DESIGN+PLUS

   This section will focus on miscellaneous designs from sectors other than weapon technology with a special focus on Formula-1 and MotoGP technology.  As always RAFAEL LASTRA ENGINEERING will focus on ground-breaking designs that will allow for greater efficiency and performance within the realm of mechanical engineering.  Below you will find a brief outline of already completed work on various motorsport innovations.   

 

A:  IMPROVED SPUR GEAR FATIGUE LIMITS

     There exist many an application in both military and civilian spheres alike whose performance regimes are directly proportional to their power throughput.  While many of them can easily accommodate larger prime-movers there remain some that cannot.  Self-propelled artillery, for example, can easily accommodate larger power units as the space required for higher horsepower drives can easily be designed in on subsequent versions.  Helicopter drivetrains like formula 1 race cars, to name just a few applications, have a much more difficult time in doing so, if any at all.  In a formula car the gearbox and final drive usually doubles as a structural member of the chassis itself.  As such, changes to the housing required for higher capacity gearing usually interfere with other critical systems therefore negating said changes during a season. 

     On rotorcraft the lifespan of individual platforms is usually measured in decades not years.  While they are designed with robustness in mind those strategies too have limitations that are way too often encroached upon as actual platform lifespans are extended beyond that originally envisioned.  The reasons for this are varied but are mainly the result of fiduciary constraints.  Significant changes to airborne structures demand both extensive and lengthy re-certification trials whose cost in treasure and manpower deem it an impossibility in many cases.  The most cost-effective avenue to take is that of continuous improvement of existing equipment.  Both of these examples require their respective design teams to ´work with what they have´, so to speak.  In an effort to stay ahead of the adversary these challenges must be met with both discerning scrutiny and expertise spanning many engineering disciplines. 

     A critical sub-system within the drivetrain are the actual gears themselves whose fatigue limits are quickly reached.  The engineer has diagnosed and repaired over 50 gearbox failures spanning the entire range of torque and speed regimes in various steel mills and can attest to the failure mechanisms involved.   Taking a closer look at the gear teeth themselves we see a cantilever beam whose maximum tensile stresses are found at the root of the tooth.  Keeping in mind that fatigue failures are mainly the result of tensile stresses the usual avenues to extend fatigue life have been the use of carburizing steels whose hardened case leaves the surface of the material at a hardness of between 58-62 HRC conferring in the layer a residual compressive stress that under load acts to reduce the resultant tensile stress thereby extending fatigue limits.  This is in stark contrast to through-hardened gears whose oil-quenched microstructure leaves the surface in a state of mild tension, ultimately reducing the fatigue life.  These mechanisms are similar to shrink-fitting barrels onto naval guns of old or the auto-frettage (self-jacketing) barrels in service today.  In the chart below we can see just how much the fatigue life can be extended in carburized gears.  With case depths between 10-20% of the normal module or addendum we can almost double the fatigue limits of gear teeth, an astounding achievement.  

 

 

     This is an excellent way to increase the load carrying capability of the material without having to vary existing geometry and/or footprint.   Another avenue taken to improve the fatigue life of highly loaded gears is the use of high alloy, carburizing steels thereby allowing for higher loads by way of their increased hardness and fracture toughness.  Questek´s Ferrium C61 comes to mind whose core hardness reaches 48-50 HRC while maintaining case hardness of, as the name implies, 61 HRC.  Such steel grades are a step up from traditional grades like 9310 or M50NiL, to name just a few.  These measures, while very effective indeed, have been known to designers for some time now and can best be defined today as ´low hanging fruit´, so to speak.  It is at this point where engineers reach their limits in extending gear life.  It is at this point where RAFAEL LASTRA ENGINEERING begins to research.

     Having reached this point, the designer must employ expertise in physics, mechanics and metallurgy, among other fields, to find new avenues to extend gear train lifespans.  Through extensive research and testing RAFAEL LASTRA ENGINEERING can report significant gains in highly loaded gears as outlined in the following chart.  These results are easily achievable using a combination of thermo-mechanical processing (not to be confused with cold rolling or forged gear teeth) along with additional means of pre-stressing the root area.  All of this is accomplished with a minimal capital outlay within the reach of any company today with additional costs of approximately 20-30% over standard processing practices. 

 

FATIGUE LIFE EXTENSION OF EXISTING PLATFORMS

                      Standard spur gears:    25-30% increases over standard

    Helical, conical & bevel gears:   20-25% increases over standard

NOTE:  Minimum normal module = 4 mm

 

 

 

 

 

B:   FORMULA-1 DESMODROMIC VALVE TRAIN

Naturally aspirated engines in top-tier motorsport categories like Formula-1 or MotoGP represent the most demanding environments for internal combustion engines. In Formula-1 engine speeds of up to 18,000+ rpm strain components to their absolute limits both in terms of mechanical stresses and fatigue lives brought on by lightning fast strain rates that cross the Rubicon into ballistic regimes. The race for more power has pushed the envelope of material science to new heights for piston/rotating assemblies as well as their corresponding valve trains. As power is directly proportional to rotational speed the quest for higher speed regimes is always on the agenda. The inertial limitations of the piston/rod assemblies can easily be addressed with subsequent reductions in crank throw placing the lions share of responsibility on the valve train. This brings into stark contrast the limitations of traditional designs and has motivated the push for novel solutions. As such, valve train operational regimes become the critical path for advances in normally aspirated engines.


Traditional valve spring layouts employ compression wound springs fit around the valve stem for valve closure relegating the opening to either tappet or finger-followers being directly actuated by the camshaft. Under these schemes the opening of the valve is under compression which represents the most reliable solution in terms of timing and geometry while the closing of the valve is the sole responsibility of the wound spring.  Unlike the opening phase the closing of the valve is not under compression of the camshaft, rather the spring must overcome the valves inertia to ensure the tappet/follower stays in contact with the cam surface. This scheme introduces a second-order spring-mass system into the equation with it´s consequent dynamic and harmonic limitations. As it could not be any other way it is here that the resonant effects inherent in any second-order system peers it´s ugly head.  Attempts to alleviate the harmonics brought on by excitation of the springs natural frequency have led to, among other solutions, the use of dual spring setups to compensate for harmonic regimes. While solving one issue such solutions introduce higher parasitic losses that ultimately reduce torque throughout the entire operating range all the while limiting operation to the 15-16,000 rpm range for specific displacements above 0.25 liters/cylinder.  While such speeds are easily attained and surpassed by smaller displacements using equivalent spring/tappet/follower designs, as seen in lower motorcycle categories using specific displacements of 0.0625 liters/cylinder or less, the effects of mass and scale make such designs the bane of categories like Formula-1 where specific displacements of up to 0.30 liters/cylinder are common.


In the never-ending quest for a competitive edge Renault introduced pneumatic spring valve technology in the 1990´s which elevated the valve trains operating regime to over 18,000 rpm. This opened up a new level of performance for naturally aspirated engines that saw 3.0 liter engines generating close to 1,000 hp become commonplace.  One of the main drawbacks of such systems is due to the inevitable leakage of the compressed gas required to run the systems as they only remain operable for a few hours before the charged accumulator pressure falls below the minimum required to run the system.  In short, these are solutions that can only be used for 1 race per accumulator charge.  As accumulator pressures reach far beyond the dieseling range for oxygen in the atmosphere nitrogen must be used meaning on-board compressors to recharge the accumulator are simply out of the question.  As such, pneumatic valve solutions are strictly for motorsport applications.  Apart from this serious limitation pneumatic valve springs do not reduce the parasitic losses of the power train as the cam must compress that gas spring during opening of the valve.  All in all, pneumatic systems for valve actuation do not represent an optimum solution in terms of power, reliability and longevity especially as far as commercial applications are concerned.


Another noteworthy design for valve train actuation takes the form of a purely mechanical solution, Desmodromic valve actuation.  These schemes are not new having been fielded by the likes of Mercedes and Porsche, among others, however the design has been made popular by it´s use in the motorcycles of Ducati of Italy.  Desmodromic valve trains provide valve actuation via finger followers for both the opening and closing phase of the valve.  As such, valve float is eliminated as the closing of the valve is under compression as in opening.  Ducati´s design does much to deal with the shortcomings of traditional schemes especially in terms of power consumption.  However, the design also has it´s share of shortcomings.  Specifically, the use of individual cams for both the opening and closing along with their followers make for a significant increase in both space and weight that make their use in Formula-1 less attractive as such design trade-offs prove too much of a compromise in a vehicle with so many other critical considerations such as center-of-gravity and aerodynamics, to name just a few.  The complexity and consequent bulkiness of the Ducati design stems from the dual cams required for both intake and exhaust valves to provide the necessary motion profiles as well as the requisite tension for valve seating.  While these shortcomings are understandable given the stringent criteria for such systems it is by no means the only solution possible.  This issue has come under the scrutiny of Rafael Lastra Engineering for an extensive period of time and is now pleased to announce the introduction of a hybrid Desmodromic valve train design of significantly reduced complexity that equals the speed regimes of pneumatically operated valves while providing class-leading reductions in parasitic losses in the smallest envelope size of all valve train designs known to date.


The novel design consists of a simple finger follower/lifter (FFL) arrangement that fits neatly under the camshaft such that it requires no more space than a traditional finger follower design. The FFL is actuated by only one cam instead of two as required in the Ducati version.  This greatly simplifies the layout requiring the least amount of space within the head.  Specifically, the mechanism does not require any more width than the swept radius of the cam lobe with no space required on the outboard side of the camshaft.  This leaves precious space available to ensure the maximum angle of attack possible for the intake runner. To deal with changes in temperature and consequent changes in dimensions the cam closes the valve to 97-98% ensuring the mechanism never interferes mechanically.  The final seating of the valve is the sole responsibility of the only spring in the system rapped around the FFL with a force equivalent to 40% of pneumatic designs which significantly reduces parasitic losses.  As such, the spring can raise and seat the valve by itself up to speeds of 10,000 rpm above which point the cam begins to share the work at an ever-increasing ratio up to redline.  This scheme alleviates the inertial load on the spring allowing it to follow the FFL while never losing contact to then regain full control and seat the valve at the end of its travel.  This load sharing feature allows for the best of all worlds ensuring parasitic losses are reduced by 60%.  The design has been inspired by the geometries employed in gear mesh applications whereby the cam surface is always in contact with either opening or closing surfaces on the FFL ensuring the valve is always constrained to follow the pre-determined motion profiles.  This arrangement allows for practically square cam profiles ensuring valve curtain area is maximized for high RPM volumetric efficiency.  Furthermore, the critical interface between the negative acceleration of the opening phase and the acceleration of the closing phase are arranged so that this critical transition avoids extreme jerks that can accelerate fatigue in the valve stem.


Another critical design feature is the interface between the FFL and the valve stem itself.  This aspect of the design has been inspired by the bones within a simple knee joint.  To this end, the valve stem is fitted with a hat providing cam surfaces on the top and bottom.  Another noteworthy aspect of the design is that of the actual camming surfaces of the FFL and the valve stem which apart from always maintaining the valve stem in balance are fitted with a slight pre-load or interference fit.  This feature allows the joint to double as a damper between both members such that oscillations which may arise over extreme speed regimes are never allowed to side load the stem on it´s corresponding bearing surfaces on the valve guide.  The scheme, while constraining both elements, still allows for the rotation of the valve on its seat so as to keep carbon build-up on that bearing surface at bay.  The arrangement also provides for the least height possible for a given valve.  For instance, a Formula-1 version requires no more than 80mm of height instead of existing designs using over 110mm.  Apart from reducing the actual weight of the valve the shorter length aids significantly in keeping flexing of the valve stem to an absolute minimum.  This is possible by employing modified valve guides extending further up from the lowest point of the FFL follower which due to their low profile wrap around the valve stem and reach below the valve guide.  As such bearing surfaces of 30mm and 22.5mm are maintained on the stem without having to protrude into the intake tract.


Below we can see one example of operational limits for a naturally aspirated engine consistent with the 3.0l-V10 and 2.4l-V8 configurations of previous Formula-1 specification.

3.0l-V10 AND 2.4l-V8 VALVE TRAIN

• Lastra Hybrid Desmodromic Valve Train Layout
• 42mm valve diameter (TiAl valve gear)
• 80mm valve length (5.5mm dia. With 2.2mm hollow)
• 15.5mm valve lift
• 20,500+ RPM at a minimum of 1,000 km at race speed
• 60% reduction in parasitic losses
• 350-360 bhp/liter capability minimum


NOTE: Motorcycle applications can exceed 23,000 RPM with Titanium valve gear and 18,000 rpm with traditional valve steel material.

 

 

 

C: INFINITELY VARIABLE DESMODROMIC VALVE TIMING & LIFT

The efficiency of an internal combustion engine is a fickle target by way of its dependence on a host of variables all of which require individual conditions for optimum performance. In the interest of reliability and cost mechanical systems traditionally used on engines do not offer the flexibility required to offer the best of all worlds. The result of which is a compromise that usually translates in solutions best defined by the adage of ´jack of all trades, master of none´. While the modern E.C.U. does much in the way of maximizing the engines output given a specific mechanical scheme, peak efficiency in terms of performance and fuel efficiency still require more adjustable mechanisms by which the combustion process can be optimized for given situations.

It is for this reason that the last 30 years have seen a plethora of systems focused on gaining an edge in performance and fuel efficiency alike. Schemes by which intake runner length, cam phasing and lift, to name just a few, have been devised with notable success. Of all these variables it is the fluctuation in the camshaft that offer a wider range of success. Companies like B.M.W. with their V.A.N.O.S. system or Honda with their V.T.E.C., among others, have proved successful to such ends as well as reliable. Nevertheless, commercial success for street applications where operating regimes rarely exceed 8,000 rpm does not necessarily translate into motorsport success where engines can routinely operate at over 15,000 rpm. At such speeds the issue of mass/inertia is exacerbated, inevitably requiring more innovative solutions.

Examples of such compromises can be found in the never-ending application of hydraulic vane-type actuators for camshaft phasing. Specifically, their bulk limit the engines response so critical for accurate positioning of the vehicle at corner entry as the assembly rotates with the camshaft. Another severe limitation of such actuators is their difficulty in properly sealing both chambers that restrict their application to merely offering two positions, hard-stop to hard-stop, as opposed to the ideal solution of being employed for continuously variable cam phasing for maximum flexibility. Another serious constraint of commercial applications is the use of hydraulically actuated pins responsible for the tying of multiple finger followers to achieve variable lift. Apart from only offering two lift profiles, the use of pins for locking/un-locking prove too fragile for high rpm use. The subsequent weight required to improve reliability only magnifies the shortcoming of such designs. Not to be outdone, there exist firms that have even gone so far as to promote fluid power solutions for the motion profiles of individual valves that can only be described as very poor engineering decision making. It would be one thing if such flexibility could not be offered by traditional mechanical means but as such solutions exist these ¨innovations¨ do not deserve a second of consideration.

Having considered all the pertinent variables Rafael Lastra Engineering has devised a means by which infinitely variable valve lift and phasing/timing is achieved for high speed motorsport applications such as Formula-1 or MotoGP. The solution comprises of a compound hydraulic cylinder (cylinder within a cylinder) of simple manufacture that allows for both continuous variability of both functions independently. Furthermore, said actuators do not rotate with the camshaft ensuring their weight/inertia does not interfere with engine dynamics. The design goes in lock-step with the desmodromic valve arrangement mentioned above offering the most efficient solution possible for top-tier motorsports and commercial applications alike. The key to the design is the use of said compound cylinder that employ the most reliable sealing systems available(circumferential) such that infinitely variable control can be used to accommodate the highest flexibility possible for the entire operating regime of the engine. By way of using low-pressure hydraulic valving simple 4/3-way directional cartridge valves feeding meter-out flow controls through pilot-operated check valves are all that are required for infinite control. In the case of even finer control requirements the actuators can be operated via proportional valves using counterbalance valves as P.O. checks to ensure smooth operation during the variable metering offered by proportional/servo valves.

Operational performance include….

  • Continuously variable lift spanning a 6mm window over a cylinder stroke of 25mm
  • Continuously variable cam phasing of 40 degrees over a cylinder stroke of 60mm
  • Operational speeds of up to 18,000 rpm (limited by the increased F.F.L. inertia required for alignment)
  • Envelope dimensions of 80mm diameter and 85mm overall length
  • Offers variable duration tied to variation in lift in a fixed format (i.e. not independently variable)

 

D:   LASTRA DESMODROMIC FOR SERIES PRODUCTION 

 

In an effort to satisfy increasingly stringent emissions requirements imposed upon by governments across the globe for passenger vehicles the last decade has been witness to a sharp rise in the use of smaller displacement engines of four and even three-cylinder configurations using turbo-chargers to maintain the performance of the larger displacement naturally-aspirated engines they replace.  This solution, while mostly acceptable in terms of performance and emissions levels, is not without drawbacks.  Forced induction induces higher strains on components leading to pre-mature wear and tear translating into increased maintenance/ownership costs.  Furthermore, the additional weight penalty brought on by turbo-chargers and intercooler circuitry work against the initial motivation of lower emissions.  Add to this the reduction in throttle response which reduces driver enjoyment, and in some cases safety, and the net effect is that of a solution full of compromises and contradictions.  Thankfully, this scenario can be optimized with the proper engineering of next-generation, naturally-aspirated engines.  

 

To this end, the Lastra Desmodromic valvetrain is a keystone element.  Using this novel valve actuation system a two-liter, four-cylinder engine can be constructed that will equal the performance of the modern, turbo-charged engines while meeting current and future emission requirements.  The key to realizing such performance is by the use of a variable that is absolutely free, speed.  Increasing engine speed regimes to generate more power in internal combustion engines is nothing new, one need look no further than the engines used in motorcycles to see this principle employed.  The transition of motorcycle engines for use in passenger cars is, however, seriously hampered by way of the sluggish low-end torque characteristics provided by such high-rpm designs.  Torque so necessary for everyday, stop-&-go driving.  Manufacturers like Honda and Toyota have had significant success with their four-cylinder engines using sophisticated variable-valve timing & lift schemes enabling them to reach or exceed the 100 hp/liter barrier.  While these accomplishments are significant they still fall short of the power and torque requirements expected by todays drivers.  As impresssive as these solutions are developing more performance exceeds the limits of the valvetrain schemes employed thus far by these trailblazing manufacturers.  Specifically, the use of traditional tappet/follower valvetrains reach their effective limits as the increased spring forces required to spin past 8,000 rpm seriously degrades low to mid-range torque.  So much so that variable valve timing & lift schemes simply cannot make up the difference.  This is why turbo-charging was chosen on recent engine designs.  

 

This is where the virtues of desmodromic valvetrains shine through.  Specifically, desmodromic designs have superior low & mid-range torque by way of their significantly reduced spring forces that in most cases is less than half of their equivalent tappet/follower counterparts.  Furthermore, desmodromic systems have the additional advantage of superior high-speed regime performance.  Add to a desmodromic valvetrain the use of infinitely-variable valve-timing & lift augmented by variable-length intake runners to maximize volumetric efficiency across the entire speed range and the result is a solution that fills the performance and emissions requirements without, in the case of a two-liter, four-cylinder engine, the need to resort to exotic materials like titanium for either the valves or connecting rods.  Below is an example of the performance characteristics of such a solution.  Remember, sound design by its very definition is inexpensive.   

 

  •          ENGINE CONFIGURATION:  4-stroke gasoline, port + direct injection, inline 4, aluminum block & head, balancer shaft,                                                                        variable intake runner length
  •                            DISPLACEMENT:  2,000 cc (122 cubic inches)
  •                 COMPRESSION RATIO:   12.5:1
  •                          BORE DIAMETER:   89.7 mm (3.53 inches)
  •                          STROKE LENGTH:   79.0 mm (3.11 inches)
  •                                         REDLINE:   9,800 RPM
  •                  MEAN PISTON SPEED:   25.8 meters/sec @ 9,800 RPM
  •                                    IDLE SPEED:   800 RPM
  •         INTAKE VALVE MATERIAL:   Stainless Steel
  •     EXHAUST VALVE MATERIAL:   Stainless Steel
  •                       PISTON MATERIAL:   Forged Aluminum
  •                   CON-ROD MATERIAL:   Forged Steel
  •                                 V ALVETRAIN:  Lastra Desmodromic + Intelligent VVT&L on intake & exhaust
  •                               PEAK TORQUE:  240 nm (178 lb-ft) from 3,000 to 7,000 RPM (85% from 2,000 RPM)
  •                                 PEAK POWER:  280 hp @ 9,500 RPM (210 nm – 155 lb-ft of torque)

 

 

E:   DESIGN CONSIDERATIONS FOR DESMODROMIC VALVETRAINS

 

Standard bucket-tappet and finger-follower valvetrain layouts have a proven track record of success over decades of application, as such they are not to be discarded on a whim. Nevertheless, these designs do have limitations. Specifically, their use in high-rpm applications for passenger cars, whose specific cylinder volumes can easily exceed 0.5 liters/cylinder, can translate into unacceptable trade-offs for the designer. These large swept volumes require large valves to fill at over 7,500 rpm, valves whose consequent inertia requires overly high spring forces to seat in order to avoid valve float. These large spring forces decrease the lubricant film thickness of the mating valve gear elements that end up causing premature wear and eventual failure. Couple this with the lower viscosity oils manufacturers prefer in order to operate variable-valve timing circuits and the issue is exacerbated. As a result, the need to find new valvetrain solutions capable of sustained operation above 7,500 rpm without having to resort to titanium valves is of ever-increasing importance as the search for energy efficiency and cost effectiveness forge ahead. This is where desmodromic valvetrain layouts gain renewed protagonism.    

The mere act of making an automotive poppet valve close positively does not in and of itself make for a viable desmodromic solution. Researching patent databases for desmodromic valvetrain designs will lead to a plethora of variations on the theme however the overwhelming majority are unworthy of mention as they do not meet the required criteria demanded of a trouble-free, operational design destined for series production. Most designs are the type that work great on paper while falling way short of solutions capable of high-speed operation in the real world where simplicity and light weight are paramount to success.

In keeping with such stringent criteria the Lastra Desmodromic solution fulfills the light weight, ultra-high speed requirements demanded in motorsport applications combined with the robust nature demanded of series production roles where high-load demands are required to seat relatively heavy, standard steel valves when operating at over 7,500 rpm. In short, the Lastra Desmodromic design represents an optimum solution for the entire spectrum of tomorrows automotive applications.  

 

 

F: MOTORSPORT CAM PHASER (VVT)

The benefits of variable valve timing and lift on power output as well as fuel efficiency are many.  The Lastra VVTL system outlined above allows the engine to have full control over its power generation allowing it to adapt to both speed and load requirements as needed.  This is of paramount importance to series production vehicles where part throttle and load requirements dictate optimum settings.  However, this flexibility has negative consequences in motorsport applications where weight/inertia, response, packaging constraints and regulations make active valve-train control difficult to justify, if not outright illegal.  Nevertheless, some racing series do allow the use of passive variable valve timing and lift schemes so long as neither hydraulic nor electrical circuits are utilized.  An example is found in top-tier MotoGP regulations.  This opens the door for savvy designers to devise purely mechanical solutions in lieu of fluid power and electrical control schemes.

 

 

An example of such a solution can be found in Suzuki´s 2017 introduction of a variable valve timing sprocket on the GSXR-1000 motorcycle (above).  The design uses purely mechanical means to produce a phase shift of approximately (11o) on a given camshaft.  Specifically, it uses the forces generated by centrifugal/centripetal action to push steel/tungsten ball bearings outward on concave tracks cut at angles that cause a phase shift between the driving and driven plates the balls are sandwiched in.  The two plates squeeze the ball bearings by way of spring washers that can be changed out to alter the changeover speed for said phase shift up or down depending on the requirements of the engine specification.  As such, a sprocket on a single cam can alter phasing of both camshafts by (11o) degrees or as much as (22o) if sprockets are used on both intake and exhaust camshafts.  The sprockets are lightweight adding just a few more ounces compared to a simple sprocket ensuring the sharp throttle response required in competition is not altered.  Suzuki´s low tech VVT solution does not alter lift nor does it alter valve timing with varying loads nevertheless phasing of valve timing is a significant advantage in motorsport where power is made so high up on the rev range that midrange torque tends to suffer drastically.  Having a simple and lightweight means of adjusting cam timing based on speed is a welcome advantage in competitions where mere tenths of a second in lap times can have drastic consequences in championship standings.  The scheme is nothing short of a cunning piece of engineering ensuring performance and reliability all within regulation.  Suzuki should be rightly proud. 

This type of mechanism opens the door to a unique niche for motorsport applications that truly pay dividends.  As such, Rafael Lastra Engineering is pleased to announce a novel solution for a purely mechanical variable valve timing solution for high rpm engines be it in motorcycle or automotive motorsport applications.  This unique solution shares all of the same virtues as the Suzuki design while shoring up the few shortcomings it suffers.  Specifically, the higher than desired hysteresis the unit experiences causing the shift point to vary when the engine speed falls below the set-point speed.  The actuation of the Suzuki solution starts with the ball bearings in the inner most position radially being pulled outward to their maximum position.  This implies that the reset rpm needs to be lower than the actual preset speed.  The engineers at Suzuki understand this which is why the concave grooves on the spring loaded exterior plate go from a deeper groove at the starting position to a shallower groove further away from the axis.  The shallower groove puts more load on the spring washers which in turn squeeze the counterweight balls harder in an effort to compensate for this effect.  This design feature certainly helps to minimize hysteresis however it is not enough to ensure the precise shift from the advanced position to the initial position.  One source of this problem stems from the high friction found on so many ball bearings squeezed under high pressure due to the fact that these counterweights slide during actuation as opposed to roll.  The second source of high friction is found in the exterior plate itself which must move axially on a splined shaft.  This exterior plate is under a constant torque load which will see it resting against the hard-stop provided by the actual splines themselves.  This means hysteresis will vary from unit to unit depending on tolerances, surface coating and lubrication.  The end result being a cam reset point below the actual set-point.  As an example, a set-point of 10,000 rpm  on the Suzuki mechanism will reset itself anywhere from 500-1,000 rpm (5-10%) lower.  This forces the engineers to choose a less than optimum shift point for transition.  This will then be subject to transmission ratios, specific track characteristics as well as pilot riding styles for final setting.  This issue is determinant in allowing race teams the highest level of flexibility and precision in cam selection and general setup for a particular race.    

Though the Lastra VVT sprocket is of a completely different physical layout the design works on the same basic principals in that centrifugal/centripetal force is used for actuation. A notable difference is that the spring compensation method for the reset speed uses wound springs that are virtually friction free.  This scheme allows a reset point of no more that 2% below the actual shift speed.  Combined with this low hysteresis figure the Lastra solution offers significantly lower costs of manufacturing as well as a much more tunable platform as spring rates are easily and reliably adjusted via set screws.  Additionally, the Lastra VVT sprocket provides a high shift torque capability.  An example shift point of 11,000 rpm crankshaft speed equates to an actuation torque of 27 nm (20 lb-ft) which is enough to shift an inline 4-cylinder camshaft of uneven firing order such as that found on the Yamaha R1 block.  This torque figure is high enough to be used with either traditional valve schemes (bucket tappet/follower) or desmodromic actuation.   The actuation of one camshaft allows for a phase shift of (13o) hence a sprocket on each camshaft generates a total phase shift of (26o) which goes a long way in controlling valve overlap.  The result of this simple solution is the extension of the usable torque curve as the engine would be ¨on the cam¨ an additional 1,250-2,000 rpm for applications using a cam profile of long duration and high lift.  All of this can be packaged in an envelope of no more than 80mm (3.15¨) diameter and a width of no more than 20mm (0.80¨) ensuring the crisp throttle response required in motorsport applications. 

EXAMPLE PERFORMANCE FIGURES:

  • Inline 4-cylinder block of uneven firing order
  • (2) Lastra phasers, (1) per camshaft (DOHC)
  • Total phase shift of (26o)
  • Shift speed of 11,000 rpm (crankshaft)
  • Reset speed at 2% under set-point (10,800 rpm)
  • Shift torque of 27 nm (20 ft-lb) per camshaft @ 11,000 rpm
  • Sprocket diameter of 80mm (3.15¨)
  • Sprocket thickness of 20mm (0.80¨)
  • Set-point adjustment via set screws (9,000-12,000 rpm)

 

G:   HIGH PERFORMANCE PISTON RING DESIGN

 

 

 

Rafael Lastra Engineering – Unparalleled Know-How