Engine exhaust particulates undergo different processes in the ambient environment such as agglomeration, coagulation, surface condensation, adsorption, and oxidation before evolving as mature particles. Conventional Cl engines emit a significant mass/number of particulates due to heterogeneous combustion. However this problem can be resolved by using an advanced combustion technology named as Homogeneous Charge Compression Ignition (HCCI), which has potential to substantially reduce particulates and NOx simultaneously and deliver efficiencies comparable to conventional Cl combustion. In the present study involving homogeneous mixture of diesel and air, an electrically heated diesel vaporizer was developed. Experiments were performed at different relative air-fuel ratios and EGR levels. Enrichment of the mixture increases the peak in-cylinder temperature, which was effectively controlled by EGR under leaner HCCI conditions. A partial flow dilution tunnel was used to collect particulate samples for trace metal content and Benzene Soluble Organic Fraction (BSOF), which is considered to be a marker of toxicity. This analysis showed that the trace metals detected were comparatively lower in HCCI particles. Trace metal concentration increased with application of EGR in the HCCI engine. BSOF content of the HCCI particulates increased when the mixture becomes leaner as well as with increasing EGR. Physical characterization of particulates was also carried out using engine exhaust particle sizer (EEPS), which measures the particle size-number distribution for the nano-particles in the exhaust. The particles collected on the filter paper were also analyzed for morphology using scanning electron microscopy (SEM). (C) 2013 Elsevier Ltd. All rights reserved.