Just interested to know why an engine rotates in a certain direction rather than another? Is it because that is the way it is initially rotated by the starter. Can it run in the opposite direction?

When I was young I had a motorcycle, small two stroke YZ80, that started up and went backwards, meaning the engine was going backwards. I shut it down that started it again and it was fine.

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87 Fiero GT (3.2 Turbo)
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well, the oil and water pumps would not function properly, and im pretty sure it would be sucking air in through the exaust and pushing it out the intake, which would mean no fuel could get in the cyls. On the other hand, that would make for a nasty jet engine coming out the throttle body...

Your little motorcycle was a two-stroke engine. As long as it's ignition system will tolerate it, they'll run eather direction.
But most car engines are four-stroke engines, with a full set of valves and a far more complex ignition system, etc. So with a four-stroke spinning the opposit direction, it would suck air in through the exaust, compress it, perhaps spark it (there'd be no fuel, so no combustion) and then blow it out the throttle body.

If you mean why do they all run clockwise vs. counterclockwise.... I'd say it's for the sake of transmissions.

If you had say Chev doing CW, and Ford doing CCW, and Dodge doing it randomly depending on the model say, it's create havoc over at Borg Warner (etc). They'd have to keep changing the design instead of producing (for example) ONE T-5 that can then get used on various platforms.

From a physics standpoint it doesn't matter. Marine engines run "backwards". Of course, they have reverse rotation cams and related parts.

quote Originally posted by RotrexFiero: Just interested to know why an engine rotates in a certain direction rather than another? Is it because that is the way it is initially rotated by the starter. Can it run in the opposite direction?

In some ways that's a loaded question, given that many imports rotate "backward" compared to GM engines.

The operation of a four stroke engine relies upon a unidirectional sequence of events. Simply reversing this sequence will not create the conditions required for power output, hence you can not start a four stroke engine by spinning the crankshaft in the opposite direction from that which the engineers intended.

Assuming that you're not familiar with the four stroke cycle, check out this article over at howstuffworks.com for a basic understanding of it. You may notice that the steps which are gone through to achieve ignition only work in a specified order.

quote When I was young I had a motorcycle, small two stroke YZ80, that started up and went backwards, meaning the engine was going backwards. I shut it down that started it again and it was fine.

Yep, two stroke engines can do that. The sequence of events required for a two stroke to run is symmetrical. Get the spark timing right (or wrong as the case may be), and it will run as happily with the crankshaft spinning in one direction as it will in the other.

In fact, the Rotax electronic reverse available in many two stroke snowmobiles actually reverses the rotation of the engine rather than engaging a reverse gear.

they do it in boats all the time. when theres twin engines, one rotates one way, the other rotates the other way.

Except in boats I bet only the props rotate opp. not the engines. That would be expensive to manufacture an left and right engine when you could just throw an extra gear into the lower unit to reverse a prop.

quote Originally posted by Pyrthian: they do it in boats all the time. when theres twin engines, one rotates one way, the other rotates the other way.

Same for some twin engine aircraft, and most piston aircraft engines are direct drive (no gearbox). The only fundamental differences between CW and CCW aircraft engines are the camshafts, the oil pumps. The accessory drives (for magnetos, vacuum pumps, etc.); the crankcases, crankshafts, cylinders, connecting rods, and cylinder heads are the same. I don't remember offhand whether the pistons are different or just reversed.

quote Originally posted by RotrexFiero: Just interested to know why an engine rotates in a certain direction rather than another?

To answer your original question, direction of rotation is just an arbitrary design choice ... like chosing English vs. metric measures, left hand vs. right hand screw threads, or driving on the left vs. the right side of the road. Usually each regional industrial or economic community quickly standardizes on one choice, which reduces overall costs and simplifies maintenance.

As an example, European piston aircraft engines run in the opposite direction vs. American piston aircraft engines, and both work equally well. (This generality ignores the reverse-rotation engines described above.)

[This message has been edited by Marvin McInnis (edited 01-04-2006).]

The engine rotates the way it does primarily because of the camshaft. The camshaft sets the sequence of valve openings and closings and this is what determines the engine's firing order, and thus, the direction of rotation. You can buy reverse-rotation camshafts for some engines, such as Corvairs and small-block Chevys. Of course, this answer is an oversimplification; other factors, such as the direction that the starter rotates, connecting-rod offset, bearing insert orientation, etc. all have to be coordinated to have an engine run in the "correct" rotation.

quote Originally posted by Dodgerunner: Except in boats I bet only the props rotate opp. not the engines. That would be expensive to manufacture an left and right engine when you could just throw an extra gear into the lower unit to reverse a prop.

no, the motors actually run oppisite each other. the engine toque is the main reason - like in the old muscle cars, when you rev/brake torque, and the car starts to lean - not fun in a boat. and I expect it the same reasoning in planes mentioned above. and yes, boat motors are expensive.

quote Originally posted by Tom Piantanida: ... other factors, such as the direction that the starter rotates, connecting-rod offset, bearing insert orientation, etc. all have to be coordinated to have an engine run in the "correct" rotation.

You are correct. Piston thrust direction is another consideration ... which in addition to the connecting rods may also affect piston choice and/or structural considerations of the cylinder block itself. Usually, cast cylinder blocks have considerable excess strength to start with, so the block is generally not a problem except in some very-high-output applications.

[This message has been edited by Marvin McInnis (edited 01-04-2006).]