Burned Gas Fraction Research Articles

Physics-based modeling of homogeneous charge compression ignition engines with exhaust throttling

Homogeneous charge compression ignition (HCCI) engines create a more efficient power source for either stationary power generators or automotive applications. Due to the absence of direct actuation of ignition, HCCI based combustion control is quite challenging. For the purpose of control synthesis and design, a crank-angle based dynamic model of an HCCI engine with internal exhaust gas recirculation (EGR) is proposed in this paper. The HCCI autoignition timing is controlled by the amount of internal EGR induced by the cost effective exhaust throttling strategy. The zero dimensional single zone model is developed based on conservation of mass and energy and ideal gas law. The model is comprised of five subsystems: cylinder volume, intake, and exhaust runners along with combustion and heat-transfer models. Inputs to the model include engine speed, intake temperature, fueling rate, intake, and exhaust throttle positions. Outputs of the model contain the pressure, temperature, mass and burned gas fraction in the intake, exhaust and cylinder, air-to-fuel ratio (AFR), heat release rate, combustion phasing, and the indicated mean effective pressure (IMEP). The model is developed based on MATLAB/Simulink and validated against experimental data under steady-state and transient conditions at various intake temperatures, fueling levels, and exhaust throttle positions. Results demonstrate that by changing the exhaust throttle position, the pressure in the exhaust runner is regulated. As a result, the residual gas fraction is altered, leading to changes in mixture temperature and thus control of the HCCI combustion phasing. In the end, closed loop simulation is conducted with a linear quadratic Gaussian (LQG) controller applied to the crank-angle based model for regulation of combustion timing CA50 using exhaust throttle during load (fueling level) changes. In the future, this model can be utilized for control development and hardware in the loop (HIL) simulation.

Development of a Hierarchical Observer for Burned Gas Fraction in Inlet Manifold of a Turbocharged Diesel Engine

Altogether with extent of automotive role in today's life, governments are approving stringent laws to lower the permitted level of automotive emissions. An important specie of automotive engines emissions is NOx. Different methods are developed to decrease the level of NOx generation; one of which is exhaust gas recirculation (EGR). Since the performance of EGR is strictly depended on burned gas fraction (BGF) in inlet manifold, the precise control of BGF is of importance. Unfortunately, due to inlet manifold temperature, no economical sensor is available to measure the BGF; therefore, estimators are employed instead. In this paper, a stable observer is designed for estimation of air fraction in inlet manifold. The governing equations are in the form of linear parameter varying (LPV). Since the LPV parameters are not directly measured, a hierarchical estimator structure is developed. Lyapunov theory is employed to design the higher level estimator, while high gain estimators and open-loop estimators are developed for estimation of the lower level parameters. Experimental test results show that the higher level estimator is able to estimate the BGF with high accuracy in both transient and steady states. Furthermore, it is shown that BGF estimator is more sensitive to aspirated gas flow estimation rather than exhaust temperature and pressure.

Air-Path Control of a Heavy-Duty EGR-VGT Diesel Engine

In this paper, a model based multi-variable control strategy is applied to the air-path control of a turbocharged Diesel engine. An exhaust gas recirculation (EGR) valve, and a variable-geometry turbocharger (VGT), are controlled using a technique called Model Predictive Control (MPC). The aim is to optimize the fuel consumption by minimizing pumping losses, while the CO2 based burned gas fraction follows a demand and constraints on the minimum air fuel ratio are imposed. The proposed technique is applied to a 13L engine, and its current software extended with the control function developed. The function developed is fitted in a production engine electronic control unit (EECU), and is validated through real-world experiments in an engine test cell and in a heavy-duty vehicle. The MPC proposed is found to work satisfactorily.

A Quasi-dimensional Model of the Ignition Delay for Combustion Modeling in Spark-Ignition Engines

Quasi-dimensional (QD) modeling of combustion in spark-ignition (SI) engines allows to describe the most relevant processes of heat release. Here, a submodel for the ignition delay is introduced and applied. The start of combustion is considered from ignition to the crank angle of 5% burned gas fraction. The introduced physical approach identifies the turbulent propagation velocity of the initiated kernel by taking into account early flame expansion and geometric restrictions of the flame propagation. The model is applied to stationary operation within an entire engine map of a turbocharged direct injection SI engine with fully variable valvetrain. Based on provided cycle-averaged input data, the model delivers good results within the margins of measured cycle-to-cycle fluctuations. Thus, it contributes to the assessment of the interplay between engine, engine control unit, drivetrain, and vehicle dynamics, hence making a step toward optimization and virtual engine calibration.

Practical delay modeling of externally recirculated burned gas fraction for Spark-Ignited Engines

This paper studies the low-pressure burned gas recirculation in spark-ignited engines and shows, at the light of supportive experimental results, that a linear delay system permits to capture the dominant effects of the system dynamics. The modeled transport delay is defined by implicit equations stemming from first principles. This model is shown to be sufficiently accurate to replace a sensor that would be difficult and costly to implement on commercial engines.

Control-oriented modelling of combustion phasing for a fuel-flexible spark-ignited engine with variable valve timing

In an effort to reduce dependence on petroleum-based fuels and increase engine efficiency, fuel-flexible engines with advanced technologies, including variable valve timing, are being developed. Fuel-flexible spark-ignition engines permit the increased use of ethanol–gasoline blends. Ethanol, an alternative to petroleum-based gasoline, is a renewable fuel, which has the added advantage of improving performance in operating regions that are typically knock limited due to the higher octane rating of ethanol. Furthermore, many modern engines are also being equipped with variable valve timing, a technology that can increase engine efficiency by reducing pumping losses. Through control of valve timings, particularly the amount of positive valve overlap, the quantity of burned gas in the engine cylinder can be altered, eliminating the need for intake throttling at many operating points. However, the presence of elevated levels of in-cylinder burned gas and ethanol fuel can have a significant impact on the combustion timing, such that capturing these effects is essential if the combustion phasing is to be properly controlled. This paper outlines a physically based model capable of capturing the impact of the ethanol blend ratio, burned gas fraction, spark timing and operating conditions on combustion timing. Since efficiency is typically tied to an optimal CA50 (crank angle when 50% of fuel is burned), this model is designed to provide accurate estimates of CA50 that can be used for real-time control efforts – allowing the CA50 to be adjusted to its optimal value despite changes in ethanol blend and burned gas fraction, as well as the variations in engine thermodynamic conditions that may occur during transients. The proposed control-oriented model was extensively validated at over 500 points across the engine operating range for four blends of gasoline and ethanol. Furthermore, the model was utilized to determine the impact of ethanol blend and burned gas fraction on the CA50, as well as their impact on the optimal spark timing. This study indicated that the burned gas fraction could change the optimal spark timing by over 20° at some operating conditions and that ethanol content could further affect the optimal spark timing by up to 6°. Leveraging the model in this manner provides direct evidence that accounting for the impact of these two inputs is critical for proper spark-ignition timing control.

A comparison between cylinder charge composition of common rail diesel engine

Using a diesel passenger-car, we measured charge composition and emissions of nitrogen oxide and soot, on the European extra-urban drive cycle (EUDC), and also on isolated transients, representative of the EUDC. Our objective was to compare emissions and charge composition, between a transient, and engine operation at specific points on the transient locus ('quasi-steadystate'). On the EUDC, we measured two nitrogen oxide spikes. The first, fast-response nitrogen oxide spike, could be ascribed to the pedal demand step input: this is small, compared to the second nitrogen oxide spike resulting from the remainder of the transient (adjustments in speed and load). The estimated charge composition, in a transient, agreed closely with the measurement, both qualitatively and quantitatively. The burned-gas fraction (BGF), estimated from a transient, revealed that the period of largest discrepancy from that demanded, is short (≈1 sec). This indicates that the use of BGF, rather than the customary mass air-flow, may be more apposite during transients encountered in real driving. However, since this discrepancy is short-lived, the realisable reduction in overall NOx emissions, using BGF as a controlled variable, at least during the EUDC, is likely to be small. BGF-based control is nevertheless attractive, insofar as boundary conditions bearing direct relationships to combustion are easily imposed.

Control of a variable-geometry turbocharged and wastegated diesel engine

This paper addresses the problem of controlling a turbocharged diesel engine so as to minimize NOx and smoke emissions while ensuring that the driver's torque demands are met. A diesel engine equipped with a variable-geometry turbocharger (VGT) and an exhaust gas recirculation (EGR) valve is considered. The technical challenges in the control design task include the multivariable non-linear dynamics of the system and the unavailability of key states for feedback. A control strategy based on non-linear control synthesis is developed and shown accurately to control the air-fuel ratio (AFR) and the burned gas fraction in the intake manifold, F1, to desired values in the presence of changing operating conditions. The variables F1 and AFR are shown to be crucial for feedback. Since neither of these variables can be measured, an observer based on flow and pressure sensor measurements is developed for their real-time estimation. Lyapunov theory is used to show that the developed observer is asymptotically stable. Simulation results confirm the performance of the observer and the observer-based feedback controller. The importance of the developed observer extends beyond the application discussed in this paper. It could be useful for a wide variety of different control and diagnostic applications in diesel engines.

Generic functions for fuel consumption and engine-out emissions of HC, CO and NOx of spark-ignition engines

The prediction of fuel consumption and engine-out emissions of spark-ignition engines from characteristic relationships has been investigated. For spark-ignition engines operating on homogeneous mixtures, indicated specific fuel consumption based on gross indicated power is related to compression ratio and spark timing relative to optimum values. The influence of burn rate is largely accounted for by differences in spark timings required to phase combustion correctly. For engine-out emissions of HC, CO and NO x, generic functions have been derived that relate the emissions index in each case to air-fuel ratio, spark timing relative to the optimum and burned gas fraction present during compression. The effects of exhaust gas recirculation and residual gas dilution are accounted for. Brake specific performance maps can be generated. This requires knowledge or predictions of calibrations, cylinder volumetric efficiency, rubbing friction, auxiliary loads and exhaust back pressure. A closed set of equations and illustrative results are given.

Prediction of Swirl Velocity of In-Cylinder Flow of an Engine by a Simplified Simulation Model. 4th Report, Evaluation of Valve Timing.

Improving combustion of the spark ignition engines contributes largely to decreasing fuel consumption, especially under the conditions of medium or low speed and low load. In this paper, valve timing is evaluated from the viewpoints of swirl velocity as well as the volumetric efficiency, burned gas fraction in a fresh mixture charge, and pumping loss at intake stroke by using the predictions of the simplified simulation model. As a result, the timing of intake valve opening and closing is considerably affected by the swirl velocity. It is also confirmed that the mean swirl velocity controlled by the valve timing is higher than that controlled by the conventional throttling system under the same volumetric efficiency condition.