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Analysis of Post-oxidation Phenomena in a Turbocharged DISI Engine Focused on Gas and PM Emissions and Gas Temperature

The detailed investigation of post-oxidation phenomena based on gas emissions, PM and gas temperature were performed on a turbocharged DISI engine. In post-oxidation phenomena, rich air-fuel mixture is used inside cylinder and this rich excursion gives rise to the production of CO, THC, PM and H2 emissions. Then the scavenged O2 and emitted H2 play a key role for post-oxidation reaction (exothermic) with CO, THC and PM in exhaust manifold and in-turn can aid in increasing the temperature along with overcoming of decrease in temperature by heat absorption reaction. The actuation time delay of reaction was also investigated with emissions and temperature with fast response devices in transient mode.


Introduction
Recently, the fuel economy and emissions have become the key research point for the automotive industries to fulfill the emissions regulation along with better performance. Considering these challenges, a wider research on these topics have been The scavenging-based post-oxidation process is defined as secondary combustion in exhaust manifold between unburned hydrocarbons and scavenged air. The in-cylinder combustion used rich air-fuel mixture and this rich excursion gives rise to the production of CO, THC, PM and H2 emissions. Then the scavenged O2 and emitted H2 play a key role for post-oxidation reaction with CO, THC and PM in exhaust manifold and in-turn can aid in increasing the temperature along with overcomming of decrease in temperature by heat absorption reaction [1][2][3][4][5]. Apart from this, TWC life time improvement and load reduction on catalyst, reduction in residual gas fraction, and knocking tendency reduction by the in-cylinder temperature decrease etc. can be additionally grabbed from the post-oxidation phenomena. Without post-oxidation, all unburned fuel emitted is treated with a catalyst, which raises the catalyst temperature. On the other hand, when post-oxidation is actuated, the temperature of the exhaust manifold rises because an oxidation reaction occurs in the exhaust manifold, but the temperature drops as the turbocharger works. At the same time, less unburned fuel is brought into the catalyst, which prevents the catalyst from rising in temperature. In past, another methodology for the post-oxidation has been also adopted such as, secondary air injection in exhaust port that can enable secondary reaction (post-oxidation) and could increase enthalpy at turbocharger upstream and hence turbo-lag can be reduced [3][4][5][6][7].
However, the scavenging-based post-oxidation phenomena (adopted in this research) is different from the secondary air injection methodology. Compared to the secondary air injection method, this method does not require pump power and the amount of supplied air can be easily controlled by VVT. In this methodology, the O2 is directly short-circuited from intake to exhaust manifold through in-cylinder by the VVT tuning and it does not participate in in-cylinder combustion compared of air injected in exhaust manifold with the separate pump in the case of secondary air injection. Due to this, the in-cylinder AFR leads toward rich and exhaust port AFR becomes lean automatically.
Then, this scavenged O2 mixed with rich unburned gas species in exhaust manifold and the post-oxidation actuated. In addition, this method can improve the exhaust efficiency of exhaust gas by 1. The engine setup is shown in Fig. 1. The dynamometer controller was used to regulate the engine speed and load. The  In addition, a two-wire thermocouple was installed at the TWC downstream to measure the fast response temperature development in post-oxidation condition. Since, the sensor is very delicate and costly, a safety system was also designed to ensure protection of the sensor by exhaust gas dynamics and vibration, and to obtain precise measurement data with allying the flow direction to the probe perpendicular. The K-type thermocouple was used to observed the mean value of temperature. The measurement positions were selected based on the required objectives as given in Table 2.

Experimental Methodology
In the experiment, the following methodology was adopted to achieve the desired research objectives.
• Firstly, the engine cooling water was warmed up till 85 deg. C temperature. The intake temperature was controlled around 30 deg.C.
• After the engine had reached the desired temperature, postoxidation reaction was actuated by tuning the engine speed, torque, VVT and spark timing.
• After a sustainable post-oxidation reaction was achieved, the emissions, PM and temperature were measured and analyzed. analysis [11] were also collected at the respective positions.
• The two-wire thermocouple was used for the fast response temperature measurement. The voltage signal was measured with the two-wire thermocouple and data was captured. After getting the raw data of voltage, post-processing was performed to convert the voltage into temperature with the calibration data of R-type thermocouple and then further analysis were performed. Then, the gas temperature were estimated with the time constant compensation [10].

Results and Discussion
In the post-oxidation phenomena, the main source of heat generation is CO oxidation. The oxidation process of CO is represented as below [2], while the reaction equations and are less effective due to the low reaction rate constant. It was also noted that hydrogen (H2) concentration plays a key role in the actuation of the post-oxidation reaction as it assists in the formation of OH radicals that is as shown in the chemical reaction below, and the production of H2O is mainly driven by For the oxidation of CO (equation (I)), OH needs to be produced mainly by equation (IV) and (V). The OH production is also influenced by equation (VI).

Steady State Analysis
In the steady state analysis, the gas emissions, PM and gas temperature with and without post-oxidation phenomena were conducted.

Emissions (CO, CO2, THC and PM)
CO, CO2 and THC emissions were analyzed at various positions (refer Table 2) in post-oxidation condition at 1600 rpm, 160 Nm and 90 deg. overlap. The FID response delay 0.9 ms (9 deg. at 1600 rpm) and the NDIR response delay 8 ms (77 deg. at 1600 rpm) were deducted from the measured data to synchronize the actual variation of THC and CO/CO2 with the engine stroke and valve dynamics. In Fig. 2, the comparison of (a) THC (b) CO and (c) CO2 contents at exhaust port of cylinder 1, TC upstream and TWC downstream positions are plotted. Moreover, the simulated gas velocity with the VVT dynamics are also shown in the same graph (2 (d)). It can be observed that the THC and CO level decreased and CO2 level increased as moving from port towards TWC downstream. This is due to the post-oxidation reaction that is taking place and also inhomogeneity exists between exhaust port 1 and TC upstream.
This engine-out THC and CO contents are participating in a chemical reaction with scavenged O2 and OH redicals (eqs . I & II) hence forming CO2 and H2O. From the graph, we can draw two conclusions. First, the THC content is decreasing in each respective stroke at exhaust port. This indicates that postoxidation reaction is taking place. Moreover, it must be kept in mind that mutual cylinder gas dynamic effects are also contributing hence the peaks are observed in every stroke at TC upstream and TWC downstream. Secondly, in the case of THC at exhaust port position, it can also be observed that one peak is rapidly declining around EVO. This is due to when the exhaust valve of cylinder 1 opens, the fresh unburned/burned gases (CO and CO2) from current cycle appeared when effective valve opened. The THC rapidly decline due to higher pressure difference between exhaust valve to port that caused higher velocity hence residing time for the THC in port drastically decreases and FID did not detect enough THC concentration that can be observed in ( Fig. 2 (a)). Further, as intake valve opens and overlap started, fresh air pushed the remaining THC towards exhaust port which was remaining inside the cylinder and near exhaust valve and hence THC increased.
The PM measurement by the EEPS 3090 system is done near atmospheric pressure to avoid volatile and nonvolatile particle's condensation. Also, the ribbon heating system were used in the sample line to avoid the water condensation in the sample line.
Moreover, the catalytic stripper (CTS) device was also used to increase the sample intake temperature to further evaporate SOF, volatile and semi-volatile particles. The gas sample was taken from various positions in the exhaust manifold i.e. exhaust port 1, TC upstream, TC downstream and TWC downstream (shown in Table 2) and passed through the EEPS with the help of in-built pump and dilutor. The gas sample and air ratio was set to 1.5:9.
The  Furthermore, the PN analysis was also performed with the ignition timing (SA) tuning. It was noted that the PN appeared higher in the exhaust manifold when ignition timing was set to 0 deg.
bTDCf (post-oxidation condition) as shown in Fig. 5. This is due to the combustion phase which was shifted to late and hence the combustion timing decreased that lead to higher unburned gases 0.00E+00

Gas Temperature
The temperature was measured at the TWC downstream position with the using of fast response two-wire thermocouple. The same operating conditions were selected to analyze the temperature development trend with and without post-oxidation condition. A two-wire thermocouple with probe thicknesses of 25 and 50 micrometers (μm) were used to observe the fast response characteristic of temperature. The probe of the thermocouple was made with the cromel/alumel material that functions on similar principles to that of R-type thermocouple [10]. Since two-wire thermocouple is considered to be an extremely sensitive and fragile device hence proper measures to ensure the device's safety was also taken into consideration. For this purpose, a protector was designed for the two-wire thermocouple which not only ensured that proper temperature measurement but also sensor's safety. A schematic diagram of the whole protection system is as shown in Fig 7 (a). Furthermore, since the sensor is able to detect precise temperature of gases when the flow is perpendicular to its wires, four holes of diameter 4 mm were drilled into the protector in such a way so that sufficient exhaust gases can enter in a perpendicular direction as shown in Fig 7 (b). The actual setup of the protection system along with the two-wire thermocouple at the test bench TWC downstream position is as shown in Fig 7 (c).
In the measurement, firstly, probe hot junction temperatures T1 and T2 were recorded and then an analysis to detect the actual gas temperatures was carried out. By adopting the use of a mathematical model [10], we were able to calculate and predict the actual gas temperature from probe hot junction temperatures that were detected by the two-wire thermocouple.  Where, Tg is the gas temperature, T is the probe hot junction temperature and τ is the time constant. Based on the above Eq.
(VIII), the gas temperature from both probes can be calculated as in Eqs. (IX) and (X).
Theoretically, the gas temperature Tg1 and Tg2 should be equal. But some difference exists due to the wires of different thermal inertia, diameter and thickness were used hence the use of time constants τ1 and τ2 were introduced in Eqs. (IX) and (X). Hence, the time constant τ1 and τ2 were calculated to overcome this limitation with the use of several measured data using the "least square method" that can minimize the error. The detailed theoretical model and its explanation can be referred in [10]. The purpose of this analysis was to determine and compensate the time constant errors that had encountered during our measurement which will help us to understand better about the actual temperature development and dynamics in the exhaust manifold during post-oxidation condition. As we can see from the experimental results ( Fig. 8 (a)), we were able to detect four peaks of temperature due to the mutual effects of all the cylinders. We can observe from these results as well that when the exhaust valves of the respective cylinders were opening, their influence was observed at the TWC downstream position.
When the exhaust valve of cylinder 1 opened, exhaust gases started to flow out towards the exhaust side due to which the peak rises. This gas was detected from the two-wire thermocouple situated at the TWC downstream position. It was also observed that T1 was showing significant peaks whereas T2's peaks are much smaller. This was due to the thickness of the wire as the 25 μm wire was quick to detect the exhaust gas sample's temperature (due to the time constant (τ) being smaller as shown in Fig. 8 (b)).
Furthermore, the gas temperature Tg1 and Tg2 were also calculated based on the Eqs. (IX) and (X) which is shown in the same Fig. 8 (a). The mathematical model took τ1 and τ2 into consideration and reduced the error and time response limitations that were observed on our actual experimental test data. The trend shows that the gas temperature response is faster than the probes temperature. This  It was also noted that the four peaks appeared in both cases in a cycle due to the four cylinder combustion and gas dynamics effects. Also, the magnitude variation appeared higher in the case of with post-oxidation condition. This had appeared due to the higher in-homogeneity in secondary combustion that leads to unstable post-oxidation in the exhaust manifold. This can be improved if the mixing of unburned gas species with scavenged O2 will be able to enhanced in the exhaust manifold [7]. The influence of the post-oxidation in the temperature development was also observed. This shows that the enthalpy generated by the post-oxidation not only increased the temperature but also the rising rate of temperature. From Figs. 8 and 9, it can be also stated that the response time of the thermocouple is enough to analyze the cylinder-to-cylinder influence on gas temperature. The four peaks shows the mutual cylinder effects on the gas temperature.
The authentication of the temperature measurement by the twowire thermocouple was also done with the comparing of data with K-type thermocouple. In this case, the mean value of the calculated gas temperature from two-wire thermocouple was compared with the K-type measured data. Slight deviation was observed in the lower overlap (w/o post-oxidation condition) and overall results of two-wire thermocouple matches with the K-type data and deviation is below 5%. The deviation might have appeared due to the extrapolation of calibration range and different material properties of two-wire (R-type) and K-type thermocouple.

Transient State Analysis
Fast response devices such as HFR500 Fast FID (for THC), NDIR500 (for CO and CO2) and two-wire thermocouple (for gas temperature) were used for the gas emissions and temperature at

Conclusions
A detailed investigation of scavenging-based post-oxidation phenomena focused on gas and PM emissions and gas temperature were performed. The following conclusions can be drawn form this research, • It was noted that the temperature (two-wire thermocouple) and gas measurement (NDIR500 and HFR500 fast FID) instruments has enough response to investigate the post-oxidation phenomena on the exhaust emissions and temperature as it can reflect the actual gas dynamics in the exhaust manifold that can possibly help in the resolving of the combustion based temperature development complexity in post-oxidation phenomena.
• It was noted that the gas and PN emissions appeared higher at the exhaust port due to the in-cylinder rich combustion in the post-oxidation condition. Then, the gas and PN emissions decreased from TC upstream onwards as effective postoxidation occurred at this stage and as a consequence temperature increased.
• It was noted that the post-oxidation does not appear effectively at the exhaust port and TC upstream as inhomogeneity effects had dominated on it. However, it appeared visibly at TWC downstream.
• When zone shifted from non-post-oxidation to post-oxidation, initially, CO gives sharp increment and then its level gradually lowers as time passes. Similarly, CO2 first decreased sharply and then it gradually increases. This phenomenon appeared due to the time delay in the actuation of the post-oxidation.
• Initially, the sharp increment of CO and decrement of CO2 appeared due to the exit of in-cylinder unburned/burned gas species level as there was no post-oxidation reaction actuated yet.
However, as time passes, CO and THC decreased and CO2 increased due to the favorable environment for post-oxidation.