Orange County California buying new natural gas power for buses

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]