Hipótesis Especificas

The outcomes of the SI engine technology comparison have highlighted the high potentiality of the WI technology, and a deep investigation has been carried out in the research activity by means of :

- experimental tests on engine dyno with a 4 cylinder Turbocharged GDI engine, equipped with a PWI system [217];

- CFD simulations, to support the experimental activities and to enlarge the understanding of phenomena in the intake manifold and in the combustion chamber [218,225];

- system analysis, to evaluate the achievements in terms of fuel economy and CO2 reduction [216].

An overview of the main results obtained by means of the first two activities is presented in this section, whereas the system analysis will be presented in Chapter #12.

Experimental Tests

Experimental tests have been conducted on a prototype Gasoline Direct Injection (GDI) turbocharged engine (Table 6-7), whose intake system has been modified in order to install port water injectors and rail.

117 Table 6-7 Specification of engine tested with PWI system

Displaced Volume 1390 cm³

Bore 76.5 mm

Stroke 75.6 mm

Compression Ratio 10.1

Architecture L4, firing order 1-3-4-2 Number of valves 4 per cylinder Exhaust Valve Open 580° BTDC @ 0.1 mm lift Exhaust Valve Close 356° BTDC @ 0.1 mm lift Intake Valve Open 358° BTDC @ 0.1 mm lift Intake Valve Close 132° BTDC @ 0.1 mm lift Maximum Torque 220 Nm @ 1500 ÷4000 rpm

Maximum Power 103 kW @ 6000 rpm

In Figure 6-34 and Figure 6-35 the modified intake manifold water rail and injectors position are visible.

Water injectors location is as close as possible to the intake valves, according to the CFD indication about water evaporation dynamics [218]. Prototype water injectors and pump were developed by Magneti Marelli and are controlled by a RCP (Rapid Control Prototyping) system developed by the research group in collaboration with Alma Automotive, controlling injection timing and rail pressure, with great flexibility.

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Figure 6-34 Detail of the experimental setup: modified intake manifold (red arrow) and water rail (green arrow) are visible. Water injectors are highlighted with blue circles.

Figure 6-35 Images of water injector position in intake manifold and spry targeting

Experimental data here presented have been obtained with constant injection pressure and timing, 10 bar and 360°CA ATDC (beginning of the intake stroke), respectively. Such values have the results of

Injecto rposition

118 preliminary sensitivity analysis. Engine operating point as an example is 3000 rpm, 1.5 bar of intake manifold pressure, and stoichiometric mixture. Additional information about operating conditions are collected in Table 6-8.

Table 6-8 Operating conditions for tests r In Cylinder Air

[mg/cycle/cylinder]

Fuel Mas [mg/cycle/cylinder]

Water Mass [mg/cycle/injector]

0 519.17 35.56 0

0.2 495.38 33.93 6.78

0.4 504.86 34.58 13.83

0.6 500.63 34.29 20.57

where r is defined as

𝑟 =𝑊𝑎𝑡𝑒𝑟 𝑀𝑎𝑠𝑠 𝐹𝑢𝑒𝑙 𝑀𝑎𝑠𝑠

(6.1)

Combustion phasing

Figure 6-36 confirms the significant water injection effects on combustion timing and larger markers identify maximum efficiency points. Another effect of the water injection is the shift towards lower values of CA50%MFB corresponding to maximum efficiency, as water quantity increases. In Figure 6-36 such value goes from about 11°CA ATDC for 𝑟 = 0 to about 6°CA ATDC for 𝑟 = 0.6.

Figure 6-36 Effect of water injection on combustion phasing

Indicated Mean Effective Pressure

Figure 6-37 shows IMEP (Indicated Mean Effective Pressure) values. It results a slight reduction in the maximum IMEP achievable as water is injected. Larger markers identify maximum IMEP (and efficiency) points.

119 Figure 6-37 Measured IMEP as a function of CA50%MFB, for different water quantities; larger markers identify maximum IMEP (and efficiency) points

Exhaust temperature

Exhaust temperature reduction, should be evaluated (at least in a first analysis) at maximum efficiency points (highlighted in Figure 6-38 by larger markers). In fact, because of water injection effect on combustion duration, exhaust temperature slightly increases as water mass increases, for the same spark advance angle.

Figure 6-38 Measured exhaust temperature reduction, depending on water ratio; bigger markers correspond to MBT.

A reduction of about 50°C is achieved with 60% of water ratio

Supposing the highlighted point for 𝑟 = 0 as knock limited (i.e., supposing knock intensity to be above the admissible threshold), the achievable gain in terms of exhaust temperature reduction is even greater than 50 °C.

Knock

Experimental knock intensities is shown in Figure 6-39. The knock intensity is evaluated by means of 98th MAPO percentile. The Knock Limit (KL) indicated in figure is based on the empirical relationship: KL = Engine Speed/2000. What is particularly interesting is the knock intensity reduction for the maximum efficiency points (larger markers) as water mass increases. Inconsistently, for 𝑟 = 0.2, knock intensity at maximum efficiency is greater than for 𝑟 = 0. This is probably due to an ambiguous identification of the

120 maximum efficiency spark advance value. The selected operating point (1500 rpm, 1.5 bar) is not considered to be knock limited, and further investigation on higher load operation is required. Anyway, a slight reduction on knock intensity as injected water mass increases is verified.

Figure 6-39 Comparison of knock intensity at different values of water/fuel mass; bigger markers correspond to MBT

Last analysis is on Brake Specific Fuel Consumption (Figure 6-40). Basically, investigated water quantities do not affect engine efficiency, confirming that effectively water enables an extension of the engine operation, with no compromises. In particular with r=0.4 and an higher SA then baseline (r=0), a quite BSFC reduction has been observed (2%), with lower sensitivity MAPO/SA .

Figure 6-40 Measured Brake Specific Fuel Consumption trends for different water ratio values

CFD Simulations

Numerical simulations can assist in designing water injection systems and in understanding the intertwined phenomena occurring from the water spray to the combustion. Some examples of CFD

stud-KL

121 ies have been presented recently in the literature [218, 269]. Two aspects combine: the realistic modeling of the water injection process and the robust knock risk prediction. Spray model validations are mandatory, for a proper description of the water and fuel distribution in the ports and in the cylinders.

Concerning SI combustion modeling, it has been clearly shown that large eddy simulation (LES) is the proper method to account for all statistical phenomena from flame propagation to autoignition [270], at the expense of costly multi-cycle simulations. Using tabulated kinetics for ignition (TKI) combined with the extended coherent flamelet (ECFM) combustion model, the authors showed high accuracy in capturing knock frequency of occurrence and intensity, over a range of conditions.

However, RANS is still the method of choice for large number of design space explorations [218, 269].

Some authors are proposing new advanced models with presumed probability distribution functions to include knock statistics upfront in RANS [271,272].

In this section we first show validations of an overall engine model, then we investigate how port water injection affects combustion, and lastly, we quantify the IMEP gain that can be achieved adding water and re-optimizing the spark timing. The main purpose is a fundamental study on water injection behavior using CFD. The water wall film dynamics in the intake ports is of the utmost importance in this problem, and several engine cycles are needed to reach a converged behavior. Due to the costly method requiring multi-cycle simulations because of the port injection, we limited our range of investigation to a single engine baseline point, under a KLSA operating condition. We explored different water injection timings and pressures to assess the effectiveness of a fixed amount of water. A well validated G-equation combustion model was used to simulate multiple-cycles efficiently. Spark sweeps were then carried out by simply restarting the simulations on the last cycle slightly before the ignition, and adopting the ECFM model for the combustion coupled with TKI to add autoignition prediction capability. The CFD simulations showed to be very useful for estimating the charge cooling effect, the reduction of the burn-ing rate and ultimately the power increase under the same KLSA criterion.

Computational setup

The engine used for the investigation is the same of the experimental activities, see Table 6-7, as well as the engine operating point, 1.5 bar of intake pressure and 2500 rpm of engine speed.

The CFD model is built for a single-cylinder configuration, cutting out just one cylinder from the original 4-cylinder geometry, as already presented in [218]. Figure 6-41 shows the geometry and how the CFD analyses were setup. Water injector and gasoline injector locations are highlighted with white cones in Figure 6-41.a and an example of spray patterns is shown with blue and red parcels, respectively, in Figure 6-41.b. The airbox has been replaced by the hemispherical region positioned at the inlet of the single runner configuration. Being aware that during the closed valve period some water droplets could travel backwards toward the inlet boundary, because of convection or pressure waves, we checked simulation results and verified that no water mass was lost in any analyzed case. Therefore, the adopted inlet region (grey in Figure 6-41.a) is considered adequate for the scope of this work. The CONVERGE CFD solver has been used in this study, which uses a modified cut-cell Cartesian gridding method to automate the creation of the mesh at runtime (Figure 6-41.c). Various fixed and adaptive meshing refinement strategies were used, ad-hoc temporally and spatially activated, to achieve 0.75 mm cell size in sprays and combustion areas. Around spark a resolution of 0.1875 mm was used. More details about computational set-up are provided in the published work [225].

122 Figure 6-41 CFD engine model: geometry with injector locations (a); water and gasoline spray visualization (b); mesh (c)

Spray Validation

In order to build a reliable engine model, the preliminary focus was on the accurate setup and validation of the spray sub-models. Magneti Marelli is the manufacturer of both water and gasoline injectors and provided experimental data collected in a constant volume vessel.

The water injector features a three-hole nozzle, with almost parallel and interacting jets lying on the same plane. All the three holes have been modeled so that the global asymmetric spray morphology is naturally reproduced. Injection rates and a population of atomized droplets based on RR distributions are prescribed as boundary conditions. Three injection pressures have been explored, namely 3, 6, and 9 bar, with an initial Sauter mean droplet diameter (SMD) of 74, 65, 60 μm, respectively. These values have been selected after a preliminary calibration based on the comparison of the simulated spray droplet sizes against measurements.

As an example, the validation results for the 6 bar case are following illustrated. The water injected is 8.39 mg at fluid temperature of 296 K in a chamber at 1bar and 373 K. Results are shown in Figure 6-42.

Figure 6-43 shows a comparison of the measured and predicted SMD (=D32) and aver-age diameter (=D10), at 15 mm from the injector tip (z direction). Simulations are able to reproduce experimental trends and values in a satisfactory manner. Only minor differences can be observed, as the experimental curves show a mild peak in the center that is less evident in the simulations.

123 Figure 6-42 (a) Spray angles definition; (b) Grid size effects on spray patterns and on tip penetration

Figure 6-43 Comparison of simulated and measured droplet diameters at various transverse locations at 15 mm from the tip

Similarly, the model of 5 holes GDI injector has been validated considering as test liquid Exxsol-D40.

More details are provided in the published work [225].

PWI Multi-Cycle Simulations

The PWI injection generally causes impingement on the port walls or on the valves, and complex film dynamics is generated. Multiple cycles are required to model these phenomena. In the current im-plementation of the ECMF combustion model it is not possible to handle more than one liquid species, therefore multicycle analyses have been conducted using the G-equation combustion model which has no restrictions in this regard and allows to have two different injectors with different liquids. The baseline SA of 14 CAD before top dead center firing (bTDCf) is knock-free, hence calculations with G-equation are correct. Within this framework, the focus of the first part of the work is on the amount of in-cylinder trapped water, on the amount of charge cooling due to evaporation, and on the peak pressure reduction with fixed SA. Later, in the second part of the study, we will switch to the ECFM+TKI combustion model to investigate knock occurrence by varying spark advance.

Water injection cases that have been analyzed are specified in Figure 6-44.a. Five water SOI timings are considered at 3 bar injection pressure, ranging from 0 CAD to 540 CAD after top dead center firing (aTDCf). In addition, two pressures with equal SOI of 300 CAD aTDCf are investigated, namely 3 vs. 9 bar cases, to assess the effect of injection pressure. Water mass is fixed at a 0.3 water-to-fuel ratio, and injection temperature is set to 308 K. Special care was devoted to prescribing wall tempera-tures for the

124 critical part of the intake port and runner surfaces. After a preliminary exploration of conditions assessed against available measurements, we set a spatially varying condition of the wall temperatures, with a linear variation from 330 K in the upstream part of the duct up to 420 K just before the intake valve.

Figure 6-44 Timing and duration of each water injection case (a), and comparison of measured pressure vs. sim-ulated in-cylinder pressure (b)

Figure 6-44.b shows the comparison between the computed in-cylinder pressure and the experimental trace for the baseline case and for a case with water injected at 3 bar and SOI = 0 CAD aTDCf. It can be observed that the reduction of peak pressure due to water injection is to some extent underestimated. The main reason is attributed to the specific dynamics of the wall film accumulation and will be discussed further in next section.

Water spray dynamics in the intake runner and ports

Focusing on the intake runner and ports, results of the liquid water dynamics for each simulation are presented in Figure 6-45. In each chart three curves are shown: the cumulated injected water mass, in red;

the cumulated mass of the water that reaches the cylinder in liquid form, in light blue; and the accumulated wall film mass, in green. Vapor phase is not shown for clarity, as it can be easily inferred from the information on the liquid phase. First, we notice that none of the cases under investigation leads to null wall film mass, meaning that the conditions are not sufficient for vaporizing or stripping off the total amount of water impinging on the walls. Also, none of the cases after ten cycles reached a steady state amount of accumulated wall film mass. Therefore, part of the water mass sticks to the walls and will not contribute to the anticipated scopes on the combustion mitigation. It is likely that in the engine point under consideration, longer times are needed to reach fully developed wall films and quasi-steady engine behaviors, which unfortunately means long and costly runs. In all the simulated cases, the water mass entering the cylinder is noticeably lower than the injected mass, decreasing the overall effectiveness and leading to an underestimation of the peak pressure reduction (cf. Figure 6-44.b). This effect can be further investigated in the future, also considering higher load points with higher operating temperatures to reduce the time scale of the phenomena. The results of all simulated conditions are provided in Figure 6-46, which shows the percentages, with respect to the injected mass, of the liquid entering the cylinder and of the liquid remaining on the wall.

In conclusion, we observe that, from the point of view of spray dynamics, the most effective timing at 3 bar injection pressure is the SOI = 180 CAD aTDCf, which produces the lower amount of wall film and the largest amount of liquid water reaching the cylinder.

125 Figure 6-45 Dynamics of liquid water in the ports and runner, focus on case (c) and (f), see the following Figure 6-46

Figure 6-46 Liquid water balance in the ports and runner; percentages refer to the injected water mass

When comparing the 9 bar injection pressure case against the low pressure case, at 300 CAD aTDCf SOI, we observe a rather effective reduction of the wall film (about 0.4x) and an increase of the in-cylinder trapped liquid water (about 2x). This is in agreement with what has been already observed in the previous published work [218] spanning various injection timings, suggesting that better atomization and shorter timings are potentially beneficial. Using higher injection pressure, the mass of liquid water entering the cylinder and evaporating with less wall contact increases substantially, therefore a larger impact is expected on the combustion.

Impact on combustion

Ultimately, the goal of water injection is to affect combustion. A comprehensive view of the water impact for the various cases is presented in Figure 6-47. The top left chart shows the reduction of the peak pressure caused by the water, normalized with respect to the baseline case without water, far all cases compared to measurements. The top right chart quantifies how much the combustion is slowed down in terms of MFB50 (crank angle of 50% mass fraction burned). The bottom chart reports the charge cooling effect, before spark, as the decrease of in-cylinder temperature w.r.t. the no-water case.

The reduction of peak pressure which is achieved is in the order of 4÷6% according to the simulations, while measurements suggest a reduction of about 7÷8%, depending on the pressure. Experimental data were acquired for several injection timings, but a clear trend was not observed, and an overall data scatter and uncertainty of about ± 2% was observed. We previously noted that the accumulated water wall film mass in the simulation is not quasi-steady after 10 cycles, therefore this can explain the reduced

(c) (f)

126 effectiveness provided by the model. Focusing on the effect of the injection pressure, we observe that due to improved atomization level, at constant SOI timing, the charge cooling effect increases, the peak pressure reduction is more significant, and the combustion slows down more, consistently. A reduction of about 2% in the peak pressure is observed in the simulations switching from 3 bar to 9 bar injection pressure, which correlates with measurements.

Concerning the effect of the injection timing, at 3 bar, we observe that the ~300 CAD aTDCf cases behave as the worst, as the peak pressure is less reduced and the MFB50 is less retarded, in agreement with the previous conclusions on the spray dynamics (see results in Figure 6-46). The case at SOI = 0 CAD aTDCf does not strictly follow this criterion though, and despite the trapped water was not the highest, it produces the strongest impact on the combustion. This can be explained including other combined side effects. In particular, the amount of vapor being formed in the ports decreases the volu-metric efficiency and alters the average equivalence ratio. In this case, being ϕ = 1 in the baseline case, in the water cases the mixture tends to be slightly rich (injected fuel is kept constant) because of lack of oxygen replaced by vaporized water. This is more pronounced at 0 CAD aTDCf SOI and it explains the reduction of the combustion rate.

In addition, as visible in Figure Figure 6-47.c, the effect of charge cooling for this case is not the largest at 3 bar, so the peak reduction is a result of a slight decrease of combustion efficiency, as the air-fuel ratio variations are not compensated in the models.

Results under similar KLSA

Because water slows down the burn rate, the comparison in terms of performance must be made after the spark timing is re-adjusted to the same knock risk margin. As mentioned in the introduction, to do this we used the ECFM combustion model with tabulated autoignition data. We run several spark sweeps mapping the flow variables from the last available cycle and restart-ing the calculations with different SA timings to cover the combustion. Starting from -14 CAD aTDCf, spark is advanced up to -28 CAD aTDCf. Results

Because water slows down the burn rate, the comparison in terms of performance must be made after the spark timing is re-adjusted to the same knock risk margin. As mentioned in the introduction, to do this we used the ECFM combustion model with tabulated autoignition data. We run several spark sweeps mapping the flow variables from the last available cycle and restart-ing the calculations with different SA timings to cover the combustion. Starting from -14 CAD aTDCf, spark is advanced up to -28 CAD aTDCf. Results