NREL Study Highlights Emissions and Fuel Economy Benefits of CNG Over Diesel Buses
15 March 2006
Buses running on compressed natural gas have been a leading cleaner alternative to diesel-powered transit, and as such CNG buses and infrastructure were adopted by a number of transit agencies. The improving baseline emissions of comparable diesel buses with advanced emission control technologies has led to some questioning over whether or not CNG still retains its clean advantage. According to a recent study by NREL, it generally does.
The National Renewable Energy Laboratory (NREL) evaluated the emissions of twelve 40-foot, low-floor buses operated by the Washington Metropolitan Area Transit Authority. These buses were of two types: CNG and low-sulfur diesel (approximately 17 ppm sulfur). All CNG buses had lean-burn natural gas engines and oxidation catalysts. All diesel buses had catalyzed particulate filters, and one group of diesel buses had exhaust gas recirculation (EGR).
The four test bus configurations included in the study were:
CNG buses with model year (MY) 2004 John Deere 6081H engines, equipped with oxidation catalysts;
CNG buses with MY 2001 Cummins Westport, Inc. (CWI) C Gas Plus engines, equipped with oxidation catalysts;
Diesel buses with MY 2004 Detroit Diesel Corporation (DDC) Series 50 engines, equipped with catalyzed particulate filters and EGR; and
Diesel buses with MY 2000 DDC Series 50 engines, equipped with catalyzed particulate filters.
The John Deere CNG buses produced 49% lower NOx emissions and 84% lower PM emissions compared with the MY 2004 DDC diesel buses, and 63% lower NOx emissions and 60% lower PM emissions compared with the MY 2000 DDC diesel buses.
The CWI CNG buses produced 6.1% higher NOx emissions and 60% lower PM emissions compared with the MY 2004 DDC diesel buses, and 23% lower NOx emissions and equal PM emissions compared with the MY 2000 DDC diesel buses.
In addition to showing the emissions advantage of CNG buses, the project showed promising fuel economy results for the CNG buses compared with the benchmark diesel buses. On a diesel-gallon-equivalent basis, the John Deere CNG buses exhibited a 9.0% fuel economy improvement compared with the MY 2004 DDC diesel buses and a 2.9% improvement compared with the MY 2000 DDC diesel buses.
The CWI buses exhibited fuel economy that was 4.2% higher than the MY 2004 DDC diesel buses and 1.6% lower than the MY 2000 DDC diesel buses. Both CNG engines use lean burn technology.
| Relative Emissions and Fuel Economy Performance, CNG vs. Diesel | ||||
|---|---|---|---|---|
| John Deere CNG | CWI CNG | |||
| MY 2004 DDC | MY 2000 DDC | MY 2004 DDC | MY 2000 DDC | |
| NOx | -49% | -63% | +6.1% | -23% |
| PM | -84% | -60% | -60% | –% |
| Fuel economy | -9% | -2.9% | +4.2% | -1.6% |
Overall, NREL concluded that the CNG buses show significant improvements in fuel economy and show progress toward meeting the increasingly stringent EPA emission regulations that all heavy-duty engines will have to meet in 2006–2010 and beyond.
Resources:
interesting. is there a percentage missing in the table, Mike?
what i don't really get is why the MY2004 DCCs had greater particulate emissions even though they had EGR in them?
bummer.
Posted by: lensovet | 15 March 2006 at 10:23 PM
PM emissions from buses are particularly bad as they occur mostly during load transitions such as pulling away from a bus stop. Passengers that have just disembarked, especially children and senior citizens, are exposed to these emissions.
Btw, the 450 city transit buses in Vienna, Austria run on liquefied petroleum gas (LPG), with similar results as the more complex CNG systems used elsewhere. Unfortunately, many refineries prefer to sell their LPG to the chemical industry as a feedstock.
An alternative to switching to LPG or CNG is to radically downsize the diesel, equip it with a particulate filter and add a powerful hybrid electric system. This is especially appropriate for city buses and other heavy vehicles that need to make frequent stops. Diesel has the advantage of an existing distribution infrastructure.
Posted by: Rafael Seidl | 16 March 2006 at 02:10 AM
No figure is missing -- there was no difference in PM between the CWI CNG bus and the MY 2000 DDC bus.
Posted by: Mike | 16 March 2006 at 08:55 AM
It looks like every alternative to petrol and diesel has better emissions.
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