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The two-stroke internal combustion engine differs from the more common four-stroke engine by completing the same (thermodynamic) cycle in only two strokes of the piston, rather than four. This is accomplished by using the beginning of the compression stroke and the end of the combustion stroke to simultaneously perform the intake and exhaust functions, which is called scavenging. This allows a power stroke for every revolution of the crank, instead of every second revolution as in a four-stroke engine. For this reason, two-stroke engines provide high specific power, so they are valued for use in portable, lightweight applications such as chainsaws as well as large-scale industrial applications like locomotives.

A two-stroke engine, in this case with an expansion pipe illustrating the effect of a reflected pressure wave on the fuel charge. This feature is essential for maximum charge pressure (volumetric efficiency) and fuel efficiency. It is used on most high-performance engine designs.

Invention of the two-stroke cycle is attributed to Dugald Clerk around 1880 whose engines had a separate charging cylinder. The crankcase-scavenged engine, employing the area below the piston as a charging pump, is generally credited to Joseph Day (and Frederick Cock for the piston-controlled inlet port).

Applications

Throughout the 20th century, many small motorized devices such as chainsaws and outboard motors were powered by two-stroke designs. They are popular due to their simple design (and resulting low cost) and higher power-to-weight ratios. However, in most designs to date the lubricating oil is mixed with the fuel, which significantly increases the emission of pollutants (due to the oil's incomplete combustion). For this reason, two-stroke engines have been replaced with four-stroke engines in many applications, though some newer two-stroke designs are as clean as four-strokes.^{citation needed}

A two-stroke minibike.

Two-stroke engines are still commonly used in high-power, handheld applications where light weight is essential, primarily string trimmers and chainsaws.

To a lesser extent, these engines may still be used for small, portable, or specialized machine applications such as outboard motors, high-performance, small-capacity motorcycles, mopeds, underbones, scooters, tuk-tuks, snowmobiles, karts, ultralights, model airplanes (and other model vehicles), chainsaws and lawnmowers. The two-stroke cycle is used in many diesel engines, most notably large industrial and marine engines, as well as some trucks and heavy machinery.

Several automobiles used two-stroke engines in the past, including the Swedish Saab and German manufacturers DKW and Auto-Union. Production of two-stroke cars stopped in the 1960s in the West, but Eastern Bloc countries continued producing Syrena in Poland, Trabant and Wartburg in East Germany with two-stroke engines until as recently as 1991. Suzuki¹ also produced them in the 1970s.

Different two-stroke design types

A Cox Baby Bee 0.049 cubic inch (0.8 cubic cm.) reed valve engine disassembled. It uses glow plug ignition. The weight is 64 grams.

Although the principles remain the same, the mechanical details of various two-stroke engines differ depending on the type. The design types of the two-stroke engine vary according to the method of introducing the charge to the cylinder, the method of scavenging the cylinder (exchanging burnt exhaust for fresh mixture) and the method of exhausting the cylinder.

Piston controlled inlet port

Piston port is the simplest of the designs. All functions are controlled solely by the piston covering and uncovering the ports as it moves up and down in the cylinder. A fundamental difference from typical four-stroke engines is that the crankcase is sealed and forms part of the induction process in gasoline and hot bulb engines.

Piston controlled inlet and exhaust in opposed piston engines

In an opposed piston engine one of the opposing pistons in a cylinder controls the exhaust port and the other controls the transfer/inlet port. Some engines have a slight angular offset between the top and bottom crankshafts to achieve early exhaust before scavenging.²

Reed inlet valve

Main article: Reed valve

This is similar to and almost as simple as the piston port but substitutes a reed type check valve in the intake tract for the piston-controlled port. Reed valve engines deliver power over a wider speed range than the piston port types, making them more useful in applications such as dirt bikes, ATVs, and marine outboard engines. Reed-valved engines do not lose fresh fuel charge out of the crankcase as do piston-port engines.

In common with many two-strokes, reed valve engines can rotate in either direction. This has been used to back up microcars such as the Messerschmitt KR200 that lacked reverse gearing, and it allows mounting model airplane engines in either tractor or pusher configuration without needing to change the propeller.

Many early two-stroke engines, particularly small marine types, employed a poppet type check valve for the same purpose, but the valve's inertia limited this arrangement to lower speeds only.

Rotary inlet valve

The intake pathway is opened and closed by a rotating member. In the most commonly used type, a thin disk attached to the crankshaft, spinning at crankshaft speed, contains an opening that allows the charge to proceed toward the crankcase at a certain portion of the cycle.

Another form of rotary inlet valve used on two-stroke engines employs two cylindrical members with suitable cutouts arranged to rotate one within the other - the inlet pipe having passage to the crankcase only when the two cutouts coincide. The crankshaft itself may form one of the members, as with the two-cylinder Maytag washing machine engine of the 1930s and 40s and is still used in most Glowplug model engines. In yet another embodiment, the crank disc is arranged to be a close-clearance fit in the crankcase and is provided with a cutout which lines up with an inlet passage in the crankcase wall at the appropriate time, as in the Vespa motor scooter.

The advantage of a rotary valve is that it enables the two-stroke engine's intake timing to be asymmetrical which is not possible with two-stroke piston port type engines. The two-stroke piston port type engine's intake timing opens and closes before and after top dead center at the same crank angle making it symmetrical whereas the rotary valve allows the opening to begin earlier and close earlier.

Rotary valve engines can be tailored to deliver power over a wider speed range or higher power over a narrower speed range than either piston port or reed valve engine. Their disadvantage is that the rotary-valve engine is more complicated.

Crossflow-scavenged

In a crossflow engine the transfer ports and exhaust ports are on opposite sides of the cylinder and a deflector on the top of the piston directs the fresh intake charge into the upper part of the cylinder pushing the residual exhaust gas down the other side of the deflector and out of the exhaust port. The deflector increases piston's weight and its exposed surface area, and also makes it difficult to achieve an efficient combustion chamber shape. This design has been largely superseded by loop scavenging method (below), although for smaller or slower engines the crossflow-scavenged design can be an acceptable approach.

Loop-scavenged

Main article: Schneurle porting

This method of scavenging uses carefully shaped and positioned transfer ports to direct the flow of fresh mixture as it enters the cylinder. Usually a piston deflector is not required, so this approach has an advantage over the cross flow scheme (above). Often referred to as "Schnuerle" (or "Schnürl") scavenging after the German inventor of an early form in the mid 1920s, it became widely adopted in that country during the 1930s and spread further afield after World War II. Loop scavenging is the most common type used on modern engines.

Uniflow-scavenged

In a uniflow engine the mixture, or air in the case of a diesel, enters at one end of the cylinder controlled by the piston and the exhaust exits at the other end controlled by an exhaust valve or piston . The gas-flow is therefore in one direction only, hence the name uniflow. The valved arrangement is common in diesel locomotives (Electro-Motive Diesel) and large marine two-stroke engines(Wärtsilä). Ported types are represented by the opposed piston design in which there are two pistons in each cylinder, working in opposite directions such as the Junkers Jumo and Napier Deltic³. The unusual twingle design also falls into this class being effectively a folded uniflow. With advanced angle exhaust timing uniflow engines can be supercharged with a crankshaft driven ( piston ⁴ or Roots ) blower.

Stepped Piston Engine

A stepped piston engine uses piston movement to provide suction and then compression to feed the charge into the cylinder. A flange, or step, around the base of the piston creates a secondary chamber which draws the fuel/air mixture in on the piston's downstroke. On the upstroke, the mixture in this chamber is passed into an adjacent cylinder. The advantage of this system is that the piston is more easily lubricated and plain bearings can be used, as with a four-stroke engine. The piston weight is about 20% heavier than a loop-scavenged piston. The patents on this design are held by Bernard Hooper Engineering Ltd (BHE).⁵

Power valve systems

Main article: 2-stroke power valve system

Many modern two-stroke engines employ a power valve system. The valves are normally in or around the exhaust ports. They work in one of two ways, either they alter the exhaust port by closing off the top part of the port which alters port timing such as Ski-doo R.A.V.E Yamaha YPVS, Honda RC-Valve, Cagiva C.T.S., Suzuki AETC system or by altering the volume of the exhaust which changes the resonant frequency of the expansion chamber, such as Honda V-TACS system. The result is an engine with better low-speed power without sacrificing high-speed power.

Direct Injection

Main article: Gasoline Direct Injection#In two-stroke engines

In modern two-strokes such as those used for outboard engines (Mercury OptiMax, Evinrude E-TEC, Nissan TLDI⁶ or Yamaha HPDI), personal water craft, scooters (such as Aprilia DiTech models), snowmobiles⁷, motorcycles, tuk-tuk⁸ and small aircraft it is no longer necessary to pre-mix the fuel and lubricating oil. The oil tank is either part of the engine or is a separate tank installed in the vehicle. The oil is injected just after the reeds, lubricating the rotating assembly of the engine. The fuel is injected directly into the cylinder. In most cases the fuel is not injected until after the exhaust port has closed, eliminating short circuiting (fuel lost out the exhaust port without being combusted). Direct injection creates more power and uses less fuel than a carbureted engine, and reduces emissions. In some cases the two-stroke engines have emission ratings as good as or better than four-stroke engines. Evinrude was even awarded for being clean with their E-TEC DI two-stroke technology⁹. LPG gas is possible to use, in this way, as well¹⁰.

Direct Injection (DI) retrofit

In order to reduce pollution caused by crankcase-scavenged carbureted two-stroke engines, the Envirofit project produces a DI retrofit kit¹¹ based on Orbital technology. Research has shown LPG or CNG gas DI to be retrofittable to existing engines in similar fashion to the Envirofit system¹².

Two-stroke Diesel engines

Unlike a gasoline engine, which employs a spark plug or glowplug to ignite the fuel/air charge in the combustion chamber, a Diesel engine relies solely on the heat of compression for ignition. Fuel is injected at high pressure into the superheated compressed air shortly before top dead center (TDC) and begins burning. Scavenging is performed with intake air alone; the combustion gases exit through conventional exhaust valves located in the cylinder head or Schneurle porting just above the piston at bottom dead center (BDC). Two-stroke Diesels are scavenged by Forced induction. A mechanically driven blower (often a Roots positive displacement blower) or exhaust-driven turbocharger(s) are used. It should be noted that the scavenging engine driven blower can not be used as a supercharger on loop scavenged engines because the exhaust ports located above the inlet ports close afterwarts bleeding off the excess presure, a turbocharger will work because it develops back pressure.
Turbocharger(s) may be added to increase mass airflow. An exhaust-driven turbocharger cannot be used by itself to produce scavenging airflow, as it is incapable of operating unless the engine is already running. Hence it would be impossible to start the engine. The common solution to this problem is to drive the turbocharger's impeller through a gear train and freewheel connector. In this arrangement, the impeller turns at sufficient speed during engine cranking to produce the required airflow, thus acting as a mechanical blower. At lower engine speeds, the turbocharger will continue to act as a mechanical blower. However, at higher power settings the exhaust gas pressure and volume will increase to a point where the turbine side of the turbocharger will drive the impeller and the connector will freewheel allowing the turbocharger to turn at higher speed, supercharging the intake air.

Lubrication

Two-stroke engines (except Diesels) usually do not have an internal lubrication system; the moving parts are lubricated by a small flow of lubricating oil (usually a special two-stroke oil) which enters the combustion chamber pre-mixed with the fuel. This oil is usually not completely burned during the power stroke, and therefore a portion of it clings to the cylinder walls, passes into the crankcase, and eventually reaches all moving parts of the engine. Handheld devices using this method of lubrication have the advantage of being able to operate in any orientation since there is no oil reservoir (which requires gravity for proper function). Honda has worked around this difficulty in their mini four-stroke engines.¹³

Depending on the engine's application, the oil can be mixed with the fuel manually each time fuel is added, or an oil pump can automatically mix fuel and oil from separate tanks as those liquids flow toward the engine.

Vehicles using two-stroke engines often have a freewheel mechanism in the powertrain, to disconnect the engine from the drive wheels when the vehicle is on the over-run (i.e. descending a hill in a low gear with a closed throttle or slowing down with the throttle closed). In these situations without the freewheel the engine is turning rapidly but no oil is able to enter the upper cylinder, so the moving parts are not adequately lubricated. If this situation continues, the engine may eventually overheat and seize. The freewheel allows the engine to return to its set idle speed, preventing seizure.

Reversibility

Large marine two-stroke and four-stroke diesel engines are able to start and run in either direction directly coupled to the propeller. Thus the engine can be run in reverse to move the vessel backwards.

Sources

See also

• Bourke Engine
• Junkers Jumo 205
• Napier Deltic
• Twingle engine
• Wärtsilä-Sulzer RTA96-C
• Kadenacy effect
• How Stuff Works: Two-Stroke Engine
• Wolfhart Engine

External links

• The Fuel and Engine Bible – A good resource for understanding different engine types and how they work

Wikipedia content modification information:

• This page was last modified on 30 November 2008, at 15:28.

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