engine run backwards?

P!-Ro

Hey I'm curious, I have a 1/3 hp motor I am interested in using as a starter/generator for my robot (seen in avatar) and I was curious as to whether it could actually start the engine backwards in order back-up. It seems alright thinking about it, the only seemingly different thing would be where the electricity flowed from on the spark plug. However, I do not know about other issues such as if the valves would be messed up with the engine running the other direction, and the flow of oil through it. Anyone with extra experience wish to help me out?

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Got 150 pounds of bot and growing.

Leon

Two-stroke engines can run backwards, FWIW.

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Leon Heller
Amateur radio callsign: G1HSM

bill190

I would think with a 4 stroke engine, it would be trying to suck in air from the exhaust and try to blow out through the carburetor?

P!-Ro

I was looking around on the web, and I don't think that's the problem. With the two valves I think they don't open at the right time when switched so the engine couldn't work. Leon got me thinking though, what if I used a two stroke to run the backing up, and thus had a smaller simpler motor to start it? Sounds possible, maybe. Bulky though.

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Got 150 pounds of bot and growing.

W9GFO

Even a two stroke will have problems running backwards. You would need to account for the spark timing being on the wrong side of TDC.

Rich H

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The Simple Servo Tester, a kit from Gadget Gangster.

Erik Friesen

Rotary valve two strokes timing doesn't work very well, at least in glow engines. It will run, but at times will restart forward, at least with airplane engines.

BradC

My 4 cyl Volvo used to diesel backwards quite frequently after a hard run when shut off, so I reckon with a bit of tweaking of the ignition you might get it to work. It's not going to play happy with all the various belts, cams and other bits designed to work in a specific manner though.

If you are using a conventional centrifugal pinion starter you might be SOL though.

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You only ever need two tools in life. If it moves and it shouldn't use Duct Tape. If it does not move and it should use WD40.

Joms

Not sure if it helps, but I know snowmobile engines do that same thing, they will almost shut down, but actually reverse the engine when you press the reverse button... Maybe Google in that area for awhile?

kwinn

Running any internal combustion engine (diesel or gas, 2 or 4 stroke) backwards is not good for it's health. Don't do it unless you want to be replacing it a lot sooner than necessary.

Leon

The main problem with running a two-stroke engine backwards is that the nut on the crankshaft will have the wrong thread, and might work loose. I remember that being mentioned as a possibility.

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Leon Heller
Amateur radio callsign: G1HSM

Mikerocontroller

Don't forget that if you have any kind of a pressurized lubrication system you might lose oil pressure. Also any belts that use tensioners might lose tension. Some cooling systems don't work backwards either. Most of that probably won't apply to an air-cooled two-stroke, though. The only two stroke I can think of that would run almost as well backwards would have to be a glow ignition with reed valve induction or a glow ignition rotary valve ported to open the crankcase as near to BDC as possible. The rotary valve engine is timed to the crankshaft (which carries the rotor) and well as the piston so the direction of rotation can be factor in port timing. The phase of the crankshaft in a reed valve motor might affect airflow inside the crankcase but reverse rotation won't change the port timing. When you add timed spark and/or rotary induction the engine might not run reversed. A "dieseling" spark ignition four stroke can actually run backwards (trivia fact).

stamptrol

2 Strokes will run happily backwards if the ignition timing is changed. The big Mercury outboards (up to 6 cylunder) of the late 60's used two starter motors to spin the engine in the desired direction. When you moved the throttle/shifter lever, the engine would idle down, stop and automatically restart in the opposite direction.

The linkage changed the ignition timing to the other side of TDC .

As for the Bot, I'd think 1/3hp might be a bit small without some serious gearing for starting which could lead to over-speeding the motor when the engine does start and rev up.

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Tom Sisk

http://www.siskconsult.com
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Phil Pilgrim (PhiPi)

Some diesel engines can run both directions. We used to have a ferry serving our community that had a direct drive from the crankshaft to the prop. When coming into the dock, the engine had to be stopped, then restarted in reverse to slow the approach.

-Phil

Leon

Diesel engines are two-strokes, of course.

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Leon Heller
Amateur radio callsign: G1HSM

W9GFO

Leon said...
Diesel engines are two-strokes, of course.

Well, there are some two stroke diesels, but they are not common. Four strokes are more efficient.

Rich H

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The Simple Servo Tester, a kit from Gadget Gangster.

Phil Pilgrim (PhiPi)

Reversing a four-stroke diesel is facilitated by a sliding camshaft that preserves the order of valve openings and closures when the direction of rotation changes. Here's a video:

www.youtube.com/watch?v=ebvD1RnX7LY&feature=related

-Phil

Junk_Bot

Yeah, I'd imagine a big part of the problem running an engine backwards is the valve timing- the valves and camshaft should be built to run in sequence- fuel in, close fuel valve, ignite fuel, open exhaust valve, let out exhaust, close exhaust valve, let in fuel, and so on.

I think that if I was in your position, I'd use the engine to turn a alternator, then use the electricity from the alternator to power the bot and power motors to turn the wheels. If you use an H-bridge the motors will be reversable AND, if you get really ambitious, you could use the alternator to charge a battery, and only run the engine when you need extra juice for the motors or when the battery needs charging.



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Never give up,·Never submit to the seemingly invincible enemy.

JS-KB3RBR

Phil Pilgrim (PhiPi)

Another advantage of the internal-combustion/electric hybrid approach is that the fuel-burning engine can always be running at its optimum RPMs.

-Phil

mikediv

FWIW I have been really big into snowmobiling for a very long time ,, all the 2 stokes now run backwards for reversing the sled I know Yamaha has been very big in R&D on trying to do the same for their line of 4 stroke sleds and as of 2011 still do not have it or offer it anyway
So I would take that as a very big indications that it would be very difficult to make this happen as another posted pointed out it would have to suck through the exhaust and blow out through the EFI the 2 stoke engine is a completely different animal
I know nothing about RC gas engines I would be curious myself if they can run backwards

Mikerocontroller

I know the smaller displacement 2-stroke powerplants are sometimes run in reverse when they are used in twin or quad prop/screw combinations. I just found out that one problem with reverse operation is that the fore/aft thrust of the piston slug is directed forward instead of the normal rearward direction. All this stress and heat is now being absorbed by the weakest and least lubricated part of the cylinder: the Swiss cheese-like exhaust ports. As kids we used to destroy each other's dirt bikes by trying to port them . I tried not to mess with the exhaust ports but I did grind up the transfer ports. I never really got the magic results I wanted and usually ended up with a dangerously abrupt powerband. The kids who had an endless supply of top-end rebuild kits made their power by skimping on the premix. What we really needed was good handling. I always felt like those 2-strokes were trying to kill us.

CannibalRobotics

Those little model airplane and helecopter engines run quite well in either direction - they are essentially 2 stroke desiel engines. If it's a reed valve engine then the timing is set by the piston gas pressure right?
I would think that as long as the oil pump is working, running it backwards would not be that big of a deal. With propper spark timing the effencies would be better.

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Send $1 to CannibalRobotics.com.

W9GFO

CannibalRobotics said...
If it's a reed valve engine then the timing is set by the piston gas pressure right?

A typical glow fuel RC engine does not use a reed valve. It uses a rotating cylinder which is part of the crankshaft. I have had them run in reverse, but not very well. I think that it because the intake port on the rotating valve is timed to one side.

A gasoline RC engine does use reed valves, and if it also uses electronic timing then it would be really easy to add one more sensor on the other side of TDC and just switch to the appropriate one when you want to change direction.

Rich H

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The Simple Servo Tester, a kit from Gadget Gangster.

AJ-9000

Why not use the motor to drive your robot backwards like the Prius car does instead of trying to reverse the engine.
You didn't mention the size of the engine but because the motor is only 1/3 HP I don't think my Idea mentioned below would be applicable but I'll post it anyway.
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looking at the drawing below notice;
The engine can only turn the drive shaft when the electric clutch #1 is
energized.
The motor-generator shaft is directly connected to the floating drive shaft and the #2 electric clutch
turns the drive sprocket when energized to·turn the wheel sprocket by chain.

1. Electric clutch #1 when energized is for generating electricity or
starting the engine.
2. Electric clutch #2 when energized is for turning the drive sprocket
forward or reverse with the motor-generator, that's necessary for going in
reverse.
3. Electric clutch #1 & 2 when energized are for turning the drive sprocket
with the engine and possibly motor assist or generating electricity with the
motor-generator.

I think my idea is somewhat of an invention because for some reason electric clutches aren't in widespread use outside of automotive air conditioning and garden tractors as far as I know so I've never seen anything else like it.

Post Edited (AJ-9000) : 5/6/2010 1:18:22 AM GMT

N0QBH

Some gasoline golf cart engines run backwards to reverse.