Truck & Off-Highway Engineering - August 2021 - 23

POWERTRAIN FEATURE
Variable valve actuation stabilizes cold start-up combustion,
shortens engine warm-up time and lowers emissions by keeping
the aftertreatment system hot during low-load operation.
lowers overall engine system costs and integrates with
minimal impact to engine overhead designs. This technology
makes real-time adjustments to valve opening
and closing, maintaining accurate control of valve motion.
By creating a hydraulic link between the cam and
the valve, VVA precisely tunes the engine across its
operating range.
Two-Step VVA, for example, enables the combustion
engineer to optimize valve timing at two operating
points instead of the traditional fixed-cam-based single-timing
option; 2-Step early or late intake valve
closing reduces fuel consumption, optimizes compression
ratio versus load, improves transient response and
start-up, and improves emissions by keeping the aftertreatment
system hot during low-load operation.
Early intake valve closing is achieved by operating
on an early closing profile main event with the auxiliary
valve motion (normal closing) deactivated. LIVC is
achieved with a late-closing cam profile activated on
the auxiliary rocker arm to hold the valve open longer.
Early exhaust valve opening enables faster warm-up
of the engine and aftertreatment system, improves
transient turbocharger response and emissions by
keeping the aftertreatment system hot during lowload
operation. This is accomplished with an earlyopening
cam profile on the auxiliary rocker arm, or
lost motion system, and actuated on-demand with
engine oil. Early exhaust valve opening also can be an
in-cylinder solution for helping with DPF regeneration
by replacing exhaust heaters and dosers.
Another aspect of VVA is internal exhaust gas recirculation.
This stabilizes cold startup combustion,
shortens engine warm-up time, improves aftertreatment
performance and lowers emissions by keeping
TRUCK & OFF-HIGHWAY ENGINEERING
Robb Janak, director of new technology at Jacobs Vehicle Systems.
Cylinder deactivation (CDA) is mostly able to improve fuel economy through the
increased load on the operating cylinders, but incremental gains also come from
reducing camshaft friction, reducing pumping losses in part-load conditions,
and reducing or eliminating the use of the intake throttle.
the aftertreatment system hot during low-load operation. This system
responds faster than EGR systems and enables the downsizing or
elimination of external EGR systems. Versions of this technology have
been in production for nearly 20 years.
Cylinder deactivation
Though CDA has been employed successfully in passenger car engines
for decades, Jacobs is the first to develop it specifically for 7to
15-L medium- and heavy-duty engines. CDA's hydraulically activated
mechanism is integrated into a collapsing-valve-bridge system
for overhead camshaft engines or with a collapsing-pushrod system
for cam-in-block engines.
August 2021 23
ALL IMAGES: JACOBS VEHICLE SYSTEMS
Truck & Off-Highway Engineering - August 2021

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