Prasad Divekar

Exhaust gas recirculation control through extremum seeking in a Low Temperature Combustion diesel engine

Low Temperature Combustion (LTC) modes in diesel engines are characterized by enhanced homogeneity of the combustion mixture resulting from a longer ignition delay when compared to conventional diesel combustion. This is enabled by charge density and dilution control, coupled with modulation of fuel injection parameters. Charge dilution is achieved by exhaust gas recirculation (EGR), while turbocharging enables in-cylinder charge density increase. The coupling between the EGR and turbocharging systems exhibits highly non-linear interactions in the engine air-path. In this work, a two part control strategy is investigated for the regulation of EGR and turbocharging in a diesel engine to direct the combustion to approach LTC without largely compromising the combustion efficiency. Firstly, a simplified engine air-path model is presented that emphasizes the correlation between the intake oxygen concentration ([O2-int]) set-point and the individual EGR and turbocharging actuator set-points at different engine operating points. Thereafter, experimental data is presented that highlights the sensitivity of engine-out NOx emissions and combustion efficiency against the [O2-int]. Secondly, an extremum seeking (ES) algorithm is used to determine the [O2-int] set-point using a cost function that results in a desirable emission and combustion performance. Finally, the coordinated execution of the ES algorithm and the air-path model to generate the air-path actuator set-points is discussed.

Study of Cylinder Charge Control for Enabling Low Temperature Combustion in Diesel Engines

Suitable cylinder charge preparation is deemed critical for the attainment of a highly homogeneous, diluted, and lean cylinder charge which is shown to lower the flame temperature. The resultant low temperature combustion (LTC) can simultaneously reduce the NOx and soot emissions from diesel engines. This requires sophisticated coordination of multiple control systems for controlling the intake boost, exhaust gas recirculation (EGR), and fueling events. Additionally, the cylinder charge modulation becomes more complicated in the novel combustion concepts that apply port injection of low reactivity alcohol fuels to replace the diesel fuel partially or entirely. In this work, experiments have been conducted on a single cylinder research engine with diesel and ethanol fuels. The test platform is capable of independently controlling the intake boost, EGR rates, and fuelling events. Effects of these control variables are evaluated with diesel direct injection and a combination of diesel direct injection and ethanol port injection. Data analyses are performed to establish the control requirements for stable operation at different engine load levels with the use of one or two fuels. The sensitivity of the combustion modes is thereby analyzed with regard to the boost, EGR, fuel types and fueling strategies. Zero-dimensional cycle simulations have been conducted in parallel with the experiments to evaluate the operating requirements and operation zones of the LTC combustion modes. Correlations are generated between air-fuel ratio (λ), EGR rate, boost level, in-cylinder oxygen concentration and load level using the experimental data and simulation results. Development of a real-time boost-EGR set-point determination to sustain the LTC mode at the varying engine load levels and fueling strategies is proposed.Copyright

An engine cycle analysis of diesel-ignited ethanol low-temperature combustion:

Modification of the fuel–air charge properties has the potential to improve the load range of low-temperature combustion with ultra-low nitrogen oxide emissions (less than 0.2 g/kW h) and ultra-low smoke emissions (less than 0.01 g/kW h). The ignition characteristics of the cylinder charge are altered by injecting the highly reactive diesel fuel into a homogeneous lean air–fuel mixture of low-reactivity fuel. The ethanol–diesel combination has been of particular recent interest since ethanol is a renewable biofuel. The additional advantages of ethanol include excellent anti-knock properties, high volatility and reduction in the compression work through charge cooling. In this work, a detailed investigation using diesel-ignited ethanol experiments was conducted on a high-compression-ratio (18.2:1) diesel engine. The emissions, the combustion performance and the thermal efficiency characteristics are analysed at different values of the exhaust gas recirculation, the intake boost pressure, the ethanol fraction and the diesel injection timing. The empirical investigations supported by detailed zero-dimensional engine cycle simulations indicate that a diesel injection timing close to top dead centre provides direct control over the ignition timing across the engine load range. The nitrogen oxide–soot trade-off of conventional diesel combustion, which is affected by exhaust gas recirculation, is minimized to achieve clean combustion over a wide load range (indicated mean effective pressure, 4–17 bar) with increased ethanol fraction and moderate intake dilution through a combination of modulation of the exhaust gas recirculation level and an increase in the intake boost pressure. The operation at low loads is constrained by the minimum diesel amount necessary for stable and efficient combustion while progressively retarded combustion phasing is necessary at higher loads to satisfy the physical engine constraints (peak cylinder pressure, less than 170 bar; peak pressure rise rate, less than 15 bar/deg crank angle). The improved understanding of this combustion strategy through experimental and theoretical research provides the necessary guidance for obtaining clean efficient full-load operation (demonstrated at an indicated mean effective pressure of 19.2 bar).

Nonlinear model reference observer design for feedback control of a low temperature combustion diesel engine

A nonlinear observer design is proposed for fuel delivery control of a low temperature combustion (LTC) diesel engine. First, a nonlinear engine model, combining the continuous time gas exchange and the event-based closed cycle processes, is presented for the diesel engine. Secondly, an optimization routine based on the extremum seeking scheme is implemented for online observer calibration on a 10 engine rotations basis to follow the engine sensor outputs. The extremum seeking algorithm efficiently minimizes the observer error, demonstrated by simulation results with embedded engine test data. Finally, a feedback control architecture is proposed based on the observer trajectory as the engine feedback.

A Study of Combustion Inefficiency in Diesel LTC and Gasoline-Diesel RCCI via Detailed Emission Measurement

Reduction of engine-out NOx emissions to ultra-low levels is facilitated by enabling low temperature combustion (LTC) strategies. However, there is a significant energy penalty in terms of combustion efficiency as evidenced by the accompanying high levels of hydrocarbon (HC), carbon monoxide (CO), and hydrogen emissions. In this work, the net fuel energy lost as a result of incomplete combustion in two different LTC regimes is studied. The first LTC strategy, partially premixed compression ignition (PPCI), is investigated using a single, high pressure, in-cylinder injection of diesel fuel along with the application of exhaust gas recirculation (EGR). The second strategy includes dual-fuel application – reactivity controlled compression ignition (RCCI) of port injected gasoline and direct injected diesel. Moderate to high levels of EGR are necessary during engine operation in either of the two LTC pathways. A detailed analysis of the incomplete combustion products was conducted while the engine was operated in the aforementioned LTC modes. Speciation analysis of hydrocarbons was performed by sampling the exhaust gas in an FTIR. The total HC and the CO emissions were simultaneously measured using an FID and an NDIR, respectively. The production of hydrogen during the combustion process was also evaluated using a mass spectrometer. Engine tests were conducted at a baseline load level of 10 bar IMEP in the PPCI and RCCI modes. Load extension tests, up to 17 bar IMEP, were then conducted in the RCCI mode by increasing the gasoline-to-diesel fuel ratio. Test results indicated that CO, H2, and light HC made up for most of the combustion in-efficiency in the PPCI mode while heavier HC and aromatics were significantly higher in the RCCI mode.Copyright

Diesel Engine Fuel Injection Control Using a Model-Guided Extremum-Seeking Method

Diesel engine fuel injection control is presented as a feedback based online optimization problem. Extremum seeking (ES) approach is used to address the online optimization formulation. The cost function is synthesized from extensive experimental investigations such that the indicated thermal efficiency of the engine is maximized while minimizing the NOx emissions under external boundary conditions. Knowledge of the physical combustion and emission formation process based on a pre-calibrated non-linear engine model output is used to determine the ES initial control input to minimize the seeking time. The control is demonstrated on a hardware-in-the-loop engine simulator bench.Copyright

An Enabling Study of Low Temperature Combustion With Ethanol in a Diesel Engine

Previous research indicates that the low temperature combustion (LTC) is capable of producing ultra-low nitrogen oxides (NOx) and soot emissions. The LTC in diesel engines can be enabled by the heavy use of exhaust gas recirculation (EGR) at moderate engine loads. However, when operating at higher engine loads, elevated demands of both intake boost and EGR levels to ensure ultra-low emissions make engine controllability a challenging task. In this work, a multi-fuel combustion strategy is implemented to improve the emission performance and engine controllability at higher engine loads. The port fueling of ethanol is ignited by the direct injection of diesel fuel. The ethanol impacts on the engine emissions, ignition delay, heat-release shaping and cylinder-charge cooling have been empirically analyzed with the sweeps of different ethanol-to-diesel ratios. Zero-dimensional phenomenological engine cycle simulations have been conducted to supplement the empirical work. The multi-fuel combustion of ethanol and diesel produces lower emissions of NOx and soot while maintaining the engine efficiency. The experimental set-up and study cases are described and the potential for the application of ethanol-to-diesel multi-fuel system at higher loads has been proposed and discussed.Copyright