2-stroke engine: All you need to know

Two-stroke motorcycles have been the most common form since the early 20th century.  However, at present, two-stroke engines have lost a lot of ground against four-stroke engines.  Even so, there are motorcycle brands that are reluctant to see this type of engine disappear and are continuing to develop them in order to adapt them to the current regulations.

What is a 2-stroke engine?

Throughout history, the two-stroke engine has been the most commonly used engine in motorcycles. However, the requirements of emission standards are harder to meet for two-stroke engines than they are for four-stroke engines. The origins of the two-stroke engine date back to 1879. In that year a Scotsman called Dugald Clerk designed the first 2-stroke engine and the following year the German Karl Benz carried out the first experiments with several models he had built himself that pushed different car models.

Chassis, brakes, tires, fairings… All the parts of a motorcycle have evolved a lot throughout history and, of course, so have the engines. The first motorcycles with two-stroke engines were not available until the early 20th century. The use of this type of engine revolutionized the world of motorcycles, opening up the market and being the engine of choice for powering the lightest and most economical models. This was because they were simpler and therefore easier to manufacture.

How does a 2-stroke engine work?

The two-stroke engine is so called because of the number of strokes the piston performs in completing a cycle. This cycle consists of four phases known as intake (also called suction), compression, combustion (or explosion), and exhaust. This cycle is performed in two piston strokes, the first a downstroke and the second an upstroke, corresponding to one crankshaft revolution.

In order for all the work to be carried out in such a short period of time, the two-stroke engine uses the piston head to compress the air in the combustion chamber, and also its lower part to compress the gases in the crankcase. The upper part of a two-stroke engine is limited at the top by the cylinder head and at the bottom by the piston, and here the combustion process takes place, in the corresponding four phases, the first one overlapping with the last one. This forces the cylinder to be partitioned so that combustion gases do not mix with fresh gases, which are regulated by a carburetor before entering the cylinder.

The lower part of a 2-stroke engine, between the lower part of the piston and the crankshaft, is responsible for the intake process and thus for introducing the air/gasoline mixture from the outside. This lower part is intermittently linked to the upper part through the transfers.

If we take the moment of combustion of the mixture at the top as a starting point, the two-stroke engine works as follows: when the piston reaches the top dead center, the mixture is burnt by the spark from the spark plug and explodes, and this explosion launches the piston down to the so-called bottom dead center.

At a certain point in its descent, the exhaust port located on the periphery of the cylinder and connecting to the outside is uncovered. Due to the high pressure in the cylinder, combustion gases escape at high speed through this port, reducing the pressure inside the cylinder.

The piston continues to move downwards and then the transfers, which are strategically located on the sides of the cylinder, are uncovered. Through them the fresh mixture enters from the lower part of the engine, which was under pressure under the piston. The transfers must be oriented in such a way that the fresh gas pushes the combustion gas into the exhaust port without them mixing with each other.

The intake and exhaust phases are carried out during the entire downward stroke of the piston and a good part of the upward stroke. As it rises, it first closes the intake port and then the exhaust port, and it is then that the compression part of the stroke begins, ending at top dead center, when the spark plug will produce the spark that causes the cycle to repeat.

These 4 stages divided in 2 strokes make the explosion of the mixture move the piston in a linear movement along the cylinder walls, this linear force is transmitted to a rotary force by means of the crankshaft which in turn carries this energy through a series of bearings and gears that pass through the clutch, and the gearbox (in manual engines), or through the variator (in automatic engines) until it reaches the driving wheel.

Thus, the two-stroke engine can be said to have two intake stages: The intake to the cylinder and the intake to the crankcase, so that when the two are added together, the intake period takes up practically the entire cycle. However, to make it simpler, we usually describe it in these two strokes: first, intake – compression, and second, combustion – exhaust.

First stroke: intake (or suction) – compression

We define the first stroke as the piston rising, which compresses the mixture of air, fuel, and oil in the cylinder, and simultaneously creates a vacuum in the crankcase. On completion of the stroke, the piston clears the suction or pre-intake port and fills the crankcase with carbureted gasoline mixture.

Second stroke: combustion (or explosion) – exhaust

The compression causes a spark in the spark plug, which ignites the compressed mixture and creates combustion, which then pushes the piston downwards with great force. The downward piston pre-compresses the mixture in the crankcase and the piston first clears the exhaust port, through which the exhaust gases exit, and then the transfer port, which connects the crankcase to the cylinder, so that the pre-compressed mixture passes through it and fills the cylinder while expelling the last traces of exhaust gases. The cylinder is then ready to start a new two-stroke cycle.

One characteristic of a two-stroke engine is that its internal lubrication cannot be performed with oil in the crankcase, since this is used as a compression chamber for the fresh gases, so the lubrication must come from the outside included in the fresh gases, which therefore consist of a mixture of air, gasoline, and oil.

Another interesting fact is that not all two-stroke engines ever built have been single-cylinder. Two-, three-, and four-cylinder motorcycle engines have also been produced.

What are the parts of a two-stroke engine?

A two-stroke engine is divided into two parts: the upper and the lower. The upper part consists of the cylinder, cylinder head, spark plug, piston, and piston rings. The lower part consists of the crankshaft, connecting rod, and crankcase. The two parts are connected by transfer lines known as transfers. In addition, the cylinder has two windows called ports. One is the inlet, through which fresh gases enter from the outside, and the other is the exhaust, which discharges the gases once they have been burned.

You can consult all the parts of an engine, but to get an idea, they can be grouped into three groups: fixed parts, moving parts, and auxiliary systems:

Fixed parts

– the cylinder: this is the engine cavity, which, as its name suggests, is cylindrical in shape and inside which the piston moves. It is covered at the bottom by the cylinder head and at the top by the crankcase. In the cylinder are the ports that allow the gas exchange.

– the cylinder head: this is the part that covers the cylinder and contains the combustion chamber. It forms the chamber in which the piston moves and may be cast in one piece together with the cylinder itself.

– the crankcase: this is the chamber where the crankshaft and connecting rod meet and which activates the gas exchange.

Moving parts

The moving parts of a two-stroke engine are:

– the pistons: these are the cylindrical parts that move inside the cylinder, carrying out the gas exchange and causing the movement that reaches the connecting rods and the crankshaft.

– the connecting rods: these are the elongated parts that connect the pistons to the crankshaft, transmitting the piston’s impulse to the crankshaft to transform the rectilinear motion into a rotary motion.

– the crankshaft: this is a shaft with angles or bends, similar to a crank, which changes the rectilinear motion of the piston into a rotary motion that pushes the transmission and thus the driving wheel.

Auxiliary systems:

– carburetor: this is the mechanical part responsible for making the air/fuel mixture in the right proportion for the gasoline to be ignited. The mixture is known as fresh gas or simply gas.

– ignition system: this is the electrical mechanism that generates a current and controls the voltage of the current to generate the spark that ignites the gas in the combustion chamber.

– exhaust: this is the part responsible for evacuating the gases produced during combustion to the outside, also helping to reduce the temperature of the fumes and to retain some polluting particles.

Evolution of 2-stroke engines

From the mid-1970s onwards, two-stroke engines began to include a power valve in the exhaust port area of the cylinder. The performance was very high in some cases and they delivered power in a way that was difficult to control, so this system was devised to try to make the delivery more progressive. The exhaust valve opens more or less depending on engine speed, and its function is to improve throttle response or, in other words, to increase torque at low and medium revolutions. The purpose of the exhaust valves is to increase the operating range of the engines and thus make them more workable. The actuation of these elements can be mechanical, electronic, or hydraulic. It can be said that each motorcycle brand has its own system, although they have all been developed for the same purpose.

In those years, a reed valve or reed block also began to be fitted in the intake manifold, which could be connected to the cylinder or directly to the crankcase, or both at the same time. This type of valve allows only the passage of gases in the engine inlet direction. The blades are mounted on a rigid support and are raised on one side only, allowing fresh gases to enter and preventing part of them from escaping due to the intake.

Another less common type of intake system used in some two-stroke engines is the rotary valve. In this system, the intake is located on one side of the engine and connects directly to the crankcase.

How is it different from a four-stroke engine?

A four-stroke engine needs four piston strokes to complete the four stages or, in other words, it has to perform two crankshaft revolutions to complete the intake, compression, combustion, and exhaust stages.

Advantages and disadvantages of 2-stroke engines

Compared to a two-stroke engine, the four-stroke engine has a lower specific power output (between 30 and 50%) and therefore, for the same displacement, the performance of a two-stroke engine is greater.

However, with the same cubic capacity, the two-stroke engine consumes more gasoline and pollutes more. As it produces more power, the two-stroke engine also heats up more and therefore requires a more effective cooling system.

Four-stroke engines have more parts than two-stroke engines. Their cylinder heads include intake and exhaust valves, driven by camshafts, and these in turn are driven by chains, toothed belts, gears, etc. This means that, as well as being more expensive to produce, they are also heavier and more complex. In their favor, four-stroke engines are generally more mechanically reliable and therefore require less maintenance.

In terms of performance, four-stroke engines vibrate less and make less noise. In addition, the oil that lubricates them is contained in the crankcase and they run on only gasoline. In this way, when using them we forget to mix them with gasoline and oil, and the combustion gases they emit are less polluting.