The Orange County Transportation Authority is buying 531 new natural gas powered engines from Cummins Westport Inc. of Vancouver, British Columbia. They're going to use them in a combination of new buses and retro-fits to existing buses. The engines incorporate stoichiometric combustion and exhaust gas recirculation allowing for the use of a catalytic converter to clean up the emissions. In order for a catalytic converter to work properly and reliably, the exhaust mixture must be kept at a certain level corresponding to optimal air/fuel mix. That's why modern engines use oxygen sensors that feed into the air/fuel mixture calculation. OCTA currently operates over 500 buses along with commuter rail as well as other responsibilities. The new engines have 250-320hp and thirty percent better low end torque than current engines.
[Source: Westport Cummins via GreenCarCongress