MECCA_21-01_web Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 1 10.14311/mecdc.2021.01.01 Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN CRITICAL SHIFTING WINDOW IN SWITCHABLE ROCKER FINGER FOLLOWER PETR KOHOUT Eaton European Innovation Center, Bo!ivojova 2380, 252 63 Roztoky, Email: petrkohout@eaton.com JAN KINDERMANN Eaton European Innovation Center, Bo!ivojova 2380, 252 63 Roztoky, Email: jankindermann@eaton.com ABSTRACT A valvetrain including switchable rocker fi nger follower is capable of discrete switching between two modes (two cam profi les). The exact moment when switching occurs is called crossover point and this paper reviews the factors that cause the shift of the crossover point from its nominal design position. The range where crossover point can shift is called critical shifting window and its size and factors infl uencing it will be adressed. KEY WORDS: CAM, CAM PROFILE, CAM DESIGN, SWITCHABLE ROLLER FINGER FOLLOWER, TOLERANCES, STACK UP, SHIFTING WINDOW, CAE SHRNUTÍ Ventilov" rozvod s p!epínateln"m vahadlem s rolnami je schopen p!epínat mezi dv#ma re$imy (p!epínání mezi dv#ma va%kov"mi profi ly). Okam$ik, kdy dojde k p!epnutí mezi jednotliv"mi va%kami, se naz"vá bod p!echodu. V tomto p!ísp#vku budou uvedeny jednotlivé faktory, které zp&sobují posun bodu p!echodu z jeho jmenovité návrhové pozice. Cel" rozsah kam se m&$e bod p!echodu posunout je ozna%ován jako okno bodu p!echodu a v p!ísp#vku bude probráno jak jednotlivé faktory ovliv'ují jeho velikost. KLÍ!OVÁ SLOVA: VA!KA, PROFIL VA!KY, NÁVRH VA!KY, P"EPÍNATELNÉ VAHADLO S ROLNAMI, TOLERANCE, TOLERAN!NÍ ANAL#ZA, OKNO P"ECHODU, CAE 1. INTRODUCTION Valvetrain mechanism between camshaft and a valve itself allows to transform camshaft rotational movement to the intake and exhaust valve translational movement. The conventional and simplest valvetrain operation allows the fresh air or air -fuel mixture to enter the cylinder during the intake stroke when intake valves are open, participate on combustion and let the combustion products leave the cylinder during exhaust stroke when exhaust valves are open. But as demands on engines increase and fulfilling prescribed emission limits is more and more challenging new technologies and innovation are being used. The valvetrain is no exception and variable valve timing (VVT) and variable valve lift (VVL) are used in vehicles nowadays. Cam phaser is the most common way for VVT implementation. It allows to shift the entire valve lift within the specified range of an engine cycle and it appears in two versions – discrete and continuous timing switching. Switching between different cams is used for the VVL realization. The axial camshaft shifting or switching the cam that controls the valve using advanced finger followers or rocker arms is used by OEMs. Combination of VVT and VVL is commonly called as variable valve actuation (VVA). Different VVA systems used by OEMs are usually called by their marketing name such as VTEC, VANOS, MultiAir, MIVEC etc. Camless valvetrains are the most variable solution but they are used mainly in experimental and research engines so far [1]. More on the topic of VVA can be found in the following publications – [2], [3], [4]. The switchable roller finger follower (SRFF) is one of the ways how to implement discrete variable valve lift. [5] That means it allows to switch between two different valve lifts. The crossover point is the moment when switch is realized, thus the moment when the valve changes cam lobe which prescribes its lift. The principle of SRFF will be explained followed by the thorough description of the critical shifting window, how it is created and influence of the specific factors on the window size. Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 2 2. SWITCHABLE ROLLER FINGER FOLLOWER VALVETRAIN The conventional valvetrain system with a standard roller fi nger follower shown in Figure 1 is often referred to as Type II valvetrain. It consists of a camshaft that acts on a roller fi nger follower through its roller. The roller fi nger follower is in contact with pivot on one side and valve stem on the other side. Improvement of such a system by replacing the roller fi nger follower by its switchable version (Figure 2) enables to switch between two different lifts on one valve. It allows to switch for example between normal mode and Miller cycle on the intake side. The same thing could be applied to the exhaust side where normal exhaust valve lift can be supplemented by small extra lift during the intake stroke, which allows to get some of the exhaust gases entering back to the cylinder and this is often referred to as internal exhaust gas recirculation (iEGR). SRFF can be used for cylinder deactivation or other advanced valve actuation strategies. Inside the SRFF there is a latch pin (Figure 3) and depending on its position the finger follower responds to the inner roller. When the pin is not latched the inner roller of SRFF makes so called lost motion. On the other hand, when the pin is latched the entire SRFF and thus also the valve reacts on the movement of the inner roller. To be able to perform two different lifts with SRFF valvetrain system a camshaft must have 3 cam lobes per SRFF (Figure 4). Two outer cam lobes are identical and act on outer rollers of the SRFF, FIGURE 1: Type II valvetrain OBRÁZEK 1: Ventilov" rozvod typ II FIGURE 2: Switchable rocker fi nger follower (SFRR) OBRÁZEK 2: P!epínatelné vahadlo FIGURE 3: SRFF section OBRÁZEK 3: (ez vahadlem FIGURE 4: SRFF cam lobes OBRÁZEK 4: Va%ky pro p!epínatelné vahadlo Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 3 the inner cam lobe acts on inner roller which is connected to the inner arm and can either perform a lost motion or transmit the cam lift into the valve lift. Function of SRFF valvetrain when the pin is not latched is as follows. On the base circle the outer cam lobes are in contact with outer roller (no lash is present because a hydraulic lash adjuster is often used). The lash between the inner cam lobe and the inner roller is present and is called mechanical lash at cam (MLC). As the camshaft rotates the valve lift is influenced only by outer cam lobes. During the camshaft rotation there is a moment when inner cam lobe gets in contact with inner roller and as MLC gets closed the impact on inner roller appears. The lift is not transferred from the inner cam lobe to the valve as pin is not latched and inner arm makes lost motion. When pin is latched the situation in the beginning is similar. Outer rollers are in contact with cam lobes, MLC is present and there is also lash between latch pin shelf and inner arm mating surface which is called mechanical lash at latching pin (MLL). During the camshaft rotation the MLC is closed first, then the inner arm starts to move and MLL is closed. At this moment the valve lift is no more controlled by the outer lobe profiles and starts to be controlled by the inner lobe profile instead. This moment is considered as the crossover point. Very similar conditions and phenomena as during crossover point happen when MLC is closed so further in the article it will be adressed as a crossover point 1 (CP1) and the actual crossover point when MLL is closed as crossover point 2 (CP2). As the lift of the inner cam lobe decreases back to the base circle, the MLL is opened first and outer cam lobes get in contact with outer rollers and valve is again controlled by the them. Further as the inner cam lobe lift goes back to base circle the MLC is opened. 3. APPROACH GT -Suite is a CAE toolset widely used in industry especially in the automotive as it has many useful features for simulation of the specific parts of the vehicles and engines. It is capable of 1-D flow simulation, kinematics, MBD etc.. Two parts of this complex software package were used in order to examine influence of various factors on width of critical shifting window. The GT -ISE where libraries for valvetrain and multibody dynamics were used and VTDESIGN where cam profiles were designed, and kinematics of the system was examined. In general, when designing cam profile, it is important to control cam velocity and acceleration. Too high velocity during opening and closing ramps results in excessive impacts in the system which result in increased wear or higher failure probability. Acceleration is controlled in order to avoid contact separation in the valvetrain. A separation could happen when inertia forces are higher than force generated by a valve spring. Acceleration has a direct influence on manufacturability as with the high acceleration the concave radius of curvature of the cam decreases. If cam concave radius is smaller than the grinding tool, it will be impossible to grind some areas on the profile. Specific limit values are usually set by internal company guidelines and are often treated as business secret. More about process of developing the cam profile can be found in [6]. The MBD model of the single valve mechanism including SRFF was built in GT -ISE in such a way that position of various components in the valvetrain can be quickly and easily changed which allows to implement manufacturing tolerances and wear of the specific parts in the system. VTDESIGN was used to design cam lobe profiles which are then input in the MBD model. Initially the simulation was performed with all the nominal dimensions and baseline cam profiles thus perfectly fulfilling the moment when CP1 and CP2 were intended to happen based on the specific requirements on the function of the valvetrain and engine. Furthermore, the position of the components was changed in order to simulate influence and sensitivity of moment CP1 and CP2 on manufacturing tolerances and other aspects that will be discussed later in appropriate chapters followed by the interpretation of the results and conclusion. The main motivation for keeping critical shifting window small is because only in this area the crossover point can happen thus only here the velocity difference needs to be controlled. If the CSW is too wide the velocity difference needs to be kept sufficiently small for a long time period and that results in restrictions for cam design of inner and outer cam lobe profile. FIGURE 5: Mechanical lashes in the SRFF OBRÁZEK 5: V&le v p!epínatelném vahadle Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 4 4. BASELINE The MBD simulation using all the nominal dimensions was performed in order to set the nominal position of the CP1 and CP2. The critical shifting window will be created around those values as different factors will be changed in the following chapters. It is also important to set up the initial position of every simulation and derived angular positions of contacts during the CP. Every simulation performed has the same layout as specified in Figure 6. From the point of view, the valve is on the left and pivot on the right, the camshaft rotates counterclockwise and all the cam lobe first points (first point that is higher than cam base circle) lies on the global negative Y axis. As the simulation time goes forward the camshaft rotates and angles !, ", # can be observed as in Figure 7. ! is the angle between negative Y axis and the cam lobe first point and gives us the information about the timing. It tells when the CP happens. " is the angle between first cam lobe point and the contact point between outer cam and roller. It gives us the information about where on outer cam profile does the CP happen. # is very similar to " but it goes from the first point of the cam lobe to the contact of inner cam and roller. All three angles will be used in description of the critical shifting windows. Big deviation in ! signs that the function of the inner profi le lift can cause not desired infl uence on the engine cycle as the prescribed CP can happen too early or too late. Angles " and # gives us the information where is the contact point on the cam when the CP happens. It is important as it gives us the information about the impact that appears in the system during CP. In order to realize CP, the velocity on the inner cam lobe has to be higher than on outer cam lobe so the lashes will get closed. But the velocity difference has to be limited so the strong impacts will not damage and wear the components and cause the system failure. Relative velocity difference is calculated as described in equation (1) and (2). 𝑣𝑣!"##@%&'�=�𝑣𝑣"(()*(𝛾𝛾%&')�−�𝑣𝑣+,-)*(𝛽𝛽%&') 𝑣𝑣!"##@%&.�=�𝑣𝑣"(()*(𝛾𝛾%&.)�−�𝑣𝑣+,-)*(𝛽𝛽%&.) (1) 𝑣𝑣!"##@%&'�=�𝑣𝑣"(()*(𝛾𝛾%&')�−�𝑣𝑣+,-)*(𝛽𝛽%&') 𝑣𝑣!"##@%&.�=�𝑣𝑣"(()*(𝛾𝛾%&.)�−�𝑣𝑣+,-)*(𝛽𝛽%&.) (2) Baseline design angles !, ", # are in the Table 1 and relative velocity difference inTable 2. CP1 happens when camshaft rotates 87,8° from the initial position and rollers are in contact at 91,3° of outer cam lobe and 91,1° of inner camlobe. CP2 design moment is at 101,9° rotation after initial state. Outer cam lobe is in contact with roller at 102,7° of its profile and inner cam lobe at 101,9° of its profile. Critical shifting window will be created around those nominal values. Same cam lobe profiles will be used if not mentioned otherwise. FIGURE 6: Simulation initial state OBRÁZEK 6: Po%áte%ní stav simulací FIGURE 7: Crossover point angles defi nition OBRÁZEK 7: Defi nice úhl& pro bod p!echodu TABLE 1: Nominal crossover points TABULKA 1: Nominální p!echodové body Baseline CP1 Baseline CP2 ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] 87,8 91,3 91,1 101,9 102,7 101,9 TABLE 2: Baseline relative velocity difference during crossover point TABULKA 2: V"chozí relativní rozdíl rychlostí na va%kách v p!echodov"ch bodech Baseline CP1 vel. difference [mm/deg] Baseline CP2 vel. difference [mm/deg] 0,0075 0,0075 Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 5 5. SRFF TOLERANCE FACTORS In an ideal case every product going from the same production line would be identical. But in practice even if material goes through same prescribed set of operations there is always some deviation in dimensions or material properties thus the final products have some level of variation. But that does not necessarily mean that the function is affected. Setting the tolerances for manufacturing processes limits the deviation in final products in a way that desired function is assured. But setting the tolerance limits has its other side as well. The tighter are the deviation limits the more accurate thus more costly steps and processes must be utilized. It is always extremely important to find a compromise between the price and tolerance levels. All the component variations are taken in account in so called stack -up analysis to see if the desired function is assured. The stack -up analysis is not the object of interest in this article thus it will not be described in detail what is the cause of position change. Only the stack- -up analysis results of parts that affects the critical shifting window will be used. Some of the cases that are discussed are artificially created but it helps to distinguish what is the real factor that moves a crossover point. It can be observed for example in first case where x position of outer rollers is changed. In real scenario the resulting change in position of outer roller would be caused by changed position of the outer roller axis and as this axis is in contact with bushing of the inner roller it would naturally change the initial position of inner roller and size of MLC. For sake of clarity and simplicity let’s consider cases where only one specific position is changed and rest stays in its nominal position. 5.1 OUTER ROLLERS POSITION Infl uence of roller position tolerance was examined in 9 cases prescribed as in Figure 8– 1 nominal position and then 8 positions of the outer roller axis on the circle with radius of 0.03 mm. Results are in Table 3 and it can be seen that values for ! (the angle describing the timing) go from 84,5° to 91° for CP1 and from 99° to 105,1° for CP2. If it is considered that 1° of cam angle rotation corresponds to 2° of crank angle (CA) rotation the shift of the CP1 in engine cycle can be shown. CP1 can happen 6,6° CA before or 6,4° CA after the designed moment and anywhere in between. CP2 can happen 5,8° CA before or 6,4° CA after the designed moment and anywhere in between. In the next chapters the result description will not be as detailed as here, but only table with results and critical shifting window expressed by the range of ! will be mentioned. 5.2 INNER ROLLER POSITION The same strategy as in the previous chapter was used and 9 cases were simulated including nominal position and 8 axis offset positions on a circle around the nominal position (Figure 9). FIGURE 8: Examined outer rolers axis positions OBRÁZEK 8: Zkoumané pozice osy vn#j)ích rolen FIGURE 9: Examined inner rolers axis positions OBRÁZEK 9: Zkoumané pozice osy vnit!ních rolen TABLE 3: Results for different outer rollers position TABULKA 3: V"sledky pro r&zné pozice vn#j)ích rolen Outer roller position tolerance CP1 CP2 X tol [mm] Y tol [mm] ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] 0 0 87,8 91,3 91,1 101,9 102,7 101,9 -0,03 0 86,4 90,4 90,0 100,4 101,5 100,6 -0,02121 0,02121 88,8 92,1 91,8 102,9 103,7 102,8 0 0,03 90,8 93,6 93,3 105,1 106,0 105,0 0,02121 0,02121 91,0 93,8 93,5 105,1 105,9 105,0 0,03 0 89,3 92,4 91,2 103,4 104,1 103,2 0,02121 -0,02121 86,7 90,3 90,2 100,9 101,8 101,1 0 -0,03 84,7 88,7 88,6 99,2 100,4 99,7 -0,02121 -0,02121 84,5 88,6 88,5 99,0 100,2 99,5 Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 6 Critical shifting window infl uenced only by the inner roller position goes from 84,5° to 91° in terms of ! for CP1 and from 99° to 105,1° for CP2. 5.3 LATCH -PIN SHELF TOLERANCE When referring to the latch -pin shelf tolerance, the position of surface compared to nominal position as shown in Figure 10 is meant. As this dimension is not anyhow involved during the CP1 its infl uence only on CP2 will be examined. The critical shifting window for CP2 in term of ! can go from 100° to 104,1° due to the latch -pin shelf tolerance. 6. CAM LOBE TOLERANCES The same as for SRFF is valid for the cam lobe profi les. The tolerances that are taken in account here are cam profi le tolerance, wear and cam profi le angular tolerance. Profi le tolerance is easy to understand as it means that the designed cam profi le can be either higher or lower by the specifi ed value. Wear is captured by adding higher value to the negative side of the profi le tolerance so the actual cam profi le can be lower than the nominal partially because of manufacturing and partially due to wear over the time. Cam angular tolerance means that the cam lobe profi le can be shifted relatively to the other cam lobe. In the baseline case, both cam lobes have their fi rst profi le point in the direction of negative Y direction but in reality, the profi les can be shifted to each other due to angular position tolerance 6.1 OUTER CAM LOBE PROFILE TOLERANCE Four cases were tested including again the nominal dimension, then two cases for ±0,03 caused by the manufacturing and then case -0,06 where the half of the value is caused by the manufacturing and half by the wear of the cam lobe. Thus the tested cases and critical shifting window are not symmetrical. Each case results are Table 6. Critical shifting window infl uenced only by outer cam profi le tolerance and wear goes from 80,3° to 91° in terms of ! for CP1 and from 96,4° to 105,0° for CP2. 6.2 INNER CAM LOBE PROFILE TOLERANCE The same cases as in previous chapter were tested for the inner cam lobe tolerances. Results are in the Table 7. Critical shifting window infl uenced only by inner cam profi le tolerance and wear goes from 84,5° to 91° in terms of ! for CP1 and from 99,3° to 105,1° for CP2. The trend is here opposite to the FIGURE 10: Latch -pin shelf tolerance OBRÁZEK 10: Tolerance obrobení plochy p!epínacího %epu TABLE 5: Reults for latch -pin shelf tolerance TABULKA 5: V"sledky pro tolerance plochy p!epínacího %epu Pin tolerance CP2 [mm] ! [deg] " [deg] # [deg] 0 101,9 102,7 101,9 -0,03 100,0 101,0 100,3 -0,02 100,5 101,5 100,7 -0,01 101,2 102,1 101,3 0,01 102,6 103,4 102,5 0,02 103,4 104,1 103,2 0,03 104,1 104,9 103,9 TABLE 6: Results for outer cam profi le tolerance and wear TABULKA 6: V"sledky pro profi lovou toleranci a opot!ebení vn#j)ích va%ek Outer cam profi le tolerance CP1 CP2 [mm] ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] 0 87,8 91,3 91,1 101,9 102,7 101,9 -0,06 80,3 85,2 85,2 96,4 98,0 97,5 -0,03 84,5 88,6 88,5 99,0 100,2 99,5 0,03 91,0 93,8 93,5 105,0 105,9 105,0 TABLE 4: Results for different inner rollers position TABULKA 4: V"sledky pro r&zné pozice vnit!ních rolen Inner roller position tolerance CP1 CP2 X tol [mm] Y tol [mm] ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] 0 0 87,8 91,3 91,1 101,9 102,7 101,9 -0,03 0 89,3 92,5 92,2 103,4 104,1 103,3 -0,02121 0,02121 86,6 90,3 90,2 101,0 101,9 101,1 0 0,03 84,7 88,8 88,7 99,2 100,4 99,7 0,02121 0,02121 84,5 88,6 88,5 99,0 100,2 99,5 0,03 0 86,4 90,1 89,9 100,5 101,5 100,6 0,02121 -0,02121 88,8 92,1 91,8 102,9 103,7 102,7 0 -0,03 90,7 93,6 93,3 105,1 105,9 104,9 -0,02121 -0,02121 91,0 93,8 93,5 105,1 105,9 105,0 Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 7 tolerances of outer cam lobe. The higher is the inner cam profi le the earlier happen both crossover points while at the outer cam lobe the higher is the profi le the later the crossover points occur. 6.3 OUTER CAM LOBE ANGULAR TOLERANCE Changing the relative angular position of the cams means shifting the profi le timing thus changing all the cam lobe characteristics including lift, velocity and other higher derivatives. It is important to check if the relative velocity difference during CP do not exceed the prescribed guideline limits so the impacts in the system are controlled. Results for outer cam lobe angle are in Table 8. Critical shifting window infl uenced only by outer cam angular tolerance goes from 83,2° to 93,9° in terms of ! for CP1 and from 95,9° to 108,5° for CP2. 6.4 INNER CAM LOBE ANGLE POSITION TOLERANCE Same cases as prescribed in previous chapter were tested for inner cam lobe angular tolerance. See the results in the Table 9. Critical shifting window infl uenced only by inner cam angular tolerance goes from 83,7° to 93,4° in terms of ! for CP1 and from 96,4° to 108,0° for CP2. It can be observed that the trends are similar as in cam lobe profi le tolerance – shifting outer cam lobe angular position clockwise (+0,5°) cause CPs occur later on the other hand shifting the inner cam lobe same direction causes that CPs occur earlier. 7. WORST CASE SCENARIO After the examination of each factor infl uence to the position of CPs the overall impact of all should be added together and see how it can infl uence the moment of CP1 and CP2. In reality such a case is highly improbable and statistical approach should be applied so the tolerances are not set too strict only for highly improbable combinations. See the results in Table 10. It can be observed that due to manufacturing tolerances set as prescribed in the previous chapters the CP1 can happen anytime from 63,8 ° to 110,5° and CP2 from 78,2° to 120° in terms of !. Critical shifting window is 46.7° wide for CP1 and 42,1°wide for CP2. Such a width of CSW and level of uncertainty when does the CP happen might not be suffi cient for some applications so in the next chapter there will be ways how to infl uence the the width of CSW. 8. CRITICAL SHIFTING WINDOW ADJUSTMENTS There are two ways how to adjust the width of CSW. First way is very obvious, and it consists of making tolerances tighter. For our case the tolerances were halved. It can be considered that for roller tolerances the more accurate machine was used to drill the holes in SRFF, and more precise turning was used for rollers. That would result in roller’s axis lying in circle of radius 0,015mm around its nominal position. Same applies for cam tolerances, TABLE 7: Results for inner cam profi le tolerance and wear TABULKA 7: V"sledky pro profi lovou toleranci a opot!ebení vnit!ní va%ky Inner cam profi le tolerance CP1 CP2 [mm] ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] 0 87,8 91,3 91,1 101,9 102,7 101,9 -0,06 93,9 96,1 95,6 108,5 110,9 110,0 -0,03 91,0 93,8 93,5 105,1 105,9 105,0 0,03 84,5 88,6 88,5 99,3 100,4 99,7 TABLE 8: Results for angular tolerance of outer cam lobes TABULKA 8: V"sedky pro úhlovou toleranci vn#j)ích va%ek Outer cam angular tolerance CP1 CP2 [deg] ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] 0 87,8 91,3 91,1 101,9 102,7 101,9 -0,5 83,2 88,0 87,5 95,9 98,1 97,1 0,5 93,9 95,7 95,7 108,5 109,5 109,3 TABLE 9: Results for angular tolerance of inner cam lobe TABULKA 9: V"sledky pro úhlovou toleranci vnit!ní va%ky Inner cam angular tolerance CP1 CP2 [deg] ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] 0 87,8 91,3 91,1 101,9 102,7 101,9 -0,5 93,4 95,7 95,7 108,0 109,5 109,3 0,5 83,7 88,0 87,5 96,4 98,1 97,1 TABLE 10: Worst case scenario results TABULKA 10: V"sledky pro kombinaci nejhor)ích mo$n"ch tolerancí Worst case superposition CP1 CP2 ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] Beginning of CSW 63,8 72,0 71,6 78,2 83,9 83,1 End of CSW 110,5 112,4 113,3 120,3 126,0 127,5 Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 8 furthermore the better material in terms of wear would be used so the peak wear decreases to 0,015mm thus cam profi le tolerance would go from -0,03 mm to +0,015 mm around nominal value and cam angle tolerance ±0,25°. Infl uence on CSW is in Table 11. The improvement is signifi cant and CSW for CP1 goes from 75° to 101,2° and for CP2 from 89,3° to 114,7° in terms of !. Then width of the CSW is 26,2° for CP1 and 25,4° for CP2. Another way how to make CSW tighter is the adjustment of cam design and its velocity specifi cally. Tolerance deviation is basically increasing or decreasing the initial size of the lashes (MLC, MLL) compared to nominal, which has to be closed. The relative velocity difference tells us how quickly get those lashes closed around CP. Adjusting cam design in a way that position of CP stays the same but relative velocity difference is higher will result in closing the lashes with their deviations faster and so decreasing the infl uence of tolerances on CSW size. The new inner cam profi le was designed wither higher relative velocity difference (Table 12) and its infl uence on CSW size is in Table 13. The results show the size of CSW can be decreased by proper cam design as well. In this case increasing relative velocity difference for CP1 from 0,0075 mm/deg to 0,0103 mm/deg decreased the size of CSW by 12,9° in terms of !. With velocity difference increase from 0,0075 mm/deg to 0,0132 mm/deg for CP2 the CSW was decreased by 10,6° in terms of !. Increasing relative velocity is not for free as well, since the higher the difference is the higher is the impact that appears in the system during CP. The advantage of making CSW tighter has to be compared with disadvantage of possible higher wear or necessity of using better material. Last case in this article will be the combination of two adjustments made above. The results for case where tolerances have the half size compared to the worst case and the relative velocity difference is as in Table 12. Critical shifting window is 16,2° wide for CP1 and 17,7° wide for CP2 in terms of !. 9. CONCLUSION Concept and principle of critical shifting window was explained and infl uence of various factors on its size was examined. Detailed study of each factor was performed and based on results the following can be stated. The presence of critical shifting window is inevitable, and its size is prescribed by the manufacturing tolerances and design of a cam lobe profi le during CP. Adjustments to the size of CSW can be done either by making manufacturing process more accurate or by increasing the relative velocity difference at cam lobes during the CP. The disadvantage of more accurate manufacturing process is the higher cost. The information about the actual tolerance classes, tolerance -based assembly and the trade -off between cost and CSW width is usually considered as a business secret and it is extremely diffi cult to reach to such information. It is important to compare the brought advantage for the increased cost. For example, if improving production process of the camshaft would TABLE 11: Results for worst case scenario with half tolerances TABULKA 11: V"sledky pro kombinaci nejhor)ích mo$n"ch polovi%ních toleranci Worst case with half tolerances CP1 CP2 ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] Beginning of CSW 75,0 81,1 80,8 89,3 92,6 91,9 End of CSW 101,2 101,9 101,5 114,7 119,2 119,8 TABLE 14: Results for half tolerances and higher relative velocity difference TABULKA 14: V"sledky pro polovi%ní tolerance a vy))í relativní rychlost mezi va%kami Worst case with tighter tolerance and higher velocity difference CP1 CP2 ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] Beginning of CSW 79,9 85,1 84,8 92,5 95,1 94,4 End of CSW 96,1 97,6 97,2 110,2 112,3 111,5TABLE 11: Results for worst case scenario with half tolerances TABULKA 11: V"sledky pro kombinaci nejhor)ích mo$n"ch polovi%ních toleranci Worst case with half tolerances CP1 CP2 ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] Beginning of CSW 75,0 81,1 80,8 89,3 92,6 91,9 End of CSW 101,2 101,9 101,5 114,7 119,2 119,8 TABLE 12: Higher relative velocity difference for new inner cam TABULKA 12: Vy))í relativní rozdíl rychlostí pro novou vnit!ní va%ku Higher CP1 vel. difference [mm/deg] Higher CP2 vel. difference [mm/deg] 0,0103 0,0132 TABLE 13: Results for new inner cam with higher relative velocity difference TABULKA 13: V"sledky pro novou vnit!ní va%ku s vy))í relativní rychlostí Higher cam velocity difference CP1 CP2 ! [deg] " [deg] # [deg] ! [deg] " [deg] # [deg] Beginning of CSW 68,5 75,9 75,7 84,7 89,2 88,6 End of CSW 102,3 102,6 102,6 116,2 120,9 121,8 Critical Shifting Window in Switchable Rocker Finger Follower PETR KOHOUT, JAN KINDERMANN MECCA 01 2021 PAGE 9 bring the same benefi t as improving the accuracy of rollers position but the cost is rapidly higher for the camshaft then focusing on SRFF manufacturing process is the way to go to. The increased relative velocity difference has also its disadvantage because the higher is the velocity difference the higher are the impacts in the system and higher wear can occur. The infl uence of the tolerances to a valve lift change and to the engine breathing was not the area of interest for this paper but as the values of tolerances are in hundredths of millimetres it is expected to have minor or almost no infl uence to the engine performance. REFERENCES [1] Kisabo A.B., Ibrahim M. J., Oluwafemi O. A., Comparative Analysis Between Cam and Cam -less Valve Actuating for Automotive System. International Journal of Systems Engineering, Vol. 1, No. 2, 2017, pp. 48 – 57. doi: 10.11648/j.ijse.20170102.12 [2] Lou Z., Zhu G., Review of Advancement in Variable Valve Actuation of Internal Combustion Engines. Applied Science. 2020, 10(4), 1216, 2020, doi:10.3390/app10041216. [3] J. R., Variable Valvetrain System Technology, SAE International, 2006, ISBN 978-0-7680-1685-7 [4] Norton L.R., Cam Design and Manufacturing Handbook – 2nd edition Reference Book, Industrial Press Inc., 2009, ISBN-13: 978-0831133672 [5] Radulescu, A., McCarthy JR, J., and Brownell, S., Development of a Switching Roller Finger Follower for Cylinder Deactivation in Gasoline Engine Applications, SAE Technical Paper 2013-01-0589, 2013, https://doi.org/10.4271/2013-01-0589. [6] Kohout P. (2020) Cam design for variable valve lift system with switchable roller fi nger follower, Conference paper at 51st International Scientifi c Conference of Czech and Slovak University Departments and Institutions Dealing With the Research of Internal Combustion Engines, “KOKA 20”, CTU in Prague, Czech Republic, pp. 138 – 148. ISBN 978-80-01-06744-4 [7] Qianfan X., Diesel Engine System Design – 1st edition, Woodhead Publishing, 2011, ISBN-13: 978-1845697150 [8] Nicholas M. P., TAKASHI M., WONJOON CH., Shape Interrogation for Computer Aided Design and Manufacturing, https://web.mit.edu/hyperbook/ Patrikalakis-Maekawa-Cho/node17.html [9] GT -Suite v2019 user manual, Gamma Technologies, LLC. SYMBOLS AND ACRONYMS CA crank angle CAE computer aided engineering CP crossover point CSW critical shifting window iEGR internal exhaust gas recirculation MLC mechanical lash at cam MLL mechanical lash at latching pin OEM original equipment manufacturer SRFF switchable roller fi nger follower VVA variable valve actuation VVL variable valve lift VVT variable valve timing 13 CSW - critical shifting window iEGR - internal exhaust gas recirculation MLC - mechanical lash at cam MLL - mechanical lash at latching pin OEM - original equipment manufacturer SRFF - switchable roller finger follower VVA - variable valve actuation VVL - variable valve lift VVT - variable valve timing 𝑣𝑣!"##@%&' - relative velocity difference at CP1 𝑣𝑣!"##@%&( - relative velocity difference at CP2 𝑣𝑣"))*+(𝛾𝛾%&') - velocity on inner cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛾𝛾%&() - velocity on inner cam lobe at contact point during CP2 𝑣𝑣"))*+(𝛽𝛽%&') - velocity on outer cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛽𝛽%&() - velocity on outer cam lobe at contact point during CP2 α - rotation angle of camshaft from initial state β - angle between cam profile first point and contact point at outer cam profile γ - angle between cam profile first point and contact point at inner cam profile relative velocity difference at CP1 13 CSW - critical shifting window iEGR - internal exhaust gas recirculation MLC - mechanical lash at cam MLL - mechanical lash at latching pin OEM - original equipment manufacturer SRFF - switchable roller finger follower VVA - variable valve actuation VVL - variable valve lift VVT - variable valve timing 𝑣𝑣!"##@%&' - relative velocity difference at CP1 𝑣𝑣!"##@%&( - relative velocity difference at CP2 𝑣𝑣"))*+(𝛾𝛾%&') - velocity on inner cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛾𝛾%&() - velocity on inner cam lobe at contact point during CP2 𝑣𝑣"))*+(𝛽𝛽%&') - velocity on outer cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛽𝛽%&() - velocity on outer cam lobe at contact point during CP2 α - rotation angle of camshaft from initial state β - angle between cam profile first point and contact point at outer cam profile γ - angle between cam profile first point and contact point at inner cam profile relative velocity difference at CP2 13 CSW - critical shifting window iEGR - internal exhaust gas recirculation MLC - mechanical lash at cam MLL - mechanical lash at latching pin OEM - original equipment manufacturer SRFF - switchable roller finger follower VVA - variable valve actuation VVL - variable valve lift VVT - variable valve timing 𝑣𝑣!"##@%&' - relative velocity difference at CP1 𝑣𝑣!"##@%&( - relative velocity difference at CP2 𝑣𝑣"))*+(𝛾𝛾%&') - velocity on inner cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛾𝛾%&() - velocity on inner cam lobe at contact point during CP2 𝑣𝑣"))*+(𝛽𝛽%&') - velocity on outer cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛽𝛽%&() - velocity on outer cam lobe at contact point during CP2 α - rotation angle of camshaft from initial state β - angle between cam profile first point and contact point at outer cam profile γ - angle between cam profile first point and contact point at inner cam profile velocity on inner cam lobe at contact point during CP1 13 CSW - critical shifting window iEGR - internal exhaust gas recirculation MLC - mechanical lash at cam MLL - mechanical lash at latching pin OEM - original equipment manufacturer SRFF - switchable roller finger follower VVA - variable valve actuation VVL - variable valve lift VVT - variable valve timing 𝑣𝑣!"##@%&' - relative velocity difference at CP1 𝑣𝑣!"##@%&( - relative velocity difference at CP2 𝑣𝑣"))*+(𝛾𝛾%&') - velocity on inner cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛾𝛾%&() - velocity on inner cam lobe at contact point during CP2 𝑣𝑣"))*+(𝛽𝛽%&') - velocity on outer cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛽𝛽%&() - velocity on outer cam lobe at contact point during CP2 α - rotation angle of camshaft from initial state β - angle between cam profile first point and contact point at outer cam profile γ - angle between cam profile first point and contact point at inner cam profile velocity on inner cam lobe at contact point during CP2 13 CSW - critical shifting window iEGR - internal exhaust gas recirculation MLC - mechanical lash at cam MLL - mechanical lash at latching pin OEM - original equipment manufacturer SRFF - switchable roller finger follower VVA - variable valve actuation VVL - variable valve lift VVT - variable valve timing 𝑣𝑣!"##@%&' - relative velocity difference at CP1 𝑣𝑣!"##@%&( - relative velocity difference at CP2 𝑣𝑣"))*+(𝛾𝛾%&') - velocity on inner cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛾𝛾%&() - velocity on inner cam lobe at contact point during CP2 𝑣𝑣"))*+(𝛽𝛽%&') - velocity on outer cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛽𝛽%&() - velocity on outer cam lobe at contact point during CP2 α - rotation angle of camshaft from initial state β - angle between cam profile first point and contact point at outer cam profile γ - angle between cam profile first point and contact point at inner cam profile velocity on outer cam lobe at contact point during CP1 13 CSW - critical shifting window iEGR - internal exhaust gas recirculation MLC - mechanical lash at cam MLL - mechanical lash at latching pin OEM - original equipment manufacturer SRFF - switchable roller finger follower VVA - variable valve actuation VVL - variable valve lift VVT - variable valve timing 𝑣𝑣!"##@%&' - relative velocity difference at CP1 𝑣𝑣!"##@%&( - relative velocity difference at CP2 𝑣𝑣"))*+(𝛾𝛾%&') - velocity on inner cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛾𝛾%&() - velocity on inner cam lobe at contact point during CP2 𝑣𝑣"))*+(𝛽𝛽%&') - velocity on outer cam lobe at contact point during CP1 𝑣𝑣"))*+(𝛽𝛽%&() - velocity on outer cam lobe at contact point during CP2 α - rotation angle of camshaft from initial state β - angle between cam profile first point and contact point at outer cam profile γ - angle between cam profile first point and contact point at inner cam profile velocity on outer cam lobe at contact point during CP2 ! rotation angle of camshaft from initial state " angle between cam profi le fi rst point and contact point at outer cam profi le # angle between cam profi le fi rst point and contact point at inner cam profi le