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Join Date: Oct Apropos 2. What happens if you do? Join Date: Jul Location: on the A Well apart from drawing air in through the exhaust valves, and out through the intake Join Date: Feb Location: Weston-sloping-Mare.
Join Date: Aug Location: Reading. Weaken or shorten the life of the timing belt due to being stretched the opposite way. Valves could contact piston crowns because of slack in timing belt.View Full Version : Car question: What makes an engine rotate backwards?
Send questions for Cecil Adams to: cecil straightdope. Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks. I don't know the correct technical term for that. When does it happen and why? How can this opposite rotation be sustained? Ignition timing would be totaly wrong.
Also, why is it so bad for the engine when it happens? Are you talking about the engine running in reverse or just turning. I do believe that it happens when the point of ignition is to close to top dead centre.4 BMX Skills to Learn First
I saw it happen to a fella at the traffic lights on a Montessa motorbike. Piss funny. He went to take off forwards but went backwards, luckily noone was behind him. The engine of the Lamborghini Muira had to be made to run in reverse to the way it was designed IIRC because of its orientation in the car.
It was done by adjusting the timing of the engine. How do they do that :confused:. Let's say that the crankshaft normally turns clockwise. You have just parked the car and put it in neutral. You then turn the ignition off. The engine's revs drop as usual, but then, instead of coming to a stop, the crankshaft starts turning in the opposite direction! A four-stroke wouldn't run - the whole induction-compression-power-exhaust cycle isn't self sustaining run backwards.
A two-stroke engine, however How do they do that :confused: If I wanted to change the direction of rotation for most four stroke automotive engines I might have to take at least the following steps. Cam - Easiest way is to grind cams that give the proper timing when going in the opposite direction.
Ignition - With a typical distributor it might be best to swap the helical gears so the rotor goes the same direction with the engine going the opposite way that it was designed.
Oil pump - This is a tough one with most engines as you may need a redesigned pump to flow the right direction with opposite engine rotation. Most use a meshing gear type pump with one side driven by the jackshaft or crank.
Accessories - engine driven accessories will have to be mounted or altered so they work correctly with opposite belt rotation.
Say, any aviation mechanics here? How about Lockheed Ps where each prop went in an opposite direction. From the pilot's POV the right prop rotates clockwise and the left counter clockwise. My WAG is that the engines were identical and both went the same direction but the reduction gear cases were different so one has an output shaft going the opposite way but I have nothing to base this on. As I recall, back in my hot rod days late '60s, early '70shot rodded automotive engines that were used in boats ran in reverse rotation to their landlubber cousins.
Why, I don't know something to do with the drivetrain, perhaps? I do remember opposite chirality camshafts being advertised. I can't easily imagine a modern four cycle engine running backwards spontaneously.If you are trying to turn the crankshaft while the timing chain is off, you may have an open valve that is hitting a piston.
This can be a big deal and you can damage the valve if you try too hard. Normally you would set the engine up on the timing marks before removing the timing chain, and not turn the crank or camshaft until the new chain is installed. If the crank and cam are out of sync the valves can interfere.
You can get everything lined back up by setting the crank in a position where all the pistons are down. Then and only then can you turn the camshaft to get it into timing position. After the cam is set, you can bring the number one piston up to TDC by turning the crank. It may be best to get professional help by contacting a mechanic and having someone look it over and help you figure out what to do. Q: What to do when crankshaft wont turn with wrench to line timing chains asked by Amanda O.
What would cause the crankshaft not to turn with a wrench to set the timing chains? My car has miles. My car has a manual transmission. John Hege Automotive Mechanic. Thank John. Was this answer helpful? Thank you for your feedback!
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You usually can tell the difference by how the leak Read more. The vehicle is put under extra load when the climate control is turned on. The engine is impacted different ways by the different systems.
Hi guys Let a single cylinder ic engine. Now when we start the enginethen depending upon the position of crank the last time we switched off the engine, it is possible it may rotate in reverse direction because piston just has to reciprocate so acco to crank position at rest it may move reverse when piston comes down. But we want movement only in forward or take clockwise in order to move car forward. Related Mechanical Engineering News on Phys.
The starter rotates the crank 1 way starter will throw it 1 way only so that a spark can make it run. If you tried rotating it the other way the valves would open at the wrong time and the spark would not ignite anything. It would be impossible for an engine to run backwards without retiming the spark and reversing the cam profiles. Last edited: Nov 22, Bob S. Im not sure about 2 strokes really. Here is a SAAB with a two-cycle engine that used all forward manual transmission gears while backing up.
Last edited by a moderator: May 4, Yes engines can run backward either by design or not by design. Two cycles can be designed to start either way with a reversible starter. Some snowmobiles have this system because they have no reverse gear. In this case the air filter becomes clogged with exhaust and the oil pump does not work, so it is not a good thing.
Reverse crankshaft movement
I recall back when I earned my spending money mowing lawns as a kid that my old mower would sometimes start in reverse. This happened when the two-cycle engine back-fired as I pulled the rope. Occasionally, this would result in reverse running. It was easy to tell when it happened because the blade would "chew" the grass rather than cutting.
As chris said, 4s cycle cant reverse start, due to inappropriate valve timing.Hi, I have 02 new Beetle tdi wk on body and 40 k on rebuilt motor from previous owner and was driving down the road when the car started to jerk and I pulled over to find out what was going on. When I stopped the engine died and would not turn over - well maybe only once! Still I go to start the car and it will turn about once maybe and thats it.
We dropped the pan and could turn the engine backwards ok but could not rotate it forward at all! Can anyone tell me what is going on with my car?
I took it to a local shop and he is not sure what it is either. What do you think? Jason is the man up there! Sounds like you may have lost the timing belt. When a TB is chaqnged, you HAVE to pull the engine through 3 complete revolutions to make sure that nothing is binding, i would do that and see, if you cant pull it through by hand, your starter certainly wont be able to!! Last edited by flyingmikey; May 10th, at Troubleshooting Low Power Having trouble searching?
You say you could turn the engine backwards, meaning the crankshaft can be rotated counter clockwise. Were you able to turn a complete revolution or partial?
If you were not able to turn a complete revolution, then what is happening is that there is contact between valves and pistons. This indicated that your timing belt problem.
The other option you have is to remove the head and see if you can then rotate the crank completely clockwise.
If you are unable to do so would require disassembling the block and rebuilding, not a difficult task. Last edited by Dodoma; May 10th, at Remove the accessory belt and try again. I have seen many times an accessory frozen, and the belt holds the entire engine from moving.Automotive Forums.
One question. Can the crank on a 2. Book says no, some say yes. Which is it? What are you doing? If you're replacing a broken timing belt, don't bother. This is an interference engine and you very likely have some damage to valves or pistons if it broke while running. If the crank won't turn, the cam is not in the proper position to allow it. There are valves hitting pistons, stopping the rotation. Thanks Cat, We've been working on the engine, changing the head gasket, new belts, water pump, etc.
I put the belt on and tried to line up the camshaft mark with the valve cover mark and it stopped moving. I know that when we put the exhaust cam back on it didn't 'set' in with the mark straight up like when it was taken out. I was hoping I could rotate the crank backward and try to get everything to line up. Will that work? Thanks again. If the crank won't turn there must be a reason why. Ignoring that could cause some problems.
I have no idea why but I follow it. I've turned both crank and can only clockwise always I just do it