Today it is impossible to imagine modern life without. Traveling in your own car, traveling on public transport, purchasing goods, flying on an airplane and other actions. These processes are, one way or another, connected with the engine.
Despite the number of different designs and types of power plants, piston engines today are more common than others. The number of strokes to complete the working cycle divides the unit into a two-stroke and a four-stroke engine. These types of motors make up the majority among the variety of manufactured equipment.
The difference between motors arises from the application point of view. For installation on automotive equipment, a four-stroke unit is more often used; a two-stroke engine is used if dimensions and weight play a decisive role.
Suzuki RM125 motorcycle with single-cylinder two-stroke engine
Creation of a two-stroke engine
There are many speculations about who was the first to create an internal combustion engine. It is known for certain that the first two-stroke engine running on gas was invented and designed by the Belgian Jean Joseph Etienne Lenoir; this event occurred in 1858.
Lenoir engine (on display in the museum)
At that time, a steam engine had already been created, and the Belgian’s invention surpassed it in characteristics. The engine is much lighter, simpler, and consumes less fuel. Despite the advantages, the power plant had many shortcomings and was inferior in reliability. After Nicholas Otto presented a four-stroke engine, which at that time was thought out in more detail, the engine operating on the two-stroke principle was forgotten, and was not used anywhere for a long period of time.
During the Great Patriotic War, the power plant was installed on aircraft. In our region, motors are known for their use on moto equipment. Three-cylinder units performing two strokes are used on motorcycles from Suzuki and Kawasaki. Today, engines are used in aviation; the leader here is the Austrian company Rotax, which produces engines for use on small aircraft.
Rotax 582 UL two-stroke twin-cylinder engine
After stricter requirements for environmental standards and emissions, the two-stroke engine was no longer used for installation in classic automobile vehicles. However, on light equipment such as scooters, snowmobiles, boats, replacing a small and light unit is not easy. Here the two-stroke unit simply has no competitors.
Features of a two-stroke engine
A power unit that uses two strokes is good because it is simple and reliable. The difference between a two-stroke and a four-stroke engine is the duty cycle. This cycle consists of two strokes: compression and expansion, whereas in a four-stroke engine there is an intake stroke of new fuel and an exhaust stroke of spent fuel. An interesting fact is that these two strokes are also present in a two-stroke power plant, otherwise the unit would not be able to work, but they are combined with the processes of compression and expansion.
The cycle being performed clearly demonstrates the difference between a two-stroke engine and a four-stroke engine. The process of a two-stroke motor takes place per revolution of the shaft. This feature achieves installation in comparison with the opponent, one and a half times. Despite the increase in power, the return rate is underestimated, which is a negative point.
In addition, this feature leads to the release of a volume of heat during operation, which greatly overheats the motor. Two-stroke power units require intensive cooling. The positive point is that when working, the piston makes half as many movements as the piston of a four-stroke mechanism, this reduces wear on parts and elements.
The peculiarity of the unit is that there is no lubrication mechanism. Oil is supplied directly with fuel. For this purpose, a mixture of gasoline and oil is added to the gas tank, the ratio is one to fifty, or the lubricant is mixed with fuel in the pipeline at the inlet. The oil burns with gasoline and is removed with waste products.
The distinctive moment is the combustion itself. In a four-stroke unit, one cycle is allocated for this. In push-pull units, combustion occurs in a fraction of a second, so the mechanism needs adjustments to achieve the effect.
Two-stroke engines have found themselves in another industry, shipbuilding. Cylinder power plants are also used on scooters produced in large quantities.
Operating principle of a two-stroke engine
To understand why four-stroke engines have replaced their younger brothers in automotive technology, let’s look at how a two-stroke engine works.
Sequence of actions of the power plant operating cycle:
Compression stroke.
The process is accompanied by the movement of the piston from bottom to top. The movement provokes the flow of fuel through the purge holes into the unit; subsequently, the piston skirt blocks these holes. Further movement is accompanied by the closure of the exhaust channels into which combustion waste was pushed out. Between the piston and the top of the cylinder, a combustion space is formed in which excess pressure is created. At the same time, a vacuum occurs in the space under the piston, and the space is used for the flow of a renewed dose of fuel. Having reached the top point, the charge lights up.
Two-stroke engine diagram
Expansion stroke.
Having ignited, the portion creates excess pressure, which presses on the bottom of the piston and forces it to move. The process is accompanied by alternate opening of the windows, first for exhaust, then for purging. The descent creates excess pressure under the piston chamber; under its influence, fuel again enters the cylinder, squeezing out the remaining waste and filling the space to repeat the previous stroke.
The operating principle of a two-stroke engine makes it possible to do without a gas distribution system, making the design of the unit lighter and more reliable. The downside is the quality of the gas exchange process. The two-stroke mode is impossible without purging, the process of which is accompanied by the release of unburned fuel along with exhaust gases to the outside. This leads to excessive consumption of fuel and increased toxicity of the unit's exhaust.
It is worth noting that the above described scheme is typical for carburetor engines. In the case of a diesel engine or, clean air is supplied to the cylinder through the purge holes. The combustible mixture is supplied through injection; this work is performed by injectors.
Methods for purging cylinders
It is obvious that the purging process, a mechanism that qualifies as complex. Properly performed purging directly affects power and efficiency indicators. To improve performance, designers are constantly trying to improve and bring the process to perfection.
How to blow out a cylinder:
- “Circuit” blowing. This type of blowing is simple and therefore widespread. The disadvantage is that the application is associated with excessive fuel consumption. Types of contour blowing: return-loop, deflector, high-altitude.
- “U-shaped” purging. The “U-shaped” principle is to be used only on engines with two cylinders. When carrying out, one cylinder participates in the process of gas intake, the second releases waste. The effect of purging is felt in fuel efficiency; the process is accompanied by uneven heating of the steam responsible for the exhaust.
- “Valve-slit” purge. It differs in that it requires a gas distribution mechanism to control the valves. The valve is used both to provide fuel and to remove exhaust vapors. Purge involves the removal of waste through a valve in the cylinder head and the flow of fuel through the holes. The advantage is that purge increases fuel efficiency and minimizes the toxicity of emitted vapors. Disadvantage, complexity of design and violation of regimes associated with an increase in the operating temperature of the unit.
- “Direct flow” blowing. Used in power plants with the number of pistons equal to two. In this case, the cylinder is located in a horizontal position. The pistons move towards each other. While moving, each piston releases and closes the valve: one piston admits a portion of fuel, the second removes a portion of waste from the cylinder. The combustion chamber is formed when the pistons approach each other. The effect of this purge option is maximum: it removes burnt gases and saves fuel. The downside is that a complex mechanism of cranks and connecting rods is required; the performance requires the use of coolers and sustainable materials for the manufacture of parts.
Two-stroke engine 5 TDF with direct-flow purge
The difference between a two-stroke engine and a four-stroke engine
Car owners are wondering: which is better, a two-stroke or a four-stroke engine. There is no definite answer; each mechanism has positive and negative sides, depending on the requirements placed on the motor.
It would seem that the power of a motor performing two strokes, in comparison with an equivalent motor performing four strokes, is greater, which means it is better. However, the reality is more complex. In practice, additional losses occur: partial ingress and mixing of waste gas with fresh fuel, release of part of the fuel during purging. The result is that when performing the same cycle, a unit that performs two cycles is inferior in terms of efficiency to a unit with four cycles.
The method of lubrication of four-stroke and two-stroke power plants is different. The two-stroke unit is lubricated by mixing engine oil and gasoline. The four-stroke unit has a lubrication mechanism using a pump, which consumes as much oil as the operation of the installation requires.
Two-stroke engines do not have valves; the role of the part is played by the piston; it opens and closes the intake and exhaust ports. The absence of gas distribution mechanisms simplifies the power unit, making maintenance simple. The power of an installation that performs two cycles is considered higher, since its cyclicity is higher. However, without fully using the piston stroke, power losses during purging and exhaust gas residues reduce the power rating.
To make it easier to determine which engine is better, two-stroke or four-stroke, we present a brief description of both power plants in the form of a table:
| Four-stroke power plant | Two-stroke power plant |
| The working process is two crankshaft revolutions. | The working process is one crankshaft revolution. |
| Ignition of the working fluid occurs every time the second revolution is made, as a result, uneven distribution of the impulse and the use of a counterweight to eliminate beats. | Ignition of the working fluid occurs every time a revolution is made, as a result, the impulse is evenly distributed, and the operation of the motor is better balanced. |
| The unit is heavy. | The unit is lightweight. |
| The complex design of the power plant includes a gas distribution mechanism. | Simplicity of design, absence of valves. |
| The unit is expensive. | The cost is lower than the four-stroke. |
| Complex devices and mechanisms lead to an underestimated mechanical efficiency. | The mechanical efficiency is higher than that of a four-cycle unit. |
| Complete removal of exhaust vapors, resulting in increased productivity. | The waste residues are mixed with new fuel, causing engine performance to be lower. |
| Operating temperature is lower. | The operating temperature of the engine is higher due to a violation of mixture formation. |
| Liquid cooling. | Air cooling. |
| Fuel consumption is lower. | The fuel consumption rate is increased, due to mixture formation and purging. |
| The dimensions of the power plant have been increased. | The dimensions of the power plant are below. |
| Requires the use of complex lubrication mechanisms. | The lubrication mechanism is simple. |
| The unit operates less noisily. | The unit operates with a lot of noise. |
| Valve timing mechanism. | The function of the gas distribution mechanism is performed by the piston and channels. |
| The heat utilization rate is efficient. | The heat utilization rate is not efficient. |
| Oil consumption is underestimated. | The oil consumption rate is overestimated, since part of the lubricant is thrown out with exhaust gases. |
It is advisable to use an engine that performs two strokes during operation in moments when fuel and lubrication savings are not at issue, but the dimensions and weight of the installation come first.
At the same time, the design of a two-stroke engine contains potential that cannot be realized in practice. The calculated indicator of power and efficiency in this unit is high; the difficulty in implementing it arises due to the subtlety of the settings. Perhaps in the near future, thanks to the use of electronic sensors and control and adjustment mechanisms, two-stroke units will be able to take a leading position in the automotive market.
The internal combustion engine operates on long-studied principles. We will consider piston engines, since rotary and other exotic units that convert combustion energy into kinetic energy are not so common.
To better understand the process, let’s define some technical terms:
- The engine operating cycle is a chain of alternating processes, as a result of which the energy of fuel combustion is transformed into rotation of the wheels (when used in a vehicle)
- Intake - filling the cylinder with a mixture of gasoline vapor and air (in gasoline engines) or air in diesel engines
- Compression - compressing the fuel mixture in the cylinder
- Power stroke - expanding gases, after ignition of the combustible mixture, rapidly drive the piston down
- Exhaust - ventilation of the cylinder cavity from a portion of exhaust gases.
During a work cycle, processes follow a strictly defined order. Each of them is called a beat. From a mechanical point of view, a stroke is the movement of the piston from one dead center to the second. Depending on the design of the motor, there may be two or four cycles.
What is a two-stroke engine - video
What is the difference between a two-stroke engine and a four-stroke engine?
For each 180° rotation of the crankshaft, two strokes are performed (different depending on the engine type). The development of the process of a two-stroke internal combustion engine is carried out in one revolution, and of a four-stroke – in two revolutions of the crankshaft. Unclear? Let's consider the question in more detail.
Important! There are passive and active beats. More precisely, there is only one active cycle - the working stroke. The remaining movements of the cylinder occur due to the inertia of the flywheel mounted on the motor axis. This applies to a single-cylinder engine, each subsequent cylinder works with a phase shift, and a heavy flywheel is no longer needed. Therefore, its role is played by the gear to which the starter clings.
A modern motor is quite complex from an engineering point of view. Processes are supported by various auxiliary mechanisms; their work must be synchronized. In addition, the engine components have a certain mass, and accordingly there is inertia.
Rubbing parts are slowed down by resistance. This slows down the process and takes away additional power. All amendments must be taken into account when designing the motor.
It is difficult to program a control algorithm; operating conditions are constantly changing. If the sequence of cycle changes fails, a loss of power or engine shutdown will occur. Therefore, for smooth operation you need so many devices around the pair: piston, cylinder.
As soon as the top of the piston closes the outlet port (1), the mixture begins to compress until the piston reaches top dead center.
Time 1. Process 2 
A spark ignites at the spark plug, the air-fuel mixture explodes, the piston receives a push and rapidly moves down.
Important! The concepts of “up”, “top dead center”, “down”, “bottom dead center” are quite arbitrary. The cylinders can be positioned horizontally (opposite engines of the Porsche and Dnepr motorcycle) or even upside down (star-type aircraft engine).
The gas pressure is maintained until the outlet (1) opens.
Working stroke
Beat 2. Process 4 
Exhaust gases flow into the open outlet. Properly calculated aerodynamics even allow you to create a slight vacuum. However, some of the exhaust remains in the cylinder.
Important! At this moment, the combustion energy of the mixture no longer acts. All movement occurs by inertia.
At this time, the piston compresses the prepared fresh charge of the fuel mixture, increasing the pressure in the crank chamber (II). At the same time, valve (3) closes.
Beat 2. Process 4 
The piston continues to descend. When the blocked purge hole (2) is released, the fuel mixture under pressure will rush into the above-piston chamber (I). Then again the dependence on aerodynamics.
The well-designed shape of the chamber and piston does not allow the fresh charge and exhaust gases to mix. However, in reality, losses of the finished mixture are inevitable. This process is also called purging.
Good day everyone, dear readers! Despite the fact that most of you are owners of four-wheeled vehicles, there are also connoisseurs of motorcycles, mopeds, and scooters among subscribers. If you don’t yet know the principle of operation of a two-stroke engine, then it’s time to familiarize yourself with this interesting topic.
This type of power unit has become the basis for various types of devices and equipment due to its simplicity and reliability. The operating cycle of such a motor has only two strokes, unlike the 4-stroke engines that are installed on most cars. This pair of bars is compression and expansion. The reader may quite rightly wonder: where does the inlet and outlet of the working mixture go? The fact is that they are combined with the compression and expansion mentioned above.
Unlike a 4-stroke engine, in a 2-stroke engine the entire working cycle occurs in just one revolution of the crankshaft. This makes it possible to increase the power quality of the engine by 1.5 times or more with an equivalent displacement. However, this leads to a decrease in efficiency, otherwise all self-propelled mechanisms without exception would be equipped with such power units. But in shipbuilding they have found the widest application. A single-cylinder two-stroke engine is also an integral component of every small-engine scooter that roams our roads with might and main.
Another important feature of such mechanisms is their tendency to overheat. This is due to the release of large amounts of heat during operation. To solve this problem, forced cooling may be required. But there are also advantages to such a motor: the work of the piston is limited to 2 strokes, which means that it makes half as many movements. Due to this, wear of key parts of the power unit is reduced.
Operating principle
Let's look at how such an engine works in practice (see video):
- The piston begins to move upward from the bottom point, which is also called the “dead” point. Simultaneously with this process, fuel is delivered along with air. The exhaust window opens slightly, and exhaust gases flow out freely through it. At the same time, the moment of compression of the working mixture occurs.
- As soon as compression begins, a space is formed in the crank chamber based on rarefied air. This frees up space for a fresh supply of fuel. When the piston reaches the top point of movement, the spark plugs produce a spark that ignites the working mixture.
- As a result of the combustion of the working mixture, energy is generated that forces the piston to move downwards. The excess pressure created in the crank chamber causes the fuel to compress. At the top point of the piston movement, the exhaust window opens, freeing the exhaust gases to exit, from where they are sent straight to the muffler.
- Further movement of the piston leads to the opening of the purge window. Fuel moves from the crank chamber to the working cylinder. As soon as the piston reaches the lowest point, this means that the full cycle of engine operation has taken place. And everything starts again, but this will be the beginning of a new cycle.
Comparison of 2 and 4 stroke engines
Since the power of an equivalent motor is two strokes greater than that of its 4-stroke counterpart, it should, in theory, be more economical. In practice, this does not happen due to additional losses that arise. Partial mixing of the exhaust gases with the newly incoming fuel occurs, and this entire mixture safely exits through the exhaust pipe. Therefore, for the same number of cycles, a two-stroke carburetor requires more fuel.
There are also differences in the lubrication system. In the case of a 2-stroke motor, it is carried out by mixing and. The 4-stroke engine has a lubrication system with a gear pump. Lubricant enters the inlet pipe of the system, and it is supplied exactly as needed.
Such engines do not have valves, which are inherent in four-stroke internal combustion engines. For them, the same work is done by a piston, which, when moving sequentially up and down, opens and closes the purge, inlet and outlet windows. Due to this, they are considered more structurally simple and easier to maintain. It is believed that their power rating is approximately 2 times higher than those designed for 4 cycles, due to the greater number of cycles passed. 
But due to insufficient use of the piston stroke, residual accumulated gases in the cylinder and partial loss of produced power for purging, the actual increase in net power will be no more than 60–70 percent. The spark on such engines appears a fraction of a second earlier than the piston reaches top dead center, and various mechanical and electronic devices are provided to change the ignition angle. On previous models, the ignition moment was set based on the optimal speed.
So, let's summarize the main advantages of the power unit in question:
- differs in small dimensions;
- has a simple device;
- produces more power with the same displacement.
At the same time, its use is limited due to design features and significant losses. However, today this type of engine is still equipped with a large number of different mechanisms that can use both a single- and two-cylinder internal combustion engine for 2 strokes. Knowing the features and operating principle of such an engine, you can independently find problems that arise in it. In some cases, such knowledge allows you to decide between a 2-stroke and 4-stroke power unit. 
In today's review, we tried to look at the design of a 2-stroke power unit, which is equipped with almost any modern motorcycle or moped, as well as other equipment. Friends, I will be grateful for your recommendations of my blog among your friends. In the upcoming blog releases, we will definitely look at new interesting topics in the field of cars. In the meantime, a few words about and what is the point in using them. Stay with us and see you again!
Piston internal combustion engines (ICEs) are widely used in various spheres of human life. However, they don't all work the same way. There is one fundamental difference between them. Depending on the design, the engine operating cycle can consist of two or four strokes. That is why it is called a two-stroke engine or a four-stroke engine, respectively. This is true for both gasoline engines and diesel engines.
Basic terms and definitions
The operating principle of all piston engines is to convert the energy of fuel combustion into mechanical energy. The transmission link is the crank mechanism. The following concepts are used to describe their work:
- Duty cycle- this is a certain sequence of interconnected events, as a result of which the energy of thermal expansion of burning fuel is converted into mechanical energy of movement of the piston and rotation of the crankshaft.
- Tact- the sequence of changes in the state of components and mechanisms that occur during one stroke of the piston.
- Piston stroke- this is the distance that the piston travels inside the cylinder between its extreme points.
- Top Dead Center (TDC)- this is the highest position of the piston in the cylinder, while the volume of the combustion chamber has a minimum volume.
- Bottom Dead Center (BDC)- the position of the piston as far as possible from TDC.
- Inlet- filling the cylinder with the air-fuel mixture.
- Compression- reducing the volume of the mixture and compressing it under piston pressure.
- Working stroke- movement of the piston under the pressure of gases from burning fuel.
- Release- pushing fuel combustion products out of the cylinder.
Operating principle of a four-stroke engine
A four-stroke engine is a piston engine in which one working cycle consists of four strokes. They have the following names:
- inlet;
- compression;
- working stroke;
- release.
In one cycle, the piston moves from TDC to BDC and back twice, and the crankshaft rotates two full revolutions. The events that occur during this time in the engine have a clearly defined sequence.
Inlet. The piston moves down to BDC. A vacuum is formed under it, due to which fuel mixed with air is drawn through the open intake valve plate from the intake manifold into the cylinder. The piston passes bottom dead center, after which the intake valve closes the intake manifold.
Compression stroke. The piston continues to move upward and compresses the air mixture.
At top dead center above the piston, the combustible mixture is ignited. When burned, it causes a significant increase in pressure on the piston. The power stroke begins. Under the influence of the pressure of the combustion gases, the piston again moves to BDC, while performing useful work.
After the piston passes BDC, the exhaust valve poppet opens. The piston, moving to TDC, pushes exhaust gases into the exhaust manifold. This is the release stroke.
Then the intake stroke begins again and so on endlessly.
Duty cycle of two strokes
A single cylinder two stroke engine works differently. Here, all four actions occur in one full revolution of the crankshaft. In this case, the piston makes only two strokes (expansion and compression), moving from TDC to BDC and back. And intake and exhaust are part of these two strokes. The operating principle of a two-stroke internal combustion engine can be described in more detail as follows.
Gases from the combustion of the fuel mixture push the piston down from TDC. Approximately halfway through the piston stroke, an exhaust hole opens in the cylinder liner, through which some of the gases are released into the muffler pipe. Continuing to move downward, the piston creates pressure, due to which a new portion of fuel enters the cylinder, while simultaneously purging it of the remaining burnt gases. Approaching TDC, the piston compresses the mixture and the ignition system ignites it. The expansion stroke begins again.
In aircraft modeling, a two-stroke diesel engine is widely used; its operating principle is the same as that of a gasoline engine. The difference is that the fuel-air mixture ignites spontaneously at the end of the compression cycle. The fuel for such engines is a mixture of ether and aviation kerosene. Ignition of this fuel occurs at a much lower compression ratio than in engines using traditional diesel fuel.
Design features and differences
A two-stroke engine differs from a four-stroke engine not only in how many cycles of operation gas exchange occurs.
The four-stroke requires a gas distribution system (intake and exhaust valves, camshaft with cam mechanism, etc.). The two-stroke does not have such a system, which makes it much simpler.
A four-stroke engine requires a complete lubrication system due to the large number of moving and rubbing parts. To lubricate a two-stroke engine, you can use oil simply by diluting it with fuel.
Performance in comparison
Comparing a two-stroke engine and a four-stroke engine, the difference between them can be seen not only in the design, but also in the performance characteristics. You can compare them based on the following indicators:
- liter capacity;
- specific power;
- efficiency;
- environmental friendliness;
- noisiness;
- work resource;
- ease of maintenance;
- price.
Liter is the power removed from a liter of cylinder volume. Theoretically, it should be twice as large for a two-stroke. However, in reality this figure is 1.5−1.8. This is due to incomplete use of the working stroke of gases, energy consumption for purging, incomplete combustion and fuel losses.
Specific power is the ratio of motor power to its weight. It is also higher for two-strokes. They require a less heavy flywheel and do not require additional systems (gas distribution and lubrication) that make the structure heavier. Their efficiency is also higher.
Efficiency (fuel consumption per unit of power) is higher for four-stroke engines. Two-stroke engines waste some of the fuel when the cylinder is purged.
The environmental friendliness of two-stroke engines is lower, again due to the loss of unburned fuel and oil. You can verify this using the example of a two-stroke outboard motor. It always leaves a thin film of unburnt fuel on the water.
The noise is higher with two-stroke engines. This is due to the fact that the exhaust gases escape from the cylinder at high speed.
The service life is higher for four-stroke engines. A separate lubrication system and lower engine speed have a positive effect on its service life.
It is certainly easier to maintain two-stroke engines due to fewer auxiliary systems. Four-stroke models have more weight. Two strokes are cheaper.
In some mechanisms, the use of two-stroke engines is clear. These are, for example, chainsaws. High power density, low weight and simplicity make it an absolute favorite here.
Two-stroke engines are also used in motorcycles, boat engines, lawn mowers, scooters, and aircraft modeling. In most homemade machines and mechanisms, craftsmen also use a two-stroke engine.
Single-stroke and three-stroke power units
There are also one- and three-stroke engines. Single-stroke engines are made with an external combustion chamber. This scheme implements all four strokes in one piston stroke. The three-stroke Wankel engine is a rotary piston engine. Due to the complexity of the design and extreme demands on the quality of surface treatment, such motors are not widely used.
Internal combustion engines are built on the same principle - the energy of fuel combustion is converted into kinetic energy of crankshaft rotation. There are two types of motors - two-stroke and four-stroke. Both have their advantages and disadvantages, let's try to figure out what the differences are.
The principle of operation of the internal combustion engine
The working cycle of a two-stroke engine consists of intake and exhaust occurring per revolution of the crankshaft, while a 4-stroke engine has the following cycles - intake, compression, power stroke, exhaust. And they leak within two revolutions of the flywheel. In a 4-stroke engine, intake and exhaust are carried out in the form of different processes; in a two-stroke engine they are combined with compression of the fuel mixture and expansion of the working gases. Operating principle of a two-stroke engine:
- The first stroke is compression. The piston moves from bottom dead center, and the purge window first closes. The spent exhaust gases are discharged through the exhaust port. At this moment, a vacuum area is formed in the crank chamber under the piston bottom, into which the enriched fuel mixture from the carburetor (injector) enters. This portion of fresh air pushes the remaining exhaust gases into the exhaust manifold. At the moment of the highest position of the piston, the mixture is ignited by the spark plug.
- The second stroke is the power stroke or expansion. The temperature and pressure of the gases in the combustion chamber increases sharply, under its action the piston begins to move to the bottom dead center, performing useful work. The increased pressure in the crank chamber closes the intake valve, preventing exhaust gases from entering the carburetor. Through the exhaust window system, exhaust gases go into the muffler, and through the purge window, a fresh combustible mixture begins to flow into the combustion chamber. At the lowest point, the action of the second stroke ends and the process repeats.
A two-stroke diesel engine works on the same principle, only it does not have a spark plug, and the fuel is ignited by compression. Therefore, the compression ratio in diesel engines is much higher than in gasoline engines.
Features of a two-stroke motor
A two-stroke engine completes a full cycle in one revolution of the crankshaft, this allows one to obtain greater specific liter power than a 4-stroke engine at the same engine speed. However, the efficiency of a two-stroke engine will be lower due to imperfections in the valve timing mechanism, inevitable losses of the fuel mixture during the purging process and incomplete piston stroke.
A two-stroke engine gets very hot because a lot of thermal energy is released during operation. Sometimes additional cooling may be required. Motorcycles rarely use two-stroke engines with a large number of cylinders; most often a single-cylinder air-cooled engine is used.
When operating on a two-stroke cycle, the piston makes fewer movements per stroke, and the load of auxiliary gas distribution, lubrication and cooling systems on the crankshaft is lower or absent altogether. Therefore, they will have less wear on the piston group. If this is not a decisive factor for light equipment, then a low-speed two-stroke diesel engine can have several times longer service life than all other internal combustion engines. Therefore, they are widely used in diesel locomotives, generators, and ship engines.
The two-stroke petrol engine reaches maximum power faster. This is actively used by motorcyclists, especially in cross-country disciplines, when an instant response to the throttle is required. In addition, it is easier to maintain, cheaper and lighter than a four-stroke.
The fuel consumption of a two-stroke engine will be 25-30% higher, as well as noise and vibration. The engine cannot comply with strict environmental standards, even if you use injection injection systems and supercharging. High air flow requires the use of special air filters.
Lubrication system and fuel preparation
The operation of a two-stroke engine requires effective lubrication of moving parts. There is no centralized separate lubrication system with an oil pump, like in four-stroke engines, so oil is added to gasoline in a ratio of 1:25 - 1:50. The resulting composition, being in the piston and crank chambers, lubricates the connecting rod bearings, cylinder walls and piston rings. When the air mixture ignites, the oil burns and is removed along with the exhaust gases.
The motor oil must be special - for a two-stroke engine, usually it is marked 2T on the canister. The use of conventional automobile oil is unacceptable for a number of reasons:
- Oil for two-stroke engines must have good solubility in gasoline;
- Has excellent lubricating properties, improving engine performance and reducing friction;
- Protection against corrosion of rubbing parts of the piston group;
- Two-stroke oil should burn without residue, without forming soot or soot. The high ash content of conventional oil leads to coking of the piston rings.
Lubricant can be supplied to a two-stroke engine in two ways. The first and simplest is to mix it with fuel in the required proportion. The second is a separate lubrication system of a two-stroke engine, when a composition of fuel and oil is prepared immediately before entering the engine in a special pipe. In this case, a separate oil tank is installed, and its supply is carried out using a special plunger pump.
This system is widely used on modern motorcycles and scooters. In addition to ease of use (now you no longer need to add oil to the tank by eye every fill-up), serious oil savings occur because its injection depends on engine speed. At idle speed the oil proportion can be as low as 1:200.
Any two-stroke engine has the ability to boost. An increase in power for the same volume is justified in sports, and in everyday use the engine becomes more flexible and more economical.
- Main ways of improvement:
- Increase the diameter of the outlet and ensure that it opens for as long as possible. This allows the maximum amount of gases to be released. This increases the engine's traction capabilities and torque.
- Ensure effective purging. To do this, you can increase the diameter of the intake window, then the combustible mixture will not linger in the crankcase and timely injection into the combustion chamber will be ensured.
- The use of a vortex diffuser on the carburetor, which supplies a larger amount of fuel mixture at the same time. Together with it, it is advisable to use a zero-resistance air filter.
- Installation of an exhaust resonator, calculated for a specific engine size. Such a device returns part of the fuel mixture back into the cylinder through the outlet.
- Refinement of the connecting rod and piston group, its lightening and careful balancing. Valves and channels must be ground in and free from burrs (burrs), braking and swirling flows. This reduces cylinder filling and reduces power.
- Installation of pressurization systems. These are usually compressor-type superchargers, and a two-stroke diesel engine can be fitted with a traditional turbocharger. With its help, the amount of air entering the cylinders increases, and accordingly, the amount of fuel can be increased.
Operation and causes of engine failure
Most often, two-stroke engines are found in motorcycles, boat engines, lawn mowers, chain saws and other devices that require the use of a lightweight and reliable engine. However, even such a simple engine can fail due to violation of operating rules.
- Low quality gasoline. Bad fuel often leads to detonation. Most often this is noticeable at low speeds when accelerating. The resulting shock loads lead to breakage of the piston partitions and excessive loads on the crankshaft bearings. Detonation can occur due to engine overheating, carbon deposits on the piston and a lean mixture.
- Low quality of parts from which the motor is assembled. This is especially true for Chinese manufacturers, who often allow defects in the production of components. This leads to early failure of the piston, crankshaft, cylinder and other parts, and then to major repairs. A simple compression test usually helps to assess the condition of the piston.
- Low quality motor oil. The fuel/oil mixture for two-stroke engines is very important. It is its quality that will determine how smoothly the engine operates, the cleanliness of the exhaust, the absence of overheating and unnecessary noise. Bad oil leads to the formation of a layer of carbon on the piston, in the main and connecting rod bearings, to scuffing on the cylinder walls and piston skirt, and the flow area of the muffler is reduced due to carbon deposits. Oils for two-stroke engines should be used synthetic or semi-synthetic; the use of mineral water is undesirable.
- Overheating on a two-stroke air-cooled engine is not uncommon. This is caused by prolonged operation with the throttle fully open, or a malfunction of the cooling system. Overheating can be short-term, when there is a loss of power and maximum speed; after reducing the load and cooling the engine, everything returns to normal. The wedge occurs as a result of very strong overheating, when the thermal gap between the piston and cylinder decreases so much that friction forces tightly grab them together. After this, repair of the CPG is required.
- The carburetor is not adjusted. The fuel mixture is too lean or too rich. Driving with an over-enriched mixture is fraught with high fuel consumption, loss of power and the formation of carbon deposits. A lean mixture can cause detonation and reduce maximum engine power.
To extend the service life and delay major repairs, you should properly break in a two-stroke boat or motorcycle engine. To do this, the proportion of oil mixed with gasoline should be slightly higher than that established for normal operation. With this mixture, let the engine run at partial power for several hours, which is equivalent to 500-1000 km for a scooter and motorcycle.
