What is a Hybrid Bus? | Demonstrations | Emissions | CertificationThe Certification Challenge for Hybrids
Emissions certification of trucks and buses presently involves testing of the engine only. The Engine Compliance Program at the Environmental Protection Agency's (EPA) Office of Transportation and Air Quality is responsible for certifying engines for heavy-duty applications. The California Air Resources Board (CARB) Mobile Sources Control Division performs a similar function for certification in the state of California. Both EPA and CARB use the same test procedures for urban bus engine certification. Engines are certified on the Federal Test Procedure (FTP) transient cycle. Emissions are measured and reported in units of grams of emissions per brake horsepower hour (g/bhp-hr) delivered by the engine under specific load regimes. The emissions are not allowed to exceed certain standards set by EPA and California. Engine manufacturers are responsible for complying with exhaust emission standards.
Industry and regulators have recognized for some time the unique challenge posed by hybrids in the emissions certification process compared to traditional transit buses. Current hybrid-electric transit buses often use new or unconventional engine technology that is smaller and different in design, control and operation from conventional engines. Although several of the first-generation hybrid engines in use today did not meet EPA urban bus standards on the FTP cycle, many demonstrated superior emissions performance in chassis testing of hybrid-electric buses. Chassis-based emissions testing in the United States only occurs on light duty vehicles, except in California where chassis-based certification of medium-duty vehicles is allowed.
The challenge is for industry and regulators to find an acceptable engine cycle on which to test hybrid engines for purposes of emissions certification. If the FTP transient cycle alone is used, will hybrid engines be able to meet the urban bus standard and at what cost? Will viable engine technologies be excluded if the FTP is used? Will the FTP allow flexibility to improve and optimize hybrid engine controls for further emission reductions in the future? Is there another engine cycle that would better represent hybrid engine operation that could be used for certification purposes? These questions were being debated in the hybrid industry when the NAVC formed the Hybrid Transit Bus Certification Workgroup.