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===Tỷ số nén===
====ĐN====
CR is the ratio of the cylinder volume at BDC to the volume at TDC.<ref name="Kreith 1998 p. 8-PA53">{{cite book | last=Kreith | first=F. | title=The CRC Handbook of Mechanical Engineering, Second Edition | publisher=Taylor & Francis | series=Handbook Series for Mechanical Engineering | year=1998 | isbn=978-1-4398-7606-0 | url=https://books.google.ca/books?id=OpD226SXKisC&pg=RA8-PA53 | page=8-PA53}}</ref>
====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>
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