Fully Flexible Valve Actuation Research Articles

Iterative Learning for Laboratory Electro-Hydraulic Fully Flexible Valve Actuation System Transient Control

<div>Fully flexible valve actuation (FFVA) is a key enabling technology of internal engine combustion research and development. Two laboratory electro-hydraulic FFVA systems have been developed and implemented in R&D test cells. These FFVA systems were designed using repetitive control (RC), which is based on internal model principle (IMP), for constant engine speed operation. With the engine operating in a steady-state condition, the valve profile input is periodic. This can be accommodated by a repetitive controller, which provides the function of flexible control to step changes in valve lift, valve opening duration, and cam phase angle position.</div> <div>During engine speed transients, as the valve reference trajectory becomes aperiodic in the time domain, the controllers based on the linear time invariant (LTI) IMP, such as RC, are no longer applicable. Engine speed transient control is a desired function to engine research and other similar applications, such as motor control. Several investigations are reported with limited results because of the assumption of IMP and periodic input.</div> <div>This article presents the control design and verification of the iterative learning control (ILC) algorithm for the laboratory electro-hydraulic FFVA system. This algorithm tracks valve lift profiles under steady-state and transient operation. A dynamic model of the plant was obtained from experimental data to design and verify the effectiveness and robustness of this approach. The simple structure of the ILC in implementation and low cost in computation are crucial benefits to recommend the ILC. It is not an IMP-based approach, and its structure does not depend on the system input. Therefore, it has higher robustness to perturbation and modeling errors than other control methods for repetitive valve lift profile tracking tasks.</div>

The effects of diluent composition on the rates of HCCI and spark assisted compression ignition combustion

In this study, an HCCI engine equipped with fully-flexible valve actuation was used to explore the effects of diluent composition on the rates of HCCI and spark assisted compression ignition (SACI) combustion. The diluent composition of the charge was characterized by the molar O2 fraction in the unburned mixture which was modified at constant charge energy content through careful control of the exhaust gas recirculation (EGR) rate. During SACI operation the amount of EGR was systematically increased (thereby decreasing the molar O2 fraction in the charge) and the temperature of the unburned gas was varied to maintain constant combustion phasing at constant spark advance. A ∼45% relative increase in the molar O2 fraction of the unburned mixture required a ∼30K decrease in the unburned gas temperature at intake valve closing (IVC) to maintain combustion phasing. Higher rates of heat release attributable to flames were observed as mixture O2 fraction increased; however, under both HCCI and SACI conditions, the auto-ignition durations were relatively insensitive to diluent composition. As the charge molar O2 fraction increased, the net indicated thermal efficiency of SACI combustion increased by 4% (relative basis) and peak rates of heat release decreased by ∼24%. However, engine-out NOx emissions during SACI operation nearly quadrupled (260% increase) for the fuel lean mixture (highest molar O2 fraction) compared to the stoichiometric EGR dilute mixture (lowest molar O2 fraction). Two-zone heat release analysis was used to better understand the contributions of flame propagation and auto-ignition to the overall SACI combustion rate.

The effects of spark timing, unburned gas temperature, and negative valve overlap on the rates of stoichiometric spark assisted compression ignition combustion

Spark assisted compression ignition (SACI) combustion is a potential means of extending the high load limit of homogeneous charge compression ignition (HCCI) while maintaining high thermal efficiency. In these experiments, an HCCI engine equipped with fully-flexible valve actuation was used to explore the effect of spark assist in controlling peak heat release rates. Fueling rate was held constant at 19mg/cycle with a net indicated mean effective pressure (IMEPn) of ∼6.5bar. The fraction of flame heat release was varied from 18% to 34% by controlling spark timing and unburned gas temperature (Tu) via changes in internal and external exhaust gas recirculation (EGR). Internal EGR was adjusted by varying the duration of negative valve overlap (NVO). This strategy also allowed mixture composition and equivalence ratio (ϕ=1.0) to be maintained. It was found that combustion phasing (CA50) could be held constant while reducing peak heat release rates by ∼40% and ringing intensity by ∼75% with no penalty in thermal efficiency. Heat release analysis showed that the effects of SACI were consistent with changes in estimated laminar flame speed near the time of spark and a relatively constant temperature at the onset of end-gas auto-ignition. For the range of NVO investigated, potential variations in thermal and compositional stratification were found to have a weak effect on burn characteristics, affirming that temperature and spark timing were the primary variables affecting SACI burn rates under the conditions studied.

Effects of Charge Preheating Methods on the Combustion Phasing Limitations of an HCCI Engine With Negative Valve Overlap

Homogeneous charge compression ignition (HCCI) has the potential to reduce both fuel consumption and NOx emissions compared to normal spark-ignited (SI) combustion. For a relatively low compression ratio engine, high unburned temperatures are needed to initiate HCCI combustion, which is achieved with large amounts of internal residual or by heating the intake charge. The amount of residual in the combustion chamber is controlled by a recompression valve strategy, which relies on negative valve overlap (NVO) to trap residual gases in the cylinder. A single-cylinder research engine with fully-flexible valve actuation is used to explore the limits of HCCI combustion phasing at a load of ∼3 bar gross indicated mean effective pressure (IMEPg). This is done by performing two individual sweeps of (a) internal residual fraction (via NVO) and (b) intake air temperature to control combustion phasing. It is found that increasing both of these variables advances the phasing of HCCI combustion, which leads to increased NOx emissions and a higher ringing intensity. On the other hand, a reduction in these variables leads to greater emissions of CO and HC, as well as a decrease in combustion stability. A direct comparison of the two sweeps suggests that the points with elevated intake temperatures are more prone to ringing as combustion is advanced and less prone to instability and misfire as combustion is retarded. This behavior can be explained by compositional differences (air versus residual gas dilution) which lead to variations in burn rate and peak temperature. As a final study, two additional NVO sweeps are performed while holding intake temperature constant at 30 °C and 90 °C. Again, it is seen that for higher intake temperatures, combustion is more susceptible to ringing at advanced timings and more resistant to instability/misfire at retarded timings.

Knock In Various Combustion Modes in a Gasoline-Fueled Automotive Engine

Homogeneous charge compression ignition (HCCI) offers great potential for improved fuel economy and dramatically reduced NOx emissions, compared to typical spark ignition (SI) combustion. However, the benefits of HCCI are limited to low and medium loads by the simultaneous occurrence of combustion instability and knock at a maximum load that is too low for conventional SI combustion. To provide smooth operation in the intermediate range between HCCI and SI requires alternative combustion strategies. One such strategy is spark-assisted compression ignition (SACI), which uses a spark plug to initiate a flame that consumes a portion of the mixture, followed by autoignition of the remaining charge. This moderates the rapid heat release and allows higher loads to be achieved without exceeding knock and stability limits. In a recent study, we have explored this region and have found that spark assist at first dramatically reduces knock as load is raised above the HCCI limit; however, with further load increase, knock returns but in a form that resembles spark ignited knock rather than the HCCI knock. This study investigates in detail the knocking conditions observed in that work. The objectives of this study are twofold: first, to explore the differences between the two forms of knock and second, to apply and compare a number of commonly used metrics for knock and noise over the range of HCCI, SACI, and SI combustion. Experimental data were acquired on a single-cylinder DI research engine equipped with a fully flexible valve actuation (FFVA) system and fueled by research-grade gasoline. Cycle to cycle results based on filtered pressure traces are shown and compared with a number of knock measures including a widely used correlation for ringing intensity for HCCI combustion. Although based on a limited set of data, the results identify important qualitative features of the two forms of knock and point out significant differences among the knock metrics. The results suggest that further investigations are needed to fully understand both the knocking phenomenon and how best to quantify it.

Transient Control of Electro-Hydraulic Fully Flexible Engine Valve Actuation System

Fully flexible valve actuation (FFVA) system, often referred to as camless valvetrain, employs electronically controlled actuators in place of the camshaft to drive the intake and/or exhaust valves for internal combustion engines. This system enables the engine controller to tailor the valve event according to the engine operating condition in real-time to improve fuel economy, emissions, and torque output performance. This paper presents the transient control of a laboratory electro-hydraulic fully flexible valve actuation system. Transient control of the FFVA system includes lift transient, duration transient, phase transient, speed transient, and mode transient. With constant engine speed, the valve profile is periodic in time domain and the lift, phase, and duration transients can be realized using robust repetitive control. When the engine speed varies, the period of the valve profile changes in real-time. This phenomenon poses a fundamental challenge to the transient control problem and repetitive control cannot be applied anymore. To overcome this challenge, we propose a new valve profile consisting of a periodic portion and a dwell portion with time-varying duration. Robust repetitive control is then applied to the periodic portion and proportional plus integral and derivative (PID) control is applied to the dwell portion. These two controls are switched in real-time to achieve asymptotic valve profile tracking performance. To demonstrate the effectiveness of the proposed control method, we show real-time valve-lift profiles used to explore homogeneous charge compression ignition (HCCI) combustion at different engine operating conditions.

Electrohydraulic Fully Flexible Valve Actuation System With Internal Feedback

Fully flexible valve actuation (FFVA) system, often referred to as camless valvetrain, employs electronically controlled actuators in place of the camshaft to drive the intake or exhaust valves for internal combustion engines (ICEs). This system offers significant fuel economy benefits, emissions reduction, and better torque output performance for the ICE. It could also enable a number of advanced combustion concepts, such as homogeneous charge compression ignition. It further provides a common platform that incorporates the functions of cam phasing, two/three step cam or continuously variable lift, cylinder deactivation, port deactivation, etc. Therefore it is desirable to develop FFVA systems for future engines. In this paper, we first outline the technical barriers for developing production-viable FFVA systems. To address those challenges, a new electrohydraulic valve actuation concept with the “internal feedback” mechanism is presented. Key technical issues, such as dynamic range capability, valve motion performance, and energy consumption, are analyzed. Experimental results based on a prototype system are shown to demonstrate the capabilities and performance of the proposed system.