Two-stroke cycle
|
|
The two-stroke cycle of an internal combustion engine differs from the more common four-stroke cycle by having only two strokes (linear movements of the piston) instead of four, although the same four operations (intake, compression, power, exhaust) still occur. Thus, there is a power stroke per piston for every engine revolution, instead of every second revolution. Two stroke engines can be arranged to start and run in either direction.
Two-stroke engines are used most among the smallest and largest reciprocating powerplants, but less commonly among medium sized ones.
The smallest gasoline engines are usually two-strokes. They are commonly used in outboard motors, high-performance, small-capacity motorcycles, mopeds, scooters, snowmobiles, karts, model airplane and motorized garden appliances like chainsaws and lawnmowers. In each application, they are popular because of their simple design (and consequent low cost) and very high power-to-weight ratios (because the engine has twice as many combustions per second as a four stroke engine revolving at the same speed). For handheld devices, they also have the advantage of working in any orientation, as there is no oil reservoir dependent upon gravity.
Two-stroke cycles have also been used in diesel engines, notably opposed piston designs, low speed units such as large marine engines, and V8 engines for trucks and heavy machinery.
| Contents |
Various two-stroke design types
To understand the operation of the Two-stroke engine it is necessary to know which type of design is in question because different design types operate in different ways.
The design types of the two-stroke cycle engine vary according to the method of intake of fresh air/fuel mixture from the outside, the method of scavenging the cylinder (exchanging burnt exhaust for fresh mixture) and the method of exhausting the cylinder.
These are the main variations. They can be found alone or in various combinations.
Piston port
Piston port is the simplest of the designs. All functions are controlled solely by the piston covering and uncovering the ports (which are holes in the side of the cylinder) as it moves up and down in the cylinder.
Reed valve
Similar to and almost as simple as the piston port but with a check valve in the intake tract. Reed valve engines deliver power over a wider RPM range than does the piston port making them more useful in most situations.
Disk rotary valve
The intake tract is opened and closed by a thin disk attached to the crankshaft and spins at crankshaft speed. The intake tract is arranged so that it passes through the disk. This disk has a section cut from it and when this cut passes the intake pipe it opens, otherwise it is closed.
Valve in head
Instead of the exhaust exiting from a hole in the side of the cylinder, valves are provided in the cylinder head. The valves function the same way as four-stroke exhaust valves do but at twice the speed.
Diesel
Spark ignition gasoline engines require a spark plug to ignite the fuel. Diesels rely on the heat of very high compression to ignite the fuel. Fuel is sprayed into the hot compressed air and ignites; therefore the scavenging is done with air only. Diesels are blower scavenged.
Blower-scavenged
Instead of using the crank case as a pump to pump fresh air/fuel mixture into the cylinder, an auxiliary air pump is used to blow air through the cylinder and clear out burnt exhaust (hence the term “blower”). Blowers are used in conjunction with diesels.
Loop-scavenged
This method of scavenging uses carefully aimed transfer ports to loop fresh mixture up one side of the cylinder and down the other pushing the burnt exhaust ahead of it and out the exhaust port. It features a flat or slightly domed piston crown for efficient combustion. Loop scavenging is by far the most used system of scavenging.
Cross flow-scavenged
In a cross flow engine the transfer ports and exhaust ports are on opposite sides of the cylinder and a baffle shaped piston dome directs the fresh mixture up and over the dome pushing the exhaust down the other side of the baffle and out the exhaust port. Before loop scavenging was invented almost all two strokes were made this way. The heavy piston with its very high heat absorption along with its poor scavenging and combustion characteristics make it an antiquated design now except where there is no way to use loop scavenging.
Basic operation
The two-stroke engine is simple in construction, but complex dynamics are employed in its operation. A typical simple two-stroke contains a piston whose face is shaped, an exhaust port on one side of the cylinder, and an intake port on the other side. The downward movement of the piston first uncovers the exhaust port, allowing most of the exhaust to be expelled, and then uncovers the intake port through which an air-fuel mixture (the fuel normally has some oil mixed in) is let into the cylinder. The piston then moves upwards, compressing the mixture which is ignited by a spark plug, driving the piston back down.
In more detail:
Intake and compression
The rising piston creates a partial vacuum in the sealed crankcase. A connection (inlet port) between the crankcase and the carburettor is uncovered by the piston as it rises, and the air-fuel mixture is sucked into the crankcase. At the same time, the air-fuel mixture already in the cylinder is being compressed.
Steps of two-stroke cycle:
Expansion stroke:
- The piston is at Top Dead Center (TDC)
- Crank is at 0 or 360°.
- In real engines the process is completed from 0 to 150° but in this model it is completed at 120°.
Intake/Compression stroke:
- The piston moves from Bottom Dead Center (BDC) to T.D.C.
- The intake port is opened and working substance flows in.
- Intake gases move inside due to partial vacuum and also blowers are used to push intake gases in.
- The vacuum opens the reed valve (thin flexible sheets made of steel, glass fiber or even carbon fiber) allowing the mixture to enter the crankcase.
- the air-fuel mixture already in the cylinder is being compressed.
- As the piston nears the top of the stroke, the ignition system ignites the charge in the combustion chamber.
- In diesel engines, at 11-13° fuel is injected in TDC Up till now only air is compressed. Fuel is injected till the last stage of compression.
Exhaust and scavenging process:
- The piston moves from TDC to BDC
- At 120° exhaust port is opened and exhaust gases move out of the cylinder due to inertia of steam.
- After 8-12° fresh scavenging gases are then let into to the cylinder.
- the air/fuel/oil mixture that was let into the cylinder pushes the exhaust out the exhaust port
- the pistion, then, compresses the air/fuel/oil mixture and lets left over exhaust out
!!jpg_reduced_25pc_no_comp_no_decrease.gif
Power and exhaust
When the piston reaches the top of its stroke, the mixture is ignited, and the piston is forced down by the rapidly expanding gases of combustion.
As the piston descends, a hole in the side of the cylinder connected to the exhaust pipe (exhaust port) is opened, and the burned gases can escape.
Furthermore, the descending piston closes the inlet port and pressurizes the crankcase. This pushes also some mixture from the crankcase back to the inlet tract, causing the reed valve to close and preventing the mixture from entering the air filter.
The air fuel mixture is forced into passageways that connect the crankcase to the cylinder. Holes connecting these passages to the upper cylinder (transfer ports) are uncovered by the descending piston and air-fuel mixture is forced into the upper cylinder. The transfer ports are just a bit lower than the top of the exhaust port, so there is a period of time when fresh air-fuel mixture is coming in while exhaust is leaving. The incoming fresh charge assists in forcing the exhaust gas out.
As the piston reaches the bottom and then starts to rise again, the transfer ports are closed by the piston and the air/fuel mixture is compressed. The next cycle starts.
Design issues
A major problem with the two-stroke engine has been the short-circuiting of fresh charge from intake to exhaust which increases fuel consumption and emissions of unburned hydrocarbons.
The cylinder ports and piston top are shaped to minimise this mixing of the intake and exhaust flows. Furthermore, a tuned pipe with an expansion chamber provides back pressure at just the right time to push fresh air-fuel mixture sneaking out the exhaust back in again.
The major components of two-stroke engines are tuned so that optimum airflow results. Intake and exhaust pipes are tuned so that resonances in airflow give better flow.
Two stroke engines mix lubricant with their fuel (either manually at refueling or by injecting oil into the fuel stream); this mixture lubricates the cylinder, crankshaft and connecting rod bearings. The lubricant is subsequently burned, resulting in undesirable emissions. An independent lubrication system from below, as is used in four-stroke designs, cannot be used in the above-described two stroke engine design, since the crank case is being used to hold the air-fuel mixture.
This problem has been addressed in newer engines which employ direct injection, similar to diesel two-strokes.
Two-stroke diesel engines
A two-stroke cycle has also been used for some diesel engines. As the fuel is injected directly into the cylinder, the lubrication of the crankshaft must be independent in these engines. There is no mixing of lubricating oil into the fuel.
There are three patterns. Some modern designs differ from the gasoline two-stroke cycle in that they have intake and exhaust valves in the cylinder head, exactly like a four-stroke engine. In these engines, the two-stroke cycle is used to improve power-to-weight ratio and/or reduce the engine speed to increase reliability. This pattern, the Clark cycle, is common in truck, railroad locomotive and machinery engines.
Other engines have used the same ported arrangement as the gasoline two-stroke, although the charge air is generally delivered under pressure from a blower through ducting rather than through the crankcase. Examples are the Junkers Jumo 205 and Napier Deltic high-speed opposed piston engines.
A third pattern uses the induction method of the gasoline two-stroke, but with an exhaust valve in the cylinder head. Large marine diesels commonly use this arrangement. These engines commonly also use a crosshead bearing, which together with a sliding seal on the piston rod allows the air path to be separated from the crankshaft while still using the piston movement as an air pump.
The simpler stroke in the fully valved diesel two-stroke cycle is the compression stroke; both valves are closed, and the rising piston compresses the air, heating it. At the top of the stroke, diesel fuel is injected into the cylinder, where it ignites and burns. The hot, high pressure gases produced by the combustion push against the piston as it descends in the initial part of the second stroke, delivering power. At this point, both valves are still closed. When the piston nears the bottom of the stroke, the exhaust valve opens, and the exhaust gases, still under pressure, rush out. The intake valve then opens. Air under pressure rushes into the cylinder, blowing out the remainder of the exhaust gases. The exhaust valve closes at that point, and shortly after that, and at about bottom dead center, so does the intake valve.
If the crankcase is not used as an air pump, some other means of forced induction is required, and is often used for efficiency in any case. The intake air must be under pressure, since the engine does not have an induction stroke and cannot suck the air in by itself. A low-pressure supercharger (blower) is needed at minimum, but many are turbocharged.
The diesel two-stroke generally lacks the inefficiency and pollution problems of the gasoline two-stroke, since no unburned fuel, only air, can get blown out of the exhaust valve before it closes. Also, there is no mixing of lubricant with the fuel.
Compared with four-stroke engines
Two-stroke engines have several marked disadvantages that have largely precluded their use in automobiles (although there was some use, such as in historic Saabs and DKWs and until recently in several automobiles produced in the Eastern bloc, including Trabants and Wartburgs, among others) and are reducing their prevalence in the above applications. Firstly, they require much more fuel than a comparably powerful four-stroke engine due to less efficient combustion. The burning oil, and the less efficient combustion, makes their exhaust far smellier and more damaging than a four-stroke engine, thus struggling to meet current emission control laws. They are noisier, partly due to the more penetrating high-frequency buzzing and partly due to the fact that muffling them reduces engine power far more than on a four-stroke engine (high-performance two-stroke engine exhausts are tuned by determining the resonant frequency of the exhaust systems and exploiting it to top-up the fuel air charge just before the cylinder port closes) Finally, they are considered less reliable and durable than four stroke engines.
A notable area of use today is in small displacement motorcycles, mostly in off-road "dirt-bikes", and scooters, where their higher power-to-weight ratio, and smaller size outweight their aforementioned disadvantages.
There are more elaborate possible two-stroke engine configurations, but these often have enough complications that they do not outperform comparable four-stroke engines. New two-stroke designs rely on electronically-controlled fuel injection, oil injection and other design tweaks to reduce pollution and increase fuel efficiency. However, such systems increase the cost of the engines to the point that for small systems simple four-stroke engines are most cost-effective. Many large manufacturers, including Ford and Honda are still actively researching ways to build practical and clean two strokes for automotive use.
External links
- Animated Engines: Two Stroke (http://www.keveney.com/twostroke.html)
- How Stuff Works: Two-Stroke Engine (http://science.howstuffworks.com//two-stroke.htm)de:Zweitaktmotor
fr:Cycle à deux temps nl:Tweetaktmotor ja:2サイクル機関 pl:Silnik dwusuwowy sv:Tvåtaktsmotor