is it possible to reverse the rotation of a single cylinder two stroke gas engine with EI?
I know the first thing to do is to remount the hall sensor so it will be at the same angle but in a CCW insted of CW.
is there anything else to do or retiming the engine is enough ?

the engine is a 26cc gas engine, with EI.
i need it to spin CCW so i can use it as a pusher with an ordinary prop (mounting the prop backwards)

Thank you.
Raviv.

Just change the position of the ignition sensor and start the engine in the other direction

Have fun

Elson

Or you can leave the sensor where it is, but flip the sensor in the housing, then install a second magnet with the polarity reversed where it should be for reversed timing. From then on to get the othere direction you just reflip the sensor.

Thank you for your answers,
It seems that the only thing needed in order to make the engine work currectly (but opposite direction) is to re-time it.

What about cooling?
in a pusher mode, the prop pulles air from the cylinder head (instead of pushing air into the cooling fins), could this be a problem?
the modified engine will be mounted as a pusher without any option for baffeling or jacketing,
further more, the plane (actually - VTOL) will spend alot of time hovering and will not have free air flow caused by straight flight.

Cooling is a BIG problem with pusher configurations. If you fail to duct the air to the cylinder, fail to maintain a tight fit of a cooling duct around the cylinder, and design the cooling outlet in a manner that will force good extraction you can ruin an engine in short order. Take the time to think the cooling system through and design it with minimum obstruction. Pay particular attention to cooling the exhaust side of the cylinder. Do not use lean oil ratios!! Pusher installations are the true test of what an oil can do so stay with the upper end of the quality selections and avoid ratios leaner than 50-1.

To assist the design of a cooling system you can set up a crude wind tunnel test by using a leaf blower to push air over the airframe. Set a screen in front of the leaf blower and plane with long thin strips of mylar attached to the screen long enough to reach to the back of the aircraft. Watch how the mylar strips flow over the airframe to determine where the natural airflow runs over the plane and use that airflow to determine where a cooling duct should begin to obtain the most volume. After you build the prototype air duct perform the airflow check again to see if it's use had a negative effect on the original flow of air.

No need to run an iar cooled engine when MVVS makes a liquid cooled 58cc. Just find a spot for the radiator in open air.

If the gas engine has a rotary valve like the MVVS or ZDZ you probably cannot reverse it.
Also if the engine has been run for any time in the normal direction, reversing it will create abnormal wear and might ruin it. You need to start with a new engine and always run it reverse.

well, i really have no intentions of buying a liquid cooled engine (although it crossed my mined), and i want to use the engine i already have.

hi "old man", thank you for your answer.
It seems that i have a problem, isnt there any other way cooling the engine while hovering?
the plan is to build a shroud that will surround the prop, kinda like a ducted fan... a prop ducted fan,
if the walls of the shroud will be high enough to contain both the prop and the engine itself, wouldnt it be sufficient to force enough air over the cylinder and keep it cool?
p.s, while serching RCU for some VTOL information, i came across some of your threads, a lot of useful information, thank you.

Raviv.

hi dirtybird, the engine is not a rotary valve type, i know that these kind of engines need a special cranckshaft with different timing in order to spin backwards.
the engine is a normal two stroke engine , XYZ26cc.
it ran flawlessly for approximately 4 gallons, always on 1:33 synthetic oil (mobile racing).

How about getting a pusher prop and running the engine in the original direction?

it is possible, but because this is going to be a (very) experimental model, i want to have the ability to change props without waiting two and a half weeks until i recived the one i orderd.
it seems rather easy changing the location of the hall sensor in ordr to get the timing right for a CCW rotating engine, the main problem is the heat.
using a pusher prop will not solve the heat problem.

Thank you.

Raviv.

The volume of air required to cool any engine is much more than most would believe, let alone the speed it needs to move at. In a hovering situation you will be forced to find a way to pump air to the engine which will present a completely different set of items to over come.

What you are doing is what modeling is all about. It's not always the easy, off the shelf, everyone does this, type of stuff where everyone has a safe haven. Someone always has to led the way and in doing so they have a lot of experimenting and research to do. You may want to look into some high volume electric mini fans. Think computer components with some specialized plenums. Better yet, brushless electric r/c motors turning fans from computer cooling systems.

Any reed valve engine is easy (easiest) to reverse running direction. Most of the piston port engines can as well. As for changing wear patterns, the only place there would be any effects would be at the bearings. Pistons and rings go in and out regardless of rotational direction.

True but there are side forces which change on the piston/cylinder.

It is never going to work . The transfer port timing (in a 2-stroke)/ valve timing (in a 4-stroke) is different when turning in reverse.

If you could redesign the engine it might be feasable, but the performance would be just plain poor.

Tell that to the hundreds of two strokes currently running every day in reverse

Hi "old man" (name?), electric fan is a good idea, i can use a 5"-8" diameter fan with high CFM, the noise is not a problem.
the added fan might restrict the airflow for the prop, but with proper design and build this can be reduced to minimum and ignored.

talked to a friend of mine that knows his way around these engines and UAVs (he design and build them for a living), he said (like most of you) that there is no problem reversing the engine, the only thing needs to be done is re-time the ignition (different location of the hall sensor or magnet).

any other suggestions regurding engine cooling are appreciate.
will look further into mounting an electric computer fan with shroud, but this will cause additional weight for the fan and battery.

"Tired old man" is a new name for an old member. The reason for the name change I don't want to go into. I do the same thing your friend does for a living, and I'm starting to think I've forgotten how to work with an engine running in the "normal" direction.

On a 4c you need a new cam and the timing must be changed.
On a 2c pistion port just change the timing. Do it all the time, some engines even run backwards better than forward.

Pat, the quote gives you up...

If this is true why do my 2c glow engines run so crappy when they start backwards? Not trying to be smart, just would like to know.

RJ

because your two stroke glow engine are rotary type and have the rotary valve as part of the cranckshaft.
it order to get a two stroke glow engie work CCW, you need a different cranckshaft with a different valve timing.

most gas engines are leaf valve type, theire timing is determined by the position of the hall sensor and magnet.

Ah yes, thank you...........RJ

On some two stroke glow engines with a rotary valve you can rotate part of the crankcase and get correct timing for reverse operation.

just came across these pictures,
this is a VTOL UAV that seems to be in a pusher setup it order to generate lift.
i think that the controls are with vains located in the propwash, deflection of the vains according to a gyro command will cause stabilized flight.
this is very similar tho the project i am planning.
it seems that there is no special element or feature for cooling the cylinder heads, is it possible to operate the engine is a pusher mode without killing it?

In this case it appears that the propulsion/lift fan is being used to draw cooling air through the engine. My tired old eyes seem to see a lack of ducting to the cylinders which would have the same effect as it would inside an unbaffled cowl. There may well be something that is attached to an outer shroud that is not noted in the pictures that would better direct the flow around the cylinders.

A little heads up for designs like this. They are not very efficient and a very large amount of available power is used up in the lifting required to move the airframe and required fuel. It doesn't leave much available for payload purposes.