Experimental Thermal and Fluid Science 34 (2010) 217–226
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Experimental Thermal and Fluid Science
journal homepage: www.elsevier.com/locate/etfs
Development of a novel passive top–down uniﬂow scavenged two-stroke GDI engine G. Ciccarelli *, Steve Reynolds, Phillip Oliver
Mechanical and Materials Engineering, Queen’s University, Kingston, Ontario, K7P 2M4 Canada
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The design and performance characteristics of a novel top–down uniﬂow scavenged gasoline direct-injection two-stroke engine are presented. The novelty of the engine lies in the cylinder head that contains multiple check valves that control scavenging airﬂow into the cylinder from a supercharged air plenum. When the cylinder pressure drops below the intake plenum pressure during the expansion stroke, air ﬂows into the cylinder through the check valves. During compression the cylinder pressure increases to a level above the intake plenum pressure and the check valves close preventing back-ﬂow into the intake plenum. The engine head design provides asymmetrical intake valve timing without the use of poppet valves and the associated valve-train. In combination with an external Roots-type supercharger that supplies the plenum and exhaust ports at the bottom of the cylinder wall, the novel head provides top–down uniﬂow air scavenging. Motoring tests indicated that the check valves seal and the peak pressure is governed by the compression ratio. The only drawback observed is that valve closing is delayed as the engine speed increases. In order to investigate the valve dynamics, additional tests were performed in an optically-accessible cold ﬂow test rig that enabled the direct measurement of valve opening and closing time under various conditions. Ó 2009 Elsevier Inc. All rights reserved.
Article history: Received 11 September 2008 Received in revised form 26 October 2009 Accepted 27 October 2009
Keywords: Two-stroke engine Uniﬂow scavenging
1. Introduction The advantages of the two-stroke engine are its high power to weight ratio and its simple, low-manufacturing cost design. The higher power density is the result of torque delivered to the crankshaft each crank revolution, as opposed to every other crank revolution as achieved in a four-stroke engine. The challenge for a two-stroke engine lies in the efﬁcient removal of exhaust gases from the cylinder while simultaneously introducing a fresh charge. The conventional two-stroke engine achieves this by using the crankcase as a fresh charge reservoir and the piston as a pump. During the compression stroke the crankcase pressure drops and a fresh premixed air–fuel charge is inducted into the crankcase through a reed valve. This air–fuel mixture is compressed in the crankcase during the expansion stroke. As the piston approaches bottom dead center (BDC) the exhaust ports located in the cylinder wall are uncovered, starting the blowdown process. Shortly thereafter the transfer port, also located in the cylinder wall, connecting the cylinder and the crankcase volumes is uncovered. The air–fuel mixture is forced into the cylinder displacing the remaining combustion products out the exhaust port. This purging of combustion
* Corresponding author. Tel.: +1 613 533 2586; fax: +1 613 533 6489. E-mail address: firstname.lastname@example.org (G. Ciccarelli). 0894-1777/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermﬂusci.2009.10.029
products is referred to as scavenging and represents the key process in a two-stroke engine. Insufﬁcient scavenging results in excessive exhaust gas residual concentration that leads to slow, inefﬁcient burning and misﬁres, the effects and remedies of which have been widely studied . Conventional two-stroke designs also suffer from lower fuel efﬁciency due to signiﬁcant amounts of air–fuel mixture being lost out the exhaust pipe. This short-circuiting effect also...
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