Any - What determines the crankshaft direction? | Technical matters | Back Room Forum

Wed 13 Feb 2013 02:09

Any - What determines the crankshaft direction? - pdboxer

I have one question I'd love to know the answer to. When a piston drives a connecting rod, making it move in a certain sideways direction, and therefore causing the crankshaft rotate clockwise or anticlockwise, what is it that makes the connecting rod move in one direction or another - ie if the piston was at Top Dead Centre, surely the downward force could make the connecting rod go in either direction, in fact how come it doesn't go straight down, what makes it go left or right? Is it a sensor that means it only fires the sparkplug when the rod is already moved to one side, in which case what sets it at this side? Or are the connecting rods or camshaft shaped so that they can only move in one direction from being pushed down?

Am guessing its not a sensor or how would it have worked in the days of the first internal combustion engine when there were no sensors.

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Wed 13 Feb 2013 07:06

Any - What determines the crankshaft direction? - craig-pd130

You can set up a 4-stroke engine to run in either direction, but you need to organise the cam & valve timing, ignition timing and fuel injection in order to do so. You can turn any car engine's crank "backwards", but then the inlet of fresh mixture to the cylinder will happen in the wrong phase, and the spark plug will fire several degrees AFTER TDC, when compression is falling, which isn't conducive to efficient running!

As an example, the Lancia Thema 832 saloon used the 3.2 litre V8 from a Ferrari 328GTB, but in order to get the wheels turning the right way (the Lancia being front-wheel drive) the crank rotated in the opposite direction, meaning different camshaft profiles, ignition & fuel mapping etc.

However, 2-stroke engines can (and sometimes do!) start "backwards", especially if they are in a relatively low state of tune. A mate once had a nasty old CZ175 2-stroke single that could be start and run backwards if you pushed it backwards in gear with ignition on. Hours of fun on the local disused railway line :)

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Wed 13 Feb 2013 09:15

Any - What determines the crankshaft direction? - Galaxy

On the Messerschmitt three and four wheel cars you could change the timing with a control on the dashboard and get the engine to start so that it was running backwards.

This gave you four reverse gears and made it possible to do 80mph in reverse, should you so wish!

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Wed 13 Feb 2013 10:07

Any - What determines the crankshaft direction? - freman

I would the inertia stored in the flywheel wouldbe sufficient to carry the crankshaft over the T.D.C. position.

I think that as all 4 stroke engines fire before T.D.C. bear this out.

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Wed 13 Feb 2013 10:09

Any - What determines the crankshaft direction? - bathtub tom

Several 2-stroke cars of that era used to be able to run 'backwards' instead of a reverse gear.

I had one. It had a 'dynastart' combined dynamo and starter motor. When used as a starter you could reverse the direction of current flow and utilising a separate set of points run it backwards.

To answer the OP's question, the starter motor determines the direction the engine will turn initially and an internal combustion engine has to be turning to run. It would be possible to start an engine from stationary by sparking a plug on a charged cylinder (I believe early Rolls Royce used this system). It would therefore be possible to give an engine an initial kick backwards, but it would stop within one cycle as the valve and ignition timing wouldn't support running in that direction.

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Wed 13 Feb 2013 10:22

Any - What determines the crankshaft direction? - FP

"On the Messerschmitt three and four wheel cars you could change the timing with a control on the dashboard and get the engine to start so that it was running backwards."

I can't speak for the four-wheel version, but the three-wheeler did not have a separate control on the dashboard (which would have been pretty odd). If you wanted reverse you switched the ignition off, waited for the engine to stop, pushed the key hard, which made the whole lock barrel move inwards, then switched on and restarted the engine - now in reverse. There were two sets of "points", one for each direction of running; that is how the timing was changed. There was no starter motor as a separate component; the flywheel included the necessary windings to act as a starter (in either direction) and dynamo, all in one. (Messerschmitt called it a "Dyna-start".)

AFAIK, this ingenious technology has only recently resurfaced in some stop-start systems.

While avoiding the need for a more complex gearbox (the Messerschmitt's was an unmodified version integrated with its two-stroke engine from a motorbike, made by Sachs) the drawback when reversing was its clumsiness. I remember taking my driving test in mine and having to do some explaining when executing the "three-point-turn" manouvre.

Edit: BT was obviously typing his response at the same time as me!

Edited by FP on 13/02/2013 at 10:23

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Wed 13 Feb 2013 14:16

Any - What determines the crankshaft direction? - TeeCee

>> (Messerschmitt called it a "Dyna-start".)

So did everyone else. My father's 1929 Riley 9 has a Dynastart rather than a seperate Dynamo and Starter motor.

>>AFAIK, this ingenious technology has only recently resurfaced in some stop-start systems.

Yes, I saw Peugeot touting it as their all new "i-StARS" (Starter-Alternator Reversible System). There was a monumental load of waffle and spin about their revolutionary new technology, but it's a Dynastart with a badge on it. Made me laugh.

Edited by TeeCee on 13/02/2013 at 14:16

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Wed 13 Feb 2013 11:14

Any - What determines the crankshaft direction? - craig-pd130

In terms of the beefy engine components in the bottom end (crankshaft, conrods, pistons etc) they are usually made so that they can turn either way happily. However camshafts don't always have a symmetrical profile on the cam lobes, which would mean using a different cam if you wanted the motor to run backwards.

Some 2-stroke motors have a slightly offset gudgeon pin to stop the piston 'rocking' in the cylinder after ignition. This helps to keep the thrust face of the piston away from the hot exhaust port, and cuts the risk of the piston skirt snagging in the port under high engine loads.

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Wed 13 Feb 2013 12:39

Any - What determines the crankshaft direction? - unthrottled

A four stroke diesel can also run backwards-albeit with very retarded injection timing. Unlike two smokers, 4 strokes have a pressurised oiling systewm, and oiling systems don't work in reverse...!

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