Edited by: Evangelos G. Giakoumis, National Technical University of Athens, Greece
Reviewed by: George Kosmadakis, Agricultural University of Athens, Greece; Efthimios G. Pariotis, Hellenic Naval Academy, Greece
Specialty section: This article was submitted to Engine and Automotive Engineering, a section of the journal Frontiers in Mechanical Engineering
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Valvetrain flexibility enables the optimization of the engine’s ability to breathe across the operating range, resulting in more efficient operation. The authors have shown the merit of improving volumetric efficiency via valvetrain flexibility to improve fuel efficiency at elevated engine speeds in the previous work. This study focuses on production viable solutions targeting similar volumetric efficiency benefits via delayed intake valve closure at these elevated engine speeds. Specifically, the production viable solutions include reducing the duration at peak lift and reducing the amount of hardware required to achieve a delayed intake closure timing. It is demonstrated through simulation that delayed intake valve modulation at an elevated speed (2,200 RPM) and load (12.7 bar BMEP) is capable of improving volumetric efficiency via a production viable lost motion enabled boot profile shape. Phased and dwell profiles were also evaluated. These profiles were compared against each other for two separately simulated cases: (1) modulating both intake valves per cylinder and (2) modulating one of the two intake valves per cylinder. The boot, phase, and dwell profiles demonstrate volumetric efficiency improvements of up to 3.33, 3.41, and 3.5%, respectively, for two-valve modulation, while realizing 2.79, 2.59, and 3.01%, respectively, for single-valve modulation. As a result, this article demonstrates that nearly all of the volumetric efficiency benefits achieved while modulating IVC via dwell profiles are possible with production viable boot and phased profiles.
Variable valve actuation (VVA) enables intake and exhaust valve modulation to directly impact the gas exchange process of an engine (Payri et al.,
Prior research has focused on understanding the impact that IVC modulation has on the engine gas exchange, particularly, advancing or delaying IVC to modulate volumetric efficiency. Vos et al. (
Modulating volumetric efficiency and effective compression ratio (ECR) via LIVC typically involve using a dwell profile, as shown in Figure
The boot profile differs from the dwell profile by shifting the dwell duration from the peak lift to a lower lift in order to delay IVC, as shown in Figure
Another method is to use added motion to implement LIVC for extending the valve closing event. This implementation would create the normal valve profile as the default configuration. LIVC could be activated by using an additional lift to create the boot shape. The event could be implemented using a switching roller finger follower (SRFF) or alternatively an additional cam profile utilizing a lost motion capsule.
Incorporating a boot for LIVC enables the more production viable cam profile shown in Figure
Intake phasing maintains the same duration as the nominal profile while incorporating a desired phase, as illustrated in Figure
In the following, relative volumetric efficiency merits of the dwell, boot, and phased intake strategies are compared and shown to be similar. As a result, production viable boot and phased profiles can be implemented instead of the often studied dwell profile.
Figure
Gamma Technologies’ engine simulation software (GT-Power) was utilized to model a six-cylinder Cummins diesel engine incorporating valve motion flexibility. The engine includes a cooled high-pressure EGR loop, a variable geometry turbine (VGT), and a high-pressure common rail fuel injection arrangement, as shown in Figure
High-level schematic of the experimental test bed setup.
An experimental test bed was used to validate the simulation model. The test bed includes a medium-duty diesel engine equipped with a fully flexible variable valve actuation (VVA) system that enables cylinder independent, cycle-to-cycle control of the valves.
Figure
A laminar flow element (LFE) is used to determine the fresh air flow rate through a conditioned combustion air system, which maintains the temperature and relative humidity. The exhaust gas flows through a passively operating aftertreatment system (i.e., no dosing).
The engine’s six cylinders use Kistler 6067 C and AVL QC34C pressure transducers to record in-cylinder pressure by means of an AVL 621 Indicom module. Necessary data are logged and monitored using a dSPACE data acquisition system that interfaces with the experimental test bed. The engine control module (ECM) is directly connected to the dSPACE system for control of air handling actuators such as the EGR valve, the VGT nozzle, and cycle-by-cycle monitoring and control of parameters such as injection timing, fuel quantity, and rail pressure.
Simulations were conducted at an elevated speed of 2,200 RPM and a load of 12.7 bar BMEP using GT-Power. The intake valves on each cylinder were modulated to simulate three independent profiles: a dwell profile, a boot profile, and a phased profile, as shown in Figures
The experimental engine test bed setup used in this study is only capable of modulating both intake valves on each cylinder simultaneously. Figures
Experimental and simulation results illustrate similar trends over an IVC timing range from nominal to 40 CAD delayed.
The relationship between volumetric efficiency and charge mass flow rate can be expressed via the “speed-density equation”:
where
This section demonstrates that nearly all the volumetric efficiency benefits achieved while modulating IVC via dwell profiles, as illustrated in Figures
The boot profile can be varied in two locations: (1) the height of the boot and (2) the IVC timing of the profile. Figure
Two-valve modulation via GT-Power simulation predicts that a majority of the volumetric efficiency benefits can be achieved using a boot height of 5 mm, as shown in Figure
Simulation results for a boot profile where the height of the boot is varied from 3 to 5 mm, and the IVC timing is varied from nominal to 40 CAD delayed. The results for different boot heights were compared to dwelling at the peak of one intake valve, represented by the parabolic-like lines.
Similarly, one-valve modulation via GT-Power simulation predicts that a majority of the volumetric efficiency benefits can be realized using a boot height of 5 mm, as shown in Figure
Figure
Simulated intake phasing profile with IVO/IVC timings ranging from nominal (0 CAD) to 40 CAD after nominal.
Simulation for the phased intake profile predicts a phase delay of 17 CAD to be optimal with a 3.41% benefit in volumetric efficiency, very similar to that achieved via LIVC with a dwell profile, as shown in Figure
Figure
Figure
Two-valve modulation for a nominal profile and dwell profiles with IVC timings of 20 CAD and 40 CAD delayed. The top image shows the valve profiles that correspond to the mass flows in the bottom image.
The boot profiles realize a similar increase to mass flow, compared to the dwell profiles, at heights of 3, 4, and 5 mm, as shown in Figure
Two-valve modulation for a nominal profile and boot profiles with an IVC timing of 20 CAD delayed and boot heights of 3, 4, and 5 mm. The top image shows the valve profiles that correspond to the mass flows in the bottom image.
The phased profiles realize a delayed increase in mass flow rate, compared to the nominal profile, per Figure
Two-valve modulation for a nominal profile and phased profiles with valve timings of 17 CAD and 40 CAD delayed. The top image shows the valve profiles that correspond to the mass flows in the middle image and the intake manifold pressures in the bottom image.
Table
Percentages are relative to the dwell profile results for two-valve modulation.
# of modulated valves | Peak Dwell | 5-mm “Boot” | 4-mm “Boot” | 3-mm “Boot” | Intake phasing |
---|---|---|---|---|---|
Two | 100% | 95.4% | 89.7% | 81.1% | 97.7% |
One | 86.2% | 79.9% | 72.8% | 63.0% | 74.2% |
Volumetric efficiency and ECR have been shown in previous studies to be related when modulating LIVC timings via dwell profiles (Modiyani et al.,
Boot profiles realize volumetric efficiency benefits, as shown in Figures
Intake valve phasing results in larger pumping loops, a s shown in Figures
In summary, intake valve phasing results in the following detrimental effects:
a sensitive IVC and volumetric efficiency relationship (Figures a complex volumetric efficiency and ECR relationship (Figure a decrease in OCE (Figures
As a result, the lost motion enabled boot profiles are the preferred production viable means for realizing volumetric efficiency improvements through LIVC timings at elevated engine speeds in cam engine.
IVC modulation is commonly cited as a strategy for altering volumetric efficiency through increased dwell at peak lift of the profile. Previous work by the authors has demonstrated the merit behind improving volumetric efficiency via LIVC dwell profiles as a means to improving fuel efficiency (Vos et al.,
Specifically, a boot profile with a height of 5 mm on both intake valves realizes 95.4% of the volumetric efficiency benefits achieved via the dwell profile at the 2,200 RPM, 12.7 bar BMEP operating condition. In addition, implementing the boot profile reduces the amount of lost motion required, improving the production viability and durability. Modulating only one of the two intake valves as a boot with LIVC realizes 79.9% of the volumetric efficiency benefit. One-valve modulation is more cost effective than two-valve modulation, primarily because half of the hardware is required. Boot heights of 3 and 4 mm yield appreciable volumetric efficiency benefits, for both one-valve and two-valve modulation scenarios.
IVC delay via intake valve phasing realizes 97.7 and 74.2% of the volumetric efficiency benefit achieved through dwell profiles for single- and two-valve modulation, respectively. However, phasing has the following disadvantages: (1) a sensitive IVC and volumetric efficiency relationship, (2) a complex volumetric efficiency and ECR relationship, and (3) a significant decrease in OCE. Lost motion enabled boot profiles are therefore the preferred production viable strategy for realizing volumetric efficiency improvements through LIVC timings at elevated engine speeds in cam engine.
BDC | Bottom Dead Center |
BMEP | Brake Mean Effective Pressure |
BTE | Brake Thermal Efficiency |
CAD | Crank Angle Degree(s) |
ECM | Engine Control Module |
ECR | Effective Compression Ratio |
EGR | Exhaust Gas Re-circulation |
EIVC | Early Intake Valve Closure |
IMP | Intake Manifold Pressure |
IVC | Intake Valve Closing |
LFE | Laminar Flow Element |
LIVC | Late Intake Valve Closure |
LVDT | Linear Variable Differential Transformer |
OCE | Open Cycle Efficiency |
RPM | Revolutions Per Minute |
TDC | Top Dead Center |
VGT | Variable Geometry Turbine |
VV | Variable Valve Actuation |
GS: graduate student mentorship and analysis of strategy development. KV: lead writer and researcher that did the analysis and wrote most of the paper. JM: industrial perspective on production-viable valve profiles to consider. LF: essential hardware and software support.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This project is funded by Cummins Inc. and Eaton, with experiments performed at Ray W. Herrick laboratories in Purdue University. The medium-duty engine was provided by Cummins Inc., along with ample technical assistance provided by both Cummins Inc. and Eaton. The authors would also like to thank their colleagues Mrunal Joshi, Alexander Taylor, Troy Odstrcil, Matthew Van Voorhis and the shop staff at Herrick labs, particularly David Meyer, for the immense support he has extended toward this work.