A review of the influence of low temperature combustion strategies on the emissions of diesel and biodiesel engines

https://www.sciencedirect.com/science/article/pii/S0196890414000648

The aim of this review is to critically assess the impact of low-temperature combustion (LTC) strategies on diesel engine emissions for both diesel and biodiesel fuels. Diesel engines have been used extensively in various applications owing to their fuel flexibility, high thermodynamic efficiency, and stability. However, increasingly stringent emission regulations over the past two decades have necessitated the adoption of alternative in-cylinder strategies for reducing pollutant emissions, particularly particulate matter (PM) and nitrogen oxides (NOx), which are known to have harmful effects on human health and the environment.

LTC strategies generally involve enhanced pre-combustion mixing to avoid the formation of locally rich regions, which leads to reduced PM formation compared to conventional diesel combustion. The use of exhaust gas recirculation (EGR) also results in pre-combustion mixing and dilution, reducing the peak combustion temperature and consequently the formation of thermal NOx. LTC strategies, including homogeneous charge compression ignition (HCCI), partially premixed combustion (PCCI), premixed charge compression ignition (PCI), and reactivity controlled compression ignition (RCCI), have been proposed and implemented using both diesel and biodiesel fuels.

The review findings indicate that LTC strategies can effectively reduce NOx emissions, with lower NOx emissions being achieved through higher ignition delay and lower combustion rate resulting in lower in-cylinder temperature. Higher use of EGR and optimized start of injection (SOI) control these parameters on LTC to keep NOx emissions low during diesel combustion. LTC modes have also succeeded in reducing NOx emissions for biodiesels, with higher oxygen content and cetane number of biodiesels enabling them to sustain higher EGR and late SOI to attain better LTC.

LTC strategies can also simultaneously reduce NOx and PM emissions, as LTC modes take the combustion temperature below the formation temperature of PM. For diesel fuel combustion during LTC modes, PM emissions decrease. For biodiesels, inherent properties such as increased oxygen content, lower stoichiometric need of air, absence of aromatics and sulfur, combustion advance, and soot structure, can spontaneously reduce PM emissions during all the LTC strategies. However, condensation of unburned fuel sometimes increases PM emissions during LTC modes for biodiesels.

While LTC of diesel generally increases the emissions of unburned hydrocarbons (UHC) and carbon monoxide (CO) due to a reduction in in-cylinder combustion temperature and oxygen concentration, biodiesels can reduce UHC and CO emissions during LTC modes because of their short premixed combustion duration and higher oxygen content. Nevertheless, the emission level remains higher than in the conventional combustion system.

In conclusion, LTC is an emerging idea of modern combustion science, and while it shows promise for reducing emissions, new ideas or strategies are needed to eliminate or decrease its inherent drawbacks. Potential areas for future research include using oxygenated fuels or additives, high injection pressure with in-cylinder swirl formation, optimized use of fuel vaporizer, and use of different fuels having a wide range of reactivity for the RCCI combustion system. Despite its challenges, LTC is on the verge of becoming the most suitable and acceptable technology for future combustion, given the need to comply with increasingly stringent emission policies while improving performance.