1、-B-Emissions from Two-Stroke Engines-/B-OtherSAEbooksonthis topic: -B-Design and Simulation ofTwo-Stroke Engines-/B- byordon!“Blair #Order No“ R- fax #724) 776-*79* e-mail: publicationssae“org-B-Emissionsfrom Two-Stroke Engines-/B- -B-Marco Nuti-/B- Society ofAutomotive Engineers+ Inc“ Warrendale+ !
2、a“-B-Library of Congress Cataloging-in-!ublication Data-/B- Nuti+ Marco !“ Emissionsfromtwo-stroke engines /MarcoNuti“ p“ cm“ Includes bibliographical referencesandindex“ ISBN*-768*-*2 and the Pa!letic engine with mechanically controlled inlet ports $Figure l.ll).-SP-7-/SP- In addition to the high d
3、isplacement two-stroke engines usually operating with a compression ignition $diesel) cycle note that at present &oth two-stroke and four-stroke solutions are utilized in SI engine technology. In Ta&le 1.1 -SP-8-/SP- an attempt is made to determine the type of engine cycle most fre*uently used in di
4、fferent fields. Hence the characteristic sectors of the application of two-stroke and four-stroke SI engines appear defined.-HEAD-ETwo-Stroke Engines -I-Fig. 1.9 Basic configurations of the stepped piston engine.-SP-6-/SP-/I- -FOOT-10-HEAD-Cpter 1 -I-Fig. 1.10 Motorcycle application of a stepped pis
5、ton engine.-SP-6-/SP-/I- -FOOT-11-HEAD-Em Two-Stroke Engines -I-Fig. 1.11 Pa!letic engine.-/I- -B-TABLE 1.1 APPLICATIONS OF TWO-ST+OKE AND FOU+-ST+OKE ENGINES-/B- -FOOT-12-HEAD-C 1 Mopeds chain saws out&oard engines and most recreational !ehicles are powered almost completely &y two-stroke cycle eng
6、ines. On the other hand in large displacement motorcycles and passenger cars four-stroke engines are primarily used. In the past few years in the field of motorcycles and also now in the emerg- ing field of out&oard engines two-stroke engines ha!e &een progressi!ely restricted to small displacements
7、 e#cept for off-road applications in which lightness tor*ue uniformity and engine +PM utilization range are critical. This trend is the result of more stringent pollution limits &eing enforced as well as concern a&out reducing fuel consumption. In addition to the well- known ad!antages the draw&acks
8、 of the two-stroke engine are: s=& High emissions resulting from un&urned hydrocar&ons s=& High fuel consumption Clearly &oth phenomena are caused &y short-circuiting of the fresh mi#ture during the sca!enging period. Following the well-consolidated hypothesis of “perfect mi#ing“ $i.e. assuming that
9、 the fresh charge and e#haust residual gases mi# together completely and that a certain amount of the mi#ed charge is pushed out according to the difference &etween the total !olume of gas and the capacity of the cylinder) a sca!enging efficiency of 100% would &e pos- si&le only with an infinite *ua
10、ntity of sca!enging mi#ture and a trapping efficiency of 100% with zero sca!enging mi#ture. These conditions are significant. More simply it is impossi&le to a!oid a certain percentage of fresh sca!enging charge &eing pushed out with the e#haust gases. Contem- poraneously a *uantity of e#haust gas r
11、emains in the cylinder reducing the purity of the charge.-B-Chapter 2-/B- -B-The Scavenging Process: Definitions and Symbols-/B- The two-stroke engine has one partic larlyn s al characteristic: the gas e!changeprocess is “atmospheric#“ which means thatthe process occ rs in a vol me directly related
12、to the atmosphere$ Th s# it may appear that# for instance# s percharging is not possible$ %ven if a h ge amo nt of fresh mi!t rehowever# at this time# the scavenging ports begin to open and the fresh gas flows into the cylinder$ The inlet flow can contin e ntil the scavenging press re is higher than
13、 the e!ternal press re rather# a vol me corresponding to s ch a stroke is dislocated o tside$ After closing of the e!ha st ports# the compression phase can commence# and th s the thermodynamic cycle proceeds with aconstant vol me orconstant-H%AD-s -I-*ig$ 2$+ Blowdown process$-/I- press re comb stio
14、n# depending onthe type of engine$ After top dead center# the e!pansion stroke finally begins and the engine is ready for a new cycle$ The process of a real scavenging phase can hardly be described easily in a schematic# b ttwo different simplified hypotheses can aidinnderstanding this tricky gas e!
15、change process$ +$ Perfect Scavenge or Displacement *ollowing this hypothesis# d ring theinletflowoffreshmi!t re &air) into the cylinder from the scavenging ports# no mi!ing can occ r between fresh charge and resid al gas$ In this case# the behavior of the incoming gas is similarto anideal piston which p shes theb rned gas completely thro gh the e!ha st port# leaving no resid al and completely filling the entire cylinder$
