ETC test and 200 s idle test were conducted on a China Ⅴ diesel engine. According to the ETC test results, the relationship between NOx volume fraction and excess air ratio at idle and nonidle conditions could be expressed as power formula. For the 200 s idle test, the relative error between estimated values and test values of NOx volume fraction gradually decreased within 0—30 s, increased within 30—170 s and remains nearly unchanged within 170—200 s. With the transient NOx estimation model built by the fitted empirical formulas, the NOx emissions of ETC cycle were verified. The results show that estimated values of NOx volume fraction and mass flow agree well with test values, but the difference at the peak is obvious. In addition, estimated value of NOx cycle cumulative mass is 4.6% higher than test value.
