What is a Two-Stroke Engine?
A two-stroke engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution. This is in contrast to a “four-stroke engine”, which requires four strokes of the piston to complete a power cycle during two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.
Two-stroke engines often have a high power-to-weight ratio, power being available in a narrow range of rotational speeds called the “power band”. Compared to four-stroke engines, two-stroke engines have a greatly reduced number of moving parts.
History of Two-Stroke Engine
The first commercial two-stroke engine involving in-cylinder compression is attributed to Scottish engineer Dugald Clerk, who patented his design in 1881. However, unlike later two-stroke engines, his had 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. On 31 December 1879, German inventor Karl Benz produced a two-stroke gas engine, for which he received a patent in 1880 in Germany. The first truly practical two-stroke engine is attributed to Yorkshireman Alfred Angas Scott, who started producing twin-cylinder water-cooled motorcycles in 1908.
Gasoline (spark ignition) versions are particularly useful in lightweight or portable applications such as chainsaws and motorcycles. However, when weight and size are not an issue, the cycle’s potential for high thermodynamic efficiency makes it ideal for diesel compression ignition engines operating in large, weight-insensitive applications, such as marine propulsion, railway locomotives, and electricity generation. In a two-stroke engine, the exhaust gases transfer less heat to the cooling system than a four-stroke, which means more energy to drive the piston, and if present, a turbocharger.
How Does a Two-Stroke Engine Work?
As the name implies, the two-stroke engine only requires two-piston movements (one cycle) in order to generate power. The engine is able to produce power after one cycle because the exhaust and intake of the gas occur simultaneously. There is a valve for the intake stroke that opens and closes due to changing pressures. In addition, due to its frequent contact with moving components, the fuel is mixed with oil to add lubrication, allowing smoother strokes.
Overall, a two-stroke engine contains two processes:
- Compression stroke: The inlet port opens, the air-fuel mixture enters the chamber and the piston moves upwards compressing this mixture. A spark plug ignites the compressed fuel and begins the power stroke.
- Power stroke: The heated gas exerts high pressure on the piston, the piston moves downward (expansion), waste heat is exhausted.
The thermal efficiency of these gasoline engines will vary depending on the model and design of the vehicle. However, in general, gasoline engines convert 20% of the fuel (chemical) energy to mechanical energy-in which only 15% will be used to move the wheels (the rest is lost to friction and other mechanical elements).
Compared to four-stroke engines, two strokes are lighter, more efficient, have the ability to use lower-grade fuel, and more cost-efficient. Therefore, the lighter engines result in a higher power-to-weight ratio (more power for less weight). However, they lack the maneuverability possible in four-stroke engines and require more lubrication. This makes a two-stroke engine ideal for ships (need to carry a lot of cargo), motorcycles, and lawn mowers-whereas a four-stroke would be ideal for automobiles like cars and trucks.
Construction of a Two-Stroke Engine
- Piston: Piston transfers the expanding force of gases to the mechanical rotation of the crankshaft through a connecting rod.
- Crankshaft: It converts the reciprocating motion to rotational motion.
- Connecting Rod: It transfers motion from a piston to crankshaft and acts as a lever arm.
- Flywheel: It is a mechanical device that is used to store energy.
- Spark Plug: It delivers electric current to the combustion chamber and in turn ignites the air-fuel mixture leading to the abrupt expansion of gases.
- Counter Weight: Counterweight on the crankshaft is used to reduce the vibrations due to imbalances in the rotating assembly.
- Inlet and Outlet Ports: These ports allow fresh air with fuel to enter and exit from the cylinder.
Types of Two-Stroke Engine
The mechanical detail of various two-stroke engine differs depending on the type. The design types vary according to the method of introducing the charge to the cylinder, the method of scavenging the cylinder, and the method of exhausting the cylinder.
- Piston-controlled inlet port.
- Reed inlet valve.
- Rotary inlet valve.
- Cross-flow scavenging.
- Loop scavenging.
- Uniflow scavenging.
- Stepped piston engine.
Piston-controlled inlet port
Piston port is the simplest of the designs and the most common in small two-stroke engines. All functions are controlled solely by the piston covering and uncovering the ports as it moves up and down in the cylinder. In the 1970s, Yamaha worked out some basic principles for this system. They found that, in general, widening an exhaust port increases the power by the same amount as raising the port, but the power band does not narrow as it does when the port is raised.
Reed inlet valve
The reed valve is a simple but highly effective form of check valve commonly fitted in the intake track of the piston-controlled port. It allows asymmetric intake of the fuel charge, improving power and economy, while widening the power band. Such valves are widely used in motorcycle, ATV, 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 (called a disc valve).
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 version, the crank disc is arranged to be a close-clearance fit in the crankcase, and is provided with a cutout that lines up with an inlet passage in the crankcase wall at the appropriate time, as in Vespa motor scooters.
In a cross-flow 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.
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.
In a uniflow engine, the mixture, or “charge 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.
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.
Applications of Two-Stroke Engine
- Two-stroke engines are preferred when mechanical simplicity, lightweight, and high power-to-weight ratio are design priorities.
- They are lubricated by the traditional method of mixing oil into the fuel, they can be worked within any orientation as they do not have a reservoir dependent on gravity. This makes them desirable for their use in handheld tools such as chainsaws.
- Two-stroke engines are found in small scales propulsion applications such as motorcycles, Mopeds, and dirt bikes.