In 1993, the US Environmental Protection Agency (EPA) began to reduce urban residents' exposure to diesel exhaust from urban buses by implementing an urban bus retrofit/rebuild program.

All of the technologies listed in this section have different characteristics in terms of energy efficiency, life-cycle and tailpipe emissions (see table below).

The liquefied petroleum gas (LPG) bus uses mostly propane gas for combustion.

Four principle technology choices for cleaning up existing diesel buses are presented in this section - inspection and maintenance (I/M), retrofitting more advanced pollution controls, replacing existing engines with cleaner engines and conversions of existing diesel engines to CNG or LPG fuels

Inspection and maintenance programs identify buses needing repair.

Hybrid electric vehicles (HEV) have two power sources. One converts fuel into useable energy, and the other, an electric motor powered by an advanced energy storage device, lowers the demand placed on the first power source.

The fuel cell bus employs a novel technology totally different from the internal combustion engine, which converts chemical energy directly into electric energy used to power the vehicle.

The ethanol bus runs on alcohol which can be produced from a variety of renewable biological resources.

The battery electric bus is powered by electrochemical batteries which are charged using electricity from stationary power plants.

Cleaner conventional buses meet more stringent emissions standards.

The Department of Transport's bus system carries over 114 million people annually through a mix of local and express bus services provided by seven private, franchised bus companies.

The compressed natural gas (CNG) bus uses a spark-ignition engine to burn natural gas, which contains mostly methane obtained from fossil resources.

Retrofit systems upgrade existing conventional diesel buses with advanced technological equipment, in many cases in conjunction with low sulfur fuels.

Replacement programs replace or repower the existing bus engine with a new engine to meet lower emissions standards.

Bus conversions refer to original diesel buses being converted to the use of alternative fuels, in most cases CNG or LPG.

The advanced diesel bus uses advanced engine systems and aftertreatment emission reduction devices, usually in conjunction with lower sulfur fuel.