RC India

This thread is dedicated to understanding of RC engines basics, 2 stroke 4 stroke, RCV etc all goes in here. Forum has veterans and experienced people who will give their bit on the engines, their performance and care and maint issues. :thumbsup:

For starters, How does a 2 stroke Glow engine work ?

Whne you connect the Glow Plug and crank the engine the fuel inside catches fire and it explosive combustion takes place.  :argue: The explosion drives the piston down, this is where the engines power comes from (the rest of the time it is just inertia keeping the engine spinning!). As the piston travel downward the crankshaft has rotated (See Image) so that the slot to the carburettor is closed therefore the piston begins to compress the fuel air mixture that is in the crankcase. As the piston travels further down the cylinder the exhaust port is exposed and the exhaust gases start to clear out of the cylinder. Next the intake ports begin to open and the fuel/air that is under pressure in the crankcases is pushed up through the transfer ports (See  RC Engine Cutaway diagram)and starts to enter the cylinder. The intake gases help push the exhaust fumes out (Some unburnt fuel gets wasted here and you see it coming out of the exhaust). As the piston gets to the bottom of its stroke (Called the Bottom Dead Centre BDC) the crank will have rotated enough to open to the slot beneath the carb and more fuel and air mixture will start getting pulled through by the vacuum left by the mixture going into the cylinder. As the piston starts to travel up the cylinder the crankcase vacuum sucks more fresh fuel into the bottom end but also slows the transfer into the upper cylinder. The piston then slides over the intake ports covering them and begins to compress the new mixture. The final vestiges of exhaust gas leave the exhaust port. The exhaust port fully closes and the new mixture is compressed into the cylinder head where eventually the temp rise due to the compression and the glow plug cause the fuel to ignite.... And the whole process begins again!. once the engine is hot the glow plug remains hot and giving the spark to the compressed fuel air mixture

Timing
5.    In theory, the opening and closing of the valves, and the supply of the spark are all timed to take place at either top dead centre (TDC) ie when the piston is at its highest point in the cylinder, or bottom dead centre (BDC) as appropriate.  In practice however, the timing is modified to take into account the following facts:-

a.   There is a limit to the speed of which valves can be made to open/close, beyond which excessive stresses would be imposed on the valve operating gear.

b.   When a valve is almost closed, the flow of gases is minimal.

c.   There is an appreciable time between the ignition of the compressed fuel/air mixture, and the build up to a maximum pressure in the cylinder head.

d.    As engine speed is increased, it is necessary for a spark to be further advanced (ie to be made to occur at a greater crank angle before TDC) so that the maximum combustion pressure will occur early in the power stroke.  However, rich mixtures, such as are necessary when an engine is developing its full power, burn faster than normal or weak mixtures, and consequently it is usual for the spark to be slightly retarded at full throttle.

Cooling
a.    If an engine were perfectly efficient all the heat produced in it would be turned into useful work, and the problem of cooling would not arise.  This is impossible, however, and in practice less than 30% of the heat generated during combustion is converted into mechanical energy; about 40% passes out with the exhaust gases, while the reminder finds its way into the engine
and, if no steps were taken to get rid of it, would cause mechanical deterioration and break-down of the oil.

b.    There are two methods of cooling the cylinders, air cooling and liquid cooling, the difference being that in one case heat is transferred directly to the air through which the engine moves whereas in the other a liquid coolant circulates continuously between the cylinders and a radiator as in a car (Fig 4b).  Each system has its merits, and each tends, though not
exclusively, to a distinctive arrangement of the cylinders.  Whichever method is used, however, a considerable degree of cooling is achieved by the lubricating oil circulating in the engine (Castor is better than syth in this regard).  The oil is pumped through an oil cooler to dispose of this absorbed

Carburation

a.   Carburation is the process by which air and fuel vapour are mixed in suitable proportions and the supply of this mixture regulated according to the requirements at any given operating condition.

b.   The mechanical means by which this mixture is achieved is by the use of a carburetter or fuel injector.  Their purpose is to supply a well atomised and correctly proportioned mixture of fuel and air to the engine, and to provide a method of limiting the power output by limiting the flow of the mixture.

c.   Liquid fuels will not burn unless they are mixed with oxygen.  For the mixture to burn efficiently in an engine cylinder, the air/fuel ratio must be kept within a certain range, around 15:1 by weight.  The ratio is expressed in weight because volume varies considerably with temperature and pressure

Engine Limitations

1.    The stresses and wear are increased at high rpm.  Consequently, maximum rpm limitations are usually imposed which apply at the basic power conditions. If the specified engine limitations are exceeded, engine may get damaged.

2.    High cylinder temperatures lead to a break-down in cylinder wall lubrication, excessive gas temperatures and distortion. Keeping mixture high after couple of sorties is a good idea.

3.    High oil temperatures cause failure of cylinder and bearing lubrication.  Accordingly, maximum cylinder limitation is also imposed for the main power conditions.(It is good idea to get one IR temp monitor for your engine)

4.    Shortage of oil (Less Castor or syth),  may result in inadequate lubrication and bearing and or cylinder lining failure. 

Very informational... stickied ! :thumbsup:

Difference between 2 stroke & 4 stroke

One complete cycle of an IC engine comprises of: inlet, compression, power and exhaust.

In a 2 stroke engine all , with some overlap, are combined in one revolution. The complete cycle is completed in 2 strokes. In a 4 stroke engine the inlet, compression, power and exhaust are distinct. The cycle is completed in 2 revolutions, i.e.in 4 strokes.

The "technically' correct terminology is 2 stroke cycle or 4 stroke cycle, commonly abbreviated to 2 & 4 stroke.

In a 4C engine, the fuel/air mixture enters and exits the combustion chamber via valves. Virtually all use tappet valves much like car and other big engines. These are operated via "pushrods" and tappets and their timing is controlled via a cam and gears. The timing, amount of valve opening/duration will all effect the performance. These engines need to have a small clearance or gap between the tappets and valve which can tend to change with time. Though modern model engines are relatively free from this.

However, particularly when using pure castor oil as lubricant, there is a tendency for carbon to form around the valve seat thus affecting its "seal". This will cause a loss of power. And I have experienced this with a high performance Laser 4C engine.

RCV Limited of the UK have developed a technology for 4C engines that is free of the regular tappet valves and their attendant complexity. In this, the valves are effectively formed by the cylinder rotating and uncovering the ports as required.

http://www.rcvengines.com/how-it-works.htm

The biggest advantage is a total freedom from leakage around the valve seats. A side - but welcome - benefit is that the glow plug is exposed only at the top of the compression stroke. This means that the engine never "kicks" back. No bruised fingers!

The power is comparable to typical 4C engines but not amongst the "performance" types.

Yet another positive is for lovers of scale aircraft. Because of the lack of tappets, the cylinder is much "shorter". This lends itself to being able to have the engine completely enclosed within the cowl.

Nice post Wingco. :)

@ Sushil Sir,

The RCV is now manufactured by ASP in China.
RCV is been one super invention for scale projects.

Thanks Sanjeev next post is on the Assembly log of Funjet and Acromaster

To Elucidate Sushil Sir's point Pictorially. Constant Volume diagram is for no loss engine, (just an illustration)

Engine Starting
 
Before  starting  the  engine,  the  following general points should be observed.  The Engine Manual will give specific instructions, for any particular engine.

a.   If practicable, the model should be facing into wind to ensure the best possible cooling.

b.   No other model, equipment people or anything else likely to be damaged should be in the path of the slipstream.

c.   The model should not be on dusty or stony ground.  Slipstreams pick up loose particles, and damage may be caused to the airframe and propellers.

d.   Chocks/Something to hold model down should be in position.


e.   If the model has not been flown for some time, the engine should be turned over, preferably by  hand, through two revolutions of the propeller to break down the oil film which will have formed.

f.   With inverted engines, the propeller must be turned by hand through at least two complete revolutions to prevent damage by hydraulicing  :headscratch: caused by oil or fuel draining into cylinder heads.  If this is indicated by a resistance to rotation, the glow plugs should be removed from the inverted cylinders, which should then be allowed to drain.

g.   Fuel on and engine controls should be set slightly ahead of the idle position.

h. Prime the engine using a Chicken Stick or the Primer Motor. If necessary, priming may be continued while the engine is turning and until it is running smoothly.  If the engine fails to start and over-richness is suspected, the engine may be cleared
by:

1.   Stopping the priming.
2.   Turning off the fuel.
3.   Opening the throttle and having the propeller turned through several revolutions.

i.   The throttle lever should never be "pumped", either before or after an engine has been started, as with many types of carburettor this will cause an unpredictable excess of fuel to be delivered to the induction system. If the engine is running, an excessively rich. This may
lead to a rich cut or an induction fire, and will also subject the engine parts to undesirable fluctuations of loading.

Keep these points in mind, while starting a Engine, Newbies can even think of taking this checklist as a print out to the field

For 4C engines, I use the following technique and it has worked perfectly, with all engines I have owned.

First connect the glow booster (YES). Set the throttle to idle and "choke" the engine whilst turning over the propeller by hand (not flicking it).  After a couple of turns you will feel a resistance , which is the engine wanting to "fire". Give it one more turn and flick. Should be up and running. This is because 4c engines need to be started rich.

With a cowled engine this is not practical so a starter becomes essential.

I would still be skeptical with the glow booster connected and turning the engine by hand. Though I have no experience with 4 strokes. May be this is normal. But sounds scary and dangerous - turning by hand with Glow connected.

As I have said, I have been following this practice for several years. And do note, that the prop should be turned over firmly but slowly. I will demonstrate this to you, if and when possible.

And Saurabh, I think you know me well enough to understand that I would not do anything
"scary and dangerous".

You know Saurab,
hand swing was the only option those days, lot of people lost their fingers, hand and even life. In fact the yell word "Clear?" "Clear" came in predominantly as a communique between the pilot and the prop swinger. Like Sushil sir says, it is an art, you swing it tangentially, that is the trick. people swing it with the rotation, Wrong you cant be faster than a prop in its direction of rotation.
Like you rightly said, if not confident, chicken stick or the primer is the way to go

Sushil bhai - The concern is that others may not get the same/right technique as you. 

Also, what is the thought process behind having the ignitor on while you are essentially priming the engine ? 

As long as the prop is firmly held in your hand while turning there is no way the engine would start. It would certainly, if you were casual while doing it. However I would agree that for many , who may not be as experienced with ways of an IC engine it is best avoided.
For cowled engine where it is difficult or simply not practical to reach the carb venturi for choking and no choke is installed, the best way to choke and prime is engine is by blocking the exhaust outlet and rotating the prop. Please note this would only work when the muffler pressure line is connected to the tank. The exhaust pressure pushes fuel in to the carb inlet and the rotation of prop distributes it around the appropriate engine cavities. And please make sure that the carb barrel is fully open while choking the engine, and remember to pull it down to idle before giving the starting flick/spin.  

To answer Anwar's query on why the igniter/booster is connected while priming...
This is done so that you get that bump from a charge ready to fire in the combustion chamber and you know it is time to flick/spin. Without the booster, the bump would not be there and it would be difficult to know if the engine is sufficiently primed

Susheel,
You may be interested to know that the RCV tech itslef is built upon the technology developed by HP engines long back for thier fourstroke.
Lot of features in todays RCV were availble in those VT 4T from HP Austria.
For people who are interested, check out HP VT49 or VT 25.
These are still manufactured by MECOA is USA  

Captain

To the best of my knowledge, the HP does have a rotary valve like a couple of the WEBRA 4 strokes. In the Webra (I owned one) the valve was driven by a toothed belt.

The difference - and a significant one - is that the RCV has the cylinder itself as the rotating part, the upper portion of which is ported . Hence the RCV bit - Rotating Cylinder Valve.

1. There is no "technique" here as the prop is NOT being flipped.

2. Answered by Captain Manish.

Flipped or not, unless the prop is firmly held and rotated very slowly, wondering if there is a chance of firing ?  This "firm hold and slow turning" would be the technique ?

Capt Manish also seems to be worried about this, as a matter of general use.

Susheel,
Yes, that is why I said RCV built upon the HP VT technology. If you look at the VT and RCV both have a horizontally split lower crank case no other model engine I know of had/has this.
In summation yes there are similarities but they are not same.

I have not used or owned the Webra, but heard a good Webra 4t barely had power to pull the skin off a rice pudding. But they do look cool!
BTW do you still have the Webra?  

Yes Sushil. You know that I am more confident of you rather than anyone else.  ;D Its just the thought that is a bit scary... :) I have never done that or tried it. I am sure if the prop is help tight there are no chances of starting the engine. However, the backfire or the back-kick is what I was scared of which is more common with 4 strokes.

1. I had a Webra 80 4C and it was awesome!

2. Don't know. Maybe hidden in some box, somewhere.

Sushil,
If you can search in your box, and see if it still there, maybe I could make it work again

Manish

There was nothing wrong with the engine. It was stored for the many years that I was in hibernation. It may well show up and will need  cleaning and a new timing belt, most likely.

And I repeat, the engine was quite a powerhouse.

Will call you about the engine

I am sure if the prop is help tight there are no chances of starting the engine. However, the backfire or the back-kick is what I was scared of which is more common with 4 strokes.
[/quote]
Actually if any one has dealt with Diesel Engines would realise that diesels are always ready to fire  :giggle: and so priming by holding the prop is a natural thing to learn. The 'bump' as mentioned will take away frustrating flicks ;D. 

Actually for diesel engines, you dont need to feel the bump, for a restart it is the click of the contra piston moving back up which you need to listen for to know that the engine is ready to be flicked

Absolutely right. I was referring to the Glow bump in this case.

I found this video which shows how to start a small sized engine (OS 0.46 LA):
http://www.youtube.com/watch?v=ZrpSbd5OVJU

hope it helps :-)

breakup and rebuild Nitro RC engine

Engine Tear Down

www.youtube.com/watch?v=7StsUUesEw8

Nitro Engine Inspection

http://www.youtube.com/watch?v=7iLOe__2bJE

Cleaning Your Nitro Engine

http://www.youtube.com/watch?v=K93yVGcevd0

Bearing Replacement

www.youtube.com/watch?v=gWDb43LLA5g

RC Engine Assembly

www.youtube.com/watch?v=xf38gMtMHfI