Partial Flow Sampling System For Transient Engine Testing

For on-highway engine manufacturers, meeting model year 2007 emissions levels represents a major measurement challenge. However, future off-road engine emissions testing requirements are anticipated to include not only lower steady-state emissions levels, but also the possibility of using transient cycle data.

As a result, engine manufacturers are seeking more cost-effective methods of developing engines to meet regula- tions r
egardless of their size and re- quired test cycles. The measurement challenge for 2007 development, com- bined with development cost concerns for both on- and off-highw a y eng ines, have provided the impetus for improving the efficiency, accuracy and flexibility of engine test cells.

At the Caterpillar Technical Center, Mossville, Ill., new partial flow sampling systems (PFSS) are being evaluated for use in accelerating on-road engine devel-opment using transient test cycles in test cells not equipped with full-flow dilution systems. The overall goal is reduction of anticipated future expenditures by substituting raw exhaust sampling systems that provide equal or better data for the more costly full dilution tests.

The challenge is that during transient test cycles, engine speed, load, airflow and fuel flow values exhibit large variations of change over very short time frames. For example, the magnitude of engine inlet air mass flow excursions can approach 10:1 in less than two seconds. The challenge for a PFSS is to maintain constant pr opor- tional flow from an exhaust stream with a highly variable mass flow rate.

In 1990, Caterpillar and Sierra Instruments entered into a technology partnership to develop emissions test equipment. Rob Graze, senior engi- neering specialist at Caterpillar, said that Caterpillar granted Sierra an exclusive worldwide license to its patented microdilution tunnel design incorporated in the Transient Dilution Airflow Control system (TDAC).