Experimental research on particulate matter emissions from gasoline direct injection engines

As the legislation on vehicle emissions is becoming more and more stringent, increasing attention has been paid to the fine particles emitted by diesel and gasoline vehicles. The high number emission of fine particles has been shown to have a large impact on the atmospheric environment and human health. Researchers have shown that gasoline engines, especially Gasoline Direct Injection (GDI) engines, tend to emit large amounts of small size particles compared to Port Fuel Injection (PFI) gasoline engines and diesel engines fitted with Diesel Particulate Filters (DPFs). As a result, the particle number emissions of GDI engines will be restricted by the EU6 legislation. The particulate emission level of GDI engines means that they would face some challenges in meeting the EU6 requirement.
This thesis undertakes research in the following area. Firstly, the filtration efficiencies of glass fibre filters were quantified using a Cambustion Differential Mobility Spectrometer 500 (DMS500) to see if all of the particles from the sampled gas can be collected by the filters. Secondly, various valve timings and different injection modes such as double injection with a second injection after compression, single early injection and split early injection were implemented to measure the Particulate Matter (PM) emissions and combustion characteristics of a GDI engine under warm-up operating conditions. Thirdly, the techniques for removing volatile particles were investigated using a catalytic Volatile Particle Remover (VPR) and an Evaporation Tube (ET) with hot air dilution under various test conditions.
The results show that for the glass fibre filters tested here, the transmission efficiencies of the particles are very low, indicating that PM sampling using fibre filters is an effective method of studying the particulate emissions from the engine.
Particle number emissions using double injection with injection after compression were much higher than those with single injection during the intake stroke. Under 1200 rpm, 110 Nm cold engine operation, no reduction effect on PM emissions was shown by using split intake injection to further facilitate homogeneous mixture formation compared with single intake injection. Valve timings showed moderate effects on particulate emissions. Properly adjusted timing for exhaust valve closure led to reduced particulate emissions by a factor of about 2 and the combustion characteristics were not adversely affected much.
The VPR temperature and exhaust residence time did not show much effect on the catalytic VPR performance once the mass flow rate of exhaust was above 0.09 g/s. Generally, the transmission efficiencies of the VPR follow the trends of the scaled PMP counting efficiency specification. Hot air dilution is effective in reducing the small size particles. At 23 nm, the transmission efficiencies are within the error range of the PMP specification. The catalytic VPR and the Evaporation Tube were all found to be effective in reducing the particle number of small size (nucleation mode) particles. Both systems have some particle loss mainly due to the physical effects of diffusion and thermophoresis.
Until now, GDI engines have not been optimised for reducing particulate emissions as the focus has been on gaseous emissions and fuel economy. With careful re-optimisation of the catalyst light-off and engine calibration (especially for transients) then there is scope for GDI engines to meet forthcoming emissions legislation.

Keywords: Combustion
Emissions
Particulate Matter
Gasoline Direct Injection
Advisor: Stone, R