case as the piston moves up on its first stroke to compress the mixture above it. Then the compressed mixture is ignited, and hot gases are produced, which drive the piston down on its second stroke. As it moves down, it uncovers an opening (port) that allows the fresh fuel mixture in the crankcase to flow into the combustion space above the piston. At the same time, the exhaust gases leave through another port.

Two-way Catalytic Converter

– A first generation catalytic converter designed to oxidize CO and HC emissions from gasoline-fueled vehicles.

Ultra-Low Emission Vehicle (ULEV)

– A vehicle meeting CARB’s California low-emission vehicle ULEV standards or EPA’s clean fuel vehicle ULEV standards. ULEVs produce fewer emissions than LEV I or LEV II vehicles.

Vehicle Miles Traveled (VMT)

– The number of miles driven by a fleet of vehicles over a set period of time, such as a day, month, or year. One vehicle traveling one mile is one vehicle mile.

Volatile Organic Compounds (VOCs)

– Organic compounds that can include oxygen-, nitrogen-, and sulfur-containing compounds. Alkanes, alkenes and aromatic hydrocarbons are all VOCs (as well as being HCs). The simple carbon-containing compounds carbon monoxide and carbon dioxide are usually classified as inorganic compounds. A volatile organic compound is one that can exist as a gas at ambient temperatures. Many volatile organic chemicals are hazardous air pollutants; for example, benzene causes cancer. (See also Appendix B.)

Zero Emission Vehicle (ZEV)

– A vehicle that emits no tailpipe exhaust emissions.