A two-stroke or two-cycle engine is an internal combustion engine that is often found in smaller, lower-power engines such as scooters, dirt bikes, jet skis, smaller outboard motors, and lawn equipment such as lawnmowers and chain saws.
Two-stroke engines contain many (but not all) of the same components of four-stroke engines common in automobiles, but they have significant differences that allow for different performance and require different types of lubrication.
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?
As the name implies, the two-stroke engine only requires two piston movements (one cycle) to generate power. The engine is able to produce power after one cycle because the exhaust and intake of the gas occur simultaneously.
Due to shifting pressures, a valve for the intake stroke opens and closes. Furthermore, because the fuel comes into contact with moving components frequently, it is combined with oil to offer lubrication, allowing for smoother strokes.
Two-Stroke Engine Cycle
Down Stroke
The piston moves from TDC (Top-Dead-Center) to BDC (Bottom-Dead-Center) letting the fresh air enter the combustion chamber. The fresh air-fuel mixture gets into the combustion chamber through the crankcase. In this stroke, the crankshaft makes the rotation of 1800.
Up Stroke
The piston is pushed from BDC to TDC. As a result, the fuel-air mixture gets compressed and the spark plug ignites the mixture. The mixture expands and the piston is pushed down. The inlet port is open during the upstroke.
While the inlet port is opened, the mixture gets sucked inside the crankcase. When the mixture is pushed up into the combustion chamber during the previous upstroke, a partial vacuum is created as no mixture is left behind in the crankcase.
This mixture is ready to go into the combustion chamber during downstroke but remains in the crankcase until the piston goes up to TDC. In this stroke, the crankshaft makes the rotation of 1800.
From the 2nd downstroke onwards the exhaust gases get expelled out from one side while a fresh mixture enters into the combustion chamber simultaneously due to a partial vacuum created in the combustion chamber after the removal of exhaust gases. This is the beauty of the engine. Both things happen at the same time which makes it a 2-stroke engine.
The exhaust gases are expelled from the 2nd downstroke onwards from one side while simultaneously a fresh mixture of air and fuel is injected into the combustion chamber due to the partial vacuum created in the combustion chamber after the removal of exhaust gases.
Construction of a Two-Stroke Engine
- Piston: The 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: The 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
The 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 they move 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-scale propulsion applications such as motorcycles, Mopeds, and dirt bikes.
Advantages of two-stroke engines
- Two-stroke engines do not have valves which are easy to construct and lower their weight.
- Two-stroke engines fire once every revolution while four-stroke engines fire every other revolution.
- Two-stroke engines can work in any position since oil flow is not a concern with any valves to worry about.
Disadvantages of two-stroke engines
- 2-stroke engines do not last as long as four-stroke engines; there is no lubrication system in a 2-stroke engine, so parts wear out a lot faster.
- 2-stroke oil is expensive; you would burn a gallon every 1000 miles if it were in a car
- 2-stroke engines use more fuel
- 2-stroke engines produce a lot of pollution, and the way the engine is designed that part of the air/fuel leaks out of the chamber through the exhaust port, which is why you see a small thin film, or sheen, of oil around any two-stroke outboard motor, and this leaking oil is a real mess for the environment. This is the reason why two-stroke engines are used only in applications where the motor is not used very often and a fantastic power-to-weight ratio is important.
Why are two-stroke engines lighter than four-stroke engines?
The crankcase is full of gasoline, air and oil, so there is no need for additional lubricating oil pumps, piping or filters. There is also no need for cooling water pumps because there are no coolant passages in the cylinder head (no cooling water system).
The two-stroke engine design also does not require push rods or exhaust valves etc., all of this leads to a large weight reduction compared to a four-stroke engine.
Why are two-stroke engines less efficient than four-stroke engines?
Four-stroke engines have more engine parts and can better control when inlet valves and exhaust valves open and close. Controlling valve timing allows the maximum amount of energy to be extracted from the power stage prior to the exhaust stage occurring; this gives an overall increase in engine efficiency.
Fuel injection timing can be more tightly controlled with a four-stroke engine compared to with a two-stroke engine. The amount and duration of injection can be controlled using a cam or common rail system, which again leads to an increase in engine efficiency.
