MAY 31th, 2018
This is an internal combustion engine in which the entire cycle (intake - compression - expansion - exhaust) is completed in one revolution of the crankshaft.
A two-stroke engine consists of:
Operating principle top
The four cycles of an internal combustion engine (intake, compression, expansion, exhaust) are completed in a single revolution of the crankshaft, or in two strokes. For this reason they are superimposed, and there is no clear distinction as in the four strokes.
The piston rises to top dead center (TDC), creating a vacuum in the crankcase. At the appropriate time, the inlet valve opens (i.e. the piston or rotating disc uncovers the inlet port, or the vacuum in the crankcase combined with the exhaust waves opens the reed valve) and the fresh mixture fills the crankcase.
In two-stroke engines, the fresh mixture is typically composed of air/gasoline and oil. This is because the crankcase acts as a "lung" and must contain the mixture that will be burned. In this mixture there must therefore be atomized oil, which will lubricate the crankcase bearings, connecting rod and cylinder.
As soon as the TDC is reached (or rather, slightly earlier) the spark plug strikes, the air/fuel mixture ignites and pushes the piston towards the bottom dead center (BDC). During this phase the piston closes the intake port and compresses the mixture contained in the crankcase. Continuing its descent it uncovers the exhaust port, the combustion gases from the previous phase begin to escape, aided by the megaphone shape of the first part of the exhaust. A few degrees of rotation later, the transfer ports are discovered, i.e. those that connect the crankcase (still full of fresh mixture) with the cylinder. Thanks to the pressure inside the crankcase, and to the depression in the cylinder due to the exit of the exhaust gases, the fresh mixture enters the cylinder, helping among other things the exit of the combustion gases. Because of its inertia, the fresh mixture continues to enter the cylinder even as the piston begins its stroke towards the TDC, until the transfer ports are closed again.
The cylinder is full of fresh mixture, the piston is moving up toward the TDC but the exhaust port is still open, allowing some of the fresh mixture to escape. Before the exhaust port is closed at all, the fresh mixture escaped returns to the cylinder, thanks to the pressure wave generated by the converging cone of the exhaust system. At the end of its stroke, the piston has compressed all of the mixture, which is burned off thanks to the spark plug, starting a new cycle.
The introduction of fresh mixture into the crankcase can be controlled by three types of valves:
Piston-controlled inlet port top
The simplest and most widely used in low-cost engines, such as engines for work tools (chainsaws, brush cutters...) or mopeds. Although this system is reliable (there is no moving part) and easy to make, it has some disadvantages:
Normally, you would want to look for a wide intake timing, to anticipate the intake of the fresh mixture. With this type of intake, however, the intake port will remain open for an excessively long time even after intake is complete, allowing the fresh mixture to escape. For this reason, the duration of the intake phase is reduced, but doing so also reduces the advance of the light opening, delaying the entry of the mixture and reducing power. To reduce (but not eliminate) this problem, the intake duct can be shaped in such a way as to allow air to flow in one direction only, creating turbulence in the other direction to slow down the flow of the mixture. However, this is a process that would defeat the economics of this system, so it is mainly used for tuning purposes.
Reed inlet valve; top
Constructively similar to the previous valve, in this case the duct is wider and is interrupted by a one-way valve. It is not possible to modify an engine born with piston-driven intake by adding a one-way valve: this solution would obstruct the intake duct, which is too narrow for this use. Moreover, the valve requires a certain amount of energy to be opened, which takes time, delaying the intake of mixture. For this reason, the reed valve is placed in front of the exhaust port, and there is an additional port connecting the inside of the cylinder with the reed valve. The vacuum wave generated by the exhaust phase passes through this port, opening the reed valve in advance. Due to its simplicity and effectiveness, this is the most widely used intake valve, both in road and sport engines. At high rpm these valves show the phenomenon of resonance, they cannot close completely and the engine loses power. By increasing the thickness of the reed petals, the resonance frequency is raised and this phenomenon occurs at higher rpm. Thicker reed petals therefore provide better operation at higher rpm, but require more energy to open, leading to a loss of power at lower rpm.
Rotary inlet valve top
This is the most refined solution. The intake duct is represented by a "hole" in the crankcase, which is closed by a disc (generally keyed on the crankshaft) with an opening, to open and close the duct at predefined intervals. This allows maximum freedom regarding the intake timing, which can be asymmetrical with respect to the PMS. The duration of the intake phase can be decided according to the prevailing use of the engine, i.e. if it needs power at high rpm or torque at low rpm, so the rotating disc works well both for "torque" and "power" engines. There is not the problem of resonance as in the blades, so there are no limits to the maximum rotation (if not the physical ones due to the resistance of the materials). There are also some disadvantages: since there are no unidirectional valves, part of the mixture can still leak out as it did with the piston-driven light. Moreover, the disc is usually splined on the shaft, so the suction is at 90° with respect to the exhaust, leading to an uneven filling of the crankcase. In some engines, the valve is connected to the shaft by means of a belt or a bevel gear, so that it can be positioned behind the cylinder, ensuring optimal filling. Clearly this solution is more complicated from a mechanical point of view.
The exhaust pipe for two-stroke engines consists of:
It remains to calculate the length of the exhaust port from the face of the piston, to the point where the wave will reflect back. The exhaust port will remain open for a period of time, during which a wave (traveling at the speed of sound) must travel the length of the pipe and return. The length of the exhaust pipe will be given by the period of time the exhaust remains open, multiplied by the speed of the wave and divided by two.
Lt = 0.5*(phase*c)/N
Where phase is the total duration of the exhaust phase [rad], c is the speed of sound [m/s], and N the number of revolutions [rad/s]. The speed of sound varies with temperature according to the (simplified) relationship:
c = 331.45 + 0.62*T
With T in °C. the typical temperatures are around 500-600°C. More in detail, for GP bikes we are around 650 °C, cross 600 °C, enduro 500 °C, road 350 °C. In the case of a street exhaust, therefore, the speed of sound will be about 550 m/s. If we want to use more appropriate units of measurement (timing in degrees, speed of sound in m/s, RPM, length in mm), the formula becomes:
Lt = 83.33*(phase*c)/N
The length from the piston face to the middle of the converging cone is thus calculated. It is possible to size the remaining part of the expansion, deciding lengths and angles in such a way as to obtain a "torque" or "power" exhaust. In case you want the maximum power, the speed of use will be very restricted. Several solutions have been designed to extend the response curve of the engine, we will describe only some of them:
Direct injection top
The fuel in this case is injected directly into the combustion chamber, with the ports closed. This prevents any possibility of unburned gases escaping, increasing engine efficiency and reducing unburned exhaust fumes, thus reducing polluting emissions. Although in theory there are only advantages, in reality it is necessary to take into account some problems:
Injection systems fall into two categories:
This is a further step towards improving the performance of two-stroke engines. In this case, only the intake ports are practiced in the cylinder, the exhaust is controlled by one or more in-head valves similar to those used in 4-stroke engines, and the scavenging is done by a volumetric compressor. Thanks to this, it is possible to control the exhaust timing at will, and scavenging can be done more efficiently. The efficiency is very high: this cycle is used to produce slow diesel engines for ships, with an efficiency of about 50%. Moreover, the crankcase does not communicate with the inside of the cylinder, there is no combustion of lubricating oil, to the benefit of polluting emissions. These advantages are obtained at the price of a high mechanical complexity, equal to that of a supercharged four-stroke.
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