How RC Planes 2 Stroke Engine Works?
If you have followed my website www.rookiercflyer.com you will know that I have tried to cover both Nitro/Glow powered planes and Electric powered planes. In either case a basic understanding of the way the power is generated is helpful for the flyer. This post will be directed at those of you who have decided to use Nitro/Glow engines and is aimed at helping you understand how a 2 Stroke Engine Works.
First of all let us ask ‘why is it called a 2 stroke engine?’ Quite simply it is because every part of the energy producing function happens as the piston goes through one full cycle and the crankshaft goes through one full rotation. This means that the piston completes ‘two strokes’, one up and one down.
Each of these ‘two strokes’ achieves two objectives. On the ‘UP’ stroke we get both Induction and Compression. Let us analyse these two actions in detail.
This is the word that describes how the fuel/air mixture is drawn into the engine. As the piston rises up in the cylinder (as shown in the first part of the diagram above) air is drawn in through the carburettor and passes into the crankcase under the piston.
This takes place because the crankshaft is hollow and has an opening that is directly in line with the intake barrel of the carburettor and is opened once in each rotation. The piston rising in the cylinder creates a vacuum within the crankcase that has to be filled.
As the hole in the crankshaft lines up with the carburettor barrel, a volume of air rushes in to fill this vacuum. As it does so it draws with it a small quantity of fuel that is controlled by the needle valve in the carburettor. This mixture of air and fuel rushes into the crankcase
At the same time that this is happening, a previous intake of air and fuel is being compressed into the cylinder head by the rising piston ready to be burned. We will look at how it gets into this space shortly.
OK, so now we reach the point where the piston has to return ‘DOWN’ the cylinder. This is where we get two more operations called Power plus Exhaust/Transfer
All of the energy produced by the engine is created as a result of burning the air/fuel mixture in the head of the cylinder. This happens because the residual heat in the glow plug filament ignites this mixture producing gases that expand very rapidly. This rapid expansion forces the piston back down the cylinder.
The descending piston compresses the fresh air/fuel mixture previously drawn into the crankcase. As it descends almost to the bottom of its downward stroke, it uncovers an outlet in the side of the cylinder called the ‘Exhaust Port’. The expanding gases resulting from the burning of the air/fuel mixture escape through this port.
Very slightly lower down the cylinder, on the opposite side another opening is uncovered called the ‘Transfer Port’. This is a passage in the side of the engine between the crankcase and the cylinder to carry charges of air/fuel mixture. This enables a new charge of air/fuel to enter the cylinder above the piston from the crankcase.
Once the burnt exhaust gases and waste lubricating oil have exited the Exhaust port and the new air/fuel charge has entered the cylinder the whole procedure begins all over again.
In just one complete revolution of the engine all of the above has taken place.
You may already be wondering why the new air/fuel mixture doesn’t escape while the exhaust port is open at the same time as the transfer port is opened. This all comes down to a bit of clever design work by the manufacturers.
Special transfer or ‘boost’ ports are incorporated that direct the air/fuel mixture away from the exhaust port and back into the cylinder. A German engineer developed this modification some years ago and it was called ‘Schnuerle’ Porting after him.
A 2 Stroke Engine’s Moving Components
In the above explanation we have talked about the ‘Piston’ and ”Crankshaft’. These are the two principle moving components but there are other items that are essential to the successful operation of these two.
Piston & Liner
These are central to the operation of our engine and the seal between them is all important. Most engines you will encounter use one of two methods to achieve this good fit:- a) Close Tolerance Fit or b) Ringed Fit
Close Tolerance Fit
The first of these relies on a very accurately engineered fit between the piston and the liner. The picture to the right illustrates such a piston and its cylinder liner.
Engines manufactured in this way are often listed as ‘ABC’ types. This designation refers to the materials used in the manufacture of the piston and its cylinder liner.
‘A’ refers to the piston which is made from an alloy of Aluminium.
‘B’ refers to the metal from which the liner is made and that is Brass.
‘C’ refers to the fact that this Brass liner is plated with Chromium.
Most modern engines of this type are manufactured with a very slight taper inside the cylinder liner, wider at the bottom than the top. This means that the fit of the piston within its liner gets better as the piston rises. The principle reason for doing this is because most of the heat from the burned air/fuel mixture is absorbed at the top of the cylinder causing this part of the liner to expand more than the lower part. The expansion causes the liner to reach a more parallel state so the piston fits the same along its full stroke.
Modern metallurgy has provided new finishes for cylinder liners so we are starting to see such acronyms as ABN where the liner has a Nickel plating instead of Chrome.
The construction of the engine is much the same as for that above apart from the fact that there is a groove running around the top of the piston just below the crown. Into this groove is fitted a spring steel ring that forms a tight fit against the walls of the cylinder liner.
In some engines this ring is manufactured so that when air/fuel is being compressed into the cylinder head or the exhaust gases from the burned charge add pressure it presses even more against the liner walls to form an even better seal.
Cylinders with ringed pistons do not normally have tapered liners as the ring is capable of flexing against the liner walls to accommodate extra expansion at the top of the cylinder.
Up and Down To Round And Round
In the photograph above right the piston is shown with a rod protruding from its inner cavity. This is the conrod (short for ‘connecting rod’) that connects the piston to the crankshaft (photo right). If you refer to the picture above you will see that a metal pin, called a gudgeon pin, passes through the piston and the smaller hole in the conrod. The gudgeon pin is retained in place by spring steel circlips, one at each end.
The bushed hole at the other end of the conrod fits over the stub on the rear face of the crankshaft balance plate (see photo below). The accurately positioned cut-out in the crankshaft is positioned so that the charge of air/fuel from the carburettor is allowed into the crankcase at exactly the right time.
This crankshaft is the part of the engine that converts the up and down motion of the piston into a rotary motion suitable for driving the propeller. It is mounted inside the crankshaft held by either bronze sleeves or ball bearings. One in front of the conrod bearing and balance plate and the other over the narrow part of the shaft.
Controlling Air/Fuel Mixture
In the discussion above the carburettor was mentioned. Now we will look more closely at this important piece of equipment and how it works. To the right here is a photograph of a standard two needle type of carburettor found attached to model two stroke engines.
It is called a two needle carburettor because it has just that, two needles, one to control the high speed running mixture and another to control the low speed running or idle. The hand adjustable high speed jet is located on the left in the photograph whilst the idle control needle adjustment screw is hidden from view in the end of the pushrod link fastening on the right.
The fuel intake nipple that takes in fuel from the tank is located on the left hand side of the main body adjacent to the high speed mixture jet. The air intake barrel is shown at the top whilst the crankcase intake is at the bottom. This fits into a matching boss on top of the crankcase and relies on a neoprene ring to form an air tight seal when the retaining screws are tightened.
The small screw to the right of the carburettor body is an adjustment for the range of movement of the rotating air/fuel intake mixture barrel (throttle barrel). In the photograph (right) the throttle barrel is shown about half open. In the fully open position the hole in the barrel will be totally in line with the air intake hole whereas when fully closed the solid part of the barrel will completely close the air intake. This movement is controlled by the throttle servo via a pushrod connected to the rotating arm attached to the barrel.
Running through the centre of the throttle barrel is a jet with a needle controlled by the ratcheted finger adjustment. This directly controls the flow of raw fuel from the tank into the carburettor. Screwing this needle in restricts the flow of fuel into the carburettor causing the air/fuel mixture to become leaner. Screwing it out increases the flow of fuel and makes the mixture richer.
Some manufacturers have this needle valve assembly (NVA) remotely mounted at the rear of the engine for safety reasons (keeps fingers further away from the propeller) where it is connected to the carburettor fuel intake nipple by a short length of fuel tubing.
At the other side of the carburettor at the centre of the fixing for the pushrod lever is the slow running jet. this only comes into action when the throttle has been closed to below halfway. It is used to fine tune the fuel flow at low throttle settings. You will notice that when you turn the pushrod lever back and forth, the barrel moves in and out of the carburettor body. This is because the movement adjustment screw runs in a ‘helical’ groove cut into the barrel.
As the throttle barrel moves inward the idle needle engages with the main fuel jet and starts to restrict the flow of fuel. This control is independent of the main needle. The more the barrel closes the more it restricts the flow of fuel. The idle needle has a very fine thread because it has very critical effects on the low speed mixture and needs only tiny adjustments. Screwing it in makes the mixture leaner whilst screwing it out richens the mixture.
Concluding How 2 Stroke Engine Works
I hope that my explanation of this complex piece of kit has helped you understand how the power is produced. The 2 stroke engine works well as a propulsion system for model planes but it has some disadvantages in certain circumstances. Firstly it is noisy and secondly it is messy, it can also prove problematic at times. Providing you are happy to accommodate these facts you should find your 2 stroke engine works well for you.
Till the next time, happy learning.