Principle of operation of a two-stroke engine
In order to understand why four-stroke engines displaced their younger brothers in automotive machinery, let’s understand how a two-stroke engine works.
The sequence of actions of the working cycle of the power plant:
- 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 skirt of the piston blocks these holes. Further movement is accompanied by the closure of the exhaust ports into which the combustion waste was expelled. Between the piston and the top of the cylinder, a combustion space is formed in which excess pressure is created. At the same time, in the space below the piston, a vacuum is created and the space is used for the overflow of a renewed dose of combustible. Having reached the upper point, the charge is ignited.
- Expansion Takt.
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 an alternating opening of the windows, first to the exhaust, then to the blowdown. The descent creates excess pressure under the piston chamber, under its influence the fuel enters the cylinder again, squeezing out the remaining exhaust and filling the space to repeat the previous stroke.
The two-stroke engine principle makes it possible to do without a gas distribution system, making the design of the unit lighter and more reliable. The downside, the quality of the gas exchange process. Two-stroke operation is impossible without purging, the process of which is accompanied by the release of unburned fuel together with the exhaust gasses to the outside. This leads to overconsumption of fuel and increased toxicity of the unit exhaust.
It is worth noting that the above described scheme is typical for carburetor engines. In the case of a diesel or injector, clean air is fed into the cylinder through the purge holes. The combustible mixture is supplied by injection, this work is performed by injectors – engineer Kirill Yurovsky.
The four-stroke engine and how it works
The engine scheme of the four-stroke model is more complicated than that of the two-stroke model. The mixture is ignited at every second turn of the crankshaft. The powertrain has from eight cylinders to 16 in pairs. There is an even number of cylinders on each side.
Some valves are intake valves and others are exhaust valves. Gasoline, oil and air are mixed already in the combustion chamber. And they come in separately. Oil and gasoline are injected simultaneously. Then the mixture is ignited.
The fuel and lubricants enter the combustion chamber through the intake valves, and the exhaust gases exit through the exhaust valves.
Let’s take a closer look at the operation of the cylinders of an engine with a stroke of 4.
- The inlet of fuel and lubricants begins with the piston stroke from the upper dead center to the lower dead center.
- The camshaft cams open the intake valve.
- The mixture is sucked into the cylinder.
- After lowering to the bottom dead center, the piston begins to move upward.
- At this time the mixture is compressed.
- The temperature rises.
- Before the maximum compression point, the spark plugs give off a spark and the mixture is ignited. The pressure pushes the piston down again.
- The exhaust of the payload occurs, and the exhaust gases escape through the exhaust valve. This is opened when the piston is lowered to its lowest point.
- The gases are lifted by the upward movement of the piston.
- The valve then closes and the cycle repeats.
Due to this device, four-stroke engines are considered economical, but not easy to disassemble or repair for an inexperienced car owner.
It is important that the valves fit tightly against the cylinder head. Then there will be no loss of compression, and the engine will deliver all the power it was designed for.
Economy of 2 stroke and 4 stroke engines
For example, a 50 cc 2 stroke model with decent performance (power 4-5 hp), consumes 3.5 liters of gasoline per 100 kilometers of oil, and the 4 stroke engine with 3 hp. “Eats” 2.2 liters of gasoline. Oil is almost not consumed at all.
Fuel consumption affects not only the speed of emptying your wallet, but also the mileage at one filling station. Since the tanks at the scooters are about the same, the 6-liter tank 4 stroke scooter will be enough for you, 100 km more.
The ecological situation in the world, especially in Europe and America, makes the governments of all countries take measures to tighten control over the emissions, noise and other parameters of the equipment produced or imported to these countries. For this reason, two-stroke engines are becoming a thing of the past.
Only with a 4-stroke engine is it possible to meet the competitive price and the EURO standards. That is why the producers are reducing the production of 2-stroke engines, because it is more difficult to get the conformity certificate, the European approval and to enter the market of developed countries. Production of 2 stroke engines is still great but scooters with this engine go mostly to developing countries. Now there are developments of 2 stroke engines which meet all ecological standards, but their mass production is still a long way off. They will not be cheap, either.
What is called a tact in an internal combustion engine
Before we talk about powertrain strokes, we need to understand what this beast is. A tact is an action produced by a piston. For example, when the piston inside the valve of a gasoline or diesel engine goes up, it is called one tact, and when it comes back down, this action is called a second tact.
So, the strokes in a two-stroke engine are dealt with. Therefore, two-stroke engines are more powerful. That is, for one revolution of the crankshaft, the valve piston has time to do two strokes. However, they also have their disadvantages. About this we will talk later.
In addition to two-stroke power units, there are one-stroke engines. The design of a one-stroke is simple as hell. They consist of a single cylinder and were widely used in the past.
All four-stroke engines run on the Otto cycle. This cycle was named after a German engineer who came up with the idea to describe the internal combustion engine workflow by a cycle comprising:
- compression of the working body;
- isochoric heat input to the working body;
- expansion of the working body;
- isochoric cooling.
Now that we know what a cycle is and what a thermodynamic cycle is, let’s look at how a four-stroke engine works.
Reliability of 2 stroke and 4 stroke engines
In a 2 stroke engine, the piston, piston rings and cylinder are actually expendable material due to the peculiarities of design (the cylinder has holes).
Many scooter owners roll out a 2 stroke engine piston in a season, and a cylinder in two. In a 4 stroke engine – 4-5 seasons on one 4 stroke engine piston.
Due to better lubrication (oil is not fed to the responsible parts in a mixture with gasoline, but by sprinkling or pressure feeding), the 4 stroke engine is designed for a longer service life. The more complex valve mechanism of the intake and exhaust gases works more clearly, requires less complex and infrequent maintenance.