**4. Influence of Idling Time between Two Accelerations on Particulate Emissions** *4.1. Setup*

The tests were carried out on the engine mounted on a Horiba bench described in Section 2.1. With this bench, it was possible to create some transients. In order to be close to the real conditions of the "stop and start" system, an acceleration from idle to 2000 rpm and 60 Nm in 5 s was selected. To reach this point, it was necessary to keep the accelerator pedal pressed at 100% for about 2 s (part a, on the lower graph in Figure 11). Then, the engine stayed at 2000 rpm and 60 Nm during 15 s (part b) before slowing down (part c) and coming back in idle position (part d).

Three configurations were tested. For case 1, there was no idling time between two accelerations. The engine slowed down to 750 rpm before accelerating for a new transient. Case 2 consisted of an idling time of 7 s between two accelerations, while case 3 consisted of an idling time of 22 s. The test conditions are presented in the Table 3 below.

**Figure 11.** Cases description.



Figure 12 shows the evolution of engine speed, effective dynamic torque, in-cylinder pressure, and intake manifold pressure, depending on the position of the accelerator pedal during this transient.

**Figure 12.** Evolution of engine parameters during a transient.

The signal "accelerator position" represents practically the position of the engine throttle, which means that it is in the fully open position for about 2 s before reaching the requested point. In Figure 12 bottom right, the dynamic effective torque during this transient is presented. The significant variations at the start are consequences of the important variations of the in-cylinder pressures between the cylinders. For example, for one cylinder, the in-cylinder pressure may be 25 bars while it may still be 6 bars for another cylinder (which is the idle in-cylinder pressure). This difference will generate a significant variation of the engine torque.

Berthome, et al., [13] exposed that particulate emissions vary greatly from test to test despite mastering both engine and bench parameters. It is, therefore, necessary to accumulate data and to proceed with a statistician approach to get a trend out, and thus evaluate the impact of the idling time between two transients. To obtain as much data as possible, 100 accelerations (strictly identical from the point of view of the control and test conditions) are carried out successively after reaching thermal stability and respecting a time of idling between each test for each configuration. The demonstration of the similarity of these transients is not the subject of this article and has already been validated in a previous article by Berthome, et al., [13,20].
