The duty cycle of an internal combustion engine (ICE) is a series of processes that result in a portion of the force (power) acting on the engine’s crankshaft.
The working cycle consists of:
- filling the cylinder with a fuel mixture;
- its compression;
- ignition of the mixture;
- gas expansion and cylinder cleaning.
Tact in ICE is the movement of the piston in one direction (up or down). In one revolution of the crankshaft, two bars are performed. The one in which the expansion of the burnt gases occurs and useful work is performed is called the piston stroke.
Engines in which the operating cycle takes place in 2 cycles (one revolution of the crankshaft) are called two-stroke. Engines in which the working cycle is completed in 4 cycles (two crankshaft turns) are called four-stroke. Two- and four-stroke engines can be either petrol (carburetor) or diesel. What are the main operational and design features of gasoline two-stroke and four-stroke engines? What is the difference between push-pull and four-stroke? To better understand this, it is necessary to become familiar with the principle of their work.
The principle of operation of a four-stroke gasoline engine.
At the inlet, the piston is lowered from the top dead center (TDC) to the lower (LDP). In this case, with the help of camshaft cams, the intake valve opens, through which the fuel mixture is sucked into the cylinder.
During the return stroke of the piston (from BDC to TDC), the fuel mixture is compressed, accompanied by an increase in its temperature.
Just before the end of the compression between the electrodes of the spark, a spark ignites, igniting the fuel mixture, which, burning, forms flammable gases pushing the piston down. Occurs working course, which is done useful work.
After the transition of the NMT piston, the exhaust valve opens, allowing the upward piston to push the exhaust gases out of the cylinder. There is a release. At the top dead center, the exhaust valve closes, and the cycle repeats.
The principle of operation of a two-stroke gasoline engine.
During compression, the piston moves from the bottom dead center to the top. After the purge window (2) is first shut off, through which the fuel mixture enters the cylinder, and then the discharge (3) window, through which the exhaust gases exit, the compression of the air-gasoline mixture begins. At the same time, a vacuum is created in the crank chamber (1), which sucks the next batch of fuel from the carburetor. When the piston approaches the top dead center, the mixture ignites from the spark of a candle, and the resulting gases push the piston down, rotating the crankshaft and doing useful work.
In the crank chamber during a working stroke, the pressure that compresses the fuel mixture that got there in the previous cycle rises. When reaching the upper surface of the piston (it’s sealing ring) exhaust port, the latter opens, releasing the exhaust gases in the muffler. Upon further movement, the piston opens the purge port, and the fuel mixture under pressure in the crank chamber enters the cylinder, displacing exhaust gas residues (purging) and filling the over-piston space. At the transition of the piston of the lower dead point, the working cycle repeats.
Operational and structural differences between two-stroke and four-stroke gasoline engines.
In a two-stroke engine, the filling and cleaning of the cylinder are performed simultaneously with the compression and expansion strokes. at a time when the piston is near the bottom dead center. For this purpose, there are two openings in the cylinder walls. the inlet or purge and outlet, through which the fuel mixture is injected, and the exhaust gas is released. Gas distribution mechanism with valves in a two-stroke engine is missing, which makes it much easier and easier.
In contrast to the four-stroke engine, in which one working stroke accounts for two revolutions of the crankshaft, in a two-stroke working stroke is performed with each revolution of the crankshaft. It means that a 2-stroke engine should have (theoretically) twice as large a liter of power (the ratio of power to engine displacement) than a 4-stroke engine. In practice, however, the excess is only 1.5-1.8 times. It is due to incomplete use of the piston stroke during expansion, the worst mechanism of cylinder release from exhaust gases, wasting part of the power for purging and other phenomena related to the gas exchange features of 2-stroke engines.
Surpassing the four-stroke engine in the liter and power density, the two-stroke engine is inferior to him in efficiency. The exhaust gases are displaced by an air-fuel mixture flowing into the cylinder from the crank chamber. In this case, part of the fuel mixture enters the exhaust channels, being removed together with the exhaust gases and not producing useful work.
Two-stroke and four-stroke engines have a different principle of engine lubrication. In 2-stroke models, it is carried out by mixing in certain proportions (usually 1: 25-1: 50) engine oil with gasoline. An air-fuel-oil mixture, circulating in the crank and piston chambers, lubricates the bearings of the connecting rod and crankshaft, as well as the mirror cylinder. When the fuel mixture ignites, the oil that exists in the form of the smallest droplets burns along with gasoline. Its combustion products are removed along with the exhaust gases.
There are two ways to mix oil with gasoline. Simple mixing before pouring fuel into the tank and separate supply, in which the fuel-oil mixture is formed in the inlet pipe located between the carburetor and the cylinder.
In the latter case, the engine has an oil tank, the pipeline of which is connected to a plunger pump, supplying oil to the inlet pipe precisely in the amount required depending on the amount of air-gasoline mixture. Pump performance depends on the position of the “gas” supply knob. The more fuel is supplied, the more oil enters, and vice versa. The separate lubrication system of two-stroke engines is more perfect. When the ratio of oil to gasoline at low loads can reach 1: 200, which leads to a decrease in smoke, reduce the formation of carbon and oil consumption. This system is used, for example, on modern scooters with two-stroke engines.
In a four-stroke engine, the oil does not mix with gasoline but is fed separately. To this end, the engines are equipped with a classical lubrication system consisting of an oil pump, filter, valves, and a pipeline. The role of the oil tank can be performed by the engine crankcase (wet crankcase lubrication system) or a separate tank (dry crankcase system).
When lubricating with a “wet” crankcase, the pump 3 sucks the oil out of the pan, pumps it into the output cavity and then feeds it through the channels to the crankshaft bearings, parts of the crank-connecting rod group and the gas distribution mechanism.
When lubricating with a “dry” crankcase, oil is poured into the tank, from where it is pumped to the rubbing surfaces with the help of a pump. That part of the oil that drains into the crankcase is pumped out with an additional pump, which returns it to the tank.
To clean the oil from the wear products of engine parts, there is a filter. If necessary, a cooling radiator is also installed, because during operation the temperature of the oil can rise to high temperatures.
Since the oil burns in two-stroke engines, but not in four-stroke engines, the requirements for its properties vary greatly. The oil used in two-stroke engines should leave a minimum of soot in the form of ash and smoke, while oil for four-stroke engines should provide stability characteristics for as long as possible.
Comparison of the basic parameters of two-stroke and four-stroke engines:
- Liter power. 2-stroke engines are 1.5-1.8 times higher than 4-stroke engines.
- Power density (power to mass ratio of the engine). Also higher in 2-stroke.
- Ensuring fuel supply and cylinder cleaning. 4-stroke engines are equipped with a gas distribution mechanism, which is absent from 2-stroke engines.
- Efficiency. Higher in 4-stroke, fuel consumption of which is approximately 20-30% lower than that of 2-stroke.