Abstract
Research and/or Engineering Questions/Objective
It is a common opinion that vehicles fuelled by gas fuels are more eco-friendly than these fuelled with conventional fuels. There are quite a few investigations on exhaust emissions from LPG/CNG light duty vehicles. But nowadays, it is the market of CNG city buses which is growing the most dramatically. For example, in Poland we have ca. 3000 CNG vehicles and this number includes 500 CNG city buses. The data on exhaust emissions from latest CNG city buses are still very limited. The objective of this study was to investigate the environmental performance of EEV CNG buses in their real operating conditions.
Methodology
The investigations were performed on two 18 m city buses fitted with a CNG (spark ignition) and a diesel engine respectively, both meeting the EEV (Enhanced Environmentally Friendly Vehicle) emissions standard. The exhaust emissions measurements were performed in the SORT (Standardized On-Road Test Cycles) tests and under actual conditions of operation, when the buses operated on regular city routes in the city of Poznan, Poland. In the exhaust emissions tests of the buses a PEMS (Portable Exhaust Emissions Analyzer) was used (Semtech DS by Sensors Inc.). The selected test route reflected typical city bus operating conditions in large agglomerations. The test route was 16 km long had 43 bus stops. A varied configuration of the test route (main city road, residential area roads and downtown roads) ensures high variability of accelerations and share of road congestions, which enables the analysis of the exhaust emissions in a wide range of vehicle operating parameters.
Results
The on-road emissions show similar trends in all SORT tests. From the comparison of these emissions we know that higher values of CO, HC and CO2 were obtained for the CNG fueled bus. It is the HC emission differences that were the highest in all the driving tests (in SORT 2 and 3 they were over 20 times higher for the CNG fueled bus compared to the diesel fueled one). The differences in the road emissions of CO and NOx were lower than those of HC. From the comparison of individual components we know that the CNG fueled bus had higher emissions of CO2 (fuel consumption) in all driving tests. These were small differences. The greatest differences in the SORT 1 test reached 15%. Based on the research performed under actual traffic conditions on a city bus route the values of road emissions were determined and compared. Similarly to the SORT tests, the CNG fueled vehicle was characterized by higher emissions of CO, THC and CO2. For the mentioned exhaust components the differences between the CNG and diesel fueled vehicles were 78%, 1843% and 18% respectively. The emission of NOx from the CNG bus was lower by 87%.
Limitations of this study
A certain limitation of the study is that the PM emissions were not measured. However it is well known, that CNG vehicles equipped with spark engines are characterized by very low PM emissions.
What does the paper offer that is new in the field including in comparison to other work by the authors?
Most of the available scientific papers on vehicles fueled with gaseous fuels concerns passenger cars and other LDVs. The unique features of this study are as follows: - test objects: heavy duty vehicles of a high emissions standard, - test conditions: on-road – SORT cycles and real bus route, - test equipment: PEMS.
Conclusions
The research revealed that CNG bus generates significantly lower NOx emission whereas its CO and HC emissions are higher. CNG buses emit significantly less PM as well. Even meeting the same emission standard (EEV), CNG buses should be regarded as more eco-friendly means of public transport than conventional buses fueled with diesel fuel.
KEYWORDS : exhaust emissions, city bus, CNG, real conditions of operation, PEMS