Effect of Injection Strategies in Diesel/NG Direct-Injection Engines on the Combustion Process and Emissions under Low-Load Operating Conditions

Jinze Li,Longfei Deng,Binyang Wu,Jie Zhang,Min Zhang,Jianjun Guo,Zhenyuan Zi

doi:10.3390/en13040990

Abstract

The direct injection of natural gas (NG), which is an important research direction in the development of NG engines, has the potential to improve thermal efficiency and emissions. When NG engines operate in low-load conditions, combustion efficiency decreases and hydrocarbon (HC) emissions increase due to lean fuel mixtures and slow flame propagation speeds. The effect of two combustion modes (partially premixed compression ignition (PPCI) and high pressure direct injection (HPDI)) on combustion processes was investigated by CFD (Computational Fluid Dynamics), with a focus on different injection strategies. In the PPCI combustion mode, NG was injected early in the compression stroke and premixed with air, and then the pilot diesel was injected to cause ignition near the top dead center. This combustion mode produced a faster heat release rate, but the HC emissions were higher, and the combustion efficiency was lower. In the HPDI combustion mode, the diesel was injected first and ignited, and then the NG was injected into the flame. This combustion mode resulted in higher emissions of NOx and soot, with a diffusion combustion in the cylinder. HC emissions significantly decreased. Compared with PPCI combustion, HPDI had a higher thermal efficiency.

Highlights

In recent decades, road traffic emission legislation has become the main driving force in the development of internal combustion engines
Transportation accounts for approximately 20% of global carbon dioxide emissions that result from the burning of fuel, of which 80% comes from road vehicles [1]
The HPDI combustion mode is achieved by setting the natural gas (NG) injection timing after the pilot ignition diesel injection timing

Summary

Introduction

Road traffic emission legislation has become the main driving force in the development of internal combustion engines. Transportation accounts for approximately 20% of global carbon dioxide emissions that result from the burning of fuel, of which 80% comes from road vehicles [1]. Natural gas (NG) is a low-life-cycle carbon fuel and has been used in engines for a long time due to its cost effectiveness. There are a lot of combustion problems that limit spread application. The lower laminar flame velocity is the intrinsic difficulty in combustion organization in engines. Engine performance using NG is characterized by low efficiency, high exhaust gas temperatures, and high methane emissions, which is problematic from the point of view of greenhouse gas emissions

Methods

Results

Conclusion