Jason Martz

Thermodynamic sweet spot for high-efficiency, dilute, boosted gasoline engines:

Recent developments in ignition, boosting, and control systems have opened up new opportunities for highly dilute, high-pressure combustion regimes for gasoline engines. This study analytically explores the fundamental thermodynamics of operation in these regimes under realistic burn duration, heat loss, boosting, and friction constraints. The intent is to identify the benefits of this approach and the path to achieving optimum engine and vehicle-level fuel economy. A simple engine/turbocharger model in GT-Power is used to perform a parametric study exploring the conditions for best engine efficiency. These conditions are found in the mid-dilution range, a result of the tradeoff between fluid property benefits of lean mixtures and friction benefits of higher loads. Dilution with exhaust gas is nearly as effective as air dilution when compared using a ‘fuel-to-charge’ equivalence ratio defined as Φ′≡Φ (1-RGF) where RGF is the total residual gas fraction. Optimal brake efficiencies are obtained over a range 0.45 ≤ Φ′ ≤ 0.65 for operation up to 3 bar manifold pressure, yielding peak temperatures under 2100 K and peak pressures under 150 bar. These conditions are intermediate between homogeneous charge compression ignition and spark-ignition regimes, and are the subject of much current research on advanced combustion modes. An engine–vehicle drive train simulation shows that accessing this thermodynamic sweet spot has the potential for vehicle fuel economy gains between 23% and 58%.

Reaction-space analysis of homogeneous charge compression ignition combustion with varying levels of fuel stratification under positive and negative valve overlap conditions

Full-cycle computational fluid dynamics simulations with gasoline chemical kinetics were performed to determine the impact of breathing and fuel injection strategies on thermal and compositional stratification, combustion and emissions during homogeneous charge compression ignition combustion. The simulations examined positive valve overlap and negative valve overlap strategies, along with fueling by port fuel injection and direct injection. The resulting charge mass distributions were analyzed prior to ignition using ignition delay as a reactivity metric. The reactivity stratification arising from differences in the distributions of fuel–oxygen equivalence ratio ( ϕ FO ), oxygen molar fraction ( χ O 2 ) and temperature (T) was determined for three parametric studies. In the first study, the reactivity stratification and burn duration for positive valve overlap valve events with port fuel injection and early direct injection were nearly identical and were dominated by wall-driven thermal stratification. nitrogen oxide (NO) and carbon monoxide (CO) emissions were negligible for both injection strategies. In the second study, which examined negative valve overlap valve events with direct injection and port fuel injection, reactivity stratification increased for direct injection as the ϕ FO and T distributions associated with direct fuel injection into the hot residual gas were positively correlated; however, the latent heat absorbed from the hot residual gas by the evaporating direct injection fuel jet reduced the overall thermal and reactivity stratification. These stratification effects were offsetting, resulting in similar reactivity stratification and burn durations for the two injection strategies. The higher local burned gas temperatures with direct injection resulted in an order of magnitude increase in NO, while incomplete combustion of locally over-lean regions led to a sevenfold increase in CO emissions compared to port fuel injection. The final study evaluated positive valve overlap and negative valve overlap valve events with direct injection. Relative to positive valve overlap, the negative valve overlap condition had a wider reactivity stratification, a longer burn duration and higher NO and CO emissions associated with reduced fuel–air mixing.

A low-order adaptive engine model for SI–HCCI mode transition control applications with cam switching strategies

This article presents a low-order engine model to support model-based control development for mode transitions between spark ignition (SI) and homogeneous charge compression ignition (HCCI) combustion modes in gasoline engines. The modeling methodology focuses on cam switching mode transition strategies wherein the mode is abruptly changed between SI and recompression HCCI via a switch of the cam lift and phasing. The model is parameterized to a wide range of steady-state data which are selected to include conditions pertinent to cam switching mode transitions. An additional HCCI combustion model parameter is augmented and tuned based on transient data from SI to HCCI mode transitions where the conditions can be significantly outside any contained in the baseline steady-state parameterization. An adaptation routine is given which allows transient data be assimilated in online operation to update the augmented parameter and improve SI–HCCI transition predictions. With the baseline steady-state parameterization and augmented mode transition parameter, the model is shown to reproduce both steady-state data and transient performance output time histories from SI–HCCI transitions with considerable accuracy.

A thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part I – Adiabatic core ignition model

This two-part article presents a model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two components: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development and validation of the homogeneous charge compression ignition model for use under a broad range of operating conditions. Using computational fluid dynamics simulations of the negative valve overlap valve events typical of homogeneous charge compression ignition operation, it is shown that there is no noticeable reaction progress from low-temperature heat release, and that ignition is within the high-temperature regime (T > 1000 K), starting within the highest temperature cells of the computational fluid dynamics domain. Additional parametric sweeps from the computational fluid dynamics simulations, including sweeps of speed, load, intake manifold pressures and temperature, dilution level and valve and direct injection timings, showed that the assumption of a homogeneous charge (equivalence ratio and residuals) is appropriate for ignition modelling under the conditions studied, considering the strong sensitivity of ignition timing to temperature and its weak compositional dependence. Use of the adiabatic core temperature predicted from the adiabatic core model resulted in temperatures within ±1% of the peak temperatures of the computational fluid dynamics domain near the time of ignition. Thus, the adiabatic core temperature can be used within an auto-ignition integral as a simple and effective method for estimating the onset of homogeneous charge compression ignition auto-ignition. The ignition model is then validated with an experimental 92.6 anti-knock index gasoline-fuelled homogeneous charge compression ignition dataset consisting of 290 data points covering a wide range of operating conditions. The tuned ignition model predictions of θ 50 have a root mean square error of 1.7° crank angle and R2 = 0.63 compared to the experiments.

Minimum Backpressure Wastegate Control for a Boosted Gasoline Engine With Low Pressure External EGR

Turbocharging and downsizing (TRBDS) a gasoline direct injection (GDI) engine can reduce fuel consumption but with increased drivability challenges compared to larger displacement engines. This tradeoff between efficiency and drivability is influenced by the throttle-wastegate control strategy. A more severe tradeoff between efficiency and drivability is shown with the introduction of Low-Pressure Exhaust Gas Recirculation (LP-EGR). This paper investigates and quantifies these tradeoffs by designing and implementing in a one-dimensional (1D) engine simulation two prototypical throttle-wastegate strategies that bound the achievable engine performance with respect to efficiency and torque response. Specifically, a closedwastegate (WGC) strategy for the fastest achievable response and a throttle-wastegate strategy that minimizes engine backpressure (MBWG) for the best fuel efficiency, are evaluated and compared based on closed loop response. The simulation of an aggressive tip-in (the driver’s request for torque increase) shows that the wastegate strategy can negotiate a 0.8% efficiency gain at the expense of 160 ms slower torque response both with and without LP-EGR. The LP-EGR strategy, however offers a substantial 5% efficiency improvement followed by an undesirable 1 second increase in torque time response, clarifying the opportunities and challenges associated with LP-EGR. INTRODUCTION Market trends and fuel economy regulations are pushing manufacturers to develop more efficient spark-ignited (SI) internal combustion engines. While engine downsizing and boosting is one well known approach for improving fuel economy, the slower air path dynamics associated with turbocharger lag can negatively impact the drivability of these engines. The best drivability is achieved with wastegate control strategies [1] that keep the wastegate closed at part load to maintain the highest possible turbocharger speed when the engine is partially throttled. The elevated turbocharger speed and rapid intake filling during throttle opening enable fast torque response during tip-in. However, this approach sacrifices fuel economy for performance. Turbo-lag becomes even more severe when a minimum backpressure strategy is used for wastegate control. This strategy, which is also known as optimal fuel economy wastegate control [1], regulates the wastegate position to minimize the turbine inlet pressure, and improves fuel economy as a result of reduced pumping losses. Transient response unfortunately degrades with this approach as the throttle must be kept as open as possible because of the diminished boost pressure. Many different controllers have been introduced for wastegate control to improve turbocharged engine response. For example, Moulin et al. [2] use a non-linear control strategy based on feedback linearization and constrained motion planning. Thomasson et al. [3] model a pneumatic wastegate and develop a controller consisting of a feedforward loop and a feedback PID loop. A multivariable throttle and wastegate controller targeting intake manifold and boost pressures is introduced by Karnik et al. [4], while a nonlinear controller adopting a minimum backpressure wastegate control strategy is presented by Proceedings of the ASME 2016 Dynamic Systems and Control Conference DSCC2016 October 12-14, 2016, Minneapolis, Minnesota, USA

Fast Computation of Combustion Phasing and Its Influence on Classifying Random or Deterministic Patterns

The classification between a sequence of highly variable combustion events that have an underlying deterministic pattern and a sequence of combustion events with similar level of variability but random characteristics is important for control of combustion phasing. In the case of high cyclic variation (CV) with underlying deterministic patterns, it is possible to apply closed loop combustion control on a cyclic-basis with a fixed mean value, such as injection timing in homogeneous charge compression ignition (HCCI) or spark timing in spark ignition (SI) applications, to contract the CV. In the case of a random distribution, the high CV can be avoided by shifting operating conditions away from the unstable region via advancing or retarding the injection timing or the spark timing in the mean-sense.Therefore, the focus of this paper is on the various methods of computing CA50 for analysing and classifying cycle-to-cycle variability. The assumptions made to establish fast and possibly on-line methods can alter the distribution of the calculated parameters from cycle-to-cycle, possibly leading to incorrect pattern interpretation and improper control action.Finally, we apply a statistical technique named “permutation entropy” for the first time on classifying combustion patterns in HCCI and SI engine for varying operating condition. Then the various fast methods for computing CA50 feed the two statistical methods, permutation and the Shannon entropy, and their differences and similarities are highlighted.Copyright

A low-order hcci model extended to capture SI-HCCI mode transition data with two-stage cam switching

A low-order homogeneous charge compression ignition (HCCI) combustion model to support model-based control development for spark ignition (SI)/HCCI mode transitions is presented. Emphasis is placed on mode transition strategies wherein SI combustion is abruptly switched to recompression HCCI combustion through a change of the cam lift and opening of the throttle, as is often employed in studies utilizing two-stage cam switching devices. The model is parameterized to a steady-state dataset which considers throttled operation and significant air-fuel ratio variation, which are pertinent conditions to two-stage cam switching mode transition strategies. Inspection and simulation of transient SI to HCCI (SI-HCCI) mode transition data shows that the extreme conditions present when switching from SI to HCCI can cause significant prediction error in the combustion performance outputs even with the model’s adequate steady-state fit. When a correction factor related to residual gas temperature is introduced to account for these extreme conditions, it is shown that the model reproduces transient performance output time histories in SI-HCCI mode transition data. The model is thus able to capture steady-state data as well as transient SI-HCCI mode transition data while maintaining a low-order cycle to cycle structure, making it tractable for model-based control of SI-HCCI mode transitions.Copyright

A Comparison of Valving Strategies Appropriate For Multimode Combustion Within a Downsized Boosted Automotive Engine—Part I: High Load Operation Within the Spark Ignition Combustion Regime

As future downsized boosted engines may employ multiple combustion modes, the goal of the current work is the definition of valving strategies appropriate for moderate to high load spark ignition (SI) combustion and for spark assisted compression ignition (SACI) combustion at low to moderate loads for an engine with variable valve timing capability and fixed camshaft profiles. The dilution and unburned gas temperature requirements for SACI combustion can be markedly different from those of SI; therefore it is important to ensure that a given valving strategy is appropriate for operation within both regimes. This paper compares one dimensional (1D) thermodynamic simulations of rated engine operation with positive valve overlap (PVO) and a baseline negative valve overlap (NVO) camshaft design in a boosted automotive engine with variable valve timing capability. Several peak lifts and valve open durations are investigated to guide the down-selection of camshaft profiles for further evaluation under SACI conditions in a companion paper.While the results of this study are engine specific, rated performance predictions show that the duration of both the intake and exhaust camshafts significantly impacts the ability to achieve high load operation. While it was noted that the flow through the exhaust valve chokes for the majority of the exhaust stroke for peak exhaust lifts less than 8 mm, the engine rating could be achieved with peak intake lifts as low as 4 mm. Therefore, camshafts with peak lifts of 8/4 mm exhaust/intake were down selected to facilitate multimode combustion operation with high levels of PVO. Analysis of high load operation with the down-selected camshafts indicates that peak unburned gas temperatures remain low enough to mitigate end-gas knock, while other variables such as peak cylinder pressure, turbine inlet temperature and turbocharger speed are all predicted to be within acceptable limits.Copyright

Combustion shaping using multivariable feedback control

As high exhaust gas recirculation (EGR) is introduced for efficiency, the combustion duration and combustion delay is elongated due to slow fuel burn rates requiring flexible and robust management of both the combustion initiation and duration (what we call combustion shaping). Combustion shaping through cylinder pressure sensing and feedback control of spark advance (SA) and EGR-valve position can be used for spark ignited (SI) engines operating within highly dilute, high efficiency regimes even where the combustion variability (CV) limits controller bandwidth. Although EGR is directly related with combustion duration, spark advance affects the start and duration of combustion simultaneously. This input/output coupling suggests a multivariable controller that coordinates the actuators. Control of SA and EGR is investigated with a coupled linear quadratic Gaussian (LQG) controller and compared with a decoupled proportional-integral (PI) controller. Simulation of the closed-loop system uses a simple engine model derived from system identification. Gain tuning was performed aiming for fast response without overshoot and considering cyclic variability reduction through a Kalman filter. Comparison of the simulated controllers shows that the LQG controller has better transients and responds better to CV.

Thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part II—Combustion model and evaluation against transient experiments:

This two-part article presents a combustion model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two parts: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development of the combustion model which is algebraic in form and is based on the key physical variables affecting the combustion process. The model is fit with experimental data collected from 290 discrete automotive homogeneous charge compression ignition operating conditions with moderate stratification resulting from both the direct injection and negative valve overlap valve events. Both the ignition model from part 1 and the combustion model from this article are implemented in GT-Power and validated against experimental homogeneous charge compression ignition data under steady-state and transient conditions. The ignition and combustion model are then exercised to identify the dominant variables affecting the homogeneous charge compression ignition and combustion processes. Sensitivity analysis reveals that ignition timing is primarily a function of the charge temperature, and that combustion duration is largely a function of ignition timing.