ASSIGNMENT NO. 1 AENG 221 - 5 ● Two-Stroke Cycle The compression and power strokes of the four-stroke cycle are carried out without the inlet and exhaust strokes in the original two-stroke cycle (as developed in 1878), requiring only one revolution of the crankshaft to complete the cycle. A rotary blower (see figure) forces the fresh fuel mixture into the cylinder through circumferential ports in a two-stroke-cycle engine of the uniflow type. The exhaust gasses are routed through poppet valves in the cylinder head, which are controlled by a cam-follower mechanism. The valves are programmed to open near the end of the power stroke, after the cylinder pressure has dropped significantly. After the exhaust opening has reduced the cylinder pressure to the inlet pressure produced by the blower, the inlet ports in the cylinder wall begin to uncover. After the inlet ports have been covered by the rising piston on the compression stroke, the exhaust valves are allowed to remain open for a few degrees of crank rotation, allowing the flow to scavenge the cylinder more thoroughly. ● Four-Stroke Cycle - four-stroke internal-combustion engine Intake, compression, combustion (power), and exhaust are the four strokes of an internal-combustion engine. The crankshaft is turned as the piston moves during each stroke. The four-stroke cycle, a concept first developed in the late nineteenth century, has been the most important technique for recovering power from the combustion process thus far. The figure depicts the four-stroke cycle. When the inlet valve is open, the piston descends first on the intake stroke. The partial vacuum created draws an ignitable mixture of gasoline vapor and air into the cylinder. With both valves closed, the mixture is compressed as the piston ascends on the compression stroke. An electric spark ignites the charge as the end of the stroke approaches.The power stroke follows, with both valves still closed and gas pressure pressing on the piston head or crown due to the expansion of the burned gas. The ascending piston forces the spent products of combustion through the open exhaust valve during the exhaust stroke. The cycle then begins again. Each cycle thus necessitates four piston strokes (intake, compression, power, and exhaust) and two crankshaft revolutions. The compression and power strokes are the same as in a four-stroke engine. ● Diesel Cycle The diesel cycle, which describes the operation of a typical compression ignition piston engine, is one of the most common thermodynamic cycles found in automobile engines. The operation of a diesel engine is similar to that of a gasoline engine. The Diesel cycle, unlike the Otto cycle, does not use isochoric heat addition. The system executing the cycle in an ideal Diesel cycle goes through four processes: two isentropic (reversible adiabatic) processes alternated with one isochoric and one isobaric process. The operation of a gasoline and diesel engine is similar, but there are significant differences. There is no fuel in the cylinder during the compression stroke, making autoignition impossible in diesel engines. Compression ignition, rather than spark plugs, is used to start a diesel engine. The high temperature produced by adiabatic compression causes the fuel to spontaneously ignite as it is injected, which is why spark plugs are no longer required. During the power stroke, injectors direct fuel into the combustion chamber. This power stroke is performed under constant pressure. In addition, diesel engines can achieve higher compression ratios than otto engines. Diesel Cycle – Processes ➢ ➢ ➢ ➢ Isentropic compression (compression stroke) Isobaric expansion (ignition phase) Isentropic expansion (power stroke) Isochoric decompression (exhaust stroke) https://www.britannica.com/technology/gasoline-engine/Twawod-stroke-cycle#ref67286 https://www.britannica.com/technology/four-stroke-cycle https://www.nuclear-power.com/nuclear-engineering/thermodynamics/thermodynamic-cycles/die sel-cycle-diesel-engine/
