Particle Characterisation of Modern CNG, Gasoline and Diesel Passenger Cars

Tailpipe particle measurements have been performed in Euro-4 passenger cars. The investigation included compressed natural gas fuelled (bifuel vehicle concepts, commercially available) spark-ignition vehicles, gasoline port-injection spark-ignition vehicles, gasoline direct injection spark-ignition vehicles as well as compression ignited, diesel vehicles with and without particle filters. Particle number and soot mass was measured. In addition particle number size distributions have been recorded at constant vehicle speed operation. In the current study, the particle emissions of Compressed Natural Gas (CNG) vehicles are significantly lower than the corresponding ones of the other sparkignition vehicles. While direct injection spark-ignition vehicles exhibited high particle emission, only topped by the diesel vehicles without a particle filter, diesel vehicles with particle filters had lowest particulate emission, provided the diesel particulate filter was not regenerated shortly prior to the measurement cycle. CNG vehicles had almost as low particulate emissions in all cycle parts with low and moderate vehicle speeds. At typical highway driving speeds CNG vehicle particle emissions were significantly higher than the corresponding ones of the diesel particulate filter equipped vehicles. The enrichment of fuel-air mixture was identified as the main reason for the particle emission of the CNG vehicles at high vehicle speeds. We believe this enrichment to be a remnant from the gasoline operation the engine was originally designed for. In gasoline engines enrichment of the mixture reduces process and exhaust gas temperatures mainly due to fuel evaporation during and following the intake, protecting the catalyst from high temperatures. In parallel it improves engine performance and torque due to increased flame velocity. Since CNG-rich mixtures only slightly reduce process or exhaust temperatures and the maximal flame speed is at nearly stoichiometric air-fuel ratios we identify there a relatively easy to capture potential which, if reaped, may lead to the cleanest possible vehicle propulsion system. INTRODUCTION A significant reduction of CO2 emissions in the mobility sector is a major challenge for the next decades. In combination with efficient powertrain technologies, the potential of natural-gas is excellent for a comparably high as well as cost effective reduction in CO2 and toxic emissions in the near future. In the recent past our laboratory in collaboration with the Federal Institute of Technology of Zurich (ETHZ) has demonstrated the potential of a natural-gas optimised engine vehicle with the Clean Engine Vehicle (CEV) project. The vehicle used was a production line small sedan (model year 2000) with a curb weight of 1020kg. The achieved goal was 30% lower CO2 emissions than the gasoline vehicle while staying in compliance with Euro-4 as well as SULEV emission limits [1]. Also recently major passenger car manufacturers have modified gasoline fuelled spark-ignition engines for bifuel operation with CNG and gasoline. As a result, some bifuel CNG-gasoline vehicles have been introduced on the European market. While the bifuel concept is an important feature for customers, given the scarcity of CNG refuelling stations, it inhibits the exploiting of the full potential of CNG powertrains. In parallel, the relative low numbers of CNG vehicles on the road still constrains the resources dedicated by automotive industries to their development. CNG vehicles have to use established parts as well as concepts from their pure gasoline counterpart models. A main issue concerns the emissions from such vehicles. Certainly, they comply with the recent legislation limits, but are they better or worse than their peers? We are not aware of any systematic recent studies and comparisons. In this paper we examine the particle emissions of typical Euro IV propulsion systems for passenger cars. Typically particles are measured with gravimetric methods. The newest vehicle technologies, though, result in emissions often in the resolution domain of gravimetric methods. We used the gravimetric as well as the particle counting method in order to achieve comprehensive results. The counted particles from internal combustion engines are usually divided into solid and volatile particles. The solid particles mainly consist of agglomerated carbonaceous primary particles, which are in general referred to as soot. The volatile nanoparticles are usually formed by nucleation of sulfuric acid, water, and other species. These two different kinds of particles form clearly distinguishable modes in the number size distributions, designated as “accumulation” and as “nucleation mode” [2]. Extensive comparison of different powertrain technologies was carried out by [3], though no CNG systems had been included. Further investigations moved in the direction of emission modeling and inventory building [4] or assessing cold start influence on diesel and/or gasoline passenger car engines [2, 5]. Again no comparisons concerning CNG system were reported there. In the present paper we aim to address the questions of particle emission from current market-available CNG vehicles and compare them to other current technologies. The comparison extends only to EURO-IV vehicles including modern direct injected (DI) gasoline powered vehicles as well as conventional multi-point intake manifold injected (MPI) gasoline vehicles. As comparable peers from the diesel side, we chose a series of vehicles equipped with, as well as without, a particulate filter.