Experimental and Numerical Study of Swirling ... - Solid Mechanics

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Experi imental and Numerical N Stud dy of Swirling g Flow in Scaveenging Processs for 2-Stroke Marin ne Diesel Engin nes Fig gure 1.4: Puri ity as a function of Deli ivery Ratio for diffe ferent types of larg ge mar rine Two-stroke Diesel D Eng gines. (Heywood, 1988 8). 1.3.3 The un niflow scaveng ging is shownn in figure 1. .3c. The air inntake ports aare located near the bott tom dead cenntre (BDC). Thhe exhaust poort is located in the cyli inder head. An A exhaust vaalve is used too open and close the exhauust port wh hereas the inta ake ports are controlled byy piston motioon. As discusseed in sectio on 1.1.1, the fresh f air enterss the cylinder near the botttom dead centtre and flow ws upward an nd removes thhe exhaust gasees though the exhaust port at the cyli inder head. Since the scavennging air flowws from bottom to top insidde the cylinder, this type of scavenginng is called as uniflow scaveenging. Most of marine diesel engines s with unifloww scavenging hhave a single exhaust port. 1.4 Uniflow Scavenging S g Chapter 1 Compari ison of Sccavenging g Types aand Ideall Engine Scavengin S ng Processs The en ngines equipp ped with crross and looop scavengingg have simpple constru uction due to less moving pparts i.e. no eexhaust valve. This results in simpler r cylinder cover design, lesss stressed camm shaft and allso prevents thhe problem m of thermal stressing s due too exhaust valvve heating-up ( (Raunek, 20099). From sc cavenging poi int of view, thhe main disadvvantage of theese two types of scaveng ging is that the t scavenginng air cannot completely remove all thhe exhaust t gases from th he cylinder. 6 Introduction
Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke Marine Diesel Engines Chapter 1 Figure 1.4 gives a comparison of scavenging performance of different aforementioned scavenging types based on two parameters (Heywood, 1988): Delivery Ratio Mass of delivered air or mixture per cycle Reference (exhuast gas) mass Mass of air in trapped cylinder charge Purity mass of trapped cylinder charge (Schweitzer P. H., 1949) describes the ideal engine scavenging process as: ‘‘The ideal engine scavenges the cylinder of all residual products of combustion, fills it with uncontaminated fresh air, and in so doing wastes no fresh air through the exhaust.’’ This is also defined as ‘perfect displacement scavenging process’ by Hopkinson (1914). In the perfect displacement scavenging process all fresh air charge entering the cylinder is retained and perfectly displaces the exhaust gases (Blair, 1990). In actually engine scavenging processes there always occurs mixing of fresh air charge with the exhaust gases. However, the level of mixing depends on the type of scavenging. In ideal scavenging, the scavenge air acts like a piston and pushes the exhaust gases out of cylinder without mixing with them (Schweitzer, 1949). From figure 1.4 it can be seen that the uniflow scavenging type is most efficient one. Uniflow flow scavenging is the most efficient scavenging system but require added cost of exhaust valve system (Pulkrabek, 2003). Also it accounts for higher engine thermal efficiency due to better air/ gas exchange (Pevzner, 1998). Today almost all of the modern large marine diesel engines use uniflow scavenging (Raunek, 2009). Earlier measurements conducted on some marine diesel engines, manufactured by MAN Diesel A/S, resulted in 98% of purity at a delivery ratio of 1.5 (MAN Diesel, 2010). This shows that the uniflow scavenging in that engine has better performance than the one given in figure 1.4. However, optimization of the amount of air used during the scavenging process is still a challenging and potential task in order to improve the overall engine system efficiency and also to reduce cost due to scavenging system delivering excess amount of air than is actually needed. 1.5 Swirling Flow during Uniflow Scavenging Process As discussed in section 1.2, in addition to exhaust gas scavenging as the main criteria, the design of uniflow scavenging process also considers to provide a ‘mixing favorable’ flow field to the injected diesel fuel at the end of compression stroke. Introducing swirl in the flow has a wide spread use in many engineering applications for enhancement of mixing process. Swirl in the in-cylinder flow is introduced by making the intake ports at an angle to 7 Introduction
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DTU Mechanical Engineering Section
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Experimental and Numerical Study of Swirling ... - Solid Mechanics

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