Gayatri Adi

Steady-State Biodiesel Blend Estimation via a Wideband Oxygen Sensor

A substantial opportunity exists to reduce carbon dioxide (CO 2 ) emissions, as well as dependence on foreign oil, by developing strategies to cleanly and efficiently use biodiesel, a renewable domestically available alternative diesel fuel. However, biodiesel utilization presents several challenges, including decreased fuel energy density and increased emissions of smog-generating nitrogen oxides (NO x ). These negative aspects can likely be mitigated via closed-loop combustion control provided the properties of the fuel blend can be estimated accurately on-vehicle, in real-time. To this end, this paper presents a method to practically estimate the biodiesel content offuel being used in a diesel engine during steady-state operation. The simple generalizable physically motivated estimation strategy presented utilizes information from a wideband oxygen sensor in the engines exhaust stream, coupled with knowledge of the air-fuel ratio, to estimate the biodiesel content of the fuel. Experimental validation was performed on a 2007 Cummins 6.7 l ISB series engine. Four fuel blends (0%, 20%, 50%, and 100% biodiesel) were tested at a wide variety of torque-speed conditions. The estimation strategy correctly estimated the biodiesel content of the four fuel blends to within 4.2% of the true biodiesel content. Blends of 0%, 20%, 50%, and 100% were estimated to be 2.5%, 17.1%, 54.2%, and 96.8%, respectively. The results indicate that the estimation strategy presented is capable of accurately estimating the biodiesel content in a diesel engine during steady-state engine operation. This method offers a practical alternative to in-the-fuel type sensors because wideband oxygen sensors are already in widespread production and are in place on some modern diesel vehicles today.

Control-variable-based accommodation of biodiesel blends

This paper introduces, and presents experimental validation for, an on-engine applicable control framework for fuel-flexible combustion of diesel–biodiesel blends. The approach is based on changing two of the closed-loop targeted control variables used by the engine control module (ECM): (1) replacing exhaust gas recirculation (EGR) fraction with combustible oxygen mass fraction (COMF); (2) replacing total injected fuel mass with total injected fuel energy, including replacing start of main injection timing with end of main injection timing. It is shown that the stock ECM control structure with pure biodiesel (B100) produces 38 per cent more brake-specific nitrogen oxides (NO x ) compared to pure conventional diesel (B0). However, new results presented here with the proposed control framework show that B100 can be made to produce lower brake-specific NO x , 2 per cent higher brake thermal efficiency, 50 per cent lower brake-specific particulate matter, and 1.2 dB lower combustion noise than B0. Benefits of, and novel contributions related to, this strategy are that it is generalizable to other engine systems, is physically based, does not require modified or additional engine calibration, and maintains the stock B0 performance. In essence, the approach presented defines the biodiesel blend control problem as closed-loop targeting of COMF and injected fuel energy, paving the way for future work in controller design to achieve these targets in real-time, on-engine situations.

Modeling Cylinder-to-Cylinder Coupling in Multi-Cylinder HCCI Engines Incorporating Reinduction

Residual-affected homogeneous charge compression ignition (HCCI) is a promising strategy for decreasing fuel consumption and NOx emissions in internal combustion engines. One practical approach for achieving residual-affected HCCI is by using variable valve actuation to reinduct previously exhausted combustion products. This process inherently couples neighboring engine cylinders as products exhausted by one cylinder may be reinducted by a neighboring one. In order to understand this coupling and its implication for controlling HCCI, this paper outlines a simple physics based model of a multi-cylinder HCCI engine using exhaust reinduction. It is based on a physics based model previously validated for a single cylinder, multi mode HCCI engine. The exhaust manifold model links exhaust gases from one cylinder to those of the other cylinders and also simulates the effect of exhaust reinduction from the previous cycle. Depending on the exhaust manifold geometry and orientation, the heat transfer in the manifold causes a difference in the temperature of the re-inducted product gas across the cylinders. The results show that a subtle difference in the re-inducted exhaust gas temperature results in a dramatic variation in combustion timing (approx. 3 degrees). This model provides a basis for understanding the steady state behavior and also for developing control strategies for multi-cylinder HCCI engines. The paper presents exhaust valve timing induced compression ratio modulation (via flexible valve actuation) as one of the approaches to mitigate the imbalance in combustion timing across cylinders.Copyright

A robust fuel flexible combustion control strategy for biodiesel with variable fatty acid composition during mixing-controlled combustion:

Biodiesel is a diesel fuel alternative which is produced from renewable and domestically available sources. The use of biodiesel generally lowers carbon dioxide, carbon monoxide and particulate matter emissions. However, there are certain challenges associated with the use of biodiesel, mainly (1) lower fuel energy density, (2) increased nitrogen oxide (NO x ) emissions and (3) fuel variability due to feedstock and processing differences. In prior efforts, the authors have demonstrated that the first two of these challenges can be overcome for different blend fractions of soy-based biodiesel by using a control algorithm incorporating energy-based fueling for torque control and combustible oxygen mass fraction control for NO x regulation. However, in addition to overcoming these combustion-related challenges, in this work, the authors consider the extension of these techniques to biodiesel generated from oils/fats of varying composition. The type of oil/fat from which the biodiesel is derived will impact the fuel properties via variation in the fuel’s fatty acid composition. The fuel’s fatty acid composition can also be altered by an additional processing done in order to change certain fuel properties. For example, the saturation level of biodiesel can be reduced in order to lower the fuel cloud point, making it suitable for colder climates. The effect of variation in the fuel fatty acid structure on the previously developed control algorithm is studied in this work. It is shown both theoretically and experimentally that the proposed control algorithms are robust to variation in the fatty acid composition of biodiesel due to the fact that biodiesels with very different fatty acid compositions exhibit minor changes in heating values and fuel oxygen mass fraction. As such, the control technique is suitable for use with variable blend fractions of biodiesel produced from different feedstocks as well as fuel processed to improve cold weather operation.

Uncertainty Analysis of Wideband Oxygen Sensor Based Strategy for Steady-State Biodiesel Blend Estimation

Real-time estimation and accommodation is critical for clean and efficient utilization of biodiesel blends in “fuel flexible” diesel engines. This paper utilizes a generalizable, physically-based, and experimentally-verified blend estimation strategy which uses exhaust oxygen sensor measurements coupled with engine control module (ECM) estimates of fuel and air flow to estimate the biodiesel blend fraction. The paper assesses the impact of uncertain variables on the estimation strategy. The strategy is essentially unaffected by biodiesel feedstock variations and, when applied to a Cummins 6.7-liter engine, is not susceptible to significant blend estimation discrepancies in response to expected fuel flow and oxygen sensor errors. However, observed errors in air flow estimates are expected to lead to large blend estimate errors. Use of direct air flow measurement or the use of a dynamic estimator (e.g., Kalman filter) synthesized from the model, the subject matter of future work, is expected to significantly reduce these errors.Copyright

Diesel Engine Control Strategy for Biodiesel Blend Accommodation Independent of Fuel Fatty Acid Structure

Abstract Growing dependence on foreign oil for transportation as well as greenhouse gas emissions have created a need for advancement in clean and renewable fuel technologies. Biodiesel, a renewable fuel produced from plant oils or animal fats, generally produces lower carbon dioxide, carbon monoxide and particulate matter emissions. However, the use of biodiesel in a diesel engine has certain challenges including lower power output and up to 40% higher nitrogen oxide (NO x ) emissions compared to diesel. Biodiesel usage in colder climates is also challenging due to its relatively high cloud point. Based on studies done to understand the effect of biodiesel combustion on engine power and NO x emissions, control strategies are developed to mitigate these challenges. This includes control of the total energy of injected fuel in order to get same the power output for diesel and biodiesel. In order to maintain consistent NO x emissions between diesel and biodiesel, the fraction of oxygen available for combustion contributed by the fuel, air and recirculated exhaust gases is maintained constant. This fuel flexible control strategy greatly reduced or completely eliminated increases in emissions of NO x of up to 30% while largely maintaining the power capacity of the engine when operating with biodiesel. It is also shown with experimental validation that these control techniques are robust to changes in the fatty acid composition of biodiesel, which could vary significantly depending on the feedstock used to produce biodiesel or additional processing done on the fuel in order to make it suitable for cold weather operation.

Closed-Loop Control Framework for Fuel-Flexible Combustion of Biodiesel Blends

This paper presents a closed-loop control framework for fuel-flexible combustion control of biodiesel blends. This framework consists of two parts: blend detection and blend accommodation. Blend detection can be accomplished by an experimentally-validated dynamic estimator using exhaust oxygen and air-fuel ratio information. Blend accommodation can be accomplished by changing the control variables that the engine control module uses, namely, replacing exhaust gas recirculation fraction with combustible oxygen mass fraction, replacing total injected fuel mass with total injected fuel energy, and replacing start of main injection timing with end of main injection timing. With the conventional control structure it is experimentally shown that pure biodiesel (B100) produced 38% more brake specific nitrogen oxides (BSNOx) than pure conventional diesel (B0). With the new proposed structure, B100 produced not only lower BSNOx than B0, but also higher torque, higher brake thermal efficiency, lower particulate matter, and lower combustion noise than B0. Comparable experimental results are also presented for B5 and B20 blends.Copyright

Optimization of the Performance and Emissions of Soy Biodiesel Blends in a Modern Diesel Engine

As the world is faced with continued petroleum demand, the need for alternative fuels which are renewable and domestically available is becoming apparent. Biodiesel is one such attractive alternative fuel which has physical and chemical properties similar to, and miscible with conventional diesel. While biodiesel does have many advantages, due to fuel property differences including oxygenation and a lower calorific value than diesel fuel, biodiesel combustion often results in higher fuel consumption and higher nitrogen oxide (NOx ) emissions than diesel combustion. Stock diesel engine design and decision making target optimal performance with conventional diesel fuel, leading to suboptimal results for biodiesel. This study aimed to determine the appropriate engine decision making for the air/fuel ratio (AFR), exhaust gas recirculation (EGR) fraction, injection (rail) pressure, and start of main fuel injection (SOI) in a modern common rail diesel engine using variable geometry turbo-charging and operating with varying blend ratios of diesel and soy-based biodiesel fuel mixtures to minimize brake-specific fuel consumption (BSFC) and adhere to strict combustion noise, NOx and particulate matter (PM) emission constraints. When operating with the stock engine decision making, biodiesel blend combustion resulted in increases in NOx of up to 39% and fuel consumption increases up to 20% higher than the nominal diesel levels but also had substantial reductions in PM. Through modulation of the AFR, EGR fracton, rail pressure, and SOI at several operating points, it was demonstrated that the optimal engine decision-making for biodiesel shifted to lower AFRs and higher EGR fractions in order to reduce NOx , and shifted to more advanced timings in order to mitigate the observed increases in fuel consumption at the nominal settings. The optimal parameter combinations for B5 (5% biodiesel and 95% diesel), B20 (20% biodiesel and 80% diesel) and B100 (100% biodiesel) still maintained substantial PM reductions but resulted in NOx and noise levels below nominal diesel levels. However, these parameter combinations had little impact on reducing the biodiesel fuel consumption penalty but did improve the thermal efficiency of biodiesel blend combustion.Copyright

An Experimental and Simulation Study of Increases in Fuel Consumption and NOX Emissions in a Biofueled Diesel Engine

Alternative fuel vehicles are gaining importance as a means of reducing petroleum dependence. One attractive option is biodiesel, a renewable diesel fuel produced from plant or animal fats, since it significantly reduces carbon monoxide, unburned hydrocarbon, and particulate matter emissions as well as carbon dioxide when considered on a full life cycle basis. However, biodiesel combustion also typically results in increased fuel consumption and nitrogen oxide (NOx ) emissions relative to petroleum diesel. In order to determine the cause of and develop mitigation strategies for increased biodiesel fuel consumption and NOx emissions, an accurate simulation model was developed and validated. Key fuel properties as well as ignition delay characteristics were implemented in a previously validated whole engine model to reflect soy-biodiesel fuel. The model predictions were within 5% of experimental results for most values at the three operating points. Using this biodiesel model, the “biodiesel NOx effect” was linked to the near stoichiometric equivalence ratios for biodiesel.Copyright