Fpt Frank Willems

Is Closed-Loop SCR Control Required to Meet Future Emission Targets?

To meet 2010 emission targets, optimal SCR system performance is required. In addition, attention has to be paid to in-use compliance requirements. Closed-loop control seems an attractive option to meet the formulated goals. This study deals with the potential and limitations of closed-loop SCR control. High NO x conversion in combination with acceptable NH 3 slip can be realized with an open-loop control strategy. However, closed-loop control is needed to make the SCR system robust for urea dosage inaccuracy, catalyst ageing and NO x engine-out variations. Then, the system meets conformity of production and in-use compliance norms. To demonstrate the potential of closed-loop SCR control, a NO x sensor based control strategy with crosssensitivity compensation is compared with an adaptive surface coverage/NH 3 slip control strategy and an openloop strategy. The adaptive surface coverage/NH 3 slip control strategy shows best performance over simulated ESC and ETC cycles. SCR catalyst dynamics, time delay in the urea injection and maximum NH 3 slip targets limit the performance of closed-loop SCR control. If new reagent dosage systems and future catalyst technology are able to relieve these limitations, closed-loop control has the potential to reduce the calibration effort and to improve the transient control performance.

Modeling and control of compressor flow instabilities

Compressors are widely used for the pressurization of fluids. Applications involve air compression for use in aircraft engines and pressurization and transportation of gas in the process and chemical industries. The article focuses on two commonly used types of continuous flow compressors: the axial compressor, where the gaseous fluid is processed in a direction parallel to the rotational axis, and the radial or centrifugal compressor, where the pressurized fluid leaves the compressor in a direction perpendicular to the rotational axis. In these machines, the entering fluid is pressurized by first accelerating it via the kinetic energy imparted in the rotors and then converting the kinetic energy into potential energy by decelerating the fluid in diverging channels. Toward low mass flows, stable operation of axial and radial compressors is constrained by two aerodynamic flow instabilities: rotating stall and surge. The article gives an overview of the current state of modeling and control of these instabilities.

Optimization of urea SCR deNOx systems for HD diesel engines

In the past decade, SCR deNOx technology with urea injection has grown to maturity. European OEMs will apply SCR deNOx to meet future heavy-duty emissions legislation starting with EURO-4 (2005/2006). Numerous research programs in Europe and the US have shown a variety of system layouts and control strategies. The main differences are formed by: the engine-out NOx calibration; the application of an NO to NO2 catalyst; and open-loop or closed-loop urea dosage control. This paper gives an overview of possible SCR system configurations that are required for different stages of future emission legislation. Engine-out NOx emission is strongly influenced by ambient conditions. Projections in this study show that a combination of cold climate and a wintergrade fuel is the most severe: it may lead to 30% lower engine-out NOx emissions with respect to laboratory conditions. The varying engine-out NOx emission requires a safety margin in the case of an open-loop-controlled system in order to avoid excessive NH3 slip. Calculations in this paper show that Euro 4 limits can be met with open-loop-controlled urea dosage; an NO to NO2 catalyst is not even required. For other future emission legislation (Euro 5, US 2007 and US 2010), it becomes difficult to safely achieve the limits with open-loop control. US 2007 and 2010 legislation require an NO to NO2 catalyst because of the low average load and consequently low exhaust gas temperatures. Engines that are developed to meet US 2010 legislation need special measures to increase the exhaust gas temperature and to reduce engine-out NOx below 2 g/kWh. In all cases, optimal hydrolysis and mixing conditions are of major importance. High-speed photography and droplet size measurements are presented as methods for characterizing and modelling aqueous urea spray patterns. The information from these measurements is used as input parameters for simulation tools. These consist of a 2D/3D CFD program for urea mixing analysis and a 1D SCR system model for development of dosage control and optimization of catalyst dimension. Finally, simulations with the SCR system model are compared with measurements on the engine test bench.

Cylinder pressure-based control in heavy-duty EGR diesel engines using a virtual heat release and emission sensor

This paper presents a cylinder pressure-based control (CPBC) system for conventional diesel combustion with high EGR levels. Besides the commonly applied heat release estimation, the CPBC system is extended with a new virtual NO x and PM sensor. Using available cylinder pressure information, these emissions are estimated using a physically-based combustion model. This opens the route to advanced On-Board Diagnostics and to optimized fuel consumption and emissions during all operating conditions. The potential of closed-loop CA50 and IMEP control is demonstrated on a multi-cylinder heavy-duty EGR engine. For uncalibrated injectors and fuel variations, the combustion control system makes the engine performance robust for the applied variations and reduces the need for a time consuming calibration process. Cylinder balancing is shown to enable auto-calibration of fuel injectors and to enhance fuel flexibility. For both Biodiesel and US diesel, the effects on NO x and PM emissions are partly compensated for by combined CA50 and IMEP control. This can be further improved by application of (virtual) emission sensors. Furthermore, it is shown that this combustion controller shows good transient performance during load changes. The virtual emission sensor is successfully implemented for real-time control. For operating conditions with high EGR rates and varying injection timing, the predictions of the virtual NO x and PM sensor are compared with measurements. NO x emission prediction inaccuracy is typically on the order of 12%, which is comparable to commercially available sensors. The predicted PM emissions show good qualitative agreement, but need further improvement for application in DPF regeneration and PM emission control strategies. Robust emission control is essential to meet future requirements for On-Board Diagnostics and In-Use Compliance.

Experimental Demonstration of a New Model-Based SCR Control Strategy for Cleaner Heavy-Duty Diesel Engines

Selective catalytic reduction (SCR) is a promising diesel after treatment technology that enables low nitrogen oxides (NOx) tailpipe emissions with relatively low fuel consumption. Future emission legislation is pushing the boundaries for SCR control systems to achieve high NOx conversion within a tailpipe ammonia (NH3) slip constraint, and to provide robustness to meet in-use compliance requirements. This work presents a new adaptive control strategy that uses an ammonia feedback sensor and an online ammonia storage model. Experimental validation on a 12-liter heavy-duty diesel engine with a 34-liter Zeolite SCR catalyst shows good performance and robustness against urea under- and over-dosage for both the European steady-state and transient test cycles. The new strategy is compared with a NOx sensor-based control strategy with cross-sensitivity compensation. It proved to be superior in terms of transient adaptation and taking an NH3 slip constraint into account.

Brief Positive feedback stabilization of centrifugal compressor surge

Stable operation of axial and centrifugal compressors is limited towards low mass flows due to the occurrence of surge. The stable operating region can be enlarged by active control. In this study, we use a control valve which is fully closed in the desired operating point and only opens to stabilize the system around this point. As a result, only nonnegative control values are allowed, which complicates the controller design considerably. A novel positive feedback controller is proposed which is based on the pole placement technique. This controller has been successfully applied to a laboratory-scale gas turbine installation. Initial experiments show that the surge point mass flow can be reduced by at least 7%. Using this efficient control strategy, stable operation in the desired operating point is maintained with small average control valve mass flow.

Experimental Demonstration of RCCI in Heavy-Duty Engines using Diesel and Natural Gas

Premixed combustion concepts like PCCI and RCCI have attracted much attention, since these concepts offer possibilities to reduce engine out emissions to a low level, while still achieving good efficiency. Most RCCI studies use a combination of a high-cetane fuel like diesel, and gasoline as low-cetane fuel. Limited results have been published using natural gas as low-cetane fuel; especially full scale engine results. This study presents results from an experimental study of diesel-CNG RCCI operation on a 6 cylinder, 8 l heavy duty engine with cooled EGR. This standard Tier4f diesel engine was equipped with a gas injection system, which used single point injection and mixed the gaseous fuel with air upstream of the intake manifold. For this engine configuration, RCCI operating limits have been explored. In the 1200-1800 rpm range, RCCI operation with Euro-VI engine out NOx and soot emissions was achieved between 2 and 9 bar BMEP without EGR. Corresponding hydrocarbon levels were high, but exhaust temperature levels hold promise for a suitable reduction through catalytic aftertreatment. Thermal efficiency was comparable to or better than diesel operation. In the load ranges tested, gas Methane Number (MN) variations between 70 and 100 have only a small effect on RCCI performance.

Integrated energy & emission management for hybrid electric truck with SCR aftertreatment

Energy management in hybrid vehicles typically relates to the vehicle powertrain, whereas emission management is associated with the combustion engine and aftertreatment system. To achieve maximum performance in fuel economy and regulated pollutants, the concept of (model-based) Integrated Powertrain Control (IPC) is proposed.

Experimental validation of extended NO and Soot model for advanced HD Diesel Engine Combustion

A computationally efficient engine model is developed based on an extended NO emission model and state-of-the-art soot model. The model predicts exhaust NO and soot emission for both conventional and advanced, high-EGR (up to 50%), heavy-duty DI diesel combustion. Modeling activities have aimed at limiting the computational effort while maintaining a sound physical/chemical basis. The main inputs to the model are the fuel injection rate profile, in-cylinder pressure data and trapped in-cylinder conditions together with basic fuel spray information. Obtaining accurate values for these inputs is part of the model validation process which is thoroughly described. Modeling results are compared with single-cylinder as well as multi-cylinder heavy-duty diesel engine data. NO and soot level predictions show good agreement with measurement data for conventional and high-EGR combustion with conventional timing.

Experimental validation of a dynamic waste heat recovery system model for control purposes

This paper presents the identification and validation of a dynamic Waste Heat Recovery (WHR) system model. Driven by upcoming CO2 emission targets and increasing fuel costs, engine exhaust gas heat utilization has recently attracted much attention to improve fuel efficiency, especially for heavy-duty automotive applications. In this study, we focus on a Euro-VI heavy-duty diesel engine, which is equipped with a Waste Heat Recovery system based on an Organic Rankine Cycle. The applied model, which combines first principle modelling with stationary component models, covers the two-phase flow behavior and the effect of control inputs. Furthermore, it describes the interaction with the engine on both gas and drivetrain side. Using engine dynamometer measurements, an optimal fit of unknown model parameters is determined for stationary operating points. From model validation, it is concluded that the identified model shows good accuracy in steady-state and can reasonably capture the most important dynamics over a wide range of operating conditions. The resulting real-time model is suitable for model-based control. Copyright