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Two-stroke engine
2017 Mattia Piron. All rights reserved.

  1. Introduction
  2. Operating principle
  3. Inlet
    1. Piston-controlled inlet port
    2. Reed inlet valve
    3. Rotary inlet valve
  4. Exhaust
  5. Direct injection
  6. Uniflow

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Immagine di copertina

MAY 31th, 2018

 

Introduction       top

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:

  1. Moving parts: crankshaft, connecting rod and piston. Although they have the same function as the same components of four-stroke engines, their shape is slightly different due to the different type of use. The crankshaft is made up of two disks; balancing is done by drilling two holes of appropriate diameter and position next to the connecting rod button. These two holes are then filled with a lighter material, typically aluminum. By doing so, the crankshaft occupies as much volume as possible, helping to reduce the free volume inside the crankcase and thus intensifying the pressure-depression effects. The piston has a more elongated shape than those of four-stroke engines, because it must also control the opening/closing of the ports on the cylinder. Also, there are no oil scraper rings, and the top surface is smooth or slightly convex. The connecting rod of a two-stroke engine is usually lighter than that of a four-stroke engine. It must withstand lower stresses, for reasons that will be discussed in detail later;
  2. Fixed parts: crankcase, cylinder and cylinder head. The crankcase has the function of containing the crankshaft. In addition, it functions as a chamber for the first intake stage. The cylinder is the component on which the piston slides. It also features the intake and exhaust ports, allowing the passage of fresh mixture and combustion gases. The cylinder head encloses the cylinder, acts as a support for the spark plug and is shaped to allow optimal combustion of the mixture;
  3. Fuel System and Exhaust System. The fuel system includes filter case, filter, carburetor (or throttle/injector valve), and inlet valve. This can be simply a light opened and closed by the motion of the piston, a one-way reed valve, or a rotating disc. They will be discussed in detail later. The exhaust system, ideally made with the classic expansion shape, to increase engine efficiency.

 

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.

Principali componenti del motore a due tempi

 

Intake       top

The introduction of fresh mixture into the crankcase can be controlled by three types of valves:

  1. Piston-controlled inlet port;
  2. Reed inlet valve;
  3. Rotary inlet valve.

 

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:

  1. Little design freedom regarding the duration of the intake phase;
  2. Symmetrical intake phase before and after TDC.

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.

 

Exhaust       top

The exhaust pipe for two-stroke engines consists of:

  1. Collector, it can be constant section or slightly conical (2-3). To obtain maximum power, its section should be 10-15% larger than the exhaust port section, while its length should be 6-8 times its diameter. If a wider range of use is desired, sacrificing some power, the section can be 1.5 times that of the exhaust port, and the length 11 times the diameter.
  2. Divergent cone, creates a wave of depression helping the emptying of the cylinder. Its main function is to intensify and lengthen the duration of the return pressure wave. Increasing the taper of this area increases the intensity of the pressure wave but reduces its duration. That is, you get more power, but in a narrow range of use. The output area of the cone should be 6.25 times the input area, while the angle will be 7-10 degrees (with respect to the axis).
  3. Cylindrical section, connecting convergent and divergent cone. Its length determines the timing between the depression wave and the pressure wave: the shorter this section is, the closer the two waves will be, reducing the range of use and relegating it to higher speeds.
  4. Converging cone, it reflects the pressure wave forcing the mixture out of the exhaust to return inside the cylinder, and operating a real supercharger. The greater the taper of this section, the more intense the pressure wave will be. In addition, the taper will influence the shape of the power curve in extension, beyond the achievement of maximum power. The taper is double that of the divergent cone, or 14-20 (with respect to the axis).
  5. Needle (tube) outlet. The vacuum wave generated inside the exhaust depends on the section and length of the needle. The narrower and longer it is, the more intense the wave will be; however, it should not be too narrow. The passage area will be 0.58 - 0.62 times that of the manifold, while the length will be 12 times the diameter. If in doubt, it is better to use a larger diameter.

 

Schema della marmitta ad espansione

 

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 500-600C. 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:

  1. De-Laval nozzle: can be installed in the initial part of the exhaust pipe pin, and thanks to it, the operating arc can be extended.
  2. Water injection: injecting water into the expansion cool down the exhaust gases, slowing down the sound waves.
  3. Resonator: Duct (or chamber) closed at one end, mounted on the exhaust manifold and parallel to it. It generates resonances that slow down sound waves at certain engine speeds.
  4. Aerodynamic brake: washer placed between the converging cone and the needle, chokes the duct increasing the gas temperature and modifying the expansion response. They are usually interchangeable, to modify behavior according to conditions.
  5. Exhaust valve: Mechanical valve that changes the exhaust timing according to the speed of rotation, thus modifying the tuning of the engine.

 

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:

  1. The injector has an extremely short time available for fuel injection, less than half of what it would have in the case of indirect injection;
  2. Given that the use would be in a two-stroke engine, the injector will have to work at twice the frequency of the same in a four-stroke, which is more difficult to accomplish. It will also have less time to cool between firing;
  3. It will have to inject gasoline while the piston is compressing the mixture, so this is an injector that will have to work at very high pressures.

Injection systems fall into two categories:

  1. Air-assisted systems: an air/fuel mixture is injected into the cylinder. In this way a better fuel atomization is obtained, moreover it is possible to use a normal injector that injects gasoline inside the cylinder of a compressor, that in turn will inject this air/fuel mixture inside the cylinder. This system has a high mechanical complexity; it is adopted by Piaggio (Piaggio Fast) and by Orbital (used on Aprilia DI Tech, Piaggio Pure Jet and in the nautical field by MERCURY and Tohatsu).
  2. Single-fluid systems: only fuel is injected. This is the most effective system, but also the one that presents the greatest problems from the point of view of the realization of the injector. Systems of this type are adopted by EVINRUDE (FICHT) and by YAMAHA (Ram Tuned).

 

Uniflow       top

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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