Thermodynamics

Initially, both the intake and the exhaust valves are closed, and the piston isat its lowest position (BDC). During the compression stroke, the piston movesupward, compressing the air–fuel mixture. Shortly before the piston reachesits highest position (TDC), the spark plug fires and the mixture ignites,increasing the pressure and temperature of the system. The high-pressuregases force the piston down, which in turn forces the crankshaft to rotate,producing a useful work output during the expansion or power stroke. At theend of this stroke, the piston is at its lowest position (the completion of thefirst mechanical cycle), and the cylinder is filled with combustion products.Now the piston moves upward one more time, purging the exhaust gasesthrough the exhaust valve (the exhaust stroke), and down a second time,drawing in fresh air–fuel mixture through the intake valve (the intakestroke). Notice that the pressure in the cylinder is slightly above the atmosphericvalue during the exhaust stroke and slightly below during the intakestroke.In two-stroke engines, all four functions described above are executed injust two strokes: the power stroke and the compression stroke. In theseengines, the crankcase is sealed, and the outward motion of the piston isused to slightly pressurize the air–fuel mixture in the crankcase, as shown inFig. 9–14. Also, the intake and exhaust valves are replaced by openings inthe lower portion of the cylinder wall. During the latter part of the powerstroke, the piston uncovers first the exhaust port, allowing the exhaust gasesto be partially expelled, and then the intake port, allowing the fresh air–fuelmixture to rush in and drive most of the remaining exhaust gases out of thecylinder. This mixture is then compressed as the piston moves upward duringthe compression stroke and is subsequently ignited by a spark plug.The two-stroke engines are generally less efficient than their four-strokecounterparts because of the incomplete expulsion of the exhaust gases andthe partial expulsion of the fresh air–fuel mixture with the exhaust gases.However, they are relatively simple and inexpensive, and they have highpower-to-weight and power-to-volume ratios, which make them suitable forapplications requiring small size and weight such as for motorcycles, chainsaws, and lawn mowers (Fig. 9–15).Advances in several technologies—such as direct fuel injection, stratifiedcharge combustion, and electronic controls—brought about a renewed interestin two-stroke engines that can offer high performance and fuel economywhile satisfying the stringent emission requirements. For a given weight anddisplacement, a well-designed two-stroke engine can provide significantlymore power than its four-stroke counterpart because two-stroke engines producepower on every engine revolution instead of every other one. In the newtwo-stroke engines, the highly atomized fuel spray that is injected into thecombustion chamber toward the end of the compression stroke burns muchmore completely. The fuel is sprayed after the exhaust valve is closed, whichprevents unburned fuel from being ejected into the atmosphere. With stratifiedcombustion, the flame that is initiated by igniting a small amount of therich fuel–air mixture near the spark plug propagates through the combustionchamber filled with a much leaner mixture, and this results in much cleanercombustion. Also, the advances in electronics have made it possible to ensurethe optimum operation under varying engine load and speed conditions.ExhaustportChapter 9 | 495SEE TUTORIAL CH. 9, SEC. 2 ON THE DVD.CrankcaseSparkplugFIGURE 9–14Schematic of a two-strokereciprocating engine.IntakeportFuel–airmixtureFIGURE 9–15Two-stroke engines are commonlyused in motorcycles and lawn mowers.© Vol. 26/PhotoDiscINTERACTIVETUTORIAL