It should be noted that the transient FTP certification
considers only the cycle average, but it does not put a limit on the peak emissions in the cycle. In other words, at certain times during the cycle, the NOx value is allowed to exceed 2.4 g/bhp-hr. This is unlike NTE certification
which puts a cap on the maximum power specific emissions. Hence, based on these standards, the limiting ratio
should be 2.4 g of NOx per 525 g (approximately) of CO2; hence the ratio for certification should be 0.0046.
This was comparable to the 2003-2005 MY truck average of 0.0051. From the earlier study by Ramamurthy and
Clark [7], the average NOx/CO2 for the older MY trucks was 0.01. In another similar study conducted by Khan
and Clark [36], the average NOx/CO2 ratio for the E-55/59 trucks from 1991-2004 MY tested on the transient mode was found to be 0.0089.