The two-stroke internal combustion engine differs from the more common four-stroke engine by completing the same four processes (intake, compression, combustion, exhaust) in only two strokes of the piston rather than four. The internal combustion engine is an engine in which the Combustion of Fuel and an Oxidizer (typically air occurs in a confined space called a Today Internal combustion engines in cars, Trucks motorcycles aircraft construction machinery and many others most commonly use a four-stroke cycle. This is accomplished by using the beginning of the compression stroke and the end of the combustion stroke to perform the intake and exhaust functions. 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. In Engineering, the term specific power can refer to power either per unit of Mass, Volume or Area, although power per unit of
Invention of the two-stroke cycle is attributed to Dugald Clerk around 1880 whose engines had a separate charging cylinder. Sir Dugald Clerk (1854 – 1932 designed the first Two-stroke engine in 1878 and Patented it in England in 1881 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). Joseph Day (born 1855 in London, died 1946 trained as an engineer at the School of Practical Engineering at Crystal Palace in London began work at Stothert
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Applications
Throughout the 20th century, many small motorised devices such as chainsaws, and outboard motors were usually powered by two-stroke designs. A chainsaw (or chain saw) is a portable mechanical motorized Saw. An outboard motor is a propulsion system for smaller Boats General uses Outboard motors for a Boat are developed as a self-contained They are popular due to their simple design (and therefore, low cost) and very high power-to-weight ratios. Power-to-weight ratio (specific power is a calculation commonly applied to Engines and other mobile power sources to enable the comparison of one unit or design to another However, varying amounts of engine oil in traditional designs mixes with the air-fuel mixture, which significantly increases the emission of pollutants. 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. Government mandates, rather than market forces, have driven manufacturers to abandoning the two-stroke in spite of its clear power and weight advantages.
Two-stroke engines are still commonly used in high-power, handheld applications where light weight is essential, primarily string trimmers and chainsaws. Weedeater redirects here for the band see Weedeater (band A string trimmer, also called a line trimmer, Weedeater A chainsaw (or chain saw) is a portable mechanical motorized Saw.
To a lesser extent, these engines may still be used for certain small, portable, or specialized machine applications. These include outboard motors, high-performance, small-capacity motorcycles, mopeds, underbones, scooters, snowmobiles, karts, ultralights, model airplanes (and other model vehicles) and lawnmowers. An outboard motor is a propulsion system for smaller Boats General uses Outboard motors for a Boat are developed as a self-contained MotorCycle is the title of a 1993 album by Rock band Daniel Amos, released on BAI Records. Mopeds are a class of low-powered (typically under 50 cc displacement) motorized vehicle generally two or three wheeled Underbones or Underbone Motorcycles are a class of small Motorcycles (also known as Mopeds in Western countries that are halfway between true scooters A scooter is a style of two-wheeled motor vehicle traditionally defined by characteristics such as a step-through frame wheels less than 16" in diameter and an engine located A snowmobile (known locally as snowmachine, snowsled or by the Brandname Ski-Doo) is a land vehicle that is commonly propelled by For other meanings see Kart (disambiguation Kart racing or karting is a variant of Open-wheel Motor sport with Model aircraft are flying or non-flying models of existing or imaginary Aircraft, often scaled down versions of full size planes using materials such as balsa A lawn mower or lawnmower is a machine that has one or more revolving blades to cut a Lawn at an even length 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. A diesel engine is an Internal combustion engine which operates using the Diesel cycle (named after Dr
Several cars used two-stroke engines in the past, including the Swedish Saab, German manufacturers DKW and Auto-Union. Saab AB is an Aerospace and defense company based in Sweden. History "Svenska Aeroplan AB (aktiebolaget" ( Swedish for Dampf Kraft Wagen (steam-driven car or DKW is a historic car and Motorcycle Marque. Auto Union was an amalgamation of four German Automobile manufacturers established in 1932 in Zwickau, Saxony, during the Great Depression Production of two-stroke cars stopped in the 60s in the West, but East block countries kept producing Syrena in Poland, Trabant and Wartburg in East Germany with two stroke engines until as recently as 1991. The Syrena was a Polish Automobile model first exhibited at the Poznań Trade Fair in 1955 and manufactured from 1956 or 1957 The Trabant is an Automobile formerly produced by East German auto maker VEB Sachsenring Automobilwerke Zwickau in Zwickau, Saxony Wartburg was a car manufactured in East Germany. It had a three- cylinder two-stroke engine with only seven moving parts And Suzuki[1] produced them in the 70s as well. is a Multinational corporation headquartered in Hamamatsu Japan that specializes in manufacturing compact automobiles a full range of Motorcycles All-Terrain
The two-stroke cycle
Two-stroke cycle engines operate in two strokes, instead of the four strokes of the more common Otto cycle. Today Internal combustion engines in cars, Trucks motorcycles aircraft construction machinery and many others most commonly use a four-stroke cycle.
- Power/exhaust: This stroke occurs immediately after the ignition of the charge. The piston is forced down. After a certain point, the top of the piston passes the exhaust port, and most of the pressurized exhaust gases escape. As the piston continues down, it compresses the air/fuel/oil mixture in the crankcase. Once the top of the piston passes the transfer port, the compressed charge enters the cylinder from the crankcase and any remaining exhaust is forced out.
- Intake/Compression: The air/fuel/oil mixture has entered the cylinder, and the piston begins to move up. This compresses the charge in the cylinder and draws a vacuum in the crankcase, pulling in more air, fuel, and oil from the carburetor. The compressed charge is ignited by the spark plug, and the cycle begins again.
In engines like the one described above, where some of the exhaust and intake charge are in the cylinder simultaneously the gasses are kept separate by careful timing and aiming of the transfer ports such that the fresh gas has minimal contact with the exiting exhaust which it is pushing ahead of itself. Inevitably there is some mixing, roughly speaking the degree of mixing is a function of the shape of the piston head, the configuration of the ports, and the volume of gas injected per cycle.
Different two-stroke design types
Although the principles remain the same, the mechanical details of various two-stroke engines may differ to a large extent and, in order to understand the operation, it is necessary to know which type of design is in question.
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. A cylinder is the central working part of a Reciprocating engine, the space in which a Piston travels
These are the main variations. They can be found alone or in various combinations.
Piston controlled inlet port
Piston port is the simplest of the designs. A piston is a component of Reciprocating engines Pumps and Gas compressors It is located in a cylinder and is made gas-tight by Piston 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. For the GI Joe character see List of GI Joe ARAH characters. For the Transformers characters see Crankcase (Transformers.
Reed inlet 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 valves restrict flow of gases to a single direction and consist of thin flexible metal or Fiberglass strips fixed on one end that open and close upon changing pressures A check valve, clack valve, non-return valve or one-way valve is a Mechanical device, a Valve, which normally allows Fluid Reed valve engines deliver power over a wider RPM range than the piston port types, making them more useful in many applications, such as dirt bikes, ATVs, and marine outboard engines. Reed valved engines do not lose fresh fuel charge out of the crankcase like piston port engines do.
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 flexibility to pull or push model airplanes with either sense pitch propellers. The Messerschmitt KR200, or Kabinenroller (Cabin Scooter was a three-wheeled Bubble car designed by the aircraft engineer Fritz Fend and produced
Many early two-stroke engines, particularly small marine types, employed a poppet type check valve for the same purpose, but the inertia of the valve made it suitable for low speed use only. A poppet valve is a Valve consisting of a hole usually round or oval and a tapered plug usually a disk shape on the end of a shaft also called a valve stem A check valve, clack valve, non-return valve or one-way valve is a Mechanical device, a Valve, which normally allows Fluid
Rotary inlet valve
The intake tract is opened and closed by a rotating member. In the most commonly used type, it takes the form of a thin disk attached to the crankshaft and spins at crankshaft speed. The crankshaft, sometimes casually abbreviated to crank, is the part of an Engine which translates reciprocating Linear The fuel/air path through 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.
Another form of rotary inlet valve used on two-stroke engines employs two cylindrical members with suitable cut-outs arranged to rotate one within the other - the inlet pipe being in communication with the crankcase only when the cut-outs coincide. The crankshaft itself may form one of the members such as was done with the twin cylinder Maytag washing machine engine of the 1930s and 40s and is still used on some model aircraft engines. In yet another embodiment, the crank disc is arranged to be a very close clearance fit in the crankcase and is provided with a cut-out which lines up with an inlet passage in the crankcase wall at the appropriate time. This type was used on 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 RPM range or higher horse power over a narrower RPM range than either piston port or reed valve engine though they are more mechanically complicated than either one of them.
Crossflow-scavenged
In a cross flow 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 the exhaust port. The deflector increases the weight of the piston and exposed surface area of the piston, also making it difficult to achieve an efficient combustion chamber shape. This type of two stroke has been largely superseded by loop scavenging method (below). With smaller size and lower piston speed the deficiencies of the cross flow design become less apparent. The last of the OMC (Outboard Marine Corporation now Bombadier Recreational Products BRP) V4 and V6 two strokes produced up to 1995 in their mid range were still cross flows. These were produced in the 90-115 horsepower V4 configuration in a 1. 6 litre as well as the 2. 4 Litre 150-175-200 Horsepower V6's. These engines remained extremely competitive on fuel use compared to their loop charged competitors due to advanced exhaust tuning by the manufacturer. These Crossflow engines produced more torque and horsepower by burning less fuel than the Japanese loop charged competitors. Eventually OMC shifted to the Spitfire series Loop charged V4 and V6's in their mid range.
The 235 horsepower 2. 6 Litre crossflow V6 (1976 - 1986) still remains today as a very high output low weight engine compared to its much heavier loop charged 2 stroke and 4 stroke replacements.
The Crossflow design produces far more low down engine torque than the slightly more fuel efficient Looper design. Many a boater replaced their Crossflow 2. 6 Litre 235 (Flywheel rated) for larger (propshaft rated HP) 2. 7 Litre and 3 Litre 225 horsepower V6's only to be disappointed with the lack of low down torque offered.
During the late 1970s 1980's OMC successfully raced the OMC CCC engine. This was a crossflow, carburettored V6 2. 6 Litre that out ran many of the oppositions Loop scavenged, fuel injected larger displacement competitors.
It lived on with the XP 2. 6 until 1986 in a much more civilised form.
Cross flows are still to be found in small engines because it is less expensive to manufacture and allows a more compact design for multiple cylinder configurations. BRP still offer the 9. 9 and 15 HP twin cylinder two stroke available through their Johnson brand as there is still no alternative to this popular lightweight high output engine.
Loop-scavenged
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 conferring considerable 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 1920's, it became widely adopted in that country during the 1930s and spread further afield after World War II. Loop scavenging is by far 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 and the exhaust exits at the other end. The gas-flow is therefore in one direction only, hence the name uniflow. Inlet and/or exhaust may be controlled by mechanically operated valves or by ports. The valved arrangement is common in large Diesel locomotive and marine two strokes, e. A Diesel locomotive is a type of Railroad Locomotive in which the prime mover is a Diesel engine. g. those made by Electro-Motive Diesel. Electro-Motive Diesel Inc (formerly the Electro-Motive Division of General Motors Corporation is currently the world's second largest builder of Railroad 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 Deltic. The term Deltic (meaning in the form of the Greek letter Delta) is used to refer to both the opposed-piston high-speed diesel engine designed and produced by The unusual 'twingle' design also falls into this class being effectively a folded uniflow. The Twingle engine is a Two-stroke engine usually of small-capacity and usually Gasoline -fueled
Power valve systems
Many modern two-stroke engines employ a power valve system. The 2-stroke power valve system is an improvement to a conventional Two-stroke engine that gives a high power output over a wider RPM range 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 snowmobile (known locally as snowmachine, snowsled or by the Brandname Ski-Doo) is a land vehicle that is commonly propelled by A. V. E Yamaha YPVS, Cagiva C. Cagiva is an Italian Motorcycle manufacturer It was founded in 1950 by Giovanni Castiglioni in Varese, originally producing small metal components 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. is a Multinational corporation headquartered in Hamamatsu Japan that specializes in manufacturing compact automobiles a full range of Motorcycles All-Terrain An Expansion chamber is an Exhaust system used on a Two-stroke cycle engine to enhance its power output by improving its Volumetric efficiency () is a Multinational corporation, engine Manufacturer and engineering corporation headquartered in Japan. The result is an engine with better low end power without losing high rpm power.
Stepped Piston Engine
A stepped piston engine uses piston movement to provide suction and then compression to feed fuel into the combustion chamber. A flange, or step, around the base of the piston creates a secondary chamber which draws the fuel air mixture in on the down stroke of the piston. On the upstroke, the fuel air mixture in this chamber is passed into the combustion chamber of an adjacent piston. 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 inceased by the step to about 20% heavier than a conventional looped scavenged two stroke piston. The patents on this design are held by Bernard Hooper Engineering Ltd (BHE).
Direct Injection
In modern two-strokes as those used for outboard engines known as Mercury OptiMax, Evinrude E-TEC, Nissan TLDI[4] or Yamaha HPDI, personal water craft, scooters (such as Aprilia DiTech models), snowmobiles[5], motorcycles[6] and small aircraft no longer require the oil and fuel to be mixed. Gasoline direct injection is a latest variant of Fuel injection employed in modern two- and four- stroke Petrol engines The petrol/gasoline A personal water craft ( PWC) is a Recreational Watercraft that the rider sits or stands on rather than inside A scooter is a style of two-wheeled motor vehicle traditionally defined by characteristics such as a step-through frame wheels less than 16" in diameter and an engine located For the Italian city see Aprilia Latina. Aprilia is an Italian Motorcycle company one of the seven marques owned by A snowmobile (known locally as snowmachine, snowsled or by the Brandname Ski-Doo) is a land vehicle that is commonly propelled by MotorCycle is the title of a 1993 album by Rock band Daniel Amos, released on BAI Records. The oil tank is either part of the engine or for larger engines a tank on 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 would as well as having better emission ratings. In some cases the two-stroke engines have emission ratings as good or better than four-stroke engines. Evinrude was even awarded for being clean with their E-TEC DI two-stroke technology[7]. Evinrude Outboard Motors is a major brand of outboard boat motors.
Two-stroke diesel engines== Unlike a gasoline engine, which employs a spark plug to ignite the fuel/air charge in the cylinder, a Diesel engine relies solely on the heat of compression for ignition. A spark plug (also very rarely nowadays in British English: a sparking plug) is an electrical device that fits into the Cylinder A cylinder is the central working part of a Reciprocating engine, the space in which a Piston travels A diesel engine is an Internal combustion engine which operates using the Diesel cycle (named after Dr Fuel is injected at high pressure into the superheated compressed air and instantly ignites. Therefore, scavenging is performed with air alone, combustion gases exiting through conventional poppet-type exhaust valves.
In order to allow the usage of a conventional oil-filled crankcase and pressure lubricated main and connecting rod bearings, modern two-stroke Diesels are scavenged by a mechanically driven blower (often a Roots positive displacement blower) or a hybrid turbo-supercharger, rather than by crankcase pumping. The Roots type Supercharger or Roots blower is a positive displacement pump which operates by pulling Air through a pair of meshing lobes not Generally speaking, the blower capacity is carefully matched to the engine displacement so that a slight positive pressure is present in each cylinder during the scavenging phase (that is, before the exhaust valves are closed). Engine displacement is defined as the total Volume of air/fuel mixture an Engine can draw in during one complete engine cycle it is normally stated in Cubic This feature assures full expulsion of exhaust gases from the previous power stroke, and also prevents exhaust gases from backfeeding into the blower and possibly causing damage due to contamination by particulates.
Early two-stroke Diesels using the crosshead layout (where the cylinder is not integral with the crankcase) employed under-piston pressure to provide scavenge air to the combustion chamber via a by-pass port as used on a conventional petrol-fueled two-stroke engine. A crosshead (or crosshead bearing) is a Bearing used in large Reciprocating engines whether Internal combustion engines or Steam engines Although the cross-head layout is still used on some large engines, greater power and efficiency, as well as lowered exhaust emissions, can be obtained with a mechanical blower or turbocharger.
It should be noted that the scavenging blower is not a supercharger, as its purpose is to supply airflow to the cylinders in proportion to their displacement and engine speed. A supercharger is an air compressor used for Forced induction of an Internal combustion engine. A two-stroke Diesel supplied with air from a blower alone is considered to be naturally aspirated. A naturally-aspirated engine or normally-aspirated engine (or "N/A" - Aspiration meaning breathing refers to an Internal combustion engine In some cases, turbocharging may be added to increase mass air flow at full throttle—with a corresponding increase in power output—by directing the discharge of the turbocharger into the inlet of the blower, an arrangement that was found on some Detroit Diesel two-stroke engines. A turbocharger, or turbo, is an air Compressor used for forced-induction of an Internal combustion engine. A turbocharger, or turbo, is an air Compressor used for forced-induction of an Internal combustion engine. Detroit Diesel Corporation (DDC is a Diesel engine producer headquartered in Redford Michigan, USA.
A conventional, exhaust-driven turbocharger cannot be used by itself to produce scavenging airflow, as it is incapable of operating unless the engine is already running. A turbocharger, or turbo, is an air Compressor used for forced-induction of an Internal combustion engine. 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 overrunning clutch. In mechanical or Automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the Driveshaft 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 overrunning clutch will disengage, resulting in true turbocharging.
Lubrication
Two-stroke engines often have a simple lubrication system in which a special two-stroke oil is mixed with the fuel, (then known as 'petroil' from "petrol" + "oil") and therefore reaches all moving parts of the engine. Two-stroke oil (also referred to as two-cycle oil, 2-cycle oil, 2T oil or 2-stroke oil) is an Engine oil intended for use in Handheld devices using this method of lubrication have the advantage of operating in any orientation since there is no oil reservoir which would be dependent upon gravity for proper function. Depending on the design of the engine system, 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.
The engine uses cylinder port valves which are incompatible with piston ring seals. This causes lubricant from the crank to work its way into the combustion chamber where it burns. Research has been conducted on designs that attempt to reduce the combustion of lubricant. This research could potentially produce an engine having very valuable properties of both high specific-power and low pollution.
Reversibility
With proper design, a two-stroke engine can be arranged to start and run in either direction, and many engines have been built to do so.
Sources
- ^ Suzuki LJ50 INFO
- ^ U.S. Patent 6,468,122
- ^ BHE - Stepped Piston Engine
- ^ Nissan Marine Outboards - Authorized Distributor Offering Boat Motor Products, Technical Information, & Dealer Locations for the U.S. & Canada."
- ^ 2009 Ski-Doo Engine Technology
- ^ Envirofit - Challenges and Solutions - 2-Stroke Retrofit
- ^ BRP - Evinrude - E-Tec
See also
External links
- Animated Engines: Two Stroke
- How Stuff Works: Two-Stroke Engine
- The Fuel and Engine Bible - A good resource for different engine types and fuels
- 2-stroke engines CDX eTextbook
- Envirofit International
Dictionary
two-stroke engine
-noun
- An engine in which the pistons perform two strokes per engine cycle (i.e., intake/compression and combustion/exhaust).
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