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Engine Braking with Lashless Valve Trains

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Engine Braking with Lashless Valve Trains Engines DEVELOPMENT ENGINes Engine Braking with Lashless Valve Trains © Hanna_photo | iStock.com / narvikk | iStock.com / shauni | iStock.com / Maksym Dragunov | iStock.com / Jacobs Vehicle Systems, Inc. Until now, hydraulic valve lash adjusters could not be used in g Lashless valve train systems, heavy-duty diesel engines in combination with an additional utilizing technologies such as automatic or Hydraulic Lash Adjusters (HLAs), engine brake, as the engine brake causes signifcant valve lash FIGURE 1, to maintain zero-clearance during the braking process. The hydraulic compensation ele- between the mechanisms to intake ments automatically readjust to take up this play, thereby pre- and exhaust valves, have been ubiqui- tous in passenger car gasoline engines venting correct seating at the end of the cycle. Jacobs Vehicle and light-duty diesel applications for Systems has now developed systems that enable the integration decades. For applications like medium (MD) and heavy-duty (HD) diesel that of an engine brake in valve trains with hydraulic lash adjusters. require the use of dedicated engine brakes to aid in downhill control, lash- less systems were simply not compatible. A conventional engine brake in opera- tion will create an increased clearance between components of the valve train. If the HLA now readjusts, the engine components may suffer. 28 www.springerprofessional.com/automotive Engines AUTHORS (TCO) and lower Noise, Vibration and release event into the exhaust, and pre- Harshness (NVH) emitted by the valve vents the energy from going back to the train. The adoption of lashless valve crank during the expansion stroke. This trains with HLAs can allow the OEMs to “energy absorption” helps to slow the have more fexibility in valve lift design, loaded vehicle or maintain control on a which helps in meeting more stringent downhill grade without wear and tear or emission targets. overheating the service brakes. Peter J. Jo, PMP is Engineering Development Manager for North America at COMPRESSION RELEASE LASHLESS OVERHEAD Jacobs Vehicle Systems, Inc. in ENGINE BRAKE VALVE DESIGN Bloomfeld, Connecticut (USA). A standard compression release brake Traditional MD and HD diesel engine modifes the motion of the engine valves, Overhead Valve designs require setting opening the exhaust valve at Top Dead the lash with an adjustable screw to Center (TDC) of the compression stroke account for the variations in the manu- and releasing the energy used to com- facturing tolerances of the components press the cylinder charge through the and assemblies while still allowing a gap exhaust system [1]. This is often imple- for thermal expansion during normal John Mandell is Senior Project Engineer and mented by adding two valve lift events engine operating temperatures. To allow Off-highway Valvetrain Expert for to the standard exhaust main event, for the gap, the cam must be designed the Product Development Team at FIGURE 2 (left). The Brake Gas Recircula- such that long ramps, FIGURE 2 (right), Jacobs Vehicle Systems, Inc. in Bloomfeld, Connecticut (USA). tion (BGR) event is around bottom dead are at the opening and closing of the center after the intake stroke to increase valve lifts to control the motions of the the charge in the cylinder with gas cam and prevent it from hitting against from the exhaust manifold [2]. The the rocker or the bridge, and keep the charge in the cylinder is then com- valves from hitting the seat and other pressed by the piston, which provides the parts when opening/closing. braking work released at TDC by open- Due to the engine components’ ing the exhaust valve with a compression variability, the lash is set during the Hilko Schmitt is Marketing and Business Development Manager in the On- and Off-highway valve train technology development department at Jacobs Vehicle Systems, Inc. in Guyancourt (France). Gabriel Roberts is Engineering Director of Product Development at Jacobs Vehicle Systems, Inc. in Bloomfeld, Connecticut (USA). Recently approved emission regula- tions in the EU and US are placing very demanding CO2 and NOx limits on FIGURE 1 Hydraulic lash adjuster OEMs starting in 2027. In order to com- (© Jacobs Vehicle Systems, Inc.) ply, OEMs are evaluating technologies that have been successfully used in other applications. A new solution from Jacobs Vehicle Systems allows a conventional HLA and engine brake to work together in a modern valve train without compro- mise. HLAs also enable other benefts, such as reduced Total Cost of Ownership ATZ heavyduty worldwide 01|2021 29 DEVELOPMENT ENGINes FIGURE 2 Typical valve lifts (shown exaggerated for illustrative purposes) (Compression Release: CR) (© Jacobs Vehicle Systems, Inc.) initial engine build and is then typically engine braking system were to be used FULCRUM BRIDGE adjusted and reset every 100,000 to in an overhead utilizing HLAs in the 300,000 km or 1500 to 5000 h (widely main event rockers, the bridge would Jacobs Vehicle Systems developed the dependent on OEM requirements and tilt and unload the HLA when the brake Fulcrum Bridge that makes it possible usage duties) as part of scheduled main- rocker moves to open one brake valve to utilize compression release engine tenance. As the overhead components during the compression release event, brakes with HLAs [3]. The device uses wear down, maintenance is necessary FIGURE 3. The HLA would then pump the principle of leverage and prevents the as the original set lash shifts from the out and extend to take up the increased lash adjuster from overextending during optimal operating gap. lash, thus overextending and keeping the a braking event. This is accomplished by An example of component wear can be valve from seating at the end of the com- actuating the braking valve slightly off its found in the valve seats. As the valve pression release event. This would lead centerline and diverting a proportion of head opens and closes against the seats to poor engine performance and could the braking load to the HLA via a linkage over millions of times in its operation, risk damage to the valve due to heat and mechanism. This simple, cost-effective the valve seats wear down, or recess, possible valve-to-piston contact. This device uses the braking valve tip as a resulting in the valves moving up over is the primary reason why HLAs have fulcrum point to impart an upward force time and reducing the lash or gap set not been adopted into applications that through the bridge. This upward force from the initial set point. When this lash require engine brakes. counteracts the pump-out force exerted gap is reduced beyond design limits due to wear, the engine will no longer per- form optimally. At operating temperature, the valve may be hung open and leak compression which can lead to reduced engine performance, increased emissions and accelerated wear. HLAs automatically adjust to variations and wear in the over- head to maintain a zero gap between the cam, rocker and valve. This eliminates the need to perform the time-consuming task of setting the lash adjuster during the initial engine build and in service. Other benefts include a reduction in NVH by eliminating the lash that would FIGURE 3 Conventional normally result in clacking noises in the braking with HLA overhead. HLAs can be functionally sen- (© Jacobs Vehicle Systems, Inc.) sitive and factors such as component cleanliness, durability, valve train load- ing, and combustion chamber “oil can- ning” effects should be considered in the valve train design. If a conventional 30 www.springerprofessional.com/automotive Bridge may accommodate these varia- tions, with the ability to incorporate the same mechanism into other valve train elements such as the rocker arm, push tube, fnger-follower, or cam follower. FIGURE 5 shows the Spring Bridge to be incorporated into a Single Overhead Camshaft (SOHC) engine with a dedi- cated cam single valve-opening rocker brake. An HLA has been incorporated into the main event exhaust rocker with Spring Bridge located between the HLA and the engine valves, allowing the HLA to function as normal during positive FIGURE 4 Braking with Fulcrum Bridge and HLA power. When in engine braking mode, (© Jacobs Vehicle Systems, Inc.) the Spring Bridge works with the main event cam design to keep the HLA loaded during bridge tilt normally asso- ciated with single-valve engine braking. The Spring Bridge operates with an inter- nal stroke-limited piston that allows the main event lift and HLA refll portions of the cam to operate normally. When in braking mode, the main exhaust cam positions the rocker, HLA, and internal piston of the Spring Bridge in a pre- by the HLA and prevents it from overex- where the engine brake actuates the loaded condition that prevents pump- tending, FIGURE 4. The fulcrum lever braking valve changes to have the ful- out when the dedicated cam brake rocker ratio is used to design the amount of crum effect on the HLA. The same valve tilts the bridge, FIGURE 6. Either Fulcrum force proportioned back to the HLA to train cannot be used in HLA and non- Bridge or Spring Bridge can be used with maintain a balance between pump-out HLA equipped valve trains, which dedicated cam rocker brakes, overhead and collapse. Another beneft of the is one of the reasons why the Spring bolt-on brakes, bleeder brakes, and even Fulcrum Bridge is the shortened brake Bridge was developed. While the operat- lost motion rocker brakes, making it rocker ratio that reduces the required ing mechanism of the Spring Bridge is possible to adopt this system for most actuation force on the braking rocker different, the fundamental objective is engines. due to the mechanical advantage pro- the same: keep the HLA loaded during vided by the lever mechanism. This the engine brake events to prevent EMISSIONS shortened length results in less contact pump-out of the HLA, which may lead stress on the brake rocker arm and to engine damage.
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