Advanced Combustion Strategies Research Articles

Control-Oriented Gas Exchange Model for Diesel Engines Utilizing Variable Intake Valve Actuation

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 multicylinder diesel engines with variable geometry turbocharging (VGT), high pressure exhaust gas recirculation (EGR), and flexible intake valve actuation. Flexibility in the valvetrain directly impacts the gas exchange process not only through the effect on volumetric efficiency but also through the combustion process and resulting exhaust gas enthalpy utilized to drive the turbomachinery. This paper describes a low-order, five state model of the air handling system for a multicylinder variable geometry turbocharged diesel engine with cooled EGR and flexible intake valve actuation, validated against 286 steady state and 62 transient engine operating points. The model utilizes engine speed, engine fueling, EGR valve position, VGT nozzle position, and intake valve closing (IVC) time as inputs to the model. The model outputs include calculation of the engine flows as well as the exhaust temperature exiting the cylinders. The gas exchange model captures the dynamic effects of the not only the standard air handling actuators (EGR valve position and VGT position) but also IVC timing, exercised over their useful operating ranges. The model's capabilities are enabled through the use of analytical functions to describe the performance of the turbocharger, eliminating the need to use look-up maps; a physically based control-oriented exhaust gas enthalpy submodel and a physically based volumetric efficiency submodel.

Reducing Cyclic Variability While Regulating Combustion Phasing in a Four-Cylinder HCCI Engine

The cyclic variability (CV) at late phasing conditions in autoignition engines with high residuals limits the operating range of such advanced combustion strategies. A model has been recently proposed by the authors that captures the experimental observations of CV in lean autoignition combustion in a single-cylinder engine. The model is here tuned to multicylinder engine data and is used to design controllers that reduce the CV. The dynamics are only stable for certain amounts of residual gas. At late phasing, with low amounts of residual gas, a cascade of period-doubling bifurcations occur leading to seemingly chaotic behavior. The deterministic dynamics are also mixed with significant levels of noise in the residual gas fraction. The approach to reduce CV is to control the fuel injection timing, which is an effective way of influencing the combustion phasing in the individual cylinders, with feedback from the combustion phasing. A PI controller and a reduced-order linear quadratic Gaussian (LQG) controller are designed and evaluated in experiments. Integral action is utilized for maintaining the average combustion phasing between open loop and closed loop, which is shown to be essential for a proper evaluation because of the high sensitivity of CV on the combustion phasing close to unstable operation. A quantitative analysis of the experimental results show that CV is notably reduced for various levels of residual gas fraction at one speed and load. The standard deviation of the combustion phasing is reduced by 17% on average over the open-loop behavior, which results in a 15% smaller coefficient of variation of the indicated work.

Scaling and dimensional methods to incorporate knock and flammability limits in models of high-efficiency gasoline and ethanol engines

Recent work has shown the utility of using simplified models with prescribed burn rates to assess the potential of advanced combustion strategies to increase engine efficiency. However, this approach can be improved by incorporating knock and flammability limits. This work incorporates such limits using a combination of simplified conceptual models that are based on theoretical understanding of knock and flame phenomenon and calibration with experimental results. Using this method, the ideal (unconstrained) and feasible (constrained by knock and flammability) potential of a high efficiency gasoline and E85 engine are compared against a baseline naturally aspirated gasoline engine. Turbocharging, dilution with EGR, and higher compression ratios are used to increase the efficiency potential of the high efficiency gasoline and E85 engines. Results demonstrate the benefit of using this simplified approach in modeling high efficiency engines: the high efficiency gasoline engine is most limited by knock while the E85 engine is limited much less; also increased EGR can be used for the E85 engine due to the higher flame speeds of ethanol. Fuel economy maps are created for each engine/fuel strategy and evaluated in a vehicle model to obtain fuel economy results. Results comparing feasible engines show that peak brake thermal efficiency (BTE) is increased by 11.4% for the high efficiency gasoline engine and 17.8% for the E85 engine, as compared to the baseline gasoline engine. Projected vehicle fuel economy (energy equivalent) improvements are 30.1% for the high efficiency gasoline, and 40.9% for the E85 engine relative to baseline.

Comparison of Low Temperature Combustion Strategies for Advanced Compression Ignition Engines with a Focus on Controllability

In the present study, various low temperature combustion strategies were investigated using single cylinder engine experiments. The combustion strategies that were investigated premix the majority of the fuel and do not require exhaust gas recirculation (EGR) to achieve ultra-low NOx and soot emissions for low- to mid-load engine operation. These types of advanced compression ignition combustion strategies have been shown to have challenges with combustion phasing control. The focus of the study was to compare engine performance and emissions, combustion sensitivity to intake conditions, and the ability to control any observed sensitivity through the fuel injection strategy. Even though these are steady state engine experiments, this will demonstrate a given combustion strategies controllability on a cycle-to-cycle basis. The combustion strategies that were investigated are fully premixed dual-fuel homogeneous charge compression ignition (HCCI), dual-fuel reactivity controlled compression ignition (RCCI), and single-fuel partially premixed combustion (PPC). The baseline operating condition was an engine load representative of a light-duty engine: 5.5 bar gross indicated mean effective pressure (IMEP) and 1500 rev/min. At the baseline operating condition, in which the boundary conditions were chosen to yield near optimal engine performance, all three combustion strategies demonstrated high gross indicated efficiency (∼47%) and ultra-low NOx and soot emissions. By perturbing the intake conditions, it was found that all three combustion strategies display similar combustion phasing sensitivities. Both dual-fuel HCCI and RCCI were able to readily correct the observed sensitivities through the global fuel reactivity with no negative implications on the NOx emissions. However, single-fuel PPC was unable to correct for the observed combustion phasing sensitivity and, in some cases, had negative implications on the NOx emissions.

Robust oxygen fraction estimation for conventional and premixed charge compression ignition engines with variable valve actuation

In-cylinder oxygen fraction serves as a critical control input to advanced combustion strategies, but is extremely difficult to measure on production engines. Fortunately, the in-cylinder oxygen levels can be estimated based on accurate estimates or measurements of the oxygen fraction in the intake and exhaust manifolds, the in-cylinder charge mass, and the residual mass. This paper outlines such a physically based, generalizable strategy to estimate the in-cylinder oxygen fraction from only production viable measurements or estimates of exhaust oxygen fraction, fresh air flow, charge flow, fuel flow, turbine flow and EGR flow. While several of these flows are accurately measured or estimated, significant errors in the turbine and EGR flows are commonly observed and can highly degrade the accuracy of any calculations which utilize these flows. An EGR flow estimator was developed to improve the accuracy of this flow measurement over the stock engine control module (ECM) method and is detailed in this paper. Furthermore, the in-cylinder oxygen estimation algorithm is developed, and proven, to be robust to turbine flow errors. Regulation of in-cylinder oxygen levels is of interest for not only in conventional combustion modes but also in advanced combustion strategies such as premixed charge compression ignition. The proposed oxygen fraction estimator is designed such that its performance and stability is ensured in both conventional and advanced combustion modes. The model-based observer estimates the oxygen fractions to be within 0.5% O2 and is shown to have exponential estimator error convergence with a time constant less than 0.05s, even with turbine flow errors of up to 25%.

Impact of Premixed Kerosene Fuel on Performance, Emission and Combustion Characteristics in Partial HCCI Engine

Homogeneous Charge Compression Ignition (HCCI) is an advanced combustion strategy that able to reduce nitric oxides (NOx) emissions and increase fuel efficiency simultaneously. This study investigated the effects of premixed kerosene fuel on the auto-ignition characteristics in a light-duty compression ignition engine. A partial HCCI engine was modified from a single cylinder, four-stroke, direct injection compression ignition engine. The partial HCCI is achieved by injecting kerosene fuel into the intake port of the engine, while maintaining diesel fuel injected into the cylinder for combustion triggering. The auto-ignition of kerosene fuel has been studied at various premixed ratios from 0 to 0.65, under constant engine speed of 1600rpm and 20Nm load. The results for performance, emissions and combustion were compared with those achieved without premixed fuel. From the heat release rate profile, it is observed clearly that two-stage and three-stage ignition occurred in some of the cases. In addition, the increase of premixed ratio to some extent significantly reduced nitric oxide (NO) emissions.

Closed-loop combustion control of biodiesel–diesel blends in premixed operating conditions enabled via high exhaust gas recirculation rates

Stringent emissions and fuel economy regulations motivate the investigation of advanced combustion strategies such as premixed charge compression ignition. However, controlling premixed charge compression ignition is challenging as there is no direct trigger for the combustion event. In addition, fuel-flexible applications must account for the impact of variable fuel properties on the premixed combustion proces. This paper identifies the control challenges in fuel-flexible premixed charge compression ignition and presents a control strategy for mitigating the differences in combustion phasing, the increases in nitrogen oxide emissions, and the decreases in power output typically encountered when operating with biodiesel in premixed operating conditions. When biodiesel is used in premixed operating conditions with the engine operating with the stock calibration for diesel fuel, the timing of the start of combustion and the peak heat release can shift by over 2° crank angle and 4° crank angle respectively, the nitrogen oxide emissions can increase by over 100%, and the torque output can drop by over 30% with respect to the engine performance with diesel. The control techniques presented in this paper use a biodiesel estimation method in conjunction with a physics-based accommodation strategy based on accounting for the differences between the fuel energy densities and the oxygen contents of diesel and biodiesel. This control method dictates fueling quantities on an energy basis instead of the traditional mass basis and maintains a nearly constant in-cylinder oxygen fraction for both diesel and biodiesel. At the premixed operating points considered in this study, this strategy is demonstrated to eliminate increases in the biodiesel nitrogen oxide emissions, decreases in the torque, and shifts in the combustion timing. The results demonstrate, first, that combustion timing control and nitrogen oxide emissions control can also be achieved through control of the in-cylinder oxygen fraction alone and, second, that energy-based fueling provides consistent in-cylinder oxygen fractions for diesel and biodiesel while also matching the torque outputs for the two fuels when considered at the same premixed operating point (the same speed, intake manifold temperature, charge flow, effective compression ratio, and combustion timing).

Characterization of Soot Particles Produced in a Transparent Research CR DI Diesel Engine Operating with Conventional and Advanced Combustion Strategies

The effect of the combustion mode on particle emission was analyzed both in the cylinder and at the exhaust of a direct injection (DI) Common Rail (CR) transparent research diesel engine by means of spectroscopic and conventional methods. The engine was equipped with a flexible electronic control unit (ECU) capable of operating up to 5 injections per cycle with different start of injection and dwell time allowing performing different combustion modes. The conventional diesel combustion, the homogeneous charge compression ignition (HCCI), and the low temperature combustion (LTC) modes were analyzed. In-cylinder broadband UV–visible scattering and extinction measurements were carried out to follow the particle formation and oxidation processes as well as to have information about their chemical nature and size distribution. The characterization of the particulate emission at the exhaust was performed by means of an electrical low pressure impactor (ELPI), for the counting and the sizing of the particles, an...

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%.

Gasoline partially premixed combustion: high efficiency, low NOx and low soot by using an advanced combustion strategy and a compression ignition engine

A light-duty engine Volvo D5 was run with gasoline to demonstrate that high-octane number fuels, in compression ignition engines, are able to achieve high effi ciency, low NOx and soot simultaneously. An advanced three stage injection strategy was developed. A load sweep was performed between 6 and 17 bar IMEP and 2000 rpm. Gross indicated effi ciency stays fairly constant at 47% with very low NOx and soot. In the second section of the paper, diesel was compared with gasoline, while in the fi nal part the commercial 140°UA nozzle was compared with the narrower one, 120°, used in the above-mentioned experiments.

Robust Oxygen Fraction Estimation for Diesel Engines Utilizing Variable Intake Valve Actuation

Advanced combustion strategies, such as pre-mixed charge compression ignition (PCCI), homogenous charge compression ignition (HCCI) and low temperature combustion (LTC), can be controlled and enabled through the use of flexible valvetrains. The in-cylinder oxygen fraction serves as a critical control input to these strategies, but is extremely difficult to measure on production engines. Fortunately, estimates or measurements of the oxygen fraction in the intake and exhaust manifold, the in-cylinder charge mass, and residual mass can be utilized to calculate the in-cylinder oxygen fraction. This paper outlines such a physically-based, generalizable strategy to estimate the in-cylinder oxygen fraction from only production viable measurements or estimates of exhaust oxygen fraction, fresh air flow, charge flow, fuel flow, turbine flow and EGR flow. The oxygen fraction estimates are compared to laboratory grade measurements available for the intake and exhaust manifolds. Furthermore, the in-cylinder oxygen estimation algorithm is developed, and proven, to be robust to turbine flow errors. The model-based observer estimates the oxygen fractions to within 0.5% O2 and is shown to have exponential convergence with a time constant less than 0.05 seconds, even with turbine flow errors of up to 25%. The observer is applicable to engines utilizing high pressure cooled exhaust gas recirculation, variable geometry turbocharging and 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.

Optical Engines as Representative Tools in the Development of New Combustion Engine Concepts

Single cylinder optical engines are used for Internal Combustion (IC) engine research as they allow for the application of qualitative and quantitative non-intrusive, diagnostic techniques to study in-cylinder flow, mixing, combustion and emissions phenomena. Such experimental data is not only important for the validation of computational models but can also provide a detailed insight into the physical processes occurring in-cylinder which is useful for the further development of new combustion strategies such as gasoline Homogeneous Charge Compression Ignition (HCCI) and Diesel Low Temperature Combustion (LTC). In this context, it is therefore important to ensure that the performance of optical engines is comparable to standard all-metal engines. A comparison of optical and all-metal engine combustion and emissions performance was performed within the present study. The objective was to investigate the principal differences between optical and all-metal engines and understand how these differences ultimately affect mixing, combustion and emissions formation processes. Experimental results reveal the significant impact of differences in combustion chamber wall temperatures between optical and standard engine piston bowls on combustion phasing and engine-out emissions. Quantitative measurements of piston wall temperatures using a laser-induced phosphorescence technique were performed which allowed the subsequent definition of appropriate engine operating strategies so as to compensate for differences in heat transfer properties. Furthermore, differences in combustion chamber geometry were also studied. Geometrical differences can arise as a result of dynamic (compressive/tensile) and thermal loading of the extended piston-liner assembly on the optical engine, potentially leading to changes in the effective Compression Ratio. In addition, intake charge dilution in optical engines is often achieved via the use of simulated Exhaust Gas Recirculation (EGR). A comparison has been made between simulated EGR (using pure nitrogen) with real EGR under Diesel LTC conditions. Finally, “pure”, single component fuels are often employed in optical Diesel engines due to laser diagnostic constraints. However, these fuels generally differ from standard Diesel fuel in terms of cetane number and fuel volatility which can significantly influence the combustion and emissions characteristics in optical engines. These aspects have also been investigated within the present study.An improved understanding of the differences between optical and all-metal engines has allowed us to develop appropriate strategies to compensate for these differences on the optical engine. It is shown here that combustion phasing (and engine-out emissions) matching between optical and all-metal engines can be achieved even for advanced LTC Diesel combustion strategies. The ability to ensure fully representative combustion and emissions behaviour of optical engines ultimately increases the value of optical engine data, highlighting the importance of using such engines as research tools for the further development of innovative, low emission combustion concepts.

HCCI/SCCI Load Limits and Stoichiometric Operation in a Multicylinder Naturally Aspirated Spark Ignition Engine Operated on Gasoline and E85

To meet demands for improvements in the CO2 emissions and fuel economy of gasoline passenger car engines advanced combustion strategies, to replace (or combine with) conventional spark ignition, must be developed and implemented. One possible strategy is homogeneous charge compression ignition (HCCI) achieved using negative valve overlap (NVO). However, several issues need to be addressed before this combustion strategy can be fully implemented in a production vehicle, one being to increase the upper load limit. One constraint at high loads is that the combustion becomes too rapid, leading to excessive pressure-rise rates and large pressure fluctuations (ringing), causing noise. A potential solution to this is to use charge stratification, but charge stratification normally gives rise to increased NO x emissions. Tests with a multicylinder engine reported here confirmed that there is significant potential to increase the upper load limit using charge stratification. In addition, the possibility of operating the engine in stoichiometric conditions, using a combination of NVO and external exhaust gas recirculation (EGR) (thus allowing the increased NO x emissions to be countered using a three-way catalyst) was investigated. Stoichiometric operation was found to be possible for both homogeneous and stratified modes, across a wide operating range, with small compromises in maximum load and fuel consumption. Nevertheless, delaying the need for a mode shift, and operating in stoichiometric conditions when entering a mode shift, should be beneficial in a drive cycle.

A Review on Diesel Soot Emission, its Effect and Control

The diesel engines are energy efficient, but their particulate (soot) emissions are responsible of severe environmental and health problems. This review provides a survey on published information regarding diesel soot emission, its adverse effects on the human health, environment, vegetations, climate, etc. The legislations to limit diesel emissions and ways to minimize soot emission are also summarized. Soot particles are suspected to the development of cancer; cardiovascular and respiratory health effects; pollution of air, water, and soil; impact agriculture productivity, soiling of buildings; reductions in visibility; and global climate change. The review covers important recent developments on technologies for control of particulate matter (PM); diesel particulate filters (DPFs), summarizing new filter and catalyst materials and DPM measurement. DPF technology is in a state of optimization and cost reduction. New DPF regeneration strategies (active, passive and plasma-assisted regenerations) as well as the new learning on the fundamentals of soot/catalyst interaction are described. Recent developments in diesel oxidation catalysts (DOC) are also summarized showing potential issues with advanced combustion strategies, important interactions on NO2 formation, and new formulations for durability. Finally, systematic compilation of the concerned newer literature on catalytic oxidation of soot in a well conceivable tabular form is given. A total of 156 references are cited. © 2010 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

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 NOx 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 NOx 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 (multipulse 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 NOx with no fuel consumption penalty over a significant portion of the engine operating range.

Development of an optimized chemical kinetic mechanism for homogeneous charge compression ignition combustion of a fuel blend of n-heptane and natural gas using a genetic algorithm

Homogeneous charge compression ignition (HCCI) is a promising technique for advanced low-temperature combustion strategies that offers a high fuel conversion efficiency and low nitrogen oxide and soot emissions. One of the major problems associated with HCCI combustion engine application is the lack of direct control for combustion timing. A proposed solution for combustion timing control is to use a binary fuel blend in which two fuels with different autoignition characteristics are blended at various ratios on a cycle-by-cycle basis. Because dual-fuel diesel—natural-gas engines have already been used, a fuel blend of n-heptane (diesel-like fuel) and natural gas (mostly methane) is one of the best available options. The objective of this study is to optimize the chemical kinetic mechanisms available for n-heptane and natural gas to be used in a binary-fuel blend scenario. Using the genetic algorithm method, a combined mechanism was optimized and modelling results were verified against experimental results. The agreement between experimental and modelling results was found to be acceptable within the examined conditions. As a result, an optimized chemical kinetic mechanism for an n-heptane—natural-gas blend is presented.