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Two-stroke cycle
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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.
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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.
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.
Two-stroke engines are commonly used in high-power, handheld applications such as string trimmers and chainsaws. The light overall weight, and light-weight spinning parts give important operational and even safety advantages. Only a two-stroke (with a specialized fuel-system) can run a chainsaw and be used in any position.
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) 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.
A number of main-stream automobile manufacturers have used two-stroke engines in the past, including the Swedish Saab and German manufacturers DKW and Auto-Union. The Japanese manufacturer Suzuki did the same in the 1970s. Production of two-stroke cars ended in the 1960s in the West, but Eastern Bloc countries continued until around 1991, with the Trabant and Wartburg in East Germany and Syrena in Poland. Lotus of Norfolk, UK, has a prototype direct-injection two-stroke engine intended for alcohol fuels called the Omnivore which it is demonstrating in a version of the Exige.
Different two-stroke design types
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. Diesel engines have mostly a roots blower or piston pump for scavinging.
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 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.
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. A familiar type sometimes seen on small motorcycles is a slotted disk attached to the crankshaft which covers and uncovers an opening, allowing charge to enter the crankcase during one 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. Portions of the rotary-valve engine are often the crankcase itself and these must not be allowed to wear.
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
This method of scavenging uses carefully shaped and positioned transfer ports to direct the flow of fresh mixture toward the combustion chamber as it enters the cylinder. The fuel air mixture strikes the cylinder head then follows the curvature of the combustion chamber then is deflected downward. This not only prevents the fuel/air mixture travelling directly out the exhaust port but creates a swirling turbulence which improves combustion efficiency, power and economy. Usually a piston deflector is not required, so this approach has a distinct advantage over the cross flow scheme (above). Often referred to as "Schnuerle" (or "Schnürl") loop 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 of fuel/air mixture transfer used on modern two stroke engines. Suzuki was one of the first manufacturers outside of Europe to adopt loop scavenged two stroke engines. This operational feature was used in conjunction with the expansion chamber exhaust developed by German motorcycle manufacturer, MZ and Walter Kaaden. Loop scavenging, disc valves and expansion chambers worked in a highly coordinated way that saw a significant increase in the power output of two-stroke engines, particularly from the Japanese manufacturers Suzuki, Yamaha and Kawasaki. Suzuki and Yamaha enjoyed success in grand Prix motorcycle racing in the 1960's due in no small way to the increased power afforded by loop scavenging. An additional benefit of loop scavenging was that the piston could be made nearly flat or slightly dome shaped. This enabled the piston to be appreciably lighter and stronger and consequently tolerated higher engine speeds. The "flat top" piston also has better thermal properties and is less prone to uneven heating, expansion, piston seizures, dimensional changes and compression losses.
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 Deltic. 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 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).
Power valve systems
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
In modern two-strokes such as those used for outboard engines (Mercury OptiMax, Evinrude E-TEC, Nissan TLDI or Yamaha HPDI), personal water craft, scooters (such as Aprilia DiTech models), snowmobiles, motorcycles, tuk-tuk 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 technology. LPG gas is possible to use, in this way, as well.
Two-stroke Diesel engines
Unlike a gasoline engine, which employs a spark plug 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.
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 afterwards 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.
Before the use of a Roots blower or a turbocharger became the standard means of supplying scavenge air, different manufacturers employed different methods. Some, such as Worthington Simpson and Mordiesel used under-piston scavenging as on a petrol 2-stroke engine. This method was also used on 2-stroke hot bulb engines. Others, such as Petter used exhaust harmonics to extract the exhaust gas and then pressurise the air charge. This latter method had a number of disadvantages. The exhaust pipe had to be 'tuned' to set up the correct presure waves- an effect that only occurred in a very narrow speed/load range. This meant that each engine had to be adapted to its specific role and could not easily be used for a different purpose. Starting the engine was accomplished by using under-piston scavenging to supply pressurised air whilst the engine was being cranked over. Once running at the correct speed a valve would be closed and the exhaust harmonic effect would take over.
Lubrication
The common two-stroke engines that consumers regularly come across (such as motorcycles and power tools) cannot use regular sump lubrication, since the crankcase is used to pump fuel-air mixture into the cylinder. Traditionally, all moving parts of the engine itself (big-ends, little-ends, main-bearings, and piston/ring assemblies) were lubricated by a pre-mixed fuel-oil mixture (at a ratio between 20:1 and 50:1).
Increasingly, even small two-stroke engines have pumped lubrication from a separate tank of oil - this remains a total-loss system with the oil passing into the combustion chamber and being burnt the same as in the older system (though at a much lower, cleaner rate).
While such a system is more environmentally friendly it is more expensive, materially larger, more cumbersome, and impractical to date for small hand-held two-stroke powered devices such as chainsaws, brush trimmers, and such. Many such tools must work in any attitude, making the pumping of oil from a reservoir problematical. However, ways have been found to feed them with fuel when running upside down, so the problem may not be insuperable.
Two-stroke engines running on a petroil mix suffer oil-starvation if over-run, eg descending long hills or when decelerating by the use of engine-braking and changing down through the gears. Some two-stroke cars were fitted with a freewheel mechanism in the powertrain to avoid this problem, allowing the engine to idle at any time the throttle is closed and requiring the use of the brakes to be used in all slowing down situations.
Reversibility
Large marine two-stroke diesel engines are able to start and run in either direction directly coupled to the propeller. The fuel injection and valve timing is mechanically readjusted by using a different set of cams on the camshaft. Thus the engine can be run in reverse to move the vessel backwards. Some Mercury outboards use this system, which they call "direct reversing".
Regular petro-oil two-stroke toy engines will run backwards with little problem, and this has been used to provide a "reversing" facility in microcars such as the Messerschmitt KR200 that lacked reverse gearing. Where the vehicle has electric starting, the motor will be turned off and re-started backwards by turning the key in the opposite direction. Pre-electronic ignition systems (eg flywheel magneto) work almost normally in reverse except that the ignition timing is retarded and ATDC instead of BTDC, in practise this is not too noticeable.
Model airplane engines can be mounted in either tractor or pusher configuration without needing to change the propeller. These motors are compression ignition, so there are no ignition timing issues and no difference between running forwards and running backwards.
Reed-valve engines will run backwards just as well as piston-controlled porting, however a rotary valve engine has asymmetrical inlet timing and will not run very well. But running any kind of modern two-stroke engine (eg those in motorcycles) backwards is a risky procedure unless it was designed to do this, since their oil-pumps may not work in reverse, leaving the engine suffering from oil-starvation within a short time.
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