inventions

Article 2- 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 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 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 Based on presumably long-forgotten and outdated patents, and the central topic of this contribution, an additional jockey rides on the inlet valve’s disk (facing away from the ) and reshapes the in-cylinder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 ’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 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 [4,5]. Nowadays, engines with the highest 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 for inlet and outlet in the 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 . 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 scavengingϕ 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 ϕvalve. 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 ϕ 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 scavengingϕ 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-relevantϕ 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 ϕϕ 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 , (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 ϕ 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 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 loss efficiency. of fresh Furthermore, air into the exhaust the exhaust demands gas volume an over-proportionally increases (necessitating increased a largerfilling volume at full load (more than the displacement). This necessitates more effort from a charger to the cost of overall efficiency. Furthermore, the exhaust gas volume increases (necessitating a larger cross-section for the ), and the exhaust’s oxygen content is increased (confusing, e.g., a lambda probe [5]). It is the goal of this contribution to (i) largely conserve the established design of 4-stroke cylinder heads, the piston, the piston drive, and the shape of the combustion chamber and (ii) to Inventions 2019, 4, 44 3 of 13 cross-section for the exhaust system), and the exhaust’s oxygen content is increased (confusing, e.g., a lambda probe [5]). It is the goal of this contribution to (i) largely conserve the established design of 4-stroke cylinder heads, the piston, the piston drive, and the shape of the combustion chamber and (ii) to nevertheless provide an efficient gas exchange for 2-stroke operation. From the viewpoint of engineering and production, every change for an established design is undesirable and prolongs or hampers implementation. With direct [15,16] (standard for diesel and widespread nowadays for gasoline engines)Inventions the 2019 concerns, 4, x regarding fuel losses appear manageable. The general promise3 of 13 is more power at relatively less internal friction for a typical heavy duty engine at low- to medium-rotational speed, wherenevertheless the limited provideϕ for scavengingan efficient gas (Figure exchange1)is for less 2-stroke critical operation. due to From more the available viewpoint time, of i.e., engineering and production, every change for an established design is undesirable and prolongs or less pumpinghampers work is implementation. demanded from With adirect slower fuel scavenginginjection [15,16] flow. (standard Camshaft for diesel phasing and widespread for dynamic adjustments ofnowadays EVO, IVO, for gasoline EVC andengines) IVC the (Figure concerns1) regard coulding be fuel particularly losses appear helpfulmanageable. and The is, general nowadays, widespread inpromise engines is formore vehicles. power at relatively less internal friction for a typical heavy duty engine at low- to medium-rotational speed, where the limited ϕ for scavenging (Figure 1) is less critical due to more 2. A Presumablyavailable Forgotten time, i.e., Solution less pumping work is demanded from a slower scavenging flow. Camshaft phasing for dynamic adjustments of EVO, IVO, EVC and IVC (Figure 1) could be particularly helpful A potentiallyand is, attractive nowadays, concept widespread for in a engines qualitatively for vehicles. advantageous 2-stroke scavenging in an almost normal 4-stroke design is presented via a modified intake valve (Figure2) which, by itself, induces 2. A Presumably Forgotten Solution a directed flow instead of changing the complete cylinder head and intake geometry [8–11] or the A potentially attractive concept for a qualitatively advantageous 2-stroke scavenging in an combustion chamber [12,13]. It must be noted that a modified inlet valve for 2-stroke operation is almost normal 4-stroke design is presented via a modified intake valve (Figure 2) which, by itself, not a new ideainduces but was a directed already flow treated instead byof changing many and the complete mostly outdatedcylinder head patent and intake applications, geometry [8–11] including a very relatedor invention the combustion from chamber Jeong et[12,13]. al. [17 It] must in 1996. be noted Nevertheless, that a modified even inlet much valve older for 2-stroke inventions point to virtuallyoperation the sameis not approach.a new idea but These was ideasalready were, treated however, by many hardlyand mostly realized. outdated Several patent other applications, including a very related invention from Jeong et al. [17] in 1996. Nevertheless, even requisites, desirable for clean and efficient 2-stroke operation, were not matured or available at that much older inventions point to virtually the same approach. These ideas were, however, hardly time: Direct injection,realized. Several camshaft other phasing,requisites, edesirablefficient for chargers, clean and and efficient electronic 2-stroke engine operation, control. were not Further concerns werematured a premature or available wear ofat thethat valvetime: ´sDirect guide injection, bush due camshaft to the phasing, asymmetrical efficient shapechargers, of theand valve, which induceselectronic oscillating engine bending control. torqueFurther concerns to the valve’s were a premature stem. wear of the valve´s guide bush due to the asymmetrical shape of the valve, which induces oscillating bending torque to the valve’s stem.

(a) (b) (c)

(d) (e)

Figure 2. (a,b)Figure schematic 2. (a,b of) schematic the almost of the original almost inlet original valve inlet with valve an with additional an additional jockey. jockey. The The complex complexshape shape of the jockey is vertically guided with, e.g., a radially pressed-on sleeve (black pieces). The of the jockey isjockey vertically is held guided in a fixed with, angular e.g., position a radially (guiding pressed-on structure sleevein the inlet (black channel pieces). not shown) The jockeyand thus is held in a fixed angularwill positionnot follow (guidingthe valve´s structurerotation. The in jockey the inletredire channelcts the fresh not gas shown) and a certain and angle thus of will the notvalve follow the valve´s rotation.disk is Thealmost jockey completely redirects obstructed. the fresh (c) Experimental gas and a certaininlet valve angle for offlow the visualizations valve disk with is almost attached 120° jockey structure. (d) Arrangement in a nearly conventional 4-stroke engine design at completely obstructed. (c) Experimental inlet valve for flow visualizations with attached 120◦ jockey TDC. (e) Schematic of scavenging near BDC; the flow-steering jockey is marked in red and the structure. (d) Arrangement in a nearly conventional 4-stroke engine design at TDC. (e) Schematic of angular guiding structure in dark blue. scavenging near BDC; the flow-steering jockey is marked in red and the angular guiding structure in dark blue. Inventions 2019, 4, 44 4 of 13

A particular concern is too large of an obstruction of the inlet flow, which then results in excessive pumping losses for scavenging: A central topic of this work. The almost unchanged intake valve is equipped with an additional part (i.e., the “jockey,” Figure2) which follows the valve’s vertical movements. The jockey is not in contact with the combustion, i.e., no distinct heat problem is expected in the cool inlet flow. Furthermore, no delicate sealing requirements apply to the jockey. The intake valve may rotate when the jockey is kept in a fixed angular position, e.g., with a guiding tongue (or pin) inserted in the inlet channel (Figure2d,e). Small graphite pins in the jockey’s guiding sleeve could provide sufficient lubrication against wear due to the slowly rotating valve. In addition, a graphite inlay in the inlet´s angular guiding structure could prevent wear. Uneven mass-distribution of the jockey with respect to the valve´s axis—inducing critical bending torque to the stem—could be countered or compensated by (a) a thickened valve stem (presumably undesirable), (b) a preferably light jockey, or (c) streamlined counterweights as shown in Figure2a,b. As already claimed in the old patents [17], the shape of the jockey obstructs the annular gap of the open valve toward the nearby and open outlet, and it entirely re-directs the inlet flow toward the cylinder wall away from the opposing outlet valve. The effectively blocked circumference is herein denoted as the obstruction angle α. The experimental jockey structure in Figure2c results to α 120 . ≈ ◦ As is schematically shown in Figure2e and closely related to the vertical intake port design [ 11], fresh gas then descends down to the piston (in this phase near BDC) and U-turns toward the outlet region (loop scavenging), thereby more effectively replacing the cylinder’s volume before entering the outlet. The directional nozzle, formed by the valve disk and jockey and effective in the last possible placement of the flow, can enforce a comparably intense tumbling, as shown below. An angular momentum or tumbling of the filling gas is achieved, which is appreciated in many engine concepts and might be further exploited (but this is not the scope of this contribution). Furthermore, the cold and distinct stream covers the piston surface and provides some extra cooling for this highly-stressed part. It should be noted that a fixed guiding structure within the intake [8–11] (instead of a jockey) cannot perform in the same way; the annular gap of the radially re-directing valve disk would still be fully opened toward the very close outlet and premature losses would still occur [10,11], as is also simulated below. Naturally, the reduced ϕ for gas exchange together with the intake’s further reduced cross-section—both with respect to a conventional 4-stroke—would inhibit high engine speeds or would demand higher charging pressures at high speed. As we will see, however, moderate pressures (say, 30 kPa or 0.3 bars) appear sufficient for medium speed. The small-capacity test engine runs at low rpm even with zero charging, filled solely by the momentum of the exhaust gas column.

3. Principal Calculations for a Symmetrical Scavenging Around BDC For an analysis, we begin with the idealized assumption that perfect gas exchange occurs within the scavenging period ϕ (here centered at BDC as indicated in Figure1), and no work is spent on internal friction or fresh gas pumping. The relative power output with respect to a 4-stroke engine with the same displacement (also with no internal friction) is then:

P  ϕ 2-stroke = 1 + cos (1) P4-stroke 2 and would be 2 for ϕ = 0◦ (an infinitesimally quick scavenging at BDC) and 1 for ϕ = 180◦ (equivalent to the halved stroke at a doubled combustion frequency). Thus, while ϕ = 0◦ is obviously not feasible, increasing ϕ too much will finally annihilate any potential advantage. When introducing an engine’s mechanical losses (internal friction, valve train, pumping work, and other auxiliary devices) a 4-stroke’s output can be assumed to be decreased to 80%. This equals a lump sum of 20% for internal friction [18,19] or 0.2 when normalizing the 4-stroke’s internal power output to 1. Inventions 2019, 4, x 5 of 13 Inventions 2019, 4, 44 5 of 13 Two systemic deviations induce higher losses in the proposed 2-stroke design. One additional loss isTwo caused systemic by the deviations valve train, induce which higher now cycles losses at in the crankshaft proposed 2-stroke´s speed.design. In a 4-stroke, One additional the valve losstrain’s is causedenergy by cost the W valveValve train,is typically which smaller now cycles than at 0.03 the crankshaft[18,19], and´s speed. we double In a 4-stroke, this number the valve for 2-stroke operation to 0.06, i.e., the herein assumed loss from the valve train approaches 50% of the train’s energy cost WValve is typically smaller than 0.03 [18,19], and we double this number for 2-stroke operationlosses from to together 0.06, i.e., the the piston herein assembly assumed lossand fromcrank the train valve (these train are approaches much more 50% massive of the losses parts fromwith togetherlarger surfaces the piston in frictional assembly contact). and train (these are much more massive parts with larger surfaces in frictionalAnother contact). mechanical loss of utmost importance here is caused by the required pumping work WP for scavenging, which is essentially determined by the flow’s time-cross-section (Figure 1b). Another mechanical loss of utmost importance here is caused by the required pumping work WP forUnlike scavenging, a 4-stroke, which the ispumping essentially work determined must be byprov theided flow’s by time-cross-sectionan additional device, (Figure e.g.,1 b).a blower Unlike or a 4-stroke,. the pumping At a given work rotati mustonal be speed provided of the by engine, an additional the WP losses device, strongly e.g., a blower increase or at supercharger. a smaller ϕ and a further reduced cross-section, as is herein applied for the intake valves: At a given rotational speed of the engine, the WP losses strongly increase at a smaller ϕ and a further reduced cross-section, as is herein applied for the180° intake valves:𝜑 𝑊 =𝑊 1 + cos (2) 𝜑 2 180 2 ϕ2 W = W ◦ 1 + cos (2) The flow’s required speed is reciprocalP 0 toϕ ϕ [in Equation2 (2), the first term in parentheses] and proportional to the remaining displacement, which decreases with ϕ (Figure 1a) (the second term in The flow’s required speed is reciprocal to ϕ [in Equation (2), the first term in parentheses] and parentheses). The kinetic energy of the flow, however, increases with the square of the speed. The proportional to the remaining displacement, which decreases with ϕ (Figure1a) (the second term referential W0 must be determined from flow simulations and/or flow measurements of the in parentheses). The kinetic energy of the flow, however, increases with the square of the speed. representative arrangements. W0 is herein defined as the required work for providing half of the The referential W must be determined from flow simulations and/or flow measurements of the displacement [ϕ =0 180°, Equation (1)] within the period of ϕ = 180° at a given rotational speed. representative arrangements. W0 is herein defined as the required work for providing half of the Values for W0 are derived from the subsequent simulation and experiment. displacement [ϕ = 180 , Equation (1)] within the period of ϕ = 180 at a given rotational speed. Values Furthermore, and◦ unlike in a 4-stroke, the gas replacement◦ (G) in a 2-stroke will never be at for W are derived from the subsequent simulation and experiment. 100%,0 i.e., all exhaust gas being perfectly replaced by fresh gas. Instead, a scavenging efficiency G of Furthermore, and unlike in a 4-stroke, the gas replacement (G) in a 2-stroke will never be at 100%, 80% and 90% is calculated herein [10,11], which will further and directly reduce the engine’s i.e., all exhaust gas being perfectly replaced by fresh gas. Instead, a scavenging efficiency G of 80% and available power. 90% is calculated herein [10,11], which will further and directly reduce the engine’s available power. With the 4-stroke’s normalized power = 1 (without friction or any mechanical losses), Equation With the 4-stroke’s normalized power = 1 (without friction or any mechanical losses), Equation (1) for the equivalent 2-stroke engine at the same speed and with mechanical losses and the limited (1) for the equivalent 2-stroke engine at the same speed and with mechanical losses and the limited scavenging efficiency 𝐺 now writes as: scavenging efficiency G now writes as: 𝜑 180° 𝜑 𝑃 = 𝐺 1 + ϕcos  0.16 𝑊 𝑊180 2 1 + cosϕ 2 (3) P = G 1 + cos 2 0.16 W W ◦𝜑 1 + cos 2 (3) 2-stroke 2 − − Valve − 0 ϕ 2 The 4-stroke’s lump sum of losses (20% or 0.2) was replaced by 0.16, since the 4-stroke’s valve The 4-stroke’s lump sum of losses (20% or 0.2) was replaced by 0.16, since the 4-stroke’s valve train train losses (0.03) and pumping work (0.01) is separately accounted as WValve = 0.06 and 𝑊 losses[Equation (0.03) (2)]. and The pumping subsequent work Figure (0.01) is 3 separatelyshows the accounted image of asEquationWValve =(3)0.06 as anda functionWP [Equation of ϕ and (2)]. a ϕ G W Thedifferent subsequent G and W Figure0. 3 shows the image of Equation (3) as a function of and a different and 0.

(a) (b)

Figure 3.3. Calculated power or torque [Equation (3)] of the proposed 2-stroke design with poppetpoppet valves, withwith respect respect to a 4-stroketo a 4-stroke with the with same the displacement. same displacement. Besides the mechanicallyBesides the pre-adjustedmechanicallyϕ, thepre-adjusted referential ϕ pumping, the referential work W pumping0 and the work scavenging W0 and effi theciency scavengingG largely efficiency determine G alargely potential determine gain or loss.a potential (a) G = gain0.9. or (b )loss.G = (0.8.a) G = 0.9. (b) G = 0.8. Inventions 2019, 4, 44 6 of 13

Inventions 2019, 4, x 6 of 13 The potential gain or loss with respect to an equivalent 4-stroke with power output = 0.8 strongly depends on theThe gas replacementpotential gain efficiency or loss withG and respect the referential to an equivalent pumping 4-stroke work W 0with. Moreover, power aoutput sufficiently = 0.8 small W0 wouldstrongly allow depends for more on the (= excessive)gas replacement fresh gas efficiency and then G aand further the increasedreferential Gpumping. work W0. SinceMoreover, the required a sufficiently pumping worksmall basicallyW0 would increases allow for with more the (=excessive) square of the fresh engine’s gas and rotational then a further speed, the W0 inincreased Figure3 can G. also be read as proportional to the square of the engine’s speed. As proposed, Since the required pumping work basically increases with the square of the engine’s rotational an advantage over the 4-stroke occurs for preferably low- to medium-speeds, equivalent to small W0. speed, the W0 in Figure 3 can also be read as proportional to the square of the engine’s speed. As For a more exact quantification, a representative number for W0 is still missing at this point. proposed, an advantage over the 4-stroke occurs for preferably low- to medium-speeds, equivalent While the absolute mechanical losses per single crankshaft revolution are inherently higher than in to small W0. For a more exact quantification, a representative number for W0 is still missing at this a 4-stroke (even for W = 0 the W is higher for 2-stroke), the relative friction F or relative mechanical point. 0 Valve loss is the hereinWhile relevant the absolute quantity mechanical for efficiency losses considerations: per single crankshaft revolution are inherently higher than in a 4-stroke (even for W0 = 0 the WValve is higher for 2-stroke), the relative friction F or relative 180 2 ϕ2 mechanical loss is the herein0.16 relevant+ W quantity+ W for efficiency◦ 1 + cos considerations: Valve 0 ϕ 2 F =   180° 𝜑 (4) 0.16 + 𝑊 +𝑊ϕ 1 + cos G 1 + cos 𝜑 2 𝐹= (4) 2 𝜑 𝐺 1 + cos 2 F should become smaller than 0.2 (the referential 4-stroke) to achieve a systemic advantage, simply more powerF should (Figure become3) is smaller not su ffithancient 0.2 in (the terms referential of fuel economy.4-stroke) to Again, achieve the a referentialsystemic advantage, pumping simply more power (Figure 3) is not sufficient in terms of fuel economy. Again, the referential work W0 is of paramount importance. Figure4 images Equation (4) and suggests an advantage at pumping work W0 is of paramount importance. Figure 4 images Equation (4) and suggests an sufficiently low speeds: Ultimately, the referential pumping work W0 becomes small enough at lower advantage at sufficiently low speeds: Ultimately, the referential pumping work W0 becomes small speeds, even at small time-cross-sections ϕ. enough at lower speeds, even at small time-cross-sections ϕ.

(a) (b)

Figure 4. RelativeFigure 4.mechanical Relative mechanical losses F atlosses (a) GF =at 0.9(a) G and = 0.9 (b )andG = (b0.8.) G The= 0.8. referential The referential pumping pumping work workW0 and the scavengingW0 and the e ffiscavengingciency G largelyefficiency determine G largely adetermine potential a advantage potential advantage or disadvantage or disadvantage with respect with to a 4-stroke.respect to a 4-stroke.

4. Flow Simulations4. Flow Simulations In-cylinderIn-cylinder flow determines flow determines the qualitative the qualitative scavenging scavenging effi ciencyefficiencyG Gand and was was studiedstudied withwith simulations (Figure 5) for a schematic gasoline cylinder head (pent roof combustion chamber) with simulations (Figure5) for a schematic gasoline cylinder head (pent roof combustion chamber) with only two valves. Arrangements with four valves are shown in the experiments below. It must be only two valves. Arrangements with four valves are shown in the experiments below. It must be noted that such 3D flow simulations are generally not very reliable for high speed and very noted thatturbulent such 3D phenomena. flow simulations The simulations are generally in Figure not very 5 (ANSYS reliable academic for high license speed simulation and very turbulentsoftware, phenomena.ANSYS, The simulationsInc., Canonsburg, in Figure USA)5 (ANSYS show the academic quickly licensepropagating simulation fresh software,gas and corresponding ANSYS, Inc., Canonsburg,millisecond USA) show time-scale the quickly for a schematic propagating 500 freshcm³ cylinder, gas and correspondingwhich is prefilled millisecond with air at time-scale 300 K. A for a schematictwo-phase 500 cmsimulation3 cylinder, with which hot is and prefilled already with moving air at 300 exhaust K. A two-phasegas is more simulation demanding, with but hot and already(nevertheless) moving exhaust cannot gaspromise is more much demanding, more reliable but results. (nevertheless) cannot promise much more reliable results.A rapid inlet flow is enforced at 100 m/s. Such speed is required to provide the full A rapiddisplacement inlet flow (500 is enforced cm³) within at 100 3 milliseconds m/s. Such speedfor the is simulated required intake’s to provide cross-section. the full displacement Therefore, in principle, at 5000 rpm (83 Hz, equivalent to 12 milliseconds for a full revolution), the complete (500 cm3) within 3 milliseconds for the simulated intake’s cross-section. Therefore, in principle, at 5000 displacement could be exchanged within 3 ms, equivalent to 90° of crankshaft angle around BDC rpm (83 Hz, equivalent to 12 milliseconds for a full revolution), the complete displacement could be (Figure 1). exchanged within 3 ms, equivalent to 90◦ of crankshaft angle around BDC (Figure1). Inventions 2019, 4, 44 7 of 13

TypicalInventions for 2-stroke2019, 4, x operation, and herein simulated, both valves are open and7 theof 13 piston is situated near theTypical BDC for (Figure 2-stroke2 e).operation, and herein simulated, both valves are open and the piston is For a conventionalsituated near the and BDC unchanged (Figure 2e). 4-stroke cylinder head without jockey (Figure5a), the inlet flow prefers the shortestFor a routeconventional to the and open unchanged outlet and 4-stroke hardly cylinder approaches head without the cylinderjockey (Figure volume, 5a), the i.e., inlet scavenging is poor. A significantflow prefers the portion shortest of route exhaust to the gas open would outlet and remain hardly in approaches the cylinder the cylinder and G isvolume, unsatisfying. i.e., The scavenging is poor. A significant portion of exhaust gas would remain in the cylinder and G is calculatedunsatisfying. pressure drop The calculated of the rapid pressure flow, drop from of the inlet rapid channel flow, from to inle exhaust,t channel is to 40 exhaust, kPa. is 40 kPa.

(a)

(b)

(c)

(d)

(e) (f) (g)

Figure 5. Flow simulations for a schematic 2-valve 500 cm3 cylinder with propagating fresh air, forced to 100 m/s through the intake; images are color coded for velocity (dark blue: 50 m/s, green: 175 m/s, yellow: 250 m/s). (a) Unchanged inlet provides poor scavenging with premature and high losses.

(b) Obstruction angle α = 90◦: Loop scavenging with less premature loss. (c) More distinct tumbling for α = 120◦.(d) Pregnant tumbling for α = 180◦.(e) Jockey for a schematic diesel cylinder with prolonged and flat head. (f,g) Fixed directors in the inlet do not perform in these simulations. Inventions 2019, 4, 44 8 of 13

G improves with a schematic jockey on the inlet valve (Figure2); a certain circumference of the valve’s ring gap is obstructed toward the outlet valve. Different obstruction angles α are simulated. An α = 90◦ (Figure5b) already enforces a reverted tumble with a prolonged U-turn path and low initial losses of the fresh gas, all the way expelling the exhaust gas. The calculated pressure drop is moderately increased to 55 kPa. An intermediate α = 120◦ (Figure5c) results in more distinctive tumbling with zero premature loss and a 70 kPa pressure drop. In Figure5d, the obstruction α was set to 180◦. A very directed and high-speed tumble occurs in the simulation. Conversely, a stronger obstruction is present and requires higher charging pressures: 90 kPa in the simulation. From simulations, we can see that the jockey also performs for diesel designs (Figure5e, here α = 180◦) with a prolonged bore and flat cylinder head. Note that (Figure5f,g) fixed directors in the inlet [ 9–11] do not perform well, at least in these simulations with various shapes; the unshielded valve disk effectively redirects a large portion directly toward the outlet. For determining the very important W0 from these simulations, the introduced cylinder in Figure5a–d with 500 cm 2 and at 5000 rpm is assumed to deliver 20 kW output in ordinary 4-stroke operation, i.e., 25 kW internal power is present before internal friction and mechanical losses. This equals 300 J output for a 360◦ crankshaft cycle. As defined for W0, at ϕ = 180◦ the required fresh gas volume is halved and the scavenging time is doubled, demanding only 1/4 of the applied 100 m/s in the simulation. The required driving pressure, therefore, reduces to 1/16 and results in 3 3.5 kPa (α = 90◦), 4.4 kPa (α = 120◦), and 5.6 kPa (α = 180◦). The scavenging energy for 250 cm is then 0.9 J (α = 90◦), 1.1 J (α = 120◦), and 1.4 J (α = 180◦). These energies have to be doubled due to the effective time-cross-section (Figure1b) of the valve. For α = 120◦, the W0 at 5000 rpm can be calculated to 2.2 J/300 J = 0.0075, and this is represented by the orange trace (W0 = 0.01) in the Figures3 and4. Thus, although additional power can be expected at 5000 rpm, the fuel efficiency—even with an ideal charger—would be lower than in an ordinary 4-stroke operation. This is changed with a reduced rotational speed: W0 becomes 0.002 (25 % of the previous value) at 2500 rpm and now corresponds to the green traces in Figures3 and4, pointing to a superior efficiency and power (or torque) at ϕ = 80 to 100◦. Although the presentations in Figure5 are instructive, flow simulations are not guaranteed to be the same as reality. Fortunately, the scavenging effects are still quick and effective with more modest charging in the subsequent experiments. This is at least partially supported by four valves and with a relatively higher cross-section in comparison to Figure5, i.e., the practical W0 for a given α is further reduced.

5. Scavenging Flow Experiments The characteristic gas exchange is visualized on a representative time scale (0, 2, 3.5 and 5 ms) and in practical geometries with cold air (Figure6). To the benefit of this cold air approach, it should be noted that (i) the real exhaust gas would have a more distinct momentum toward the outlet, and (ii) its density is significantly lower than the herein present cold air due to its higher temperature. The gas exchange G is generally higher in terms of relative masses than in relative volumes. A conventional and unchanged 4-valve cylinder head (pent roof gasoline 4-stroke with horizontal intake ports) (Figure6a) was completed with a transparent (Perspex) 500 cm 3; cylinder (Figure6b) with a flat top (=the surface of the piston). The four valves are all open with the characteristic stroke of the valves, which is 8 mm. The inlet air flow was driven with a 15 kPa blower and the intake was instantaneously smoked with a small firecracker. Note that the density of the smoke deviates from shot-to-shot. Relevant here is the trace of propagating smoke (the “fresh gas”), which more or less effectively expels the clean air (the “exhaust gas”). As a further difficulty, the optimum angular orientation of a jockey is not clear for four valves, as it was for the two opposing valves in Figure5. Inventions 2019, 4, x 9 of 13

(Figure 6b) with a flat top (=the surface of the piston). The four valves are all open with the characteristic stroke of the valves, which is 8 mm. The inlet air flow was driven with a 15 kPa blower and the intake was instantaneously smoked with a small firecracker. Note that the density of the smoke deviates from shot-to-shot. Relevant here is the trace of propagating smoke (the “fresh gas”), which more or less effectively expels the clean air (the “exhaust gas”). As a further difficulty, the Inventionsoptimum2019 angular, 4, 44 orientation of a jockey is not clear for four valves, as it was for the two opposing9 of 13 valves in Figure 5.

(a) (b)

(c)

(d)

(e)

Jockey First Smoke 2 ms 3.5 ms 5 ms

FigureFigure 6. 6.Flow-visualization Flow-visualization inside inside aa conventionalconventional 500500 cmcm3³ 4-valve4-valve cylindercylinder (4-stroke gasoline) at 0 0 ms, 2 ms,ms, 3.53.5 andand 55 ms.ms. ((aa)) CylinderCylinder head head equipped equipped with with one one outlet outlet valve valve and and one one inlet inlet valve valve with with experimentalexperimental jockey jockey structure. structure. Characteristic Characteristic valve valve stroke stroke is is 8 8 mm. mm. ( b(b)) Transparent Transparent perspex perspex cylinder, cylinder, scalescale in in cm. cm. (c ()c Unchanged) Unchanged and and original original inlet inlet valves valves (left (left pair); pair); poor poor scavenging scavenging withwith highhigh loss.loss. ((d)) α =

=120°120◦ andand ( e) α == 180180°.◦. The redirected flowflow muchmuch betterbetter approachesapproaches cylindercylinder volumevolume withinwithin 5 5 ms. ms. Direct shortcut into the exhaust is entirely suppressed in (e) and very effective in (c). The charging occurred with 15 kPa, which is less than calculated in the simulations in Figure5. Inventions 2019, 4, 44 10 of 13

The propagating smoke was recorded with a high-speed camera (0.2 ms time resolution). The inlet valves were partly equipped with a jockey structure (Figure2c) formed from sealing putty and su fficient for this cold experiment without significant forces. As is clearly visible in the high-speed photographs in Figure6c, the original inlet valves without jockeys provide almost no gas exchange in the cylinder within 5 ms, and G is poor. The smoky “fresh” gas effectively exits through the nearby exhaust and is hardly apparent in the cylinder’s volume, related to Figure5a. Scavenging is improved with jockeys on the inlet valves (α = 120◦ for Figure6d and 180 ◦ for Figure6e). As presumed from the simulations (Figure5c,d), the smoke performs a reverted tumbling (more distinct in Figure6e with α = 180◦) and within 5 ms, approaches a significant portion of the cylinder’s capacity, although generally driven with just 15 kPa (150 mbar). However, the volumetric flow metering for the most obstructed case (Figure6e) indicated less than 500 cm 3: with 15 kPa, just 380 cm3 were replaced in 5 ms. The theoretically required charging power for such a single cylinder in 2-stroke operation at 3000 rpm and with 380 cm3 scavenging can be calculated to 0.5 kW. We use the data from the most obstructed case in Figure6e for determining the referential W0: 3 at ϕ = 180◦ (equivalent to 10 ms at 3000 rpm) the provided 380 cm within 5 ms is more than generous for the remaining 250 cm3 and just 1/3 of the speed is required and then 1/9 of the charging pressure is sufficient, i.e., 1.7 kPa. The required scavenging energy becomes 0.45 J, to be doubled to 0.9 J due to the valves’ reduced time-cross-section (Figure1b). The referential W0 at 3000 rpm is then 0.9 J/300 J = 0.003. This number points to the light green traces in Figures3 and4. Thus, even at 3000 rpm, increased torque and increased efficiency can be expected for ϕ = 80 to 100◦. Remarkably, the derived values for W0 from the simulation and the experiment are very similar: The simulated data (W0 = 0.002) is calculated at 2500 rpm, α = 120◦, and only a single intake valve. The experiment (W0 = 0.003) is calculated at 3000 rpm, a more obstructing α = 180◦, and two intake valves. The circumferential speed of the practical tumble in Figure6e can be estimated to 8 cm /2 ms = 40 m/s. Similar results are obtained when tracing the multiple high-speed photographs in more detail. This is far less than the 175 m/s in Figure5d with the same α; the pressure drop (or applied pressures) in Figure5d is several times higher, and the speedy flow reaches the outlet after a U-turn in distinctively less than 2 ms. In Figure6e, the first smoke arrives at the outlet after 4 ms. Nevertheless, all three of these more practical scenarios appear to correlate to the simulations in Figure5a,c,d even though four valves are present.

6. Experimental Engine The purpose of this section is the demonstration of the 2-stroke engine’s principle feasibility and the jockey’s principal benefit. A well-engineered and mature engine cannot be realized in this stadium and within our limited capabilities. In addition, the applied measurement (self-construction) for torque, and particularly fuel consumption, is not very accurate. A small (196 cm3), single-cylinder, and simple 4-stroke engine was modified in our laboratory (Figure7). Most changes apply to the self-made valve train and camshaft which now has to rotate at double speed (2-stroke). The valve overlap must be much higher than in 4-stroke applications and approaches 80%. The valves’ opening angle ϕ is set to 120◦. The adjacent jockey on the (here) non-rotating inlet valve was formed from a heat resistant (200 ◦C) epoxy sealant. Direct fuel injection would be very desirable for the concept but was not feasible for this small engine and our limited capabilities. Instead, a metered propane flow was continuously injected into the inlet channel. A gradually adjustable and mild charging (max. 30 kPa or 0.3 bar) provided the fresh air. Inventions 2019, 4, 44 11 of 13

Inventions 2019, 4, x 11 of 13

(a) (b)

(c)

FigureFigure 7.7. (a) 1:1 camshaft drivedrive forfor thethe 2-stroke.2-stroke. ((b)) Camshaft withwith highlyhighly overlappingoverlapping lobes.lobes. ((cc)) HeadHead withwith elevatedelevated valvesvalves andand aa jockeyjockey mademade fromfrom heatheat resistantresistant epoxyepoxy attached attached to to inlet inlet valve. valve.

TheThe stablestable idleidle speedspeed ofof thisthis smallsmall 2-stroke2-stroke engineengine isis asas lowlow asas 400400 rpm.rpm. AtAt 10001000 rpm,rpm, aa torquetorque ofof moremore thanthan 1515 NmNm isis alreadyalready available,available, whichwhich isis equivalentequivalent toto 1.651.65 kW.kW. TheThe original,original, andand muchmuch moremore mature,mature, 4-stroke4-stroke designdesign ofof thisthis engineengine deliversdelivers justjust 1313 NmNm atat aa higherhigher speedspeed ((>2000>2000 rpm).rpm). HighHigh speed was not not approached approached with with the the demonstrator; demonstrator; the the valve valve train train appears appears too too delicate. delicate. At At1.65 1.65 kW kW output output (1000 (1000 rpm), rpm), the themetered metered propane propane accounts accounts for about for about 7 kW 7 kWworth worth of combustion of combustion heat heatand an and overall an overall efficiency efficiency of 23% of is 23% the is result, the result, which, which, however, however, appears appears quite quiteoptimistic optimistic for this for small this smallengine, engine, even without even without direct injection. direct injection. The potentially The potentially largest accounting largest accounting inaccuracy inaccuracy is in the ispropane in the propanemetering. metering. AsAs toto bebe expectedexpected (Figures(Figures5 5and and6), 6), the the modified modified engine engine performs performs much much worse worse without without the the jockey—atjockey —at least least two two times times as as much much fresh fresh gas, gas, including including stoichiometric stoichiometric amounts amounts of propane,of propane, have have to beto providedbe provided to achieve to achieve only only 8 Nm 8 ofNm torque of torque at 1000 at rpm.1000 Furthermore,rpm. Furthermore, with the with applied the applied jockey, jockey, the engine the runsengine at variousruns at valvevarious angles valve even angles with zeroeven chargewith ze pressurero charge (blower pressure off), filled(blower by theoff), momentum filled by the of themomentum exhaust gases.of the Aexhaust self-sustaining gases. A runself-sustainin without theg run jockey without was hardlythe jockey possible. was hardly possible.

7.7. Summary and Conclusions InIn theory,theory, thethe experiment,experiment, andand asas alreadyalready claimedclaimed inin oldold patentspatents [[17],17], aa suitablesuitable jockeyjockey onon thethe inletinlet valvevalve considerablyconsiderably reshapesreshapes thethe inletinlet flowflow andand therebythereby allowsallows anan advantageousadvantageous gasgas exchangeexchange inin 2-stroke2-stroke enginesengines built built on on the the basis basis of conventional, of conventional, well-established, well-established, and minimally-modified and minimally-modified 4-stroke cylinder4-stroke heads.cylinder The heads. flow canThe be flow realized can withbe realized even zero with initial even shortcuts zero initial (Figure shortcuts6e) and (Figure would provide6e) and somewould additional provide coolingsome additional for the piston. cooling A fixedfor the streaming piston. guideA fixed inside streaming the inlet guide channel inside is inferior the inlet to thechannel jockey is (Figure inferior5f,g). to the As thejockey required (Figure charging 5f,g). pressures—indicatedAs the required charging by W 0pressures—indicated—are still small enough, by the jockey promises efficient and high-torque 2-stroke engines at low- to medium-rotational speeds, W0—are still small enough, the jockey promises efficient and high-torque 2-stroke engines at low- to pointingmedium-rotational to heavy duty speeds, applications. pointing to heavy duty applications. TheThe relativelyrelatively high high speed speed of of the the valves valves (doubled (doubled frequency) frequency) is, is, to ato large a large portion, portion, compensated compensated for byforthe by addressedthe addressed lower lower rotational rotational speed. speed. The conceptThe concept presumably presumably applies applies for all for kinds all ofkinds of when fuels directlywhen directly injected injected (Figure (Figure5e). 5e). The jockey itself, as the only new part (all other components are well-known and in mass-production), is not endangered by heat or distinct sealing requirements, and a light and balanced solution (Figure 2) or shaped sheet may be sufficient. Inventions 2019, 4, 44 12 of 13

The jockey itself, as the only new part (all other components are well-known and in mass-production), is not endangered by heat or distinct sealing requirements, and a light and balanced solution (Figure2) or shaped sheet may be sufficient. A variable camshaft phasing or valve control, also in mass-production, appears particularly attractive for optimized operation over extended engine loads and speeds. Valves’ timing (Figure1) could be tuned for . As previously suggested by other workers [9,10], the engine could be modified to be switched between 2-stroke (for low rpm, more torque and efficiency) and 4-stroke (for high rpm, full load). In 4-stroke operation, the jockey could be halted in an upper position which would then not distinctively obstruct the valve’s annular inlet gap. Important for long-lasting and heavy duty machines and different to 4-strokes, the piston pin in a 2-stroke design bears uninterrupted and unidirectional pressures and this may cause problems with the lubrication film. This well-known problem is sometimes countered with roller bearings.

Acknowledgments: The author wishes to thank Philipp Noc, Brahim Bouddine, and Nasraoui Hicham who elaborated the experimental engine, flow simulations, and flow visualizations, respectively. These students passed their bachelor and master’s thesis in this research field, initiated and instructed by the author. No third-party funding was applied for this work. There are no competing interests. Conflicts of Interest: The authors declare no conflict of interest.

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