The engine block that you see has a stroke of 77mm, which means that the piston covers 77mm of travel during the full length of it’s up and down motion. By this logic, rotating the crankshaft 90 degrees from top dead center should set the piston at half the stroke, in our example the piston should cover exactly 38.5 mm. Yet, as you can clearly is here it doesn’t’. During the first 90 degrees of the crankshafts travel the piston covers more than half the stroke. So why is this happening? It’s happening because the piston is connected to the crankshaft by a connecting rod and the connecting rod doesn’t just go up and down in a simple reciprocating motion. Instead the connecting rod steps out from a linear path of travel both left and right. What is primary engine balance? The source of primary balance or imbalance is simply the mass of the reciprocating parts of your enigine, which means that your pistons are the greatest potential source of a primary engine imbalance. An engine with an odd number of pistons can have a primary imbalance if the reciprocating mass of the odd piston isn’t properly canceled out by the other pistons. We’re starting with one that has the least number of cylinders the inline three engine. When it comes to inline three engines most of them have their crank throws 120 degrees angled away from each other. Such an angle distributes the crank throws evenly across the crankshaft which then enables an even distribution of cylinder firings. The inline three engine has good primary balance due to the even spacing of it’s crank throws. If you look at the crank from the front this becomes even more apparent. What about the secondary balance? Wel l that’s actually pretty good too, because the three pistons are always in different parts of the upper and lower half of the cranks rotation and no two pistons move together. This sort of disperses the negative effects of the unequal piston speeds in the different halves of the cranks rotation. But if your gut feeling is telling you there has to be a problem with an odd number of cylinders, you were right. And the issue is discovered by drawing a line across the middle of the inline three cylinder. Notice anything weird? The force on this side is obviously unequal to the force on this side. the engine’s center of gravity is at the middle of cylinder two and the unequal forces at different sides of the center of gravity mean that the engine rocks back and forth or end to end. So how do we fix the unbalance? Well the solution to fixing an unbalance in any kind of reciprocating piston engine can come in the form of a balancing shaft. In the case of the inline three we need a single balance shaft with weights which moves in the direction opposite to the piston travel to balance out the the end to end rocking of the inline three. But this doesn’t mean that all inline three engines have a balancing shaft. A balancing shaft adds cost, weight and friction which is why most manufacturers will try to avoid it whenever possible. Ford’s 1.0 liter three cylinder ecoboost engine is an example of a pretty smooth inline three cylinder that has no balancing shaft. Instead the engine uses an unbalanced flywheel and crank pulley and highly engineered engine mounts to cancel out most of the front to back rocking. Now let’s add one more cylinder to the mix and talk about the inline four cylinder engine. The inline four also has perfect primary balance as you can see each upward motion of a piston is canceled out by the downward motion of another piston. When two go up, two pistons go down. As you can see when two piston are at the top two pistons are at the bottom this means that the secondary balance forces associated with unequal speeds at the top and bottom part of the cranks rotation aren’t just present in the inline four they are in fact augmented by the fact that the pistons move in pairs. The easiest way to understand the balance associated with the inline 5 engine is to think of it as the inline three’s big brother. They share the same balance genetics, with the inline 5 being the larger more powerful version. What about he inline six? Well, it’s an inline three standing in front of a mirror. It’s that simple. Unlike any of our previous configurations the inline six has a perfect primary and secondary balance. We have an even number of pistons and no two pistons occupy the same position of the stroke at one time
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