Heavy Duty Hybrid Vehicle Testing:

Carbon Monoxide (CO) Emissions

What is the importance of carbon monoxide?

Carbon monoxide (CO) is generally a local emission issue with the impact typically occurring in low lying areas such as urban canyons. CO affects the ability of blood to carry oxygen and results in impaired cardiovascular, pulmonary and nervous systems. While most areas of the U.S. are in attainment for CO, many areas in the Northeast, such as New York City, have been designated as moderate CO non-attainment areas.

Excess CO emissions are usually associated with cold engine startups and engine operation in open loop mode. Once the engine has warmed to operating temperature the oxidation catalyst is usually sufficient to complete at least partial combustion of excess HC and CO into CO2.

Overview of Results

The hybrid electric buses exhibited the lowest CO emission of any of the buses tested, representing a 70 percent reduction from a conventional diesel bus. It appears that a majority of this benefit is attributable to reduced transient operation of the engine with the remainder attributable proportionately to increased fuel economy and potentially more effective aftertreatment control due to reduced engine idle.

As was the case with HC emissions, the CO emissions from the CNG buses are roughly 300 percent higher than the diesel buses. The CNG buses tested under this project all employ lean burn combustion strategies that result in excess CO emissions when optimizing for low NOx emissions. This type of combustion strategy typically maintains more than sufficient oxygen in the catalyst for oxidation; however, the catalyst operating parameters as well as the catalyst washcoat must be optimized for reducing CO. Catalysts do not approach 100 percent efficiency and have operational temperature requirements (i.e., need to reach operating temperature before they become effective).

CO emission rates do trend with average cycle speed with increased emission rates associated with lower average speed cycles. The percentage difference between the CNG buses and the diesel buses was generally proportional to the changes in vehicle fuel economy.

Back to Emissions Results

More on heavy-duty hybrids:

Introduction to Heavy-Duty Hybrid Buses

What is a Hybrid Bus?

Hybrid Bus Demonstrations

Certification of Hybrid Buses