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Smoke is the most obvious part of the exhaust emitted from Diesel engines. The methods for smoke testing currently used in Taiwan are the rapid acceleration test under no load and the constant speed test under full load. The smoke concentration is measured with the light reflection method. According to the test procedure, the smoke meter sampling time in the rapid acceleration test is 1∼2 seconds. The current range in sampling time may cause discrepancies in test results. In these tests, free acceleration of engine is achieved by actuating the pedal rapidly. However, the speed of engine acceleration has not been defined clearly in the test procedure. The objective of this research is to study the effect of the operating conditions upon the results of smoke tests. Experiments were carried out in this study. The results of measurements showed that the opacity of the engine exhaust increased rapidly after a delay of about 0.3 seconds and then fell.