Characterization of Nanoparticle Emissions from Diesel and Advanced Dual Fuel Compression Ignition Engines

Abstract

Diesel engines are widely used for transportation and power generation due to their high fuel efficiency, torque output, durability, and reliability. Major problems with diesel engines are emissions of particulate matter and NOx, which negatively affect human health and the environment. Stringent emissions norms are therefore introduced for diesel engines to limit the pollutants from diesel engines. NOx and PM emitted by diesel engines are significant issues to solve because of the NOx-PM trade-off, in which simultaneous reduction of these two pollutants is highly challenging. Revolutionary in-cylinder combustion strategies and emissions after-treatment systems are required for diesel engines to meet stringent emission norms such as Euro VI. Emissions after treatment systems require frequent maintenance, increasing overall vehicle cost and fuel consumption. Thus, to reduce after-treatment costs and fuel consumption, it is necessary to avoid the generation of these pollutants during combustion. Reactivity-controlled compression ignition is a dual fuel low-temperature combustion strategy that has the potential for simultaneous reduction of NOx and PM emissions with high thermal efficiency without the need for a costly after-treatment system. This study investigates PM characteristics of diesel and RCCI engines. A numerical investigation is conducted to understand the effect of injection timing and injection pressure (relevant to diesel and RCCI combustion strategy) on in-cylinder soot precursor formation and incylinder particle emission characteristics using a detailed soot model based on method of moments available in ANSYS FORTE CFD software. For the experimental study of RCCI combustion concept, suitable hardware, and instrumentation was done on existing automotive single-cylinder diesel engine. Gasoline, methanol, and CNG are used as low reactivity fuel, and diesel is used as high reactivity fuel. The low-reactivity fuel is injected in the intake manifold using a solenoid-based port fuel injector developed in the laboratory. High reactivity fuel (HRF) is injected using the common rail direct injection (CRDI) technique. An open ECU is used to vary the diesel injection timing. RCCI combustion mode experiments were conducted at 1.5 bar (lower engine load) and 3 bar BMEP (medium engine load) at constant 1500 rpm. For the characterization of solid particle emissions from RCCI engines, a thermodesorption system was developed in the laboratory for particulate sampling. The main objective of the research is to investigate the PM characteristics of diesel and RCCI engines. The impact of injection timings and injection pressure relevant to diesel and RCCI combustion strategy on soot precursor species mass fraction, particle number density, size and volume fraction is investigated. The results show that soot precursor species mass fraction and particle number density decreased with advanced injection timing from 6° bTDC to 18° bTDC and increased injection pressure from 500 bar to 1000 bar. It is observed that acetylene mass fraction start rising after the start of combustion and then decreases and remain constant at injection timings employed in RCCI experiments (30°bTDC). The impact of low-temperature heat release (LTHR) and high-temperature heat release (HTHR) on particle emission was investigated for RCCI engines with single and double injection strategies and different port fuel-injected CNG mass at low and medium engine loads. Later, empirical correlations were developed to study the relationship between LTHR, HTHR, and particle emissions. Results show that the amount and location of LTHR and HTHR significantly influence the formation of particle number emission in RCCI combustion. The developed empirical correlations show a good correlation between diesel SOI and the ratio of HTHR to LTHR to estimate total particle number concentration. Solid particle emissions from the RCCI engine were investigated with a developed thermodesorption system at different premixing ratios. The bimodal shape of the particle size distribution curve was changed to a unimodal shape when sampling was performed with a thermodesorption system for both gasoline diesel and CNG diesel RCCI operation. Thermodesorption system showed high volatile particle removal efficiency for gasoline diesel RCCI operation compared to CNG diesel RCCI operation.