Fig. 4(a) and (b) shows variations of CO emissions depending on the speed of the engine. CO emission is high at low speed in both the UE (a) and the CE (b) for all test fuels; it decreases at medium speed and starts to increase above medium speed. For both engines,the lowest CO emission occurred in CME-35, compared with that of the 2D reference fuel. Decreases in CO emission in the CE (b),compared with those of the UE (a), were 25% for 2D, 22% for CME-100, 23% for CME-20, and 24% for CME-35.