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Originally Posted by aarganesh
I've a question on multipl cylinder engines. Plz let me know if I can post it here or I will start a new thread.
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This is a great thread, let's keep it going
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Lets take an example of a 4 cylinder engine (be it inline or V shape). I lately came to know that the pistons will be at different stages and each connecting rod will rotate the crank shaft at different point in time. At no point of time all four will rotate the crank shaft (which I was thinking).
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That is correct.
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a. For this type of engine, I guess, the maximum speed of all the 4 connecting rod will be the same. So how does increasing the number of cylinders will increase the speed of vehicle or is this just my assumption that increasing the number of cylinders will increase the speed of vehicle (assuming all other factors remain the same like weight of car, # of passengers etc).
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Increasing the number of cylinders is a way of increasing engine displacement (among other things). Its just that there are certain points at which designing more or fewer cylinders makes sense. For instance in large commercial trucks which need a lot of engine displacement, a four cylinder doesn't make sense. The cylinders would have to be so big that the pistons would be very heavy, and the vibrations the engine would put out would not be pleasing. In a small car, a four cylinder makes sense as a way of maximizing displacement in a small area.
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b. Why is that the engines are designed in such a way that all four connecting rods rotate the crank shaft at different point in time rather than all of them rotating at one time?
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Two main reasons: 1) so that the force from one piston can move the other three through their other strokes. If they all fired at once, the engine would only operate at a speed fast enough that its inertia would carry it through all four strokes. However, this does occur in one-cylinder, 4 stroke engines. Since there is only one cylinder, it has to use inertia to carry it through. Because of this they have a narrower range of applications. 2) because of #1, mostly for the sake of smoothness. An engine with all of its rotating weight moving in the same direction would cause pretty violent shaking. Not only would it make you angry, it would shorten the life of the engine
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c. I think (or assume), if all 4 connecting rods rotate the crank shaft at one point in time the load on the engine is distributed to all 4.
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While your logic is correct, most of the load on the engine is carried in the crankshaft. So in this case, spreading out the loads from the pistons into four distinct events helps even out the load instead of all four hitting at once.