The engine in your Bantam is firstly a piston engine. It has a piston inside a cylinder, into which a gas is introduced. Secondly it is an internal combustion engine, because the gas is a flammable fuel/air mixture which we will ignite inside the cylinder. The hot gases expand, pushing the piston to the bottom of the cylinder. The piston is attached to a short rod by a pivot within the piston. The rod is further attached to a shaft which has a pin off centre, to which the rod is attached. Thus the linear movement of the piston is changed into a rotating movement of the shaft.
The engine needs a system to allow the mixture to be introduced to the cylinder; the burnt gases to be exhausted, and the mixture to be ignited.
This can be very a complex system of valves, or we can simply use the piston itself, and introduce the fuel & release the burnt gas through ports in the cylinder.
We call this a piston-ported engine. Using this method, we can achieve the induction, compression, power & exhaust stages of the cycle within two movements of the piston.
We call this a two-stroke engine:
So, the diagram above can help us understand how the two-stroke cycle works. It’s helpful to remember that a ‘stroke’ refers to one movement of the piston from top to bottom or from bottom to top – thus, for a two stroke the complete cycle occurs once for every rotation of the crankshaft.
So to begin:
1. In picture 1 the upward movement of the piston away from the crankcase means that the air pressure drops in the crankcase, sucking air/fuel mixture from the carburetter through the intake port
2. When the piston comes down again, as shown in picture 4, the intake port is blocked. The piston falling increases pressure in the crankcase, and blows the air/fuel mixture through the transfer ports and into the cylinder, above the piston. This important event requires a couple of design changes not necessary in a four stroke engine – one is that the thermodynamic efficiency of the engine is increased if the volume of the crankcase is minimised when the piston is down, and maximised when it is up – the difference is called the ‘crankcase compression ratio’. The second factor is that the fuel mixture needs to be retained in the crankcase, necessitating the use of seals around the crankshaft. Failure of these seals results in the mixture leaking out of the crankcase and air, or gearbox oil, leaking in which means you have an uncertain fuel/air mixture, leading to very poor running.
3. The action of the air/fuel mixture coming into the cylinder has the effect of blowing the burnt mixture through the exhaust port, shown in picture 3. The expanding gases go into the exhaust as a pressure wave, and the reflection of that pressure wave off the structure of the exhaust is used to prevent the incoming charge from passing unburnt into the exhaust as well.
4. As the piston rises again, it closes off the exhaust port and compresses the mixture in the cylinder. At a point where the piston is almost at the top of the stroke, the spark plug ignites the mixture and the mixture starts to burn. As the piston passes over its highest point (‘top dead centre’) the pressure in the cylinder from the burning mixture rises rapidly and forces the piston down again, as shown in picture 2.
5. As the piston moves down, the piston uncovers the exhaust port and the remaining gas pressure pushes part of the burnt remains of the mixture charge into the exhaust
You might have realised that with all this up and down several things are going on at once – we only have two strokes to get all this done! So, when we are sucking mixture into the crankcase in item 1 above, we are also compression mixture on the other side of the piston, in item 4; and at the end of our power stroke described in item 4, we are pushing mixture through the transfer ports in item 2.
The engine needs a system to allow the mixture to be introduced to the cylinder; the burnt gases to be exhausted, and the mixture to be ignited.
This can be very a complex system of valves, or we can simply use the piston itself, and introduce the fuel & release the burnt gas through ports in the cylinder.
We call this a piston-ported engine. Using this method, we can achieve the induction, compression, power & exhaust stages of the cycle within two movements of the piston.
We call this a two-stroke engine:
So, the diagram above can help us understand how the two-stroke cycle works. It’s helpful to remember that a ‘stroke’ refers to one movement of the piston from top to bottom or from bottom to top – thus, for a two stroke the complete cycle occurs once for every rotation of the crankshaft.
So to begin:
1. In picture 1 the upward movement of the piston away from the crankcase means that the air pressure drops in the crankcase, sucking air/fuel mixture from the carburetter through the intake port
2. When the piston comes down again, as shown in picture 4, the intake port is blocked. The piston falling increases pressure in the crankcase, and blows the air/fuel mixture through the transfer ports and into the cylinder, above the piston. This important event requires a couple of design changes not necessary in a four stroke engine – one is that the thermodynamic efficiency of the engine is increased if the volume of the crankcase is minimised when the piston is down, and maximised when it is up – the difference is called the ‘crankcase compression ratio’. The second factor is that the fuel mixture needs to be retained in the crankcase, necessitating the use of seals around the crankshaft. Failure of these seals results in the mixture leaking out of the crankcase and air, or gearbox oil, leaking in which means you have an uncertain fuel/air mixture, leading to very poor running.
3. The action of the air/fuel mixture coming into the cylinder has the effect of blowing the burnt mixture through the exhaust port, shown in picture 3. The expanding gases go into the exhaust as a pressure wave, and the reflection of that pressure wave off the structure of the exhaust is used to prevent the incoming charge from passing unburnt into the exhaust as well.
4. As the piston rises again, it closes off the exhaust port and compresses the mixture in the cylinder. At a point where the piston is almost at the top of the stroke, the spark plug ignites the mixture and the mixture starts to burn. As the piston passes over its highest point (‘top dead centre’) the pressure in the cylinder from the burning mixture rises rapidly and forces the piston down again, as shown in picture 2.
5. As the piston moves down, the piston uncovers the exhaust port and the remaining gas pressure pushes part of the burnt remains of the mixture charge into the exhaust
You might have realised that with all this up and down several things are going on at once – we only have two strokes to get all this done! So, when we are sucking mixture into the crankcase in item 1 above, we are also compression mixture on the other side of the piston, in item 4; and at the end of our power stroke described in item 4, we are pushing mixture through the transfer ports in item 2.
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