Abstract
Keywords exhaust emissions, city buses, chassis dynamometer, transient load, testing
Abstract - City buses, both diesel and CNG (compressed natural gas) fuelled, operating within the Helsinki Metropolitan area in Finland, are being tested at VTT under transient load conditions. The emissions of CO, THC, NOX and total particulate mass and size distribution are measured using an up-to-date transient-type heavy-duty chassis dynamometer, with roller diameter of 2.5 m, and a full-size CVS device. Both the urban part of the certification test cycle for European passenger cars (ECE 15) and the Braunschweig urban bus cycle were used as driving schedule.
The diesel buses represent different stages of the European emission legislation (Euro 1, w/o aftertreatment, and Euro 2 or Euro 3, out of which most were fitted with some exhaust after-treatment,
OEM or retrofitted). The CNG buses were of stoichiometric and lean-burn type representing two generations of engine technology and two stages of emission regulations (pre-Euro3 and Euro 3).
The first set of results indicated that as expected, NOX and PM emissions were lower towards newer engine types (from Euro 1 to Euro 2 and Euro 3). However, the differences between the two driving cycles (relatively smooth ECE 15 and more aggressive Braunschweig) were quite small, which was somewhat surprising. Also according to our expectations, PM emissions from CNG-fuelled buses were significantly lower than from regular diesel buses without any aftertreatment devices.
The objective of this 3-year evaluation programme is to assist Helsinki Metropolitan Area Council (YTV) and Helsinki city authorities, when they contract bus revenue service from transportation companies. Both of these authorities are quite pro-active regarding environmental issues, and when evaluating the tenders, the authorities not only set a minimum level of accepted emissions performance, but also give incentives for service provided with less polluting vehicles. Before this programme was initiated, the only grounds available to determine these incentives were the normal type approval values [g/kWh] of the engines, derived from only steady-state operation, which is quite non-representative of urban bus operation. Now more life-like, transient emission data (in [g/km]), independent of engine or bus manufacturers' information, is gathered to facilitate more righteous incentives for each type of bus.
As a parallel study for the International Association of Natural Gas Vehicles (IANGV), VTT is evaluating diesel and CNG buses using even more comprehensive emissions assessment. Overall, chassis dynamometer testing facilitates easy monitoring of the in-use emission stability and durability of different powertrain and exhaust emission control technologies.