What is a Two-Stroke Engine?- Types, And Working

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 or two-stroke cycle engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston one up and one down movement in one revolution of the crankshaft in contrast to a four-stroke engine which requires four strokes of the piston in two crankshaft revolutions to complete a power cycle.

During the stroke from bottom dead center to top dead center, the end of the exhaust/intake (or scavenging) is completed along with the compression of the mixture.

The second stroke encompasses the combustion of the mixture, the expansion of the burnt mixture and, near bottom dead center, the beginning of the scavenging flows.

Two-stroke engines often have a higher power-to-weight ratio than a four-stroke engine, since their power stroke occurs twice as often. Two-stroke engines can also have fewer moving parts, and thus be cheaper to manufacture and weigh less.

In countries and regions with stringent emissions regulations, two-stroke engines have been phased out in automotive and motorcycle uses. In regions where regulations are less stringent, small-displacement two-stroke engines remain popular in mopeds and motorcycles.

They are also used in power tools such as chainsaws and leaf blowers.

Parts 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.

How Does a Two-Stroke Engine Work?

In a two-stroke engine, the spark plug fires every revolution, which is different from four-stroke engines. Similar to a four-stroke engine, fuel and air are mixed in the cylinder.

The fuel/air mixture is compressed and pushed toward the spark plug by the piston – this is known as the compression stroke.

As the fuel/air mixture is compressed in the piston, it creates a vacuum that opens a reed valve that pulls air, fuel, and oil from the carburetor. The spark plug ignites the mixture and the piston is driven down by the explosion of the spark plug.

As the piston returns to its lower position, the exhaust gases are released from the cylinder and the cycle start all over again with fresh fuel entering the chamber. This is known as the combustion stroke.

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.

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.

1. Piston-controlled inlet port.
2. Reed inlet valve.
3. Rotary inlet valve.
4. Cross-flow scavenging.
5. Loop scavenging.
6. Uniflow scavenging.
7. Stepped piston engine.

#1. 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.

#2. 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.

#3. 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.

#4. 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.

#5. 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.

#6. 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.

#7. 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

You find two-stroke engines in such devices as chain saws and jet skis because two-stroke engines have three important advantages over four-stroke engines:

• Two-stroke engines do not have valves, which simplifies their construction and lowers their weight.
• Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost.
• Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem can add complexity to the engine.

These advantages make two-stroke engines lighter, simpler and less expensive to manufacture.

Disadvantages of two-stroke engines

Two-stroke engines often have advantages for use in small engines; but they do have issues that make them not as effective in other engine operations. Some of the disadvantages of two-stroke engines are:

• Fuel use is not as efficient in two-stroke engines – fuel economy is significantly reduced as compared to four-stroke engines.
• Two-stroke engines produce significantly more emissions and pollution as a result of oil combustion and release of hydrocarbons from the burning of fuel with the oil. The combustion of oil also can result in an oily smoke cloud from the two-stroke engine.
• The cost of running a two-stroke engine is more expensive than running a four-stroke engine. This is a result of the reduced fuel economy and the fact that oil is being burned with the gasoline in a two-stroke engine. Typically about 4 ounces of oil is burned per gallon of gasoline in a two-stroke engine.
• The fact that engine lubrication is so different in a two-stroke often leads to reduced engine life.

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.

FAQs.