Wednesday, April 23, 2008
The Two Cycle Engine
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. 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.
Invention of the two-stroke cycle is attributed to Dugald Clark 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).
Throughout the 20th century, many small motorised devices such as chainsaws, and outboard motors were usually powered by two-stroke designs. They are popular due to their simple design (and therefore, low cost) and very high power-to-weight ratios. 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.
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. 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 cars used two-stroke engines in the past, including the Swedish Saab, German manufacturers DKW and Auto-Union. 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 two-stroke cycle
Two-stroke cycle engines operate in two strokes, instead of the four strokes of the more common Otto cycle.
1. 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.
2. 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.
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. 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.
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 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.
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.
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 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 1930's and 40's 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.
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 1970's 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.
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 1930's and spread further afield after World War II. Loop scavenging is by far the most common type used on modern engines.
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.g. those made by Electro-Motive Diesel. 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.
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, 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 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).
Modern two-strokes as those used for outboard engines no longer require the oil and fuel to be mixed. The oil tank is either part of the engine or for larger engines a tank on the boat. 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.
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. 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. 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). 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. 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 two-stroke Diesel supplied with air from a blower alone is considered to be naturally aspirated. 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 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. 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 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.
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. 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.
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.
Nonpoint source pollution
According to the United States Environmental Protection Agency, some forms of water recreational activities contribute to nonpoint source pollution or "pollution runoff," and "the old 2 cycle motors have been said to cause more pollution in two hours than a car running for an entire year."
Because fuel leaks through the exhaust port each time a new charge of air/fuel is loaded into the combustion chamber, oil pollution is a problem at many National Parks and outdoor recreation areas that allow four-wheelers, snowmobiles, dirt bikes, and small watercraft. Modern direct injection technology, and active engine oil management systems, have bypassed many of these problems.
To address these problems, some organizations have begun to offer biodegradable two-stroke engine oil*, and newer fuel injected are more fuel efficient, and produce less nonpoint source pollution.
Pollution from small engines is also a significant source of air pollution. US emission standards specifically limit emissions from small engines. For some equipment, an electric motor alternative is available, which produces no emissions at the point of use, but may shift pollution to power plants. Emissions may still be reduced by the use of renewable energy in grid generation, or because central power plants generally must have stricter emissions control equipment installed.
*I have read reports from some people who are using biodiesel as the lube oil in existing gasoline 2 cycle engines, in the ratio of 20:1, gasoline: biodiesel. This has a cost advantage compared with using store bought 2-stroke oil, and it is claimed to also increase power and smoothness of running, attributed to the fact that biodiesel is a fuel as well as a lubricant. I may try this, but I'll start with something cheap in case it causes rapid engine wear. Perhaps my 2 stroke portable generator; that would seem to be a good application.