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The cylinder clearance is the ratio of fixed clearance to swept volume.
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The cylinder designer must make a trade-off between compression efficiency and VE by optimizing valve flow area and clearance. A given cylinder diameter with more and/or larger compressor valves will compress less gas, but compress that gas with better energy efficiency (lower power per capacity). Larger fixed clearance results in lower VE which is lower capacity. But something else happens with the compressor valves being larger-the fixed clearance becomes larger. Courtesy of Ariel Corporation.įundamentally, a compressor cylinder is made more efficient by using larger compressor valves for a given cylinder bore diameter (everything else being equal). Drawing showing the space available for compressor valves in a typical compressor cylinder design. These (very real) additional losses are discussed subsequently.įig. It should be noted that in this initial ideal valve loss discussion, the assumption is that the gas at the cylinder flange is at a constant pressure, and that pulsation bottle and orifice plate pressure losses are ignored. This energy is represented by the areas 1-4-4A-1 (suction valve loss power (VLP)) and 2-2A-3-2 (discharge VLP) in Fig. Overcoming this pressure drop requires energy. Inefficiency in the P- V diagram is the pressure drop incurred in moving the gas from the inlet flange of the cylinder into the compression chamber and in moving the gas from the compression chamber to the outlet flange. Yes, the gas does get hot as it is compressed but not from heat being transferred to the gas. That is not much time for any significant amount of heat to transfer, therefore lending credibility to the adiabatic assumption. Assuming each of the four events of the P- V cycle take equal time, that is 0.05 s (or 50 ms) per event. How valid is the assumption that the compression and expansion events are adiabatic? For a compressor with a rotating speed of 300 rpm (a slow rotating speed) one P- V cycle takes only 0.2 s to complete. Pressure-volume diagram highlighting suction and discharge valve loss power. In-cylinder fuel injection (high pressure, low pressure, or air-assisted injection) is one of the most important and remarkable solutions to minimizing fuel losses through short circuiting.įig. The port’s timing, scavenge ducts geometry, exhaust tuning, and the crank-case to cylinder volume ratio have crucial effects on the HC and NO x emissions by eliminating short circuiting of fresh charge (and in some cases fuel) through the exhaust port and controlling the amount of residuals retained. A separate lubrication system is by no means an important modification to decrease HC emission level. In this case the oil undergoes a partial burning process and may contribute a significant portion to the HC emission. In traditional crankcase-scavenged engines, the lubricating oil is introduced to the cylinder through the fuel system. For this reason, dual spark plugs contribute to a lower HC emission and a higher NO x emission. At the same time it results in higher HC emission due to relatively uncompleted combustion. A longer combustion duration is associated with lower maximum pressure and temperature and, thus, lower NO x emission. The position of the spark plug in the combustion chamber affects the flame travel distance and, hence, the combustion duration and the formation of various species. High turbulence intensity ensures good fuel-air and residuals mixing and high flame speed, which are significant to minimizing cycle-by-cycle variations and, hence, HC emission.
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Combustion chamber and scavenge port-entrance assembly are also important in the determination of the turbulence intensity prior and during combustion. For this reason, compact combustion chambers with high volume to surface area ratios are preferable. The design of the combustion chamber shape also determines crevice volumes and, therefore, has an important influence on the emission level of HC. Alternatively, demands for a higher-octane-number fuel may introduce other pollutant components to the fuel and thus to the exhaust gases. Preignition is associated with extremely high temperatures, which further increase the NO x formation. However, it affects the exhaust gas composition by two means: A high compression ratio increases the maximum temperature in the combustion chamber prior to combustion and thus enhances the NO x formation during combustion however, preignition of some portions of the cylinder charge may result. The compression ratio is a decisive factor for the thermodynamic efficiency of the cycle. Eran Sher, in Handbook of Air Pollution From Internal Combustion Engines, 1998 13.2.1 Engine Design Factors