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===Tỷ số nén===
====Compression ratio (CR)====
The compression ratio (CR) is defined as the ratio of the volume of the cylinder and its head space (including the pre-combustion chamber, if present) when the piston is at the bottom of its stroke to the volume of the head space when the piston is at the top of its travel (‘top dead centre’, tdc). Typically, petrol engines have a CR of 8–10, while diesel engines have a CR of 15–20. The CR of petrol engines is limited by the requirement that the fuel burns uniformly in the cylinder and does not ignite thermally prior to the spark (so-called ‘engine knocking’). In a spark-ignition engine, the CR at which pre-ignition takes place is determined by the octane number of the petrol; see Box 4.2. High-octane fuel permits a high CR. Until about 30 years ago, lead tetraethyl was added to petrol as an anti-knock agent. This was phased out for environmental reasons and non-toxic additives are now sometimes used. Improvements in engine design over recent years have, however, led to satisfactory compression ratios with lower octane fuel.<ref name="Dell Moseley Rand 2014 pp. 109–156">{{cite book | last=Dell | first=Ronald M. | last2=Moseley | first2=Patrick T. | last3=Rand | first3=David A.J. | title=Towards Sustainable Road Transport | chapter=Development of Road Vehicles with Internal-Combustion Engines | publisher=Elsevier | year=2014 | isbn=978-0-12-404616-0 | doi=10.1016/b978-0-12-404616-0.00004-9 | pages=109–156}}</ref>
====16.6.1 Effect of Compression Ratio on Engine Combustion====
The compression ratio, in this case for a realistic engine it is the effective one, has two effects on combustion. The first, and obvious effect, is on the thermodynamic cycle. The pressure and temperature at the end of compression will be affected by the compression ratio, with a higher compression ratio increasing both these parameters. Compression ratio will also have a significant effect on the geometry of the combustion chamber, and a higher compression ratio will often result in a combustion chamber of narrower aspect ratio. This means that the flame will contact the piston earlier (see Fig. 16.11(b)), and this will tend to reduce the rate of heat release. The compression ratio of the base engine was both increased and decreased as shown in Table 16.2.<ref name="Winterbone Turan 2015 pp. 345–379">{{cite book | last=Winterbone | first=Desmond E. | last2=Turan | first2=Ali | title=Advanced Thermodynamics for Engineers | chapter=Reciprocating Internal Combustion Engines | publisher=Elsevier | year=2015 | isbn=978-0-444-63373-6 | doi=10.1016/b978-0-444-63373-6.00016-2 | pages=345–379}}</ref>
====13.2.1 Engine Design Factors====
The compression ratio is a decisive factor for the thermodynamic efficiency of the cycle. 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 NOx formation during combustion; however, preignition of some portions of the cylinder charge may result. Preignition is associated with extremely high temperatures, which further increase the NOx formation. Alternatively, demands for a higher-octane-number fuel may introduce other pollutant components to the fuel and thus to the exhaust gases.
The design of the combustion chamber shape also determines crevice volumes and, therefore, has an important influence on the emission level of HC. For this reason, compact combustion chambers with high volume to surface area ratios are preferable. Combustion chamber and scavenge port-entrance assembly are also important in the determination of the turbulence intensity prior and during combustion. 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.
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. A longer combustion duration is associated with lower maximum pressure and temperature and, thus, lower NOx emission. At the same time it results in higher HC emission due to relatively uncompleted combustion. For this reason, dual spark plugs contribute to a lower HC emission and a higher NOx emission.<ref name="Ikeda Nakajima Sher 1998 pp. 441–476">{{cite book | last=Ikeda | first=Yuji | last2=Nakajima | first2=Tsuyoshi | last3=Sher | first3=Eran | title=Handbook of Air Pollution From Internal Combustion Engines | chapter=Air Pollution from Small Two-Stroke Engines and Technologies to Control It | publisher=Elsevier | year=1998 | isbn=978-0-12-639855-7 | doi=10.1016/b978-012639855-7/50052-1 | pages=441–476}}</ref>
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