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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Applied Thermal Engineering, Volume 23, Issue 5, April 2003, Pages 511–521
One of the major alternatives of the Otto cycle has been examined to determine its potential for increased efficiency and net work power in the spark ignited internal combustion engine is to shorten the compression process relative to the expansion process by early close or late of intake valve. The modified Otto cycle is called Miller cycle. This paper deals with the analysis of a supercharged Otto engine adopted for Miller cycle operation. The Miller cycle shows no efficiency advantage and suffers a penalty in power output in the normally aspirated version. In the supercharged Otto engine adopted for Miller cycle version, it has no efficiency advantage but does provide increased net work output with reduced propensity to engine knock problem. Sensitivity analysis of cycle efficiency versus early close of intake valve and that of cycle net work versus early close of intake valve are performed. Optimization on the cycle efficiency is obtained.
The spark ignited internal combustion engine is modeled as an Otto cycle. The four stroke ideal Otto cycle models the intake of fuel–air mixture (isobaric process 1–2) as the piston moves from top dead center (TDC) to bottom dead center (BDC) position with the intake valve open. Then the mixture is compressed (isentropic process 2–3) as the piston moves back to TDC with the intake valve closed. At TDC, the spark instantaneously ignites the fuel–air mixture (isochoric process 3–4) providing a heat input. The mixture is expanded (isentropic process 4–5) as the piston moves from TDC to BDC. At BDC the exhaust valve opens and pressure drops so that the exhaust process (isochoric process 5–6) followed by positive displacement pumping out of the products of combustion as the piston moves from BDC to TDC (isobaric process 6–1) with the exhaust valve open. At TDC the exhaust valve closes and the intake valve opens so the cycle can repeat as shown in Fig. 1. Fig. 1 is a pressure (p) versus volume (v) diagram on which the areas underneath the processes represent work (w) done or added. The net output work for one cycle is represented by (w45−w23), with w12 and w61 ideally equal and opposite and so cancelled. Notice that the length of the compression stroke (isentropic process 2–3) and that of the expansion stroke (isentropic process 4–5) are equal in the Otto cycle. Full-size image (2 K) Fig. 1. Four-stroke Otto cycle. Figure options The major four functions (process 2–3, process 3–4, process 4–5, and process 5–6) described above are executed in just two strokes: the power stroke and the compression stroke. The traditional thermodynamic analysis of the four-stroke Otto cycle is the same as that of a two-stroke Otto cycle as shown in Fig. 2. The Otto cycle is assumed executing in a closed system. The energy balance for any of the processes is expressed, on a unit mass basis, as equation(1) Full-size image (2 K) Fig. 2. Two-stroke Otto cycle. Figure options Assuming constant specific heats, the efficiency of the Miller cycle can be expressed as equation(2) η=1−r(1−k) where r is the compression ratio and k is the specific heat ratio cp/cv. Vehicles powered by gasoline Otto engines have been designed for many years with fixed camshaft timing. The simplicity, compactness, and rigidity of this design make it cost effective, and engineers have resisted redesigning to improve fuel economy and reduced polluting emissions. Concerns about energy conservation, pre-ignition engine knock, emission of pollutants, and carbon dioxide production have led to modifications in the internal combustion Otto engine. There are several ways proposed by Simmons  to altering the spark-ignited internal combustion Otto cycle. One of the major alternatives of the Otto cycle has been examined to determine its potential for increased efficiency and net work power in the spark ignited internal combustion engine is to shorten the compression process relative to the expansion process by early close of intake valve. The modified Otto cycle is called Miller cycle . The potential for increased efficiency and net work power in the spark ignited internal combustion Miller engine are examined in the following sections.
نتیجه گیری انگلیسی
The Miller cycle has the compression process shortened by either closing the intake valve early or late. The expansion process is unaltered. Computation in this paper illustrates that the Miller cycle has no inherent potential for improving efficiency, but that using it to increase pressure boost in supercharging or turbo-charging can increase work output with reduced danger of pre-ignition and less air pollution, and that the design of the cycle can be achieved by sensitivity analyses. The Miller cycle is a possible frontier of the internal combustion engine industry. The greatest obstacle for the cycle to overcome is the additional costs it incurs from the supercharger. In a consumer-dominated world, the problem must be dealt with in order to make the Miller cycle engine truly competitive.