*4.3. Test Results and Analysis*

Figure 13 shows the evolution of the average current carried by the particles of 100 strictly identical transients and for each case. The blue dash dotted-line curve represents the position of the accelerator pedal. At idle, the level of particulate emissions generated by the engine is very low and amounts to about 1 pA. Then, during acceleration, the current level rises sharply. In fact, to obtain rapid acceleration, it is necessary to increase the equivalence ratio to counteract friction and especially inertial effects. These rapid variations of equivalence ratio generate many particles reaching a maximum around 2.2 s. Then, from 2.5 s, the acceleration slows down, which favours stoichiometric conditions and therefore generates fewer particles, and the current measured by the PPS decreases. Cases one and

two have almost the same trends. The current peak reached 112 pA for case 1 and 149 pA for case 2. However, case 3 is more important, and the current peak reached around 415 pA.

**Figure 13.** Evolution of particle emissions during the transient for each case.

The sum of current averaged for 100 transients (Integ**I**) for the three cases are shown below, in Table 4. The percentage standard deviation (σ) is calculated from the standard deviation of Integ**I** of 100 transients divided by the mean. For case 1, i.e., without an idling phase, but only a deceleration phase between two transients (see part c Figure 11), the sum of the current averaged per 100 transients is 105 pA. For case 2, with an idling phase of 7 s between two transients, the sum of the current averaged per 100 transients is 133 pA, an increase of about 126% compared to case 1. For case 3, with an idling phase of 22 s between two transients, the sum of the current averaged per 100 transients is 329 pA, an increase of about 313% compared to case 1. The deviations between the three cases are very large even though the transients are strictly identical (cf part 4-2). Since it is always the same transients, the deviations in richness between each test remain very small and are not the cause of these differences. The only fluctuating phenomenon is the idling time between each acceleration. Consequently, it is the absence of oil sweeping by the blowby gases and its duration that leads to an accumulation of oil towards the crown of the piston. This oil is then projected towards the cylinder and is oxidised during strong acceleration, causing a peak in particle emissions. Therefore, the oil accumulation on the piston crown is proportional to the idling time between two accelerations. These experimental tests confirm the results put forward by the blowby simulation model.


**Table 4.** Results.
