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2-Stroke Scavenging in Conventional and Minimally-Modified 4-Stroke

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2-Stroke Scavenging in Conventional and Minimally-Modified 4-Stroke inventions Article 2-Stroke Scavenging in Conventional and Minimally-Modified 4-Stroke Engines for Heavy Duty Applications at Low to Medium Speeds Dirk Rueter Institute of Measurement and Sensor Technology, University of Applied Sciences Ruhr-West, D-45479 Muelheim an der Ruhr, Germany; [email protected] Received: 14 June 2019; Accepted: 7 August 2019; Published: 9 August 2019 Abstract: The transformation of a standard 4-stroke cylinder head into a torque-improved and gradually more efficient 2-stroke design is discussed. The concept with an effective loop scavenging via an extended inlet valve holds promise for engines at low- to medium-rotational speeds for typical designs of conventional 4-stroke cylinder heads. Calculations, flow simulations, and visualizations of experimental flows in relevant geometries and time scales indicate feasibility, followed by a small engine demonstration. Based on presumably long-forgotten and outdated patents, and the central topic of this contribution, an additional jockey rides on the inlet valve’s disk (facing away from the combustion chamber) and reshapes the in-cylinder flow into a reverted tumble. A quick gas exchange with a well-suppressed shortcut into the open exhaust is approached. For overall mechanical efficiency, the required charge pressure for scavenging is of paramount importance due to the short scavenging time and the intake’s reduced cross-section. Herein, still acceptable charging pressures are reported for scavenging periods equivalent to low or medium rotational speeds, as characteristic for heavy-duty applications. Using widely available components (charger, direct injection, variable camshaft angles) an increased engine efficiency is suggested due to the 2-stroke’s downsizing effect (relatively less internal friction as well as the promise of more torque and a decreased size). Keywords: two-stroke engine; scavenging; in-cylinder flow 1. Introduction It is well known [1–3] that a more “ideal” combustion engine would alternatingly compress and expand (2-stroke) rather than execute an additional idle stroke for gas exchange with no net output (on the contrary, internal friction is realized), i.e., the common 4-stroke. However, a well-known issue for 2-stroke operation is the demand for a quick and well-separated gas replacement (scavenging) around the bottom dead center of the piston [4,5]. Nowadays, engines with the highest fuel efficiency (above 50%) are 2-stroke, for example, large ship diesels with their relatively long stroke utilizing uniflow scavenging with inlet ports in the lower cylinder wall [6]. In addition, very compact 2-stroke engines with poor fuel efficiency and poor exhaust composition (for handheld machinery: Leaf blowers or chainsaws, etc.) are based on inlet and outlet ports in the cylinder walls. For a number of reasons such as wear [7], lubricant film in contact with the ports’ high-speed flow (particle emission, oil consumption), thermal problems, prolonged piston with more mass required, etc., such ports in the cylinder walls [8] are also undesirable for medium-sized, long-lasting, preferably clean and efficient engines. Inventions 2019, 4, 44; doi:10.3390/inventions4030044 www.mdpi.com/journal/inventions Inventions 2019, 4, 44 2 of 13 Therefore, 2-stroke concepts with well-established poppet valves for inlet and outlet in the cylinder head have been investigated. The figure below (Figure1) schematically displays the three operational sectorsInventions of 2019 this, 4, approach x over a full rotation of the crankshaft. 2 of 13 (a) (b) FigureFigure 1.1. (a) 360° 360◦ crankshaft cycle for aa 2-stroke2-stroke engineengine withwith conventionalconventional poppetpoppetvalves. valves. TheThe fullfull rotationrotation can can be be separated separated into into three three sectors: sectors: Compression Compression (including (including ignition), expansion,ignition), expansion, and scavenging. and Asscavenging. a challenge, Asthe a challenge, scavenging the angle scavenging' is rather angle limited ϕ is rather to leave limited a sufficient to leave compression a sufficient strokecompression C and expansionstroke C and stroke expansion E with respect stroke to E the with fully respect available to pistonthe fully stroke. available (b) Normalized piston stroke. valve elevation(b) Normalized within 'valve. The elevation effective cross-section within ϕ. (blue)The effective for scavenging cross-section is smaller (blue) than thefor geometricalscavenging valve is smaller stroke (red)than andthe accountsgeometrical for onlyvalve 50% stroke of an ideal(red) (=andunrealistic) accounts valve for withonly instantaneous 50% of an openideal/ close(=unrealistic) characteristics valve (black). with Theinstantaneous abbreviations open/close are as follows: characteristics TDC = piston’s (black). top The dead abbreviations center, E = areexpansion as follows: stroke TDC, EVO = piston’s= exhaust top valvedead opening,center, E = IVO expansion= intake stroke, valve opening,EVO = exhaustBDC = vabottomlve opening, dead center IVO ,= EVC intake= exhaustvalve opening, valve closing, BDC = IVCbottom= intake dead valvecenter, closing, EVC = and exhaust C = compression valve closing, stroke. IVC The= intake scavenging valve closing, angle ' andis determined C = compression by the camshaftstroke. The profile scavenging and strongly angle determines ϕ is determined the whole by approach.the camshaft profile and strongly determines the whole approach. While compression, ignition, and expansion do not principally deviate from 4-stroke engines, the scavengingWhile compression, process poses ignition, (a) quantitative and expansion and (b) do qualitative not principally challenges. deviate An from increased 4-stroke scavenging engines, anglethe scavenging' over-proportionally process poses reduces (a) thequantitative power-relevant and (b) strokes qualitative C and E, challenges. and the advantage An increased over a 4-strokescavenging can beangle easily ϕ lost,over-proportionally as calculated below. reduces Thus, thethe applicablepower-relevant' must strokes be much C shorter and E, (definitely and the lessadvantage than 180 over◦) than a 4-stroke an intake can stroke be easily of a 4-stroke lost, as engine.calculated On below. the other Thus, hand, the too applicable short of a ϕ' mustdirectly be reducesmuch shorter the eff (definitelyective time-cross-section less than 180°) (Figurethan an1 b)inta forke scavenging stroke of a 4-stroke and demands engine. a stronglyOn the other increased hand, pumpingtoo short workof a (pressure)ϕ directly fromreduces a charger, the effective reducing time-cross-section the engine’s net output(Figure and 1b) e ffiforciency. scavenging and demandsThe scavenging a strongly increased process is pump also delicateing work in (pressure) terms of quality. from a Unlikecharger, the reducing well-separated the engine’s exhaust net andoutput intake and stroke efficiency. in a 4-stroke engine, the provided fresh gas (i) partially mixes with the exhaust gas, (ii) doesThe not scavenging completely process expel is the also residual delicate exhaust in terms gas, of and quality. (iii) partially Unlike entersthe well-separated the exhaust withoutexhaust contributingand intake stroke to the in engine’s a 4-stroke combustion. engine, the These provided effects fresh are stronglygas (i) partially determined mixes by with the in-cylinderthe exhaust flow gas, during(ii) does scavenging. not completely expel the residual exhaust gas, and (iii) partially enters the exhaust without contributingThis contribution to the engine’s addresses combustion. the quantitatively These effects limiting are strongly scavenging determined angle ' and by the qualitativein-cylinder in-cylinderflow during flow. scavenging. AThis conventional contribution 4-stroke addresses cylinder the quantitatively head without limiting modifications scavenging is not angle very ϕ suitable and the for qualitative 2-stroke operation;in-cylinder the flow. loss of fresh gas into the simultaneously open outlet (a shortcut) is very strong, as is well-knownA conventional [5] and shown 4-stroke below. cylinder The issuehead ofwithout poor-quality modifications scavenging is not was very gradually suitable relieved for 2-stroke with modifiedoperation; geometries the loss of for fresh the gas cylinder into the head simultaneously and the piston. open These outlet geometries, (a shortcut) e.g., smalleris very strong, inlet valves as is inwell-known a vertical intake[5] and port shown [8–11 below.] and/ Theor edges issue inof thepoor combustion-quality scavenging chamber was and pistongradually crown relieved [12,13 with] or steppedmodified pistons geometries [14] are, for however,the cylinder not head very and attractive the piston. in terms These of (i)geometries, considerable e.g., re-engineering smaller inlet valves with relativelyin a vertical higher intake engine port designs [8–11] and/or and (ii) edges a potentially in the combustion affected combustion chamber (due and topiston rugged crown and increased[12,13] or innerstepped surfaces pistons of [14] the combustionare, however, chamber). not very attractive in terms of (i) considerable re-engineering with relativelyA significant higher lossengine of freshdesigns air intoand the (ii) exhaust a potentially demands affected an over-proportionally combustion (due increased to rugged filling and volumeincreased at inner full load surfaces (more of thanthe combustion the displacement). chamber). This necessitates more effort from a charger to the costA significant of overall
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