A New Proportional Collection System for Extremely Low Emission Measurement in Vehicle Exhaust

A new proportional collection system for extremely low tailpipe emission measurement in transient conditions has been developed. The new system can continuously sample a minute flow of exhaust gas, at a rate that is proportional to the engine exhaust rate. A zero grade gas dilution technique is utilized to prevent the influence of pollutants in atmospheric air that are the same concentration level as those in the exhaust gas. The system has accuracy within ±5%.

For the direct exhaust gas flow meter, a pitot tube type flow meter is utilized as it is simple, heat resistant, sufficiently accurate and has low flow-resistance characteristic. For the collection and dilution controllers, two mass flow controllers (MFC) were adopted. The MFCs' output can be adversely influenced by variation of the specific heat of the sample gas, resulting in flow reporting error. To correct this error a new corrective apparatus, which is able to sense variation of specific heat of the sample gas, was added to the new system. To compensate for collection errors that were caused by “ramp-up” time delay of the MFC, it was necessary to set the gas and signal relative delay correctly. Additionally, a new delay adjustment method enabled the system to be optimized for proper transient time-alignment, thus resulting in good overall system performance.