What is a Two-Stroke Engine?
A two-stroke engine is a type of internal combustion engine that completes a power cycle of two strokes (up and down movements) of the piston during one power cycle, which cycle is completed in one revolution of the crankshaft.
A four-stroke engine requires four strokes of the piston to complete a power cycle over two revolutions of the crankshaft. In a two-stroke engine, the end of the combustion stroke and the start of the compression stroke occur simultaneously, with the intake and exhaust (or scavenging) functions occurring simultaneously.
Two-stroke engines often have a high power-to-weight ratio, with the power being available in a narrow speed range, the so-called power band. Two-stroke engines have fewer moving parts than four-stroke engines.
How Does a Two-Stroke Engine Work?
Just in case some of you aren’t sure how two-stroke engines work, here is some review. In a four-stroke engine, each of the four essential steps of the power-producing cycle is given its own piston stroke:
- Compression
- Power
- Exhaust
- Intake
A two-stroke engine performs all the same steps but in just two piston strokes. The simplest two-stroke engines do this by using the crankcase and the underside of the moving piston as a fresh charge pump. Such engines carry the official name “crankcase-scavenged two-strokes.”
As the two-stroke piston rises on compression, its underside pulls a partial vacuum in the crankcase. An intake port of some kind (cylinder wall port, reed valve, or rotary disc valve) opens, allowing air to rush into the crankcase through a carburetor.
As the piston nears Top Dead Center, a spark fires the compressed mixture. As in a four-stroke, the mixture burns and its chemical energy becomes heat energy, raising the pressure of the burned mixture to hundreds of psi. This pressure drives the piston down the bore, rotating the crankshaft.
As the piston continues down the bore, it begins to expose an exhaust port in the cylinder wall. As spent combustion gas rushes out through this port, the descending piston is simultaneously compressing the fuel-air mixture trapped beneath it in the crankcase.
As the piston descends more, it begins to expose two or more fresh-charge ports, which are connected to the crankcase by short ducts. As the pressure in the cylinder is now low and pressure in the crankcase higher, the fresh charge from the crankcase rushes into the cylinder through the fresh-charge (or “transfer”) ports.
These ports are shaped and aimed to minimize direct loss of fresh charge to the exhaust port. Even in the best designs, there is some loss, but simplicity has its price! This process of filling the cylinder while also pushing leftover exhaust gas out the exhaust port is called “scavenging.”
While the piston is near Bottom Dead Center, the mixture continues to move from the crankcase, up through the transfer ports, and into the cylinder. As the piston rises, it first covers the transfer ports, leaving only the exhaust port still open. If there were no way to stop it, much of the fresh charge would now be pumped out the exhaust.
But there is a simple way to stop it using exhaust pressure waves in the exhaust. If we shape and dimension the exhaust pipe right, a reflection of the original pressure pulse, generated as the exhaust port opened, will bounce back to the port just as the fresh charge is being pumped out of it. This pressure wave stuffs the fresh charge back into the cylinder just as the rising piston covers the exhaust port.
Because the fuel-air mixture is constantly being pumped by the crankcase, it is not practical to lubricate piston and crank by pumped circulating oil it would be swept away by the mixture rushing in and out.
Therefore, we must either mix a little oil with the fuel (2 to 4 percent) or inject it very sparingly into the bearings with a tiny metering pump. The fact that there is so little oil dictates that such simple two-stroke engines must employ rolling bearings, whose need for oil is very small.
More complicated two-stroke engines exist. Instead of using the crankcase and underside of the piston as a fresh-charge pump, we can use a separate rotary blower, directly connected to the transfer ports in the cylinders.
We don’t have to place the exhaust port in the cylinder wall it can take the form of four overhead poppet exhaust valves, as it does in two-stroke marine, rail, and truck diesel. Because such engines do not use their crankcases as fresh charge pumps, they can employ long-lasting plain bearings, lubricated conventionally by pumped recirculating oil.
Two-stroke diesel is scavenged with pure air, not a fuel-air mixture. Their fuel is injected only after all ports have closed, preventing any loss. Certain crankcase scavenged two-strokes do the same, and are called “DI,” or Direct Injection two-strokes. They can be made as fuel-efficient and low in exhaust emissions as four-strokes.
The world’s most efficient piston engines are in fact the giant, slow-turning marine diesel that carries the world’s international shipping trade they are twice as efficient as the usual four-stroke spark-ignition engines found in cars and motorcycles.
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 a 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 engines 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 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 tract of the piston-controlled port. It allows the asymmetric intake of the fuel charge, improving power and economy while widening the power band. Such valves are widely used in motorcycles, 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 (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.
Cross-flow scavenging
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.
Loop scavenging
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 swirling turbulence that 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.
Uniflow scavenging
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.