Jacob Larimore

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.

Experiments and analysis of high cyclic variability at the operational limits of spark-assisted HCCI combustion

During combustion mode switches, between homogeneous charge compression ignition (HCCI) and spark ignition (SI) combustion, the engine will operate at throttled and stoichiometric conditions where high cyclic variability (CV) is typically observed. To analyze and eventually model the engine behavior at the high CV condition we perform measurements on a four-cylinder HCCI engine with negative valve overlap and describe a cycle-resolved analysis method that enables the characterization of cycle-to-cycle variations at such conditions. The dynamic behavior observed is characterized by the recycling of thermal and chemical energy between cycles. We quantify the cyclic exchange and relate it to the dynamic patterns that emerge from this high CV condition. We also clarify the contributions of the spark at these conditions, where advancing the spark can significantly reduce the variability. It is our conclusion that the dynamic patterns observed can be characterized by the cycle-resolved combustion efficiency as it is an essential non-linearity in the dynamic evolution.

Quantifying Cyclic Variability in a Multi-Cylinder HCCI Engine With High Residuals

Cyclic variability (CV) in lean HCCI combustion at the limits of operation is a known phenomenon, and this work aims at investigating the dominant effects for the cycle evolution at these conditions in a multi-cylinder engine. Experiments are performed in a four-cylinder engine at the operating limits at late phasing of lean HCCI operation with negative valve overlap (nvo). A combustion analysis method that estimates the unburned fuel mass on a per-cycle basis is applied on both main combustion and the nvo period revealing and quantifying the dominant effects for the cycle evolution at high CV. The interpretation of the results and comparisons with data from a single-cylinder engine indicate that, at high CV, the evolution of combustion phasing is dominated by low-order deterministic couplings similar to the single-cylinder behavior. Variations, such as in air flow and wall temperature, between cylinders strongly influence the level of CV but the evolution of the combustion phasing is governed by the interactions between engine cycles of the individual cylinders.Copyright

Reference Governor for Load Control in a Multicylinder Recompression HCCI Engine

This paper presents a model-based control strategy designed to regulate combustion phasing during load transitions in a recompression homogeneous charge compression ignition (HCCI) engine. A low-order discrete-time control-oriented model for recompression HCCI combustion is developed that represents the strong thermal and composition coupling between engine cycles. A baseline two-input single-output controller is designed to regulate combustion phasing, using the amount of negative valve overlap and the fuel injection timing as actuators. This controller is augmented by a reference or fuel governor, which modifies transient fuel mass commands during large load transitions, when future actuator constraint violations are predicted. This approach is shown in experiments to improve combustion phasing and load responses, preventing engine misfires in some cases. The fuel governor enables larger load transitions than were possible with the baseline controller alone. The governor acts only when future actuator constraint violations are predicted. The complexity and computational overhead of the governor are reduced by developing a linearized fuel governor. Satisfactory performance is demonstrated experimentally for a range of engine speeds.

Quantifying Cyclic Variability in a Multicylinder HCCI Engine With High Residuals

Cyclic variability (CV) in lean HCCI combustion at the limits of operation is a known phenomenon, and this work aims at investigating the dominant effects for the cycle evolution at these conditions in a multi-cylinder engine. Experiments are performed in a four-cylinder engine at the operating limits at late phasing of lean HCCI operation with negative valve overlap (nvo). A combustion analysis method that estimates the unburned fuel mass on a per-cycle basis is applied on both main combustion and the nvo period revealing and quantifying the dominant effects for the cycle evolution at high CV. The interpretation of the results and comparisons with data from a single-cylinder engine indicate that, at high CV, the evolution of combustion phasing is dominated by low-order deterministic couplings similar to the single-cylinder behavior. Variations, such as in air flow and wall temperature, between cylinders strongly influence the level of CV but the evolution of the combustion phasing is governed by the interactions between engine cycles of the individual cylinders.

Controlled Load and Speed Transitions in a Multicylinder Recompression HCCI Engine

A model-based control strategy to track combustion phasing during load and speed transitions in the homogeneous charge compression ignition (HCCI) operating region of a multimode combustion engine is presented in this paper. HCCI transitions can traverse regions of high cyclic variability (CV), even if the steady state transition end points are stable with low CV. A control-oriented HCCI model for both the stable, low CV region and the oscillatory, high CV, late phasing region is used to design a controller that uses valve and fuel injection timings to track combustion phasing. Novel aspects of the controller include nonlinear model-inversion-based feedforward and gain scheduled feedback based on unburned fuel that reduces transient CV. Transitions tested on a multicylinder HCCI engine include load transitions at fixed engine speeds, simultaneous load, and speed transitions, and select FTP75 drive-cycle transitions with high load slew rates. Good combustion phasing tracking performance is demonstrated, and misfires are prevented.

Enabling large load transitions on multicylinder recompression HCCI engines using fuel governors

The fuel governor control design methodology presented in [1] is extended and experimentally validated on a multicylinder recompression homogeneous charge compression ignition (HCCI) engine. This strategy regulates desired combustion phasing during load transitions across the HCCI load range. A baseline controller tracks combustion phasing by manipulating valve and fuel injection timings. A reference governor is then added on to the compensated system to modify the fuel injection amount by enforcing actuator constraints. Experimental results show improved transient responses of combustion phasing and load during load transitions, when the possibility of constraint violations exists. The nonlinear fuel governor predicts future model trajectories in real-time, and enables larger load transitions than were possible with the baseline controller alone. The complexity and computational overhead of this strategy are reduced by developing a linearized fuel governor, which is shown to work well in the entire HCCI load range and for small variations in engine speed.

Controlling combustion phasing variability with fuel injection timing in a multicylinder HCCI engine

Reduction of combustion phasing cyclic variability (CV) in homogeneous charge compression ignition (HCCI) engines operating lean with late autoignition is experimentally demonstrated. A three-state discrete time model developed in [1] is used for controlling the fuel injection timing and is applied to a multicylinder engine. A key objective of this work is to reduce cyclic variability without advancing the mean combustion phasing. Specifically, if late combustion phasing can be made less variable without advancing the operating point then areas where high CV is typically encountered could be made less variable. Examples include load transition down, when the residual temperature drops more rapidly than can be manipulated by the valve timing, or during mode transitions. Experimental results are presented to gauge the effectiveness of two control schemes, namely proportional and state feedback which have been tuned using the three-state model. Each of these controllers have been augmented with an integrator to maintain the late combustion phasing requirement. This is also done to draw a fair comparison of the controllers ability to reduce CV. The controllers are tested at various levels of CV and it is found that simple control can reduce the standard deviation of combustion phasing an average of 17% over open loop behavior. In addition, because of the simplicity of the control, this offers a viable solution for commercial applications.

Adaptive Control of a Recompression Four-Cylinder HCCI Engine

An adaptive controller is presented for the control of combustion phasing in a multicylinder homogeneous charge compression ignition engine. Adaptive parameter estimation is used to modify a model-based feedforward controller for each cylinders start of injection (SOI) timing in an effort to mitigate model errors and increase the feedforward control accuracy. In-cylinder pressure measurements are used to calculate combustion phasing, which is compared with the prediction of an online nonlinear engine model to drive the parameter estimation that adapts the feedforward controller. It is demonstrated through experiments that the adaptive parameter can reduce the parameterization effort by allowing the model to adapt and match the response of each cylinder. It is also shown that the adaptive feedforward control is more accurate in the sense that load transitions can be achieved with less correction from the feedback controller. Overall, an average reduction of 41% in the absolute value of the SOI feedback component at steady state is achieved.

Real-time internal residual mass estimation for combustion with high cyclic variability

In this work, a physics-based method of estimating the residual mass in a recompression homogeneous charge compression ignition engine is developed and analyzed for real-time implementation. The estimation routine is achieved through in-cylinder pressure and exhaust temperature measurements coupled with energy and mass conservation laws applied during the exhaust period. Experimental results on a multicylinder gasoline homogeneous charge compression ignition engine and dynamic analysis demonstrate the estimation routine’s ability to perform across a wide range of operating conditions as well as on a cycle-by-cycle basis for highly variable combustion phasing data.