Urban air quality in California can have a large impact on the
state's economy, natural and managed ecosystems, and human
health and mortality. The use of alternative, low-carbon fuels is
considered to be an effective measure to meet strict emissions
regulations of particulate matter (PM) and oxides of nitrogen
(NOx). Natural gas may be a potential alternative to
conventional liquid fuels for use in automotive internal combustion
engines, and can be used in fulfilling these requirements.
The primary objective of this study is to evaluate the impact of
varying natural gas composition on the exhaust emissions from a
transit bus equipped with a 2003 Cummins C Gas Plus, lean-burn,
spark-ignited natural gas engine and an oxidation catalyst while
operating on the Central Business District (CBD) cycle on a chassis
dynamometer. The vehicle was tested on five different fuel gas
blends with varying compositions of light hydrocarbon species and
inerts, resulting in different properties in terms of methane
number (a measure of fuel knock resistance) and Wobbe number (a
measure of fuel interchangeability). For this study, emissions of
NOx, total hydrocarbons (THC), nonmethane hydrocarbons
(NMHC), methane (CH₄), carbon monoxide (CO), carbon dioxide (CO₂)
emissions, and PM, as well as fuel economy were measured. The
unregulated exhaust emissions were characterized by determining
formaldehyde, acetaldehyde, ammonia, and total concentration and
sizing of ultrafine particles. The results showed that natural gas
composition had a strong influence on a number of emission
components. Blends with higher methane contents showed lower
NOx, CO, and NMHC, but higher THC, CH₄, and formaldehyde
emissions. PM, CO₂, and NH₃ emissions and energy equivalent fuel
economy did not show consistent trends between the fuels
tested.