Hua Zhao

Investigation of SI-HCCI Hybrid Combustion and Control Strategies for Combustion Mode Switching in a Four-Stroke Gasoline Engine

Homogeneous charge compression ignition (HCCI) combustion, also known as controlled autoignition (CAI) combustion, has been realized over a range of engine speed and load conditions in a single cylinder four-stroke gasoline engine equipped with a 4VVAS mechanical variable intake and exhaust valve lift and timing devices system. The engine has the capability to operate either in the spark ignition (SI) or HCCI combustion in order to cover the complete engine operational range. Therefore, smooth switching between SI and HCCI modes is required. In this work, a systematic investigation has been carried out to study the transition between the conventional spark ignition combustion and HCCI combustion and develop the control strategies for optimized mode switch. Results show that dynamic transitions between HCCI and SI can be stably achieved around the boundaries of HCCI operation region through the rapid and effective control of the in-cylinder residual gas concentration by a Four Variable Valve Actuation System (4VVAS). It is found that switching from HCCI to SI operation is less problematic than that from SI to HCCI combustion operation, and certain regions of residual concentrations should be avoided. During the switching process, the spark timing and exhaust valve closing timing have the largest effect and can be optimized for mode switching. Two major approaches have been developed and applied to controlling the HCCI-SI transitions, through the dynamic management of residual gas and the management of hybrid heat release processes.

Effect of recycled burned gases on homogeneous charge compression ignition combustion

ABSTRACT Homogeneous Charge Compression Ignition (HCCI) also known as Controlled Auto-Ignition (CAI) combustion, has recently emerged as a viable alternative combustion process to the conventional spark ignition (SI) gasoline and compression ignition (CI) Diesel engines, due to its potential for extremely low emissions and good fuel economy. One of the most important means of achieving and controlling HCCI combustion is to use recycle burned gases (EGR). In order to understand better the effects of recycled burned gases on such combustion process, detailed analytical studies were performed. Since HCCI combustion is a process dominated by chemical kinetics of the fuel-air mixture, an engine simulation model with detailed chemical kinetics has been developed and applied to a four-stroke gasoline engine fuelled with isooctane. After calibration and validation, the engine simulation mode was used to investigate the effects of EGR on HCCI combustion in a four-stroke gasoline engine. The characteristics of HCCI combustion investigated include the autoignition timing, the partial burning and knocking combustion, and NO emission. The heat capacity, dilution, chemical, and charge heating effects on the HCCI combustion process were studied individually by means of a series of analytical studies. When isothermal EGR is used, such as in HCCI combustion with diesel type fuels, the heat capacity has the largest effect on extending the combustion duration and slowing down the rate of the combustion. The larger heat capacity of EGR gases tends to retard the start of autoignition. The dilution of oxygen by EGR has little effect on autoignition but it has a similar effect on extending the combustion duration to that of the dilution effect. The dilution effect of EGR on lowing heat release rate is only noticeable at high concentrations. Finally, the chemical effect of CO2 and H2O was found to be negligible. When hot EGR is used in HCCI combustion, the charge heating effect of EGR has much greater effect on ignition timing and some less significant effect on the combustion duration and heat release rate than the other effects.

Numerical simulation and validation of SI-CAI hybrid combustion in a CAI/HCCI gasoline engine

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In this study, a SI-CAI hybrid combustion model (HCM) has been constructed on the basis of the 3-Zones Extended Coherent Flame Model (ECFM3Z). An ignition model is included to initiate the ECFM3Z calculation and induce the flame propagation. In order to precisely depict the subsequent auto-ignition process of the unburned fuel and air mixture independently after the initiation of flame propagation, the tabulated chemistry concept is adopted to describe the auto-ignition chemistry. The methodology for extracting tabulated parameters from the chemical kinetics calculations is developed so that both cool flame reactions and main auto-ignition combustion can be well captured under a wider range of thermodynamic conditions. The SI-CAI hybrid combustion model (HCM) is then applied in the three-dimensional computational fluid dynamics (3-D CFD) engine simulation. The simulation results are compared with the experimental data obtained from a single cylinder VVA engine. The detailed analysis of the simulations demonstrates that the SI-CAI hybrid combustion process is characterised with the early flame propagation and subsequent multi-site auto-ignition around the main flame front, which is consistent with the optical results reported by other researchers. Besides, the systematic study of the in-cylinder condition reveals the influence mechanism of the early flame propagation on the subsequent auto-ignition.

Numerical study of the effect of piston shapes and fuel injection strategies on in-cylinder conditions in a PFI/GDI gasoline engine

The study is a part of the National Science Fund project (Grant 51206118) supported by the National Science Fund Committee of China, and State Key Project of Fundamental Research Plan (Grant 2013CB228403) supported by the Ministry of Science and Technology of China. The authors would also like to acknowledge the China Scholarship Council (CSC), which funded the first author for one year study at Brunel University.

Gas Sampling Analysis of A Low-Temperature Compression Ignition Reaction Process in an Engine Fueled with Methanol and Gasoline

With an in-cylinder gas sampling system that was modified from a gasoline direct injection injector, a series of in-cylinder gas sampling experiments were performed in a Ricardo Hydra four-stroke single-cylinder gasoline engine to study the components and concentrations of chemical substances in the hot residual gas by using methanol and gasoline, respectively. And then the effects on low-temperature oxidation process of these chemical substances were analyzed with the experimental data and chemical reaction mechanisms. The experimental results show that there were some chemical substances, such as formaldehyde, acetaldehyde in the trapped hot residual gas achieved by negative valve overlap method when fueled with methanol or gasoline. These components activated the chemical reactions in the cylinder and promoted the low-temperature oxidation of the fuel. The concentration of formaldehyde in the hot residual gas of methanol was significantly higher than that under the same experimental conditions of gasoline because of the easier production process of formaldehyde in methanol according to reaction mechanisms. And the concentration of formaldehyde decreased in the prophase of the compression process and then increased rapidly, which indicated that formaldehyde in the mixture accelerated the oxidation process by participating in the reaction and that is one of the reasons why temperature increased rapidly and ignition timing advanced in the methanol-controlled auto-ignition combustion process. With the increasing residual gas fraction, formaldehyde concentration in the mixture decreases for lower reactant concentration and this affected the ignition timing as well as the heat release rate accompanied by the temperature.

The effect of intake valve lift load control on gasoline engine fuel economy

The conventional SI engine with throttle-driven load control, has a disadvantage in fuel economy dominantly caused by its pumping loss. In the paper, throttle-free load control strategy, namely the application of valve lift adjustment and phase optimization, was employed to reduce the pumping loss and improve the fuel economy. Tests were carried out on a single-cylinder research engine equipped with 4VVAS(4 Variable Valve Actuation System) cylinder head, which can independently control the intake and exhaust valve lifts and timings. Results show that pumping loss can be reduced 20∼30% by using the valve lift load control, and a 3∼12% of fuel consumption benefit can be obtained.