Francisco Espinosa-Loza

HCCI Engine Combustion-Timing Control: Optimizing Gains and Fuel Consumption Via Extremum Seeking

Homogenous-charge-compression-ignition (HCCI) engines have the benefit of high efficiency with low emissions of NOx and particulates. These benefits are due to the autoignition process of the dilute mixture of fuel and air during compression. However, because there is no direct-ignition trigger, control of ignition is inherently more difficult than in standard internal combustion engines. This difficulty necessitates that a feedback controller be used to keep the engine at a desired (efficient) setpoint in the face of disturbances. Because of the nonlinear autoignition process, the sensitivity of ignition changes with the operating point. Thus, gain scheduling is required to cover the entire operating range of the engine. Controller tuning can therefore be a time-intensive process. With the goal of reducing the time to tune the controller, we use extremum seeking (ES) to tune the parameters of various forms of combustion-timing controllers. In addition, in this paper, we demonstrate how ES can be used for the determination of an optimal combustion-timing setpoint on an experimental HCCI engine. The use of ES has the benefit of achieving both optimal setpoint (for maximizing the engine efficiency) and controller-parameter tuning tasks quickly.

Spatial Analysis of Emissions Sources for HCCI Combustion at Low Loads Using a Multi-Zone Model

We have conducted a detailed numerical analysis of HCCI engine operation at low loads to investigate the sources of HC and CO emissions and the associated combustion inefficiencies. Engine performance and emissions are evaluated as fueling is reduced from typical HCCI conditions, with an equivalence ratio f = 0.26 to very low loads (f = 0.04). Calculations are conducted using a segregated multi-zone methodology and a detailed chemical kinetic mechanism for iso-octane with 859 chemical species. The computational results agree very well with recent experimental results. Pressure traces, heat release rates, burn duration, combustion efficiency and emissions of hydrocarbon, oxygenated hydrocarbon, and carbon monoxide are generally well predicted for the whole range of equivalence ratios. The computational model also shows where the pollutants originate within the combustion chamber, thereby explaining the changes in the HC and CO emissions as a function of equivalence ratio. The results of this paper contribute to the understanding of the high emission behavior of HCCI engines at low equivalence ratios and are important for characterizing this previously little explored, yet important range of operation.

Modeling Iso-octane HCCI Using CFD with Multi-Zone Detailed Chemistry; Comparison to Detailed Speciation Data Over a Range of Lean Equivalence Ratios

Multi-zone CFD simulations with detailed kinetics were used to model iso-octane HCCI experiments performed on a single-cylinder research engine. The modeling goals were to validate the method (multi-zone combustion modeling) and the reaction mechanism (LLNL 857 species iso-octane) by comparing model results to detailed exhaust speciation data, which was obtained with gas chromatography. The model is compared to experiments run at 1200 RPM and 1.35 bar boost pressure over an equivalence ratio range from 0.08 to 0.28. Fuel was introduced far upstream to ensure fuel and air homogeneity prior to entering the 13.8:1 compression ratio, shallow-bowl combustion chamber of this 4-stroke engine. The CFD grid incorporated a very detailed representation of the crevices, including the top-land ring crevice and headgasket crevice. The ring crevice is resolved all the way into the ring pocket volume. The detailed grid was required to capture regions where emission species are formed and retained. Results show that combustion is well characterized, as demonstrated by good agreement between calculated and measured pressure traces. In addition, excellent quantitative agreement between the model and experiment is achieved for specific exhaust species components, such as unburned fuel, formaldehyde, and many other intermediate hydrocarbon species. Some calculated trace intermediate hydrocarbon species do not agree as well with measurements, highlighting areas needing further investigation for understanding fundamental chemistry processes in HCCI engines.

Fuel and Additive Characterization for HCCI Combustion

This paper shows a numerical evaluation of fuels and additives for HCCl combustion. First, a long list of candidate HCCl fuels is selected. For all the fuels in the list, operating conditions (compression ratio, equivalence ratio and intake temperature) are determined that result in optimum performance under typical operation for a heavy-duty engine. Fuels are also characterized by presenting Log(p)-Log(T) maps for multiple fuels under HCCl conditions. Log(p)-Log(T) maps illustrate important processes during HCCl engine operation, including compression, low temperature heat release and ignition. Log(p)-Log(T) diagrams can be used for visualizing these processes and can be used as a tool for detailed analysis of HCCl combustion. The paper also includes a ranking of many potential additives. Experiments and analyses have indicated that small amounts (a few parts per million) of secondary fuels (additives) may considerably affect HCCl combustion and may play a significant role in controlling HCCl combustion. Additives are ranked according to their capability to advance HCCl ignition. The best additives are listed and an explanation of their effect on HCCl combustion is included.

Development and Testing of a 6-Cylinder HCCI Engine for Distributed Generation

This paper describes the technical approach for converting a Caterpillar 3406 natural gas spark ignited engine into HCCI mode. The paper describes all stages of the process, starting with a preliminary analysis that determined that the engine can be operated by preheating the intake air with a heat exchanger that recovers energy from the exhaust gases. This heat exchanger plays a dual role, since it is also used for starting the engine. For start-up, the heat exchanger is preheated with a natural gas burner. The engine is therefore started in HCCI mode, avoiding the need to handle the potentially difficult transition from SI or diesel mode to HCCI. The fueling system was modified by replacing the natural gas carburetor with a liquid petroleum gas (LPG) carburetor. This modification sets an upper limit for the equivalence ratio at φ∼0.4, which is ideal for HCCI operation and guarantees that the engine will not fail due to knock. Equivalence ratio can be reduced below 0.4 for low load operation with an electronic control valve. Intake boosting has been a challenge, as commercially available turbochargers are not a good match for the engine, due to the low HCCI exhaust temperature. Commercial introduction of HCCI engines for stationary power will therefore require the development of turbochargers designed specifically for this mode of operation. Considering that no appropriate off-the-shelf turbocharger for HCCI engines exists at this time, we are investigating mechanical supercharging options, which will deliver the required boost pressure (3 bar absolute intake) at the expense of some reduction in the output power and efficiency. An appropriate turbocharger can later be installed for improved performance when it becomes available or when a custom turbocharger is developed. The engine is now running in HCCI mode and producing power in an essentially naturally aspirated mode. Current work focuses on developing an automatic controller for obtaining consistent combustion in the 6 cylinders. The engine will then be tested for 1000 hours to demonstrate durability. This paper presents intermediate progress towards development of an HCCI engine for stationary power generation and next steps towards achieving the project goals.Copyright

A Flexure-Based Mechanism for Precision Adjustment of National Ignition Facility Target Shrouds in Three Rotational Degrees of Freedom

Abstract This paper presents the design of a flexure-based mount allowing adjustment in three rotational degrees of freedom (DOFs) through high-precision set-screw actuators. The requirements of the application called for small but controlled angular adjustments for mounting a cantilevered beam. The proposed design is based on an array of parallel beams to provide sufficiently high stiffness in the translational directions while allowing angular adjustment through the actuators. A simplified physical model in combination with standard beam theory was applied to estimate the deflection profile and maximum stresses in the beams. A finite element model was built to calculate the stresses and beam profiles for scenarios in which the flexure is simultaneously actuated in more than one DOF.

D2 and D-T Liquid-Layer Target Shots at the National Ignition Facility

Abstract Experiments at the National Ignition Facility (NIF) using targets containing a deuterium-tritium (D-T) fuel layer have, until recently, required that a high-quality layer of solid D-T (herein referred to as an ice layer) be formed in the capsule. The development of a process to line the inner surface of a target capsule with a foam layer of a thickness that is typical of ice layers has resulted in the ability to field targets with liquid layers wetting the foam. Successful fielding of liquid-layer targets on NIF required not only a foam-lined capsule but also changes to the capsule filling process and the manner with which the inventory is maintained in the capsule. Additionally, changes to target heater power and the temperature drops across target components were required in order to achieve the desired range of shot temperatures. These changes and the target’s performance during four target shots on NIF are discussed.

Technique for Forming Solid D2 and D-T Layers for Shock Timing Experiments at the National Ignition Facility

Abstract Capsule implosion experiments on the National Ignition Facility (NIF) are driven with a carefully tailored laser pulse that delivers a sequence of shocks to the ablator and fuel. To ensure the shocks converge at the desired position, the shock strength and velocity are measured in experimental platforms referred to as keyhole targets. Shock measurements have been made on capsules completely filled with liquid deuterium for the solid deuterium tritide (D-T) layer campaigns. Modeling has been used to extend these results to form an estimate of the shock properties in solid D-T layers. To verify and improve the surrogacy of the liquid-filled keyhole measurements, we have developed a technique to form a solid layer inside the keyhole capsule. The layer is typically uniform over a 400-μm-diameter area. This is sufficient to allow direct measurement of the shock velocity. This layering technique has been successfully applied to 13 experiments on the NIF. The technique may also be applicable to fast-igniter experiments since some proposed designs resemble keyhole targets. We discuss our method in detail and give representative results.

CFD Analysis of a Cryogenic Pressure Vessel Hydrogen Leak

This paper reports the numerical simulation of the sudden hydrogen release from a cryogenic pressure vessel due to a broken tube where hydrogen vents into the vacuum jacket. Real gas effects are considered and the specific “vessel within vessel” geometry of cryogenic vessels. For practical reasons, this study focuses on hydrogen release from 34.5 MPa, with initial temperatures of 62 K and 300 K. The high pressure vessel internal volume is 151 L. Pressure versus time graphs indicate that the vacuum jacket resist the pressure build up until reaching the rupture disc setting to finally release into the atmosphere, and a comparative of this result with the ASME Code burst pressure calculation is presented.Copyright

Development of Dual Volume Cryogenic Hydrogen Storage System

We are developing cryogenic capable pressure vessels with thermal endurance at least 5 times longer than conventional liquid hydrogen (LH2 ) tanks that can eliminate evaporative losses in routine use while providing high hydrogen storage weight and volume performance. The generation 2 vessel, installed onboard a Toyota Prius hydrogen hybrid experimental vehicle, has demonstrated the longest driving distance on a single LH2 tank and the longest LH2 hold time without evaporative losses. In addition to presenting the basic concept and recent progress on cryogenic capable pressure vessels, this paper describes the potential for greatly exceeding the hydrogen storage density of the generation 2 vessel through conformable dual volume containers that can better occupy box-shaped spaces onboard a vehicle. We present a design and analysis of a dual volume vessel that shows promise for delivering unprecedented hydrogen storage weight and volume performance. Future work will focus on demonstrating the technology onboard the experimental vehicle.© 2010 ASME