Unlike a two-stroke engine, in which the crankshaft, crankshaft bearings, compression rings, piston, piston pin and cylinder are lubricated by adding oil to the fuel; the crankshaft of the four-stroke engine is in the oil bath. Due to this, you do not need to mix gasoline with oil or add oil to a special tank (on models of two-stroke scooters with a separate lubrication system). Enough to fill the clean petrol in the fuel tank and you can go. Thus there is no need to buy special oil for 2-stroke engines. Also on the piston mirror and the walls of the muffler and the exhaust pipe is formed much less soot. Besides, in a 2-stroke engine, the fuel mixture is released into the exhaust pipe, which is explained by its design. The minor drawbacks, which are more than worth the benefits, include work on adjusting the thermal clearance of the valves and the scooter acceleration time from a place that is somewhat longer than that of two-stroke mopeds. The last problem can be eliminated by the optimal setting of the variable speed drive and centrifugal clutch.
So, we proceed to the description of the device and operation of a four-stroke engine.
A drive sprocket is installed on the crankshaft, providing (for example: through a chain) the rotation of the camshaft located in the cylinder head. This shaft determines when one of the two valves (intake and exhaust valves) should be opened or closed, depending on the position of the piston. There are cams on the camshaft that engage the rocker arms of the valves. (the diagram shows the camshaft)
Rocker push on a particular valve, opening it. Between the adjusting bolt of the rocker arm and the valve should be a gap, the so-called thermal gap. When heated, the metal expands, and if the thermal hole is small or not at all, then the valves will not tightly close the intake or exhaust channels, so it is essential to adjust the valve clearance. (read the article “Adjusting the clearance of valves”) Exhaust gases are hotter than the fuel mixture, and the exhaust valve heats up (and therefore expands) more than the intake. It explains the difference in the gaps in the intake and exhaust valves.
The principle of operation of the internal combustion engine is studied in school, but I still describe it.
The first tact (the inlet).
The piston goes down, the intake valve opens, and the fuel mixture flows from the carburetor into the cylinder. When the piston reaches the bottom position, the intake valve closes.
Second tact (compression).
The piston goes up, and the fuel mixture is compressed. When the piston is a few millimetres from the top dead center ( TDC ), the candle ignites the fuel compressed by the piston.
The third tact (working stroke).
After the fuel ignites, it burns, hot gases quickly expand, pushing the piston down (both valves are closed).
Fourth tact (release).
By inertia, the crankshaft continues its rotation (for uniform rotation, weights are installed on the crankshaft. crankshaft cheeks), the piston goes up. At the same time, the exhaust valve opens, and the exhaust gases go out into the exhaust pipe. Upon reaching the piston TDC, exhaust valve closes.
Then all four measures are repeated.
I would like to briefly describe the principle of operation of a two-stroke engine, for comparison. As the name suggests, this motor has only two bars.
First beat The piston goes up, compressing the fuel mixture in the combustion chamber. The mixture ignites (not reaching TDC). When the piston is located at TDC, the intake ports in the cylinder wall are open, thanks to which the fuel mixture enters the crank chamber (due to the pressure difference, it is lower in the chamber)
The second clock stroke. Expanding gases push the piston down. When it is at the bottom, it opens the outlet and inlet (here. the window of the channel connecting the crank chamber and the cylinder) window. Since the gases go towards less resistance, i.e. in the exhaust pipe, their place is occupied by the fuel mixture coming from the crank chamber, where the mixture is under pressure.