<div class="section abstract"><div class="htmlview paragraph">Most of the recent clean diesel engines are equipped with an exhaust gas recirculation (EGR) technology in order to meet the strict criteria of NOx and particulate matter (PM) as required in the current emission regulations. More attention to strict EGR control is becoming required. Accurate and fast transient EGR ratio operation is becoming very critical in the field of the emission control.</div><div class="htmlview paragraph">The EGR ratio is typically monitored by CO₂ trace method, in which CO₂ emitted from engine, is utilized as a tracer gas. The EGR ratio can be obtained from CO₂ concentration measured at engine intake and engine out at the same time. In this study, authors have developed a new EGR analyzer consisting of two CO₂ detectors, to achieve required performance for transient measurement, i.e., short delay time and quick response, negligible difference between two CO₂ detectors, and capability of wet measurement. For this new analyzer, heated NDIR detectors which can measure CO₂ in engine emission under wet condition were adopted. The sample lines for each CO₂ detector were configured to have small volume and also almost same design as each other, to avoid transient errors caused by the difference in response time.</div><div class="htmlview paragraph">We have evaluated the performance of the transient EGR analyzer through some comparison tests with a conventional exhaust gas analyzers system. In consequence, the faster response has been observed from the transient EGR analyzer without significant transient error under several test cycles. Furthermore it has proved that effects on response time from the sample inlet pressure change are significantly small. Finally the EGR ratio measured by the transient EGR analyzer followed up the EGR valve operation well. These results show the advantages of the transient EGR analyzer for measuring EGR ratio under transient state.</div></div>