the crankshaft.
  • Due to its short length, the great rigidity of the block, its heavy crankshaft, and attractive low profile (for a car with a low hood). 
  • This type of engine lends itself to very high compression ratios without block distortion under load, resistance to torsional vibration, and a shorter car length without losing passenger room.

But, the power an line-four can provide can be as much or more than a V setup with a much smoother power delivery.Another benefit on an inline engine is that it can easily accommodate odd number of pistons which is something a V setup doesn't allow to happen. Odd number of cylinders in a V configuration is almost impossible to make smooth as the uneven number of cylinders on both the ends will result in vibrations which might need some kind of balancer to negotiate and the power delivery is expected to be very uneven as well.

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