Two and Four-Stroke Engines
Two and Four-Stroke Engines Definition – Internal combustion engines usually operate on the four-stroke (one power stroke every two revolutions) or two-stroke (one power stroke every revolution) mechanical cycle.
- The induction stroke.
The inlet valve is open, and the piston travels down the cylinder, drawing in a charge of water. In the case of a spark ignition engine, the fuel usually is premixed with the air.
- The compression stroke.
Both valves are closed, and the piston travels up the cylinder. In the case of compression ignition engines, the fuel is injected to the end of the compression stroke. As the piston approaches top dead center (tdc), ignition occurs either by means of a spark or by auto-ignition.
- The expansion, power, or working stroke.
Combustion propagates throughout the charge, raising the pressure and temperature, and forcing the piston downward. At the end of the power stroke, as the piston approaches bottom dead center (bdc), the exhaust valve opens, and the irreversible expansion of the exhaust gases is termed “blow-down.”
- The exhaust stroke.
The exhaust valve remains open, and the piston travels up the cylinder and expels most of the remaining gases. At the end of the exhaust stroke, when the exhaust valve closes, some exhaust gas residuals will remain. These will dilute the next charge.
The four-stroke cycle
The four-stroke cycle sometimes is summarized as “suck, squeeze, bang, and blow.” Because the cycle is completed only once every two revolutions, the valve gear (and any in-cylinder fuel injection equipment) must be driven by mechanisms operating at half engine speed. Some of the power from the expansion stroke is stored in a flywheel, to provide the energy for the other three strokes.
The two-stroke cycle
The two-stroke cycle eliminates the separate induction and exhaust strokes, so that between the expansion and compression processes, a scavenging process occurs. The simplest scavenging arrangement is under-piston scavenging. In the case of compression ignition engines, the fuel is injected toward the end of the compression stroke.
- The compression stroke.
The piston travels up the cylinder, compressing the trapped charge. If the fuel is not pre-mixed, the fuel is injected toward the end of the compression stroke; ignition should again occur before top dead center. Simultaneously, the underside of the piston is drawing in a charge through a reed valve.
- The power stroke.
The burning mixture raises the temperature and pressure in the cylinder and forces the piston downward. The downward motion of the piston also compresses the charge in the crankcase. As the piston approaches the end of its stroke, the exhaust port is uncovered, and blow-down occurs. When the piston is even closer to bottom dead center , the transfer port also is uncovered, and the compressed charge in the crankcase expands into the cylinder. Some of the remaining exhaust gases are displaced by the fresh charge. Because of the flow mechanism, this is called loop scavenging. As the piston travels up the cylinder, first the transfer port is closed by the piston, and then the exhaust port is closed.
For a given size of engine operating at a particular speed, a two-stroke engine will be more powerful than a four-stroke engine because the two-stroke engine has twice as many power strokes per unit time. Unfortunately, the efficiency of a two-stroke engine is likely to be lower than that of a four-stroke engine, and there is the difficulty of controlling the gas exchange processes when they are not undertaken with separate strokes of the piston.
The problem with two-stroke engines is ensuring that the induction and exhaust processes occur efficiently, without suffering charge dilution by the exhaust gas residuals. The spark ignition engine is particularly troublesome because at part throttle operation, the crankcase pressure can be less than atmospheric pressure. This leads to poor scavenging of the exhaust gases, and a rich air-fuel mixture becomes necessary for all conditions, with an ensuing low efficiency
These problems can be overcome in two-stroke direct injection by supercharging engines (either with spark ignition or compression ignition), so that the air pressure at the inlet to the crankcase is greater than the exhaust back-pressure. This ensures that when the transfer port is opened, efficient scavenging occurs.
If some air passes straight through the engine, it does not lower the efficiency because no fuel has so far been injected. Two-stroke engines are not widely used in automotive applications, and even with two-wheeled vehicles, emissions legislation is reducing their prevalence