Experimental investigation on the potential of passive prechambers for use in 2S engines

Design improvement of Internal Combustion Engines (ICEs) is required by the continuous update of European exhaust emission standards and vehicle registration protocols. This has pushed research and industry efforts towards the development of Low Temperature Combustion (LTC) systems. Among various LTC technologies, the so called “jet ignition” is claimed to be well-suited for light engines, since it provides a more uniform and rapid combustion while ensuring the mixture ignition process. In this context, the adoption of prechambers was found well-suited for two-stroke engines, for which low-pressure direct injection technologies have been developed in the last decade to reduce the fuel short circuit phenomenon. The present paper experimentally investigates the use of different passive combustion prechambers in a Low-Pressure Direct Injection (LPDI) 2-stroke engine, with the main purpose to understand the relationship between prechamber geometrical parameters and engine performance. A 50-cc single cylinder LPDI motorcycle engine is chosen as the test case and it is re-arranged to run in jet ignition mode. The experimental analysis focuses on the evaluation of benefits provided by jet ignition combustion compared to the baseline LPDI propulsion unit at different engine operating points. Design criteria for prechambers development and the tuning process of engine combustion parameters (such as start of injection, ignition time and throttle valve opening) are described in detail. The results of experimental activity are finally shown and critically discussed, highlighting advantages and disadvantages of this application in terms of engine performance, efficiency and cycle-to-cycle variation.


Introduction
The evolving legislation on polluting emissions and vehicle type-approval procedures has posed challenges for car manufacturers in recent years.With the advent of electric and hybrid vehicles, the automotive market aims at reducing emissions on the tank-to-wheel side.Hybrid cars still incorporate Internal Combustion Engines (ICEs), both to power the vehicle and to assist the electric motor.REEV (Range Extender Electric Vehicle) technology represent an innovative solution to address the electrification challenge in transportation.These vehicles enable an extension of the electric-only range, reducing dependence on fossil fuels and helping mitigate direct CO2 emissions.However, they are not exempt from emission-related challenges, as they often feature ICEs that serve as generators to recharge the batteries.These ICEs can still produce pollutant emissions, even though at lower levels compared to traditional vehicles.Therefore, while REEVs represent a step in the right direction towards more sustainable mobility, it is essential to develop strategies for minimizing emissions from ICEs in such vehicles to maximize the environmental benefits of hybridization.To address these challenges, the sole adoption of advanced exhaust treatment systems is not sufficient, necessitating the introduction of efficient and clean combustion technologies.Low Temperature Combustion (LTC) holds significant potential in resolving issues related to ICEs.LTC involves the use of lean and homogeneous mixtures to reduce fuel consumption and emissions of nitric oxides (NOx) and particulate matter (PM) [1].That said, LTC faces challenges such as ignition instability, cycle-to-cycle variation, and occasionally increased carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions.Various LTC swirl angle is not adopted because the swirl motion inside the prechamber may lead to the stagnation of exhaust gases, as shown by Roethlisberger and Favrat [16].When considering prechambers employed in Port Fuel Injection (PFI) engines, the existing literature presents a wide range of values for both the orifice area (At) and the At to prechamber volume ratio (Vp).This variability makes it challenging to establish a definitive baseline value for these parameters.Conversely, the literature offers limited data regarding Direct Injection (DI) engines.For instance, Fu et al. [18] uses a three-hole prechamber with a 12mm diameter, resulting in an At of 37.7 mm 2 , for a fourstroke engine application.Bosi et al. [19] experiments a six-hole prechamber featuring a 1mm diameter, initially set at 4.7 mm 2 and later increased to 10.6 mm 2 , in the context of a small two-stroke engine.The approach of Fu et al. [18] is characterized by a relatively large At, notably enhanced scavenging and facilitated efficient fuel filling.However, this strategy may exhibit an excessively high At for optimal combustion processes within the primary combustion chamber.Existing literature [15] suggests that smaller overall section areas tend to enhance combustion within the main chamber.Hence, values used by Bosi et al. [19] are adopted as baseline.Given that the prechamber volume is smaller than the one investigated by Bosi et al. [19] a reduced flow area is selected.Specifically, the baseline configuration is characterized by an overall flow passage area of 3.14 mm 2 .Additionally, an investigation is carried out using a value of 7.1 mm 2 to assess the impact of flow passage area on engine performance.These values correspond to At/Vp ratios of 0.018 mm -1 and 0.040 mm -1 , equating to diameters of 1.2 mm and 1.5 mm for four-orifices prechambers, and 1.2 mm for six-orifices prechambers.It is worth noting that these surface areas may appear insufficient if the intention is to directly inject fuel into the prechamber, as exemplified by Fu [18].Conversely, the objective is to swiftly vaporize the fuel within the cylinder, aligning with the principles of High-Pressure Direct Injection (HPDI).Subsequently, the prechamber is filled by capitalizing on the tumble vortex and compression motion within the engine.However, it is crucial to acknowledge that while the tumble vortex aids in scavenging the prechamber from exhaust gases and guiding fuel into it, it may also expel a portion of fuel that has entered before ignition.Consequently, the precise configuration of the injection window assumes paramount importance to mitigate the risk of misfiring.This article aims at assessing the technical feasibility of adopting a passive prechamber in a small 50cc LPDI Piaggio engine, with the perspective of potentially applying this technique in REEV.Several passive prechamber designs are tested on the engine at laboratory scale.The study focuses on establishing the relationship between prechamber geometrical characteristics and the combustion process, to identify the optimal prechamber configuration for the selected engine.Combustion parameters are finely tuned, to achieve optimal engine performance and efficiency across a range of operating points which is typical for motorcycle operation.The structure of the paper begins with the design criteria for passive prechambers based on a review of existing literature.The role of direct injection and LTC in improving performance follows.The paper then presents the prechambers tested in the project, along with the configuration of the 2S engine used for experimentation.Results from tests conducted at various engine speeds, exhaust CO percentages, and prechamber configurations are discussed, focusing on indicated efficiency and IMEP values for comparison between JI and the Standard engine (STD).Finally, the paper reports key findings of the research and outlines potential areas for future investigation.

Test Case
The experimental study took place at the LINEA Laboratory's engine testing facility, situated within the Department of Industrial Engineering at the University of Florence.The primary objective was to assess the practicality of implementing passive prechambers on small 2S engines.A prototype LPDI engine (Figure 1), derived from the Piaggio 2S engine -a single-cylinder, air-cooled motor with a displacement of 49.2 cm 3 , was used for the tests.The baseline LPDI engine features a bore-stroke of 40x39.3mmand a compression ratio of 10.5:1.Similar to previous investigations conducted by Bosi et al. [20] [19] an IC02NIj Continental injector was installed on the cylinder liner.This injector operated at an injection pressure of 5 bar and it was positioned a few millimeters above the central transfer duct.Gasoline was supplied to the injector via an external electric pump, while a Dell'Orto reciprocating pump, driven by electricity, introduced separately lubricating oil into the intake manifold.The oil pump's operation was managed by the Engine Control Unit (ECU), following a specific map that adjusted the oil quantity based on engine speed and load conditions.To optimized engine performance, the Spark Timing (IGA) and Start of Injection (SOI) for the baseline LPDI engine underwent meticulous calibration.The aim was to achieve peak Indicated Mean Effective Pressure (IMEP) levels under full-load conditions.Conversely, during the calibration phase at lower loads, the focus shifted towards maximizing engine efficiency.In the case of the prechamber configuration, the baseline engine head accommodated a BR8ES M14 spark plug, while the prechamber itself was threaded into the existing aperture (as depicted in the figure).The prechamber was designed to host an ER9EH-6N M10 spark plug.
The engine was securely mounted on a 110-kW eddy current test bench, capable of reaching speeds of up to 13,000 rpm.Torque and power measurements were carried out at the gearbox output, connected to the wheel axle, simplifying the measurement setup.To measure the average fuel mass flow rate, the AVL PLUTRON CLASSIC system was employed.Additionally, CO and O2 concentrations were analyzed using the ASSEMBLAD INFRAGAS 309 gas analyzer (prior to the silencer).These measurements facilitated an indirect evaluation of the average air-fuel ratio within the combustion chamber and exhaust duct, respectively.Temperature readings were collected using K-type and T-type thermocouples (Figure 2)., monitoring the average temperature of the crankcase and exhaust manifold.Furthermore, the test cell's average temperature and pressure were continually monitored to standardize the output data.Data acquisition during the experiments is carried out using the CoBa software, ), using a NI Compact RIO 9024 device [21].The dynamic in-cylinder pressure is measured using an AVL GR12D piezoelectric sensor, and the instantaneous angular position of the crankshaft is recorded with an AVL 365X optical encoder.Measured data are acquired by means of the AVL IndiMicro measurement system and processed using the AVL Indicom software [22].

Prechambers used in experiments and engine configuration
The experimental investigation encompassed the meticulous examination of twelve distinct prechamber configurations to discern their notable impact on engine performance.These configurations introduced variations in orifice diameter and count, prechamber volume, and the extent of penetration into the cylinder head (Table 1).Notably, the latter characteristic had an impact on the volumetric compression ratio of the engine.Among the configurations, C-#.2 uniquely managed to preserve an unaltered volumetric compression ratio, aligning with the STD.This was achieved by reducing the volume of the main combustion chamber to compensate for the additional prechamber volume.In contrast, configurations C-#.1 and C-#.2 exhibited the most compact prechamber volumes since the spark plug has been threaded until it reached the end of the internal prechamber thread.Configurations C-#.3 and C-#.4 featured a tactically inserted dedicated washer beneath the mechanical stop of the spark plug to amplify the prechamber's volume Visual representations delineating these diverse prechamber assemblies are thoughtfully presented in Figure 3. Noteworthy, the prechambers' construction material was AMPCOLOY 972 Cu-Cr-Zr alloy, meticulously machined via sophisticated five-axis milling techniques (Figure 4).
This choice of material was guided by the requirements of high thermal conductivity and resistance to high-temperature oxidation, two pivotal attributes indispensable for prechambers.Previous analyses [19] demonstrated the tendency of aluminum to undergo thermal failure when utilizing JI combustion without a dedicated cooling system.The experimental campaign evaluated engine performance at three specific operating points, simulating real-world driving conditions.These points included engine speeds of 5000 rpm and 2 bar BMEP (OP1), 6500 rpm at full charge (OP2) where maximum torque is achieved, and 7750 rpm at full charge (OP3) where maximum vehicle speed is attained.Before conducting the actual experiments, a series of preliminary tests were conducted to discern the optimal points characterized by maximum efficiency and IMEP.The tests were based on a specific degree of the start of injection (SOI) and spark timing (IGA) for the C-1.2 prechamber configuration.For all operating points and prechamber configurations, the initial focus was on ascertaining the IGA that yielded the best efficiency for partial loads and the highest IMEP for full loads.Once the optimal IGA values were determined, the search for the best SOI values commenced.Subsequently, various tests were performed using the baseline prechamber configuration C-1.2, with IGA variations, to achieve specific CO percentages in the exhaust gas.The reported results highlight the achieved CO percentages of 2% relative to the exhaust gas mass, providing valuable insights into the equivalent air-to-fuel ratio.However, it's noteworthy that no analysis was conducted on lower CO percentage values.For both the prechambers and the standard engine, no discernible enhancement in performance materialized in alignment with the predetermined performance criteria.

Results and discussion
This paragraph reports results of the experimental activity on the 49.2 cc Piaggio LPDI engine in the JI configuration.The various trends of the engine's significant output parameters (IMEP, indicated efficiency, cycle-to-cycle variation, and so on) are presented depending on the operating point and the pre-chamber configuration.It is worth to note that values coming from the analysis (dynamic pressure sensor inside the cylinder head) are collected using the AVL Indicom software by averaging 100 engine cycles.As already mentioned, the first part of the experimental campaign concerns the study of the best SOI and IGA values for the JI engine.For operating points below 5000 RPM, it is observed that the prechamber requires a larger throttle opening compared to the standard engine.This increase is aimed at enhancing volumetric efficiency while maintaining a 2 % CO level, consequently leading to a reduction in indicated efficiency and an increment in fuel consumption.As explained in [20] to achieve this enhanced efficiency, a delay in the ignition advance (IGA) is required.Such a delay, in turn, shifts the point of peak pressure away from the ideal timing, thus resulting in a reduction of the useful work provided by the piston and an increase in the temperature of exhaust gases.That said, it is important to note that these effects should not be interpreted as discrediting the use of passive prechambers.From the perspective of implementing this technology in REEV, it is crucial to consider operating conditions where partial load is not a concern, typically with a constant engine speed.Therefore, when assessing the tested engine, the data obtained from operating points above 5000 RPM become more pertinent and valuable.Below are the selected operating points for the tests: 1 -At 5000 rpm and 2% of CO, the C-1.2 achieves the highest efficiency at SOI 220 IGA 30, while the STD engine at IGA 22. So, the operating points at IGA 22 and IGA 30 will be analyzed.2 -From 6500 rpm the IMEP is selected as benchmark parameter: the C-1.2 leads to the maximum IMEP SOI 240 IGA 18 and the STD engine at IGA 22.The chosen operating points are IGA 18 and IGA 22 3 -At 7750 and CO 2%, both prechambers and STD engines have maximum IMEP at SOI 240 IGA 34.In this case, only the operating point at IGA 34 will be analyzed.
Values related to the STD engine are represented by black bars, the engine equipped with C-1.# prechambers by red bars, C-2.# prechambers by blue bars, and C-3.# prechambers by yellow bars in the upcoming graphs.

OP1:5000 rpm, 2 bar BMEP and CO 2% set.
This section reports the results of the OP1 configuration, which provides that the engine is set to run at 5000 rpm, with a BMEP set at 2 Bar and an exhaust CO of 2% compared to the mass measured at the exhaust.Such a case is borderline: the efficiency results to be not sensitive to the change of IGA.
Anticipating IGA involves that the prechamber work at high peak pressures but also at low pressures during the expansion phase while postponing IGA does not allow to exploit the optimum of the peak point position and there is an efficiency loss due to the high exhaust temperatures.
Results show that the JI engine requires in general a lower throttle opening than the STD engine to achieve the same BMEP.The STD engine needs an opening of 38.5 % while the pre-chamber with the best efficiency, C-1.3 IGA 30, needs an opening of 34 % and the one with the worst efficiency (C-2.3IGA 22) needs an opening of 36.5 %.Compared to partial loads, the higher BMEP setpoint leads to an increase in the IMEP (as shown in Figure 5), and thus to an increase in the fuel required.This feature corresponds to an increase in the opening of the throttle valve.In particular, the C-1.3 prechamber, which has an indicated efficiency of 31.52%(Figure 6) has the lowest throttle opening and fuel consumption, measured at 0.524 kg/hr.The prechamber with the worst efficiency, C-2.3, consumes 0.572 kg/hr and shows an efficiency of 28.32%.The STD engine, with an indicated efficiency of 30.87%, consumes 0.576 kg/hr of fuel.The C-2.# and PC-3.# prechamber series show the lowest efficiencies, which are around 29%, but have a fuel consumption of about 0.550 kg/hr.The graph in the Figure 7 reports specific CO2 emissions per hour in relation to the effective power output by the engine.Consequently, we will have CO2 emissions per unit of emitted energy.In the graph it can be observed that the prechamber with the highest indicated efficiency also has the lowest specific CO2 emissions, with 2.051 kg/(kWh) against 2.211 kg/(kWh) of STD .Consequently, the prechamber with the highest efficiency is the one that can ensure better combustion stability, reducing the SD of the peak pressure position and therefore decreasing cycle-to-cycle variability.In this case, the C-1.3 prechamber exhibits an SD comparable to that of the STD engine As a confirmation, measurements do not reveal any misfire in case of the C-1.As regards the C-2.3, during the expansion phase it almost overlaps with the STD engine, but due to a lower peak pressure and a bigger delay it does not provide optimal work, as well as it causes higher exhaust temperatures (the STD engine with C-2.3 has a temperature of 618 C° while the STD engine with C-1.3 has an exhaust temperature of 578 C°).

OP2:6500 rpm, WOT and CO 2% set.
This section reports the results of the OP2 configuration, which provides that the engine is set to run at 6500 rpm, with a WOT set and an exhaust CO of 2% compared to the mass measured at the exhaust.In such a configuration the prechambers with the highest IMEP are considered, instead of those with the highest efficiency since the analysis is targeted at investigating motor points at full load.Looking at Figure 9 shows that all the prechambers have approximately the same IMEP as the STD engine.The STD engine shows an IMEP of 7.30 bar, the prechamber with the best IMEP is again the C-1.2 IGA 18 °CA with 7.53 bar, while the worst configuration is the C-1.4 IGA 22 °CA with 6.89 bar. Figure 10 shows that the efficiency trend is almost the same as in the IMEP graph: the STD engine has an efficiency of 25.55 % with the C-1.2 (26.96 %), while the C-1.4 is characterized by the lowest value (24.62 %).On the other hand, the prechamber that turns out to be the most efficient in terms of indicated efficiency is the C-3.2 IGA 18°, with an efficiency of 27 %.
Figure 9 IMEP at 6500rpm, WOT set, CO target 2%  11, shows an improvement in emissions with reduced fuel consumption.As a confirmation, the consumption in the C-1.2 configuration is 1.14 kg/hr, and 1.105 kg/hr in the C-3.2 configuration, while the STD engine consumes 1.16 kg/hr.Tests show that the C-1.4 pre-chamber consumes 1.14 kg/hr despite its lower efficiency, since it operates with less fuel demand and higher combustion speed, but it fails to exploit combustion at the optimal point.The in Figure 12 and Figure A 5 reveal that the C-1.4 configuration has an extremely faster heat release than the STD engine, the C-1.2 and the C-3.2; consequently, the C-1.4 prechamber has an earlier pressure peak.More specifically, the C-1.4 prechamber shows the pressure peak at 12 °CA, with a SD of 1.63°, while the STD engine reaches the pressure peak at 15.5 ° CA with a SD of 1.7° (the C-1.2 at 15.5 °CA with a SD of 1.43°, the C-3.2 at 16.5°CA with a SD of 1.30°).The lower value of SD of the JI engine is indicative of a decrease in the cycle-to-cycle variation.From the above, it can be said that prechamber performs better at high engine loads: this is due to the improved flushing of internal volume, which can be associated with the stronger flow motion and the wider compression variation, which also increases the combustion stability.Despite the higher peak pressure, the C-1.4 prechamber presents a pressure line in the expansion phase (see Figure A 5), which is below the STD engine , C-1.2 and C-3.2, thus enabling to obtain less useful work than the others, and therefore reducing the overall efficiency.This result is in line with (19), which states that the use of a prechamber provides a lower cycle-to-cycle variation and a greater combustion stability.At the same time, as the combustion speed is higher, the JI engine needs a delay in the ignition: indeed, in case of anticipated ignition, the pressure peak occurs too early, resulting in sub-expansion effects or blow-by effects.

Figure 11 Specific CO2 emissions relative to the effective energy output of the
Figure 12 shows results related to the C-3.2 prechamber, with a heat release similar as the STD engine: the combustion begins later, but it is so fast that there is a faster heat release 5 ° of crank angle after the TDC.The C-3.2 has a mean peak pressure of 37.8 bar, with a 3.31 bar SD, lower than the C-1.4 (43.20 bar and 3.96 bar SD) and C-1.2 (40.1 bar with a 4.48 bar SD), but higher than the STD engine (37.55 bar and 5.2 bar SD).A sufficient pressure peak centered at an optimal point also provides more useful work during the expansion phase: in Figure A 5 shows that the C-1.2 and C-3.2 curves have the highest pressures during the expansion phase.This is also confirmed by the fact that the C-1# and C-3# ensure a lower exhaust temperature than the STD and therefore a better exploitation of the fuel potential.

OP3:7750 rpm, WOT and CO 2% set.
In this subchapter the results of the OP3 configuration are presented: the engine is set to run at 7750 rpm, with a WOT set and an exhaust CO of 2% compared to the mass measured at the exhaust.During preliminary experiments, it was noted that both the STD and the JI engine had the highest IMEP performance with an IGA of 34 °CA.Therefore, the results reported below are all at the same IGA.Figure 13 and Figure 14 report the trend of IMEP and indicated efficiency.As expected, by increasing the engine rpm and moving away from the area where the engine delivered maximum torque, the IMEP decreases in all configurations.The STD engine goes from an IMEP of 7.30 bar at 6500 rpm and 2% CO to an IMEP of 5.61 bar.Similarly, the PC-1B pre-chamber, which is also the best at 6500 rpm, passes from an IMEP of 7.53 bar to an IMEP of 5.64 bar.However, the different prechamber  The worst case is represented by the C-3.1, with an IMEP value of 5.11 bar.With regards to the indicated efficiency, the STD engine is found to achieve the best results, (22.14 %), consuming 1.03 kg/h of fuel, while the C-1.2 shows an efficiency of 21.73 %, with a consumption of 1.05 kg/h.Finally, the worst configuration is found to be the C-3.1, achieving a 21.50 % efficiency, with a 0.97 kg/h fuel consumption.The configuration with the highest efficiency is the C-2.2 (21.9 % efficiency), with a consumption of 0.99 kg/h.This is confirmed by CO2 emissions data reported in Figure 15: C-2.2 emits 1.93 kg/(kW*h), C-1.2 emits 2.04 kg/(kW*h), C-3.1 emits 2.00 kg/(kW*h) and STD emits 2.01 kg/(kW*h).The higher amount of fuel consumption of the C-1.2 leads on the one hand to a higher IMEP, and on the other hand, to a lower indicated efficiency.On the contrary, the C-3.1 shows a lower IMEP, but also a lower efficiency, despite the lower fuel consumption.The prechambers are found to have a lower IMEP since the peak pressure is not at the optimum point.
The JI engine at 7750 rpm shows more or less the same issues as in the lower operating point (engine speeds below 5000 rpm): prechambers, other than C-1.2, show a pressure peak position that is delayed than the STD engine, as it can be seen in   It can also be noted that during the expansion phase the pressure line of the C-1.2 remains above the curve of the STD, thus providing a higher IMEP.It can be also noted that all the prechambers have a higher exhaust temperature than the STD engine, due to the delayed pressure peak.From the above considerations, it can be stated that running the prechambers at higher engine speeds than the maximum torque point (6500 rpm) involves high total area-to-volume ratio and big orifice diameters.However, results highlight that this affects negatively prechamber efficiency at lower speeds, by postponing the peak pressure point.

Conclusion
The paper presents a preliminary study on the application of different passive prechambers to the LPDI Piaggio 2S 49.2 cm3 engine, as literature review highlights the suitability of "stumpy" multi-orifice prechambers for use in 2S engines, in particular GDI ones.Three different prechamber geometries are tested: C-1# features four holes with a diameter of 1.2 mm, C-2# features four holes with a diameter of 1.5 mm, C-3 #features six holes with a diameter of 1.2 mm.The volume and sinkage within the main combustion chamber are changed during tests, to assess the variation in terms of indicated efficiency and IMEP.The analysis stresses that the JI combustion allows increasing efficiency and IMEP when specific pressure conditions occur within the main combustion chamber.For engine speed lower than 5000 rpm, the JI engine cannot reach the same performance as the STD engine; this is due to the fact that the engine fails to fire properly, and it is necessary to increase the throttle valve opening to maximize the amount of fuel injected.The C-1# prechamber proves to be the best JI setup for the vast majority of tests carried out, exceeding the performance of the STD engine, especially in terms of IMEP, but still failing to reach the efficiencies of the STD motor at low loads.To achieve higher efficiencies, a greater number of orifices should be used for 6500 rpm, to achieve better combustion by reducing the cycle-to-cycle variation.Meanwhile, for 7750 rpm, a larger orifice diameter is needed to counteract the ignition delay inside the combustion chamber.The other prechambers tested can reach no more than the performance of the STD engine.Results above show that a prechamber with a volume as small as possible is preferable, as big volumes generate more energy jets, thus involving problems of heat exchange and consequent efficiency loss.On the other hand, the deepness of the prechamber inside the main combustion chamber has no consequences on efficiency and IMEP, since the scavenging occurs due to the compressions (not the charge motions), and therefore there are no benefits by sinking more the prechamber.In the light of previous considerations, it can be concluded that prechambers tested do not show a significant increase in performance and efficiency, since they provide better performances only under specific conditions that make favorable the scavenging of the prechamber volume; in all the other cases, the prechambers worsen the performance of the engine, unless a dedicated design is carried out to make that the prechamber operate at speeds other than those of the engine maximum efficiency.Nevertheless, the adoption of prechambers for 2S ICEs in application to REEV can be deemed a path worth exploring: since ICEs are used under constant operating conditions (solely tasked with recharging batteries), the adoption of a prechambers to enhance combustion efficiency and performance is a valid option.Future research can be focused on finding the optimal configuration of SOI and IGA for each prechamber, to achieve the best performance also at partial loads.Another interesting pint is exploring the possibility of using active/passive scavenging (e.g., designing a pigtail connected to the exhaust to promote prechamber scavenging), as well as applying other types of materials with low thermal conductivity (such as ceramic materials to achieve better results in terms of heat exchange).

4 Figure 1
Figure 1 Test-bench with Piaggio 2S engine installed.The arrow indicates the location of the engine's spark plug, thus indicating the placement of the prechamber.

5 Figure 2
Figure 2 Scheme of sensor system

Figure A 3
FigureA 3  shows the heat release of the STD engine, C-1.3 and C-2.3.The opener with the highest efficiency is the one characterized by a similar heat release as the STD engine, despite the STD engine has a higher IGA value.Such an outcome substantially confirms that prechambers can provide a faster burning process.The lowest efficiency prechamber has a more delayed start of combustion, causing lower and more delayed peak pressures.In Figure A 4 provides the P-V diagram, which shows how the C-1.3 expansion stage has a lower pressure, even than the C-2.3 prechamber, thus involving on one hand a decrease in the useful work of the piston and on the other hand a lower IMEP.As regards the C-2.3, during the expansion phase it almost overlaps with the STD engine, but due to a lower peak pressure and a bigger delay it does not provide optimal work, as well as it causes higher exhaust temperatures (the STD engine with C-2.3 has a temperature of 618 C° while the STD engine with C-1.3 has an exhaust temperature of 578 C°).
an IMEP value which is comprised within a relatively small range (5.15-5.40bar).

Figure A 6 ,Figure A 7 and
Figure

Figure A 1 Figure A 2 Figure A 3 Figure A 4
Figure A 1 Pressure Max Average Position in cylinder at 5000 rpm, BMEP set 2 bar, CO target 2%

Table 1 .
Details of the several prechamber configurations tested during the experimental campaign.
Specific CO2 emissions relative to the effective energy output of the Piaggio engine at 7750 rpm, WOT set, CO target 2%In addition, all the JI configurations show a lower pressure peak than the STD engine.The STD engine has a peak pressure of 33.47 bar, with a 4.22 bar standard deviation, positioned at 11.5° crank angle, with a 1.64 °CA standard deviation.The C-1.2 has a peak pressure of 31.20 bar, with 3.96 bar SD, positioned at 13° crank angle, with a 1.57 °CA standard deviation, and the C-3.1 27.03 bar, with 3.55 bar, positioned at 14.5°, with a 1.50° SD.Despite the lower efficiencies, the prechambers under consideration have a higher combustion stability than the STD engine, as they present lower SD.The prechamber with the highest efficiency (C-2.2) has a peak pressure of 27.89 bar with 3.45 bar SD positioned at 14.5°, with 1.45° SD.Due to the delayed peak position than the STD, the JI configurations are found to have a delayed start of combustion.Figure16shows that the combustion of the STD engine starts earlier, and JI configurations have almost the same combustion rate of the STD, the only exception being the C-2.2, which instead is characterized by a higher rate.In the curve of the C-2.2, which substantially has the same shape of the STD engine, except for the lower pressure peak, is 2.27 bar lower. 15Figure15 Maximum pressure distribution over 100 cycles for STD (black), best IMEP prechamber (red), worst IMEP prechamber (yellow) and best efficiency prechamber (blue) at 7750rpm, WOT set, CO target 2%, 0°=TDC Indicated cycle for for STD (black), best IMEP prechamber (red), worst IMEP prechamber (yellow) and best efficiency prechamber (blue) at 7750rpm, WOT set, CO target 2%, 0°=TDC 18 Figure A 5 Indicated cycle for STD engine (black), best IMEP prechamber (red), worst IMEP prechamber (purple) and best indicated efficiency prechamber (yellow) at 6500rpm, WOT set, CO target 2% Figure A 6 Average Position Pressure Max in cylinder at 7750rpm, WOT set, CO target 2% Figure A 7 Pressure Max Average in cylinder at 7750rpm, WOT set, CO target 2%