汽油均质混合气压缩着火(HCCI)燃烧技术，可提高汽油机的燃油经济性并减少有害排放量，但燃烧机理和燃烧控制技术十分复杂。 本文针对汽油HCCI发动机着火控制难度大和对运行参数过分敏感的问题，提出了分层混合气和火花点火辅助两个着火基本控制方法，在此基础上设计了以缸内直喷、两段喷射、火花点火和负阀重叠为特点的ASSCI燃烧系统。该系统通过调节单次或两次喷油时刻和喷油量在缸内形成不同燃烧性质的可燃混合气，采用活塞上的回转燃烧室形状配合高压旋流喷雾以及进气涡流保证合理的混合和稳定的燃烧，减少了HCCI燃烧对发动机运行参数的敏感度。以一台柴油机为基础进行了ASSCI燃烧系统、电控供油系统的设计试制，完成了一台多功能缸内直喷HCCI复合燃烧模式原理性试验样机的开发。 在GDI-HCCI原理性试验样机上，通过对发动机动力性、经济性、排放特性、示功图以及放热规律的分析，研究了用两段喷射控制混合气形成的基本规律，优化了喷油策略，确定了发动机HCCI工况平面特性。试验结果表明，经优化后的复合燃烧模式发动机在0.1MPa~0.53MPa指示平均压力的HCCI工作模式范围内，比常规进气道喷射汽油机的NOx排放降低95％～99％，燃油消耗率改善19％～31％。从而验证了以混合气浓度为主要控制手段的ASSCI燃烧系统的基本思路是可行的。 为深入理解GDI-HCCI发动机进气－喷雾－混合气形成－燃烧－排放的工作过程，本文在CHEMKIN源代码程序基础上开发了单区详细化学动力学的高辛烷值汽油HCCI发动机数学模型。将该模型嵌入三维流体力学程序中，利用详细化学反应动力学计算三维燃烧过程，建立了三维CFD耦合详细化学反应动力学模型。通过模拟计算和模拟试验的对比验证了模型的准确性。在此基础上建立了缸内直喷HCCI发动机进气－喷雾－混合气形成－压缩－燃烧工作过程的模型，利用试验方法确定了计算边界条件，研究了喷油策略对混合气形成和燃烧排放过程的影响，为发动机试验结果提供了理论支持。
Homogeneous Charge Compression Ignition (HCCI) combustion has advantages in high thermal efficiency and low emissions with lean burn, premixed compression ignition. But it is still facing challenges in combustion control for practical application.An Assisted Spark Stratified Compression Ignition (ASSCI) gasoline combustion system was developed based on the Gasoline Direct Injection (GDI) and aimed to solve the problem of combustion control in HCCI engine. Negative Valve Overlap (NVO) was adopted to obtain the internal Exhaust Gas Recirculation (EGR). A two-stage fuel injection strategy including flexible injection timing and quantity was utilized as a main approach to form controllable mixture in cylinder. The combustion system with the central-mount gasoline high-pressure swirl injector combined with the cylindrical combustion chamber and the helical intake port was designed to stabilize the mixture formation, which can reduce the HCCI combustion sensitivity. A dual-cylinder 4-stroke ASSCI prototype engine was modified from a diesel engine. The gasoline high-pressure common rail system and the electronic unit for controlling the injection timing and fuel quantity were also researched and designed.The ASSCI engine could be operated in HCCI combustion mode in a range of load from 0.1 to 0.53 MPa IMEP. The HCCI combustion characteristics were investigated under different A/F ratios, EGRs, injection strategies, engine speeds, as well as spark ignition advances. The effects of stratified charge, NVO and spark ignition on the engine power, fuel economy, emissions, cylinder pressure and heat release were studied in detail. After optimization, the GDI-HCCI engine fuel consumption can be reduced about 19%~31%, NOx emission can be reduced about 95%~99% compared to the conventional PFI gasoline engines at low-middle load. In order to better understand the physicochemical phenomena in cylinder, a High-RON Gasoline (HRG) HCCI model, including detailed chemical kinetics, gas exchanging loss, heat transfer, internal EGR, was established based on the original CHEMKIN code. A three-dimensional CFD code was developed by coupling with the HRG model to simulate the intake charge, two-stage spray, compression and combustion process of the HCCI engine. A three-dimensional mesh of the ASSCI combustion system including the helical intake port was generated. The experimental results obtained from the engine test bench were used as simulation boundary conditions. The effects of different injection strategies, A/F ratios on mixture formation, combustion and emission in cylinder were simulated in detail to provide theoretical explanation for the experimental results.
