## 2 Experiment Description
### Detailed Analysis of the US06 Cycle  
Since both city and highway mode characteristics and the increase in CO\({}_{2}\) concentration levels as the vehicle's cumulative mileage increases were clearly demonstrated in the US06 driving cycle, CO\({}_{2}\) and CO productions discharged from the driving cycle are closely investigated by separating city and highway modes of US06 cycles into several regions, wherein the vehicle is operated with acceleration and deceleration in the city mode, and only with acceleration in the highway mode. Figure 9 shows the velocity profile of the city mode in the US06 driving cycle. The speed profile is largely divided into three regions in terms of the relative magnitude of load applied to the engine. The regions of A, B, and C correspond to the areas wherein middle, high, and low load are applied to the engine, respectively. As mentioned previously, since the measurement of CO\({}_{2}\) concentration levels made during one driving cycle is largely affected by driving behavior, specifically through instantaneous speed, all comparisons of changes in CO\({}_{2}\) and CO due to aging of the engine were made from test data that revealed the most similar velocity profile in the time domain corresponding to rapid acceleration and deceleration regions among test data tested according to cumulative mileage of each vehicle. Figure 10 represents the concentration of CO\({}_{2}\) and CO measured during the A, B and C regions in the city mode of the US06 driving cycle. In the (a) part of the A region corresponding to the beginning of the cycle, the tendency of increase in CO\({}_{2}\) and CO productions was confirmed as the cumulative mileage of the vehicle increases. In contrast, in the final part of the US06 cycle corresponding to the (b) part of the A region wherein comparable load is required as in the (a) part of A region, there is nearly  
Figure 9: Velocity profile for city mode corresponding to the US06 driving cycles. (**a**) trace 0–130 s (**b**) trace 496–596 sno variation in CO\({}_{2}\) production with each mileage. Since exhaust gas measurement starts in the US06 driving cycle after the engine has stopped for 70 s at the end of the preconditioned process, the (a) region is thought to be colder than the (b) region. Also, it is assumed that the cylinder wall is worn out to some degree due to aging of the engine, so that the lower the ambient temperature inside the cylinder of the engine, the greater the heat transfer loss in the aging engine. In the B region, the CO\({}_{2}\) and CO emissions are analyzed by dividing the B region into two sections according to whether the load of the engine is taken at a stop, or at high rpm. In the B region, it can be observed that, as the cumulative mileage of the vehicle increases, the amount of CO\({}_{2}\) emissions also increases. In addition, when the overall growth rate of CO\({}_{2}\) according to the increase of the cumulative running distance of the vehicle is examined, it can be observed that as cumulative mileage of the vehicle increases, rate of increase of CO\({}_{2}\) emission becomes larger as the load of the engine gets heavier. Because the larger the load, the more heat energy is generated, it is estimated that heat loss is proportional to amount of heat energy generated in the worn cylinder wall. Interestingly, when the load is applied to the engine at high rpm in the (b) part of the B region, growth rates of CO\({}_{2}\) and CO emissions were higher, in comparison to when applied to an engine with stationary state in the (a) part of the B region. It is assumed that the rapidly incoming fresh charge may play a role in increasing the heat transfer rate on the worn cylinder wall, which makes it difficult for the wall to be maintained in steady state at a high temperature. So, the aging engine at high rpm is faced with more difficulty in achieving complete combustion as the load gets higher to accelerate, which leads to the production of more CO emissions, an indicator of incomplete combustion.