NOx/CO2 ratio can be useful in representing the emissions as a mass fraction of the burnt fuel. It differs from
brake-specific NOx because it takes into account the reduced engine efficiency during low power operation.
Figure 7 presents the data from CRC-34 loaded at 56,000 lb and tested on three different cycles. Figure 8
represents the data from the five trucks (described in Table 1 and from reference [8]) loaded at 56,000 lb and
tested on THE HHDDT_S cycle. In both of these plots, the data did not follow a trend. This suggests that the ratio is independent of the test cycle and the vehicle. The ratio was found to be higher at lower axle power because
of the advanced injection timing at idle and lighter loads. The average value of the ratio for the 2003-2005 MY
trucks was 0.0051. The corresponding ratio for all of the vehicles considered in the earlier study [7] by Ramamurthy and Clark was 0.0141. This decrease of the ratio could be attributed to the more stringent standards and
NOx reduction technologies after October 2002 (The 2004 standards were brought forward to October 2002 under the Consent Decree). For 2004 and later MY engines, the limiting average value of NOx (including non methane hydrocarbons) for transient FTP is 2.4 g/bhp-hr.