Two-stroke engine
A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crankshaft (an up stroke and a down stroke of the piston, compared to twice that number for a four-stroke engine).
This is accomplished by using the end of the combustion stroke and the
beginning of the compression stroke to perform simultaneously the intake
and exhaust (or scavenging) functions. In this way, two-stroke engines often provide high specific power,
at least in a narrow range of rotational speeds. The functions of some
or all of the valves required by a four-stroke engine are usually served
in a two-stroke engine by ports that are opened and closed by the
motion of the piston(s), greatly reducing the number of moving parts.
Gasoline (spark ignition)
versions are particularly useful in lightweight (portable)
applications, such as chainsaws, and the concept is also used in diesel compression ignition engines in large and weight insensitive applications, such as ships and locomotives.
The first commercial two-stroke engine involving in-cylinder compression is attributed to Scottish engineer Dugald Clerk,
who in 1881 patented his design, his engine having a separate charging
cylinder. The crankcase-scavenged engine, employing the area below the
piston as a charging pump, is generally credited to Englishman Joseph Day.
Applications
The two-stroke engine was very popular throughout the 20th century in motorcycles and small-engined devices, such as chainsaws and outboard motors,
and was also used in some cars, a few tractors and many ships. Part of
their appeal was their simple design (and resulting low cost) and often
high power-to-weight ratio.
The lower cost to rebuild and maintain made the two stroke engine
incredibly popular, until the EPA mandated more stringent emission
controls in 1978 (taking effect in 1980) and in 2004 (taking effect in
2005 and 2010). The industry largely responded by switching to
four-stroke engines, which emit less pollution than two stroke engines .
Many designs use total-loss lubrication, with the oil being burned in
the combustion chamber, causing "blue smoke" and other types of exhaust
pollution. This is a major reason for two-stroke engines being replaced
by four-stroke engines in many applications.
Two-stroke engines continue to be 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. For example, only a
two-stroke engine that uses a gasoline-oil mixture can power a chainsaw operating in any position.
These engines are still 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.
Different two-stroke design types
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 scavenging.
Reed inlet valve
The reed valve is a simple but highly effective form of check valve
commonly fitted in the intake tract of the piston-controlled port. They
allow asymmetric intake of the fuel charge, improving power and
economy, while widening the power band. They are widely used in ATVs and
marine outboard engines.
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 in the end of the crankcase, allowing charge to enter 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 in most glow plug model engines. In 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 it enables the two-stroke engine's intake timing to be asymmetrical,
which is not possible with piston port type engines. The 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. Where a portion of the rotary valve is
a portion of the crankcase itself, it is particularly important that no
wear is allowed to take place.
Crossflow-scavenged
In a crossflow engine, the transfer 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 piston's weight and exposed surface area,
and also makes it difficult to achieve an efficient combustion chamber
shape. This design has been largely superseded by the 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, and then is deflected downward.
This not only prevents the fuel/air mixture from traveling directly
out the exhaust port, but also 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 to significantly increase 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 1960s due in no small way to the increased
power afforded by loop scavenging.
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 scavenging 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 once-popular split-single design 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
The piston of this engine is "top-hat" shaped; the upper section
forms the regular cylinder, and the lower section performs a scavenging
function. The units run in pairs, with the lower half of one piston
charging an adjacent combustion chamber.
This system is still partially dependent on total loss lubrication
(for the upper part of the piston), the other parts being sump
lubricated with cleanliness and reliability benefits. The piston weight
is only about 20% heavier than a loop-scavenged piston because skirt
thicknesses can be less. Bernard Hooper Engineering Ltd (BHE). are one
of the more recent engine developers using this approach.
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, Kawasaki K.I.P.S., Cagiva C.T.S. or Suzuki AETC systems, or by altering the volume of the exhaust, which changes the resonant frequency of the expansion chamber, such as the Honda V-TACS system. The result is an engine with better low-speed power without sacrificing high-speed power.
Direct injection
Direct injection has considerable advantages in two-stroke engines, eliminating some of the waste and pollution caused by carbureted two-strokes where a proportion of the fuel/air mixture entering the cylinder goes directly out, unburned, through the exhaust port. Two systems are in use, low-pressure air-assisted injection, and high pressure injection.Since the fuel does not pass through the crankcase, a separate source of lubrication is needed.
Two-stroke diesel engines
Diesel engines rely solely on the heat of compression for ignition. In the case of Schnuerle ported and loop-scavenged engines, intake and exhaust happens via piston-controlled ports. A uniflow diesel engine takes in air via scavenge ports, and exhaust gases exit through an overhead poppet valve. Two-stroke diesels are all scavenged by forced induction. Some designs use a mechanically driven Roots blower,
whilst marine diesel engines normally use exhaust-driven turbochargers,
with electrically-driven auxiliary blowers for low-speed operation when
exhaust turbochargers are unable to deliver enough air.
Marine two-stroke diesel engines directly coupled to the propeller are able to start and run in either direction as required. 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.
Marine two-stroke diesel engines directly coupled to the propeller are able to start and run in either direction as required. 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.