Karla Stricker

Physically based volumetric efficiency model for diesel engines utilizing variable intake valve actuation

Advanced diesel engine architectures employing flexible valve trains enable emissions reductions and fuel economy improvements. Flexibility in the valve train allows engine designers to optimize the gas exchange process in a manner similar to how common rail fuel injection systems enable optimization of the fuel injection process. Modulating valve timings directly impacts the volumetric efficiency of the engine since it directly controls how much mass is trapped in the cylinders. In fact, it will be shown that the control authority of valve timing modulation over volumetric efficiency, that is, the range of volumetric efficiencies achievable due to modulation of the valve timing, is three times larger than the range achievable by modulation of other engine actuators such as the exhaust gas recirculation valve or the variable geometry turbocharger. Traditional empirical or regression-based models for volumetric efficiency, while suitable for conventional valve trains, are therefore challenged by flexible valve trains. The added complexity and additional empirical data needed for wide valve timing ranges limit the usefulness of these methods. A simple physically based volumetric efficiency model was developed to address these challenges. The model captures the major physical processes occurring over the intake stroke, and is applicable to both conventional and flexible intake valve trains. The model inputs include temperature and pressure in both the intake and exhaust manifolds, intake and exhaust valve event timings, engine cylinder bore, stroke, connecting rod lengths, engine speed, and effective compression ratio. The model is physically based, requires no regression tuning parameters, is generalizable to other engine platforms, and has been experimentally validated using an advanced multi-cylinder diesel engine equipped with a fully flexible variable intake valve actuation system. Experimental data were collected over a wide range of the operating space of the engine and augmented with air handling actuator and intake valve timing sweeps to maximize the range of conditions used to thoroughly experimentally validate the model for a total of 286 operating conditions. The physically based volumetric efficiency model will be shown to predict the experimentally calculated volumetric efficiency to within 5 per cent for all cases with a root mean square error of less than 2.5 per cent for the entire dataset. The physical model developed differs from previous physical modelling work through the novel application of effective compression ratio, incorporation of no tuning parameters, and extensive validation on a unique engine test bed with fully flexible intake valve actuation.

Effect of intake valve closure modulation on effective compression ratio and gas exchange in turbocharged multi-cylinder engines utilizing EGR

Advanced combustion strategies including premixed charge compression ignition, homogeneous charge compression ignition, and lifted flame combustion are promising approaches for meeting increasingly stringent emissions regulations and improving fuel efficiency in next generation powertrains. Variable valve actuation and closed-loop control promise to play a key role in the promotion and control of these advanced combustion modes. For example, modulation of intake valve closure timing dictates the effective compression ratio and influences the total amount of charge trapped inside the cylinder, and in so doing allows manipulation of the in-cylinder reactant concentrations and temperature prior to and during the combustion process. The effort described here uses data from, and an experimentally-validated simulation model for, a multi-cylinder engine with variable geometry turbocharging, cooled exhaust gas recirculation, and fully flexible variable valve actuation. This effort’s intent is to determine the control authority over the gas exchange process and effective compression ratio when intake valve closure timing modulation is included on a modern turbocharged diesel engine, as well as to lay the groundwork for closed-loop control design for the promotion and control of advanced combustion modes. The engine testbed at Purdue provides a unique opportunity to pursue these objectives for turbocharged engines with exhaust gas recirculation, as it is the only such engine system in academia outfitted with multi-cylinder fully-flexible valve actuation. A method to estimate in-cylinder temperature at top dead centre is also described. Candidate control architectures for both steady state and transient operation are introduced.

Estimation of effective compression ratio for engines utilizing flexible intake valve actuation

Modulation of the effective compression ratio, a measure of the amount of compression of in-cylinder gases above intake manifold conditions, is a key enabler of advanced combustion strategies aimed at reducing emissions while maintaining efficiency, and is directly influenced by modulation of intake valve closing time. To date, the effective compression ratio has most commonly been calculated from in-cylinder pressure data, requiring reliable in-cylinder pressure sensors. These sensors are generally not found on production engines, and thus a method is needed to determine effective compression ratio without in-cylinder pressure data. The work presented here outlines an estimation scheme that combines a high-gain observer with a physically-based volumetric efficiency model to estimate effective compression ratio using only information available from stock engine sensors, including manifold pressures and temperatures and air flows. The estimation scheme is compared to experimental engine data from a unique multi-cylinder diesel engine test bed with flexible intake valve actuation. The effective compression ratio estimator was tested transiently at five engine operating points and demonstrates convergence within three engine cycles after a transient event has occurred, and exhibits steady-state errors of less than 3%.

Oxygen fraction estimation for diesel engines utilizing variable intake valve actuation

Advanced diesel engine architectures employing flexible valve trains enable emissions reductions and fuel economy improvements through advanced combustion strategies. These combustion strategies, such as pre-mixed charge compression ignition (PCCI), homogenous charge compression ignition (HCCI) and low temperature combustion (LTC), are controlled and enabled through the use of flexible valve trains. The in-cylinder oxygen concentration serves as a critical input in controlling these strategies. Unfortunately, the in-cylinder oxygen concentration is extremely difficult to measure on production engines. However, the oxygen concentrations in the intake and exhaust manifold can be utilized to calculate the in-cylinder oxygen concentration when the charge and residual in-cylinder mass are available. A model-based observer is developed to estimate the oxygen concentration in the intake and exhaust manifolds. The oxygen concentration estimates will be sensitive to errors in the mass flows of the manifold filling dynamics. To improve the EGR flow measurement, a high-gain observer is implemented to provide a more accurate EGR flow estimate. The observer estimates the oxygen concentrations to within 0.5% O2 and converges in less than 0.5 seconds.

Control-oriented modeling of diesel engine gas exchange

Modeling and control of the gas exchange process in modern diesel engines is critical for the promotion and control of advanced combustion strategies. However, most modeling efforts to date use complex stand-alone simulation packages that are not easily integrated into, or amenable for the synthesis of, engine control systems. Simpler control-oriented models have been developed, however in many cases they do not directly capture the complete dynamic interaction of air handling system components and flows in multi-cylinder diesel engines with variable geometry turbocharging and high pressure exhaust gas recirculation. This paper describes a simple, low order model of the air handling system for a multi-cylinder turbocharged diesel engine with cooled exhaust recirculation, validated against engine test data.

PCCI Control Authority of a Modern Diesel Engine Outfitted With Flexible Intake Valve Actuation

Premixed charge compression ignition (PCCI), an advanced mode combustion strategy, promises to simultaneously deliver the fuel efficiency of diesel combustion and the ultralow NO x emissions that usually require advanced exhaust aftertreatment. A flexible, computationally efficient, and whole engine simulation model for a 2007 6.7 l diesel engine with exhaust gas recirculation (EGR), variable geometry turbocharging (VGT), and common rail fuel injection was validated after extensive experimentation. This model was used to develop strategies for highly fuel-efficient and ultralow NO x emission PCCI. The primary aim of this modeling investigation is to determine the PCCI control authority present on a modern diesel engine outfitted with both conventional actuators (multi-pulse fuel injectors, EGR valve, and VGT) and flexible intake valve closure modulation, which dictates the effective compression ratio. The results indicate that early fuel injection coupled with ECR reduction and modest amounts of EGR yield a well-timed PCCI exhibiting 70% + reductions in NO x with no fuel consumption penalty over a significant portion of the engine operating range.

Physically-Based Volumetric Efficiency Model for Diesel Engines Utilizing Variable Intake Valve Actuation

Advanced diesel engine architectures employing flexible valve trains enable emissions reductions and fuel economy improvements. Flexibility in the valve train allows engine designers to optimize the gas exchange process in a manner similar to how common rail fuel injection systems enable optimization of the fuel injection process. Modulating valve timings directly impacts the volumetric efficiency of the engine. In fact, the control authority of valve timing modulation over volumetric efficiency is three times larger than that due to any other engine actuator. Traditional empirical or regression-based models for volumetric efficiency, while suitable for conventional valve trains, are therefore challenged by flexible valve trains. The added complexity and additional empirical data needed for wide valve timing ranges limit the usefulness of these methods. A physically-based volumetric efficiency model was developed to address these challenges. The model captures the major physical processes occurring over the intake stroke, and is applicable to both conventional and flexible valve trains. The model inputs include temperature and pressure in the intake and exhaust manifolds, intake and exhaust valve timings, bore, stoke, connecting rod length, engine speed and effective compression ratio, ECR. The model is physically-based, requires no regression tuning parameters, is generalizable to other engine platforms, and has been experimentally validated using an advanced multi-cylinder diesel engine equipped with a flexible variable intake valve actuation system. Experimental data was collected over a wide range of the operating space of the engine and augmented with air handling actuator and intake valve timing sweeps to maximize the range of conditions used to thoroughly experimentally validate the model for a total of 217 total operating conditions. The physical model developed differs from previous physical modeling work through the novel application of ECR, incorporation of no tuning parameters and extensive validation on unique engine test bed with flexible intake valve actuation.Copyright

Control-oriented premixed charge compression ignition combustion timing model for a diesel engine utilizing flexible intake valve modulation

This paper describes a simple, analytical, control-oriented model for prediction of combustion timing during premixed charge compression ignition combustion with early fuel injection. The model includes direct dependence on in-cylinder temperature, in-cylinder pressure, and the total in-cylinder O2 mass fraction, including the contribution of recirculated exhaust gas, residual burned gas, and backflow during the valve overlap period. The model is extensively validated against experimental premixed charge compression ignition data from a multi-cylinder diesel engine utilizing high-pressure recirculated exhaust gas, variable geometry turbocharging, and flexible intake valve actuation, which allows control over the engine’s effective compression ratio. The results show that across a wide range of input conditions, the model predicts the start of combustion within ±2° crank angle of the experimental values for all but three of the 180 data points (98%+ accuracy), with a root mean square error of 0.86° crank angle. The experimental start of combustion ranges from as early as −19.3° crank angle to as late as +0.6° crank angle by heavily exercising the control actuators, specifically the commanded start of injection timing SOIecm, the intake valve closure timing, and the engine’s air-handling system (recirculated exhaust gas valve position and variable geometry turbocharger position). Analysis is performed to isolate the effects and control authority of each actuator on the ignition delay and start of combustion timing. During these actuator sweeps, the model captures complex relationships and predicts the start of combustion within ±2° crank angle of the experimental values. This premixed charge compression ignition combustion timing model can be coupled with a gas exchange model for control algorithm design and analysis.

Turbocharger Map Reduction for Control-Oriented Modeling

The gas exchange process in a modern diesel engine is generally modeled using manufacturer-provided performance maps that describe mass flows through, and efficiencies of, the turbine and compressor. These maps are typically implemented as look-up tables requiring multiple interpolations based on pressure ratios across the turbine and compressor, as well as the turbocharger shaft speed. In the case of variable-geometry turbochargers, the nozzle position is also an input to these maps. This method of interpolating or extrapolating data is undesirable when modeling for estimation and control, and though there have been several previous efforts to reduce dependence on turbomachinery maps, many of these approaches are complex and not easily implemented in engine control systems. As such, the aim of this paper is to reduce turbocharger maps to analytical functions for models amenable to estimation and control.Copyright

Control-oriented PCCI combustion timing model for a diesel engine utilizing flexible intake valve modulation and high EGR levels

This paper describes a simple, analytical, control-oriented and physically-based model for prediction of combustion timing during PCCI combustion. The model includes direct dependence on in-cylinder temperature, in-cylinder pressure, and the total in-cylinder O2 mass fraction. It is extensively validated with experimental PCCI data from a multi-cylinder engine with almost exclusively stock hardware (stock pistons, injectors/nozzles, turbocharger, etc.) and variable valve actuation. The results show that across a wide range of input conditions the model predicts the start of combustion (SOC) within ±2°CA of the experimental values for all but one of the 119 data points. The experimental SOC ranges from as early as -19.3°CA to as late as +0.6°CA by heavily exercising the control authority over SOC provided by SOIecm, IVC/ECR modulation (ECR ranges from 12:1 to 18:1), and the engines air-handling system. This PCCI combustion timing model can be coupled with a gas exchange model for control algorithm design.