**3. Results**

The effect of WiGE on the engine performance was investigated through the in-cylinder pressure analysis. As aforementioned and further discussed in the following, with reference to the TIT results, the gasoline case at stoichiometric A/F mixture, and SA = −10 CAD ATDC is missing because it is not feasible with the maximum TIT target. Figure 2 shows a comparison between the gasoline and WIGE in-cylinder pressure traces at the reference engine operating condition (λ = 0.9, SA = −10 CAD ATDC). As aforementioned, this condition is representative of commercial ECU map and the spark timing under rich air/fuel mixture corresponds to the gasoline knock limit. When switching to WiGE, the injection duration and the plenum pressure were adjusted to keep the A/F ratio and IMEP load constant. The first effect of WiGE is the charge cooling, due to the evaporation of water droplets; moreover, the water in the combustion chamber acts as an inert during the combustion process; this causes a slowdown of the rate of energy release with a reduction in the pressure peak proportional to the water content.

Figure 3 shows the relationship between water content and combustion duration and phasing, at different spark timings and relative A/F ratios and WiGEs, considering a representative engine cylinder (Cyl #2). For each spark timing sweep, a clear indication of knock-limit (KL) point is depicted in Figure 3a. This indication of KL points extends to the other figures proposed below. In agreemen<sup>t</sup> with the pressure traces shown in Figure 2, at SA = −10 CAD ATDC and λ = 0.9, the use of WiGE prolongs the combustion duration (Figure 3a) and delays the combustion phasing (Figure 3b). On the other hand, proportionally to the water content, the cooling and dilution effects of WiGE on the incylinder charge mitigate the knock tendency and allow us to advance the spark timing up to −13 and −16 CAD ATDC (for WiGE 10 and WiGE 20, respectively); consequently, also the combustion center is advanced with respect to gasoline reference case. At stoichiometric condition, the knock-limited spark timing can be further advanced with the result of a better combustion phasing (MFB50 = 12.4 CAD ATDC @ SA = −19 for WiGE20).

**Figure 2.** In-cylinder pressure trace and rate of heat release at SA = −10 CAD ATDC, λ = 0.9.

Figure 4 shows the trend of TIT against spark timing and relative air-to-fuel ratio. As shown, at a reference spark timing of −10 CAD ATDC, the turbine inlet temperature is almost the same for gasoline and WiGE in rich mixture condition. Of course, the rich A/F mixture at the reference point was selected in the manufacturer calibration to avoid, with a certain safety margin, the knock onset and the excessive thermal stress to the turbine. On the other hand, in the case of the engine mounted on the test bench, a lower heat transfer can be realized for the turbine compared to the case of an engine on the real vehicle. Based on this consideration, even if the allowable maximum TIT is 950 ◦C, a target of 819 ◦C, which is representative of the gasoline reference condition, was set for WiGE. In light of the above discussion, the criterion followed for the engine tests is the identification of knock-limited operations with WiGE and stoichiometric mixture realizing a TIT level equal or lower to the one achieved at the reference ECU condition and a lower ISFC. By increasing the spark advance, the earlier combustion phasing allowed by WiGE results in a significant reduction in the turbine inlet temperature, because the temperature of the in-cylinder gases is lower at the opening of the exhaust valves if the combustion takes place early in the engine cycle. The use of WiGE10 does not allow us to reach the TIT target at stoichiometric condition (TIT = 827 ◦C at knock limit; SA = −14 CAD ATDC), while WiGE20 resulted in a lower TIT than gasoline reference case at the two most advanced spark timing: 818 ◦C at SA = −16 CAD ATDC, and 804 ◦C at SA = −19 CAD ATDC.

The Indicated Specific Fuel Consumption (ISFC) for all the investigated WiGE fuels, spark timings and relative air-to-fuel ratio is shown in Figure 5. It is worth pointing out that fuel consumption is estimated by considering only the gasoline content of WiGEs, as the water is inert.

As expected, introducing water in combustion chamber prolongs the combustion duration with a worsening in efficiency. Hence, at fixed spark timing ( −10 CAD ATDC) and at the same air/fuel ratio (λ = 0.9), fuel consumption associated to WiGE is higher than the one measured in the reference condition. When leaning WIGE/air mixture the fuel efficiency is improved. On the other hand, this improvement is not large enough to eliminate the gap with gasoline reference case. Therefore, at fixed spark timing ( −10 CAD ATDC), the WiGE fuel consumption at stoichiometric condition is still higher than the one measured for gasoline at rich condition (λ = 0.9).

**Figure 3.** Combustion duration (2xMFB10-50) (**a**) and combustion center (MFB50) (**b**) against spark timing and relative air-to-fuel ratio for WiGE 10 and WiGE 20.

**Figure 4.** Turbine inlet temperature against spark timing and relative air-to-fuel ratio.

Thus, to achieve the same IMEP as the reference case, it is necessary to inject a higher amount of gasoline. Higher consumption is measured for WiGE20 compared to WiGE10. This behavior is probably ascribed to the water-induced lengthening of combustion process.

A decreasing trend of ISFC with advancing the spark timing is observed for both air-to-fuel ratios and both emulsions. At λ = 0.9 and reference spark timing (SA = −10 CAD ATDC), WiGE employment induces a worsening in the ISFC values compared to gasoline reference condition, due to combustion duration extension induced by water. On the other hand, the improved knock resistance related to water presence allows us to optimize the combustion phasing. The discussed water effects are mutually conflicting for the engine efficiency, and ultimately the ISFC level of KL points (for WiGE 10 and 20) under the rich A/F mixture never reaches the one attained by the ECU-reference rich gasoline case. At engine operation under stoichiometric A/F mixture, the combined effect of WiGE knock mitigation and the leaner mixture allows for an ISFC reduction of about the 3.7% and 7.1% compared to ECU-reference gasoline point at rich mixture condition, for WiGE 10 and WiGE 20, respectively.

**Figure 5.** Indicated Specific Fuel Consumption against spark timing and relative air-to-fuel ratio.

Figure 6 shows the correlation between the CO and CO2 exhaust emissions and emulsion fueling. For the rich A/F mixture condition, coherently with fuel consumption results, an increase in CO2 is measured for emulsions (both with WiGE 10 and 20) if compared with the ECU reference gasoline case, while the CO is almost similar. In stoichiometric A/F mixture condition, CO2 emissions for emulsions are higher than the ones measured under rich condition, due to a more complete combustion process.

Figure 6a also shows that the main CO variations have to be attributed to the modification of the A/F mixture quality. As expected, a strong reduction in CO of one order of magnitude is measured when switching from the rich to stoichiometric A/F mixture.

Figure 7 shows the variation of HC and NO emissions against the spark advance at the selected relative air-to-fuel ratios and for all the investigated fuels. At the rich mixture condition, the WiGE combustion produces larger HC, proportional to the water content, almost independent of the spark timing. Similarly, at stoichiometric condition, the increase in water content in the emulsion induces slightly higher unburned HC emissions. On the other hand, for a fixed water-in-gasoline emulsion, a reduction in the unburned HC is observed passing from the rich to stoichiometric A/F mixture.

Referring to NO emissions, at rich condition, the switch from full gasoline to WiGE involves a reduction in NO. This reduction is more marked for WiGE 20 than WiGE 10, and it is due to the combined effect of charge cooling and thermal dilution.

It is the case to highlight that, even with WiGE 20, the water quantity contained in the in-cylinder charge is low and the corresponding vapour conversion (evaporation process) is constrained by thermodynamic conditions, available time and full humidity conditions. However, the high latent heat of vaporization for water has a relevant role on the reduction of in-cylinder mixture temperature. As reported in Reference [19], at the spark timing, a reduction in charge temperature of 13 ◦C can be achieved at 3500 rpm and 17 bar BMEP when injecting 17%w of water in the intake manifold.

In stoichiometric conditions, the increase in oxygen content and the higher combustion temperature due to the lack in any diluent (excess fuel or water) promote NO formation, with a maximum NO level at KLSA for both WiGE 10 and WiGE 20, due to the increase of in-cylinder peak temperature by advancing the spark timing.

**Figure 6.** CO (**a**) and CO2 (**b**) emissions against spark timing and relative air-to-fuel ratio.

**Figure 7.** HC (**a**) and NO (**b**) emissions against spark timing and relative air-to-fuel ratio.
