NTRD-02 Final Project Report The Effects of Low Rolling Resistance ...

micro-trip was defined as a contiguous speed trace of vehicle driving and is made up of an idle
followed by all non-idle driving until the next idle begins. A single vehicle trip, defined as the
period from engine-on to engine-off, typically comprised numerous micro-trips. Therefore, the
driving was divided into trips and sub-divided into micro-trips before the cycle was built.
Each trip was assigned a unique trip number, and each micro-trip was assigned a unique microtrip
number. The smoothing of the raw data using a five second average, as described above,
eliminated any idle periods that were less than five seconds long, since the zero speeds just
before and just after non-zero speeds were averaged to became non-zero speeds during
smoothing. As a result, some micro-trips based on the smoothed data are longer than they would
have been if they had been based on the raw data.
The second-by-second observations for all of the micro-trips in the edited dataset were placed
into bins according to speed and acceleration. Only those longer than 20 seconds were used to
eliminate repetitive non-representative operation of the vehicle.
4.2 Cycle Development
To develop the drayage cycle we used pieces of real driving, the micro-trips from the drayage
truck activity database, which when connected together would exhibit emissions behavior similar
to a drayage truck driving on the road. The cycle was built around parameters of vehicle
operation and usage that were known to be important to exhaust emissions. By using the
approach of matching vehicle operation between measured driving behavior and the candidate
cycle, the emissions behavior of vehicles over the cycle would be similar to the emissions
behavior of drayage trucks on the road. Vehicle speed and acceleration were chosen as the
variables most important to exhaust emissions. These variables together provided a measure of
the load on the engine, which is an important variable associated with exhaust emissions. The
cycle was developed only for warmed up operation of drayage trucks. That is, we assumed that
all data in the datasets represent warmed-up driving and did not build special cycles for cold
starts and warm starts.
4.2.A General Methodology
The cycle was created by combining micro-trips taken from the recorded operation of Trucks A-
C. The two critical variables (speed and acceleration) were used for selection of micro-trips for
the cycle. To identify specific segments of vehicle driving for inclusion in the cycle, the entire
activity dataset was converted to a set of micro-trips as described in the previous section. A
strategy based on a minimizing the difference between a cycle vector C representing the driving
in the candidate cycle and a target vector T representing the driving in the activity database was
used to select micro-trips from the database for inclusion in the cycle. Enough micro-trips were
used to build-up a candidate cycle so that the difference between the two vectors was minimized
to an acceptable level; the two vectors were substantially the same, and the duration of the cycle
met the length criterion for SAE J1321 testing. For full details on the building process and the
multi-dimensional vectors space used to build the cycle, see Palacios and DeFries (2007).
4.2.B Specific Details of Non-Idle Cycle Generation
After the dataset containing all of the second-by-second driving activity for the three drayage
trucks was edited and prepared, the micro-trip information was used to find those micro-trips
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