Hydrogen-Rich Gaseous Mixture for Enhanced Combustion in a Flex-Fuel Engine: An Experimental Analysis

Abstract

This experimental study examines the effect of adding a hydrogen-enriched synthetic gaseous mixture (HGM’) on the combustion and fuel conversion efficiency of a single-cylinder research engine (SCRE). The work assesses the viability of using this mixture as a supplemental fuel for flex-fuel engines operating under urban driving cycling conditions. An SCRE, the AVL 5405 model, was employed, operating with ethanol and gasoline as primary fuels through direct injection (DI) and a volumetric compression ratio of 11.5:1. The HGM’ was added in the engine’s intake via fumigation (FS), with volumetric proportions ranging from 5% to 20%. The tests were executed at 1900 rpm and 2500 rpm engine speeds, with indicated mean effective pressures (IMEPs) of 3 and 5 bar. When HGM’s 5% v/v was applied at 2500 rpm, the mean indicated effective pressure of 3 bar was observed. A decrease of 21% and 16.5% in the ISFC was observed when using gasoline and ethanol as primary fuels, respectively. The usage of an HGM’ combined with gasoline or ethanol, proved to be a relevant and economically accessible strategy in the improvement of the conversion efficiency of combustion fuels, once this gaseous mixture could be obtained through the vapor-catalytic reforming of ethanol, giving up the use of turbochargers or lean and ultra-lean burn strategies. These results demonstrated the potential of using HGM’ as an effective alternative to increase the efficiency of flex-fuel engines.

1. Introduction

The increasing global discussion about sustainability has demanded higher incentives for the usage of renewable energy alternatives, characterized by causing less environmental degradation compared to traditional fossil fuels. The growing awareness of the depletion of natural resources has motivated many nations to review their energy strategies, prioritizing actions focused on efficiency and waste reduction. The United Nations Climate Change Conference (COP 28) of 2023, for example, stipulated the implementation of the global energy transition through the creation of fund of 420 million dollars to support the countries impacted by global warming, besides other measures that aimed to restrict the increase in the global mean temperature to 1.5 °C [1]. This new scenario exerts a significant influence on the automotive sector, which drives innovations in spark-ignited internal combustion engines and their fuels, in line with increasingly stringent international regulations regarding pollutant emissions [2].

Despite the significant advances in the electrification of the vehicle fleet, it is expected that internal combustion engines will continue to play a crucial role in the transition to more sustainable mobility, primarily when paired with the use of fuels with a lower or no carbon footprint, such as hydrogen gas [3]. It is notable that vehicle electrification will only bring tangible benefits to the environment if the energy used is from renewable sources. When analyzing the complete life cycle of a vehicle, the conventional ones that operate with 100% renewable fuels could present environmental performance compared or even superior to the electrical, once their batteries need up to 83 kg of copper [4] for their manufacture, for example, increasing the environmental impacts from the minerals’ processing activities [5].

Ethanol emerges as a viable alternative to create sustainable, long-term mobility solutions, while Brazilian gasoline, enriched with ethanol proportions, could act as an option during the energy transition period. To mitigate the environmental impacts associated with gasoline use, it is essential to optimize engine performance, considering not only technical viability but also costs [6].

It is essential that the engines achieve high levels of efficiency when operating with ethanol, aiming to avoid consumer resistance, as this fuel naturally exhibits higher consumption compared to gasoline [7]. As an energy renewable source with low pollutant emissions, ethanol is considered an advantageous environmental alternative, particularly when evaluating the entire carbon cycle.

Beyond ethanol, the use of hydrogen gas is highlighted, either as the sole fuel or as the primary fuel in gaseous mixtures that can be added to liquid fuels, such as ethanol and/or gasoline. Hydrogen is a fuel that does not emit carbon subproducts, such as CO₂, and exhibits favorable physico-chemical conditions for the combustion reaction [8]. Hydrogen gas can be generated from the catalytic steam-reforming process of ethanol, eliminating the need for electrical energy, unlike the water electrolysis process. In the reactor, the fuel (gasoline or ethanol) reacts with water to make hydrogen gas or a mixture rich in the gas [9]. This process can be performed onboard the vehicle, thus eliminating the need for hydrogen filling stations, which are currently scarce in Brazil’s infrastructure.

As hydrogen gas has a wider ignition range compared to conventional fuels, it is challenging to control combustion precisely in spark-ignited engines. The adjustment of the air–fuel mixture and the progression of injection to optimize fuel conversion efficiency and reduce pollutant emissions can be more complex when using hydrogen as the primary fuel [10]. Thus, hydrogen gas and its mixtures could be better used as supplementary fuels, combined with other primary fuels such as ethanol and gasoline. Combining a gaseous mixture rich in hydrogen gas with ethanol or gasoline requires modifications to existing engines.

The combination of ethanol and hydrogen aims to meet the emission standards for vehicles in Brazil. The Air Pollution Control Program for Motor Vehicles (PROCONVE) [11] limits the maximum pollutants allowed, according to the FTP75 (Federal Test Procedure) cycle, which is the road cycle used to assess North American vehicles. These data are summarized in Table 1 [11].

Table 1. Vehicle emissions limits in Brazil.

Pollutants	Emissions Limits (mg/km)
Carbon monoxide (CO)	1000
Particulate matter (MP)	6
Aldehydes	15
Nitrogen oxides (NOx) + Non-methane organic gases (NMOGs)	80

To meet the demands imposed by the PROCONVE and other international agencies, while ensuring the best quality of the environment, the use of flex-fuel engines and a hydrogen-rich gaseous synthetic mixture (as a secondary fuel) is an essential advance making this research possible. Machado et al. [12], for example, analyze the combustion parameters of a flex-fuel engine with direct gasoline injection operating with hydrated ethanol and two different gasolines, one from Brazil and the other imported. The authors concluded that it is possible to use fuels from various countries, but this requires improved projects and calibrations for flex-fuel engines to achieve better performance.

Kroyan et al. [13] developed a last-generation mathematical model that enabled the precise estimation of fuel consumption in flex-fuel vehicle engines, accounting for the impact of the most significant fuel properties. The fuel’s most relevant properties were sensitivity to octane rating, vapor pressure, inferior calorific power, and density. The results showed that, across all alternative-fuel cases, flex-fuel vehicles achieved better fuel economy than conventional spark-ignition light vehicles.

Beyond flex-fuel engines, in the literature, several studies have been conducted on the addition of high-purity hydrogen gas to primary fuels in spark-ignited internal combustion engines.

Masaki et al. [14] added hydrogen to gasoline to investigate the results of this kind of mixture in the combustion process. The authors tested a multi-cylinder engine at full charge, with a volumetric compression ratio of 15:1 and lambda coefficients of 1.80, 1.84, 1.98, 2.04, and 2.28, combined with hydrogen fractions of 2%, 4%, 10%, and 20%, respectively. All the tests were performed at 2000 rpm. For the lambda coefficient of 1.8, the thermal efficiency indicated at the detonation limit was the best. In this case, the tests were performed with the addition of hydrogen fractions of 2 and 4%, which indicates efficacy in the addition of hydrogen, even in impoverished mixture conditions. The addition of hydrogen reached maximum thermal efficacy during ignition when the detonation limit was reached at a lambda value of 2.04 and the hydrogen fraction was 10%.

Teng et al. [15] experimentally investigated improvements in the combustion of a naturally aspirated rotating engine under two speeds (2400 and 4500 rpm), using hydrogen addition (99.99% pure) to the combustion process in different fractions, through an electronic injection system. Gasoline and hydrogen are injected according to the signal from the integrated electronic control module, which is connected to a central computer. The air and hydrogen fraction controllers adjust the ratios of hydrogen and gasoline injections. Hydrogen was used in varying fractions, ranging from 0% to 6% in volume. At the end of the tests, it was possible to conclude that the speed of 4500 rpm, associated with ignition advances, increased percentages of hydrogen, and the use of impoverished mixtures are beneficial in reaching the maximum thermal efficiency.

Greenwood et al. [16] conducted an experimental investigation to observe the effects of hydrogen enrichment in ethanol for ultra-lean combustion conditions, with lambda varying from 1.5 to 2.1 in a spark-ignition internal combustion engine, featuring two cylinders and a total displacement volume of 0.745 L. The study was conducted at a fixed speed of 3000 rpm and with hydrogen mixtures in three different volumetric concentrations: 0%, 15%, and 30%. They collected data for NO and HC emissions, potency, temperature of exhaust gases, thermal efficiency, volumetric efficiency, fuel-specific consumption, and burn fraction curves. With the tests, it was possible to verify that ethanol enrichment with hydrogen fractions has the capacity to decrease NOx and accelerate the combustion process. The experiments showed that operating with 15% and 30% hydrogen in volume decreases NOx emissions from the engine by more than 95% compared to stoichiometric operations with gasoline. This reduction allows for an alternative to the current technologies for NOx reduction. The potency, thermal efficiency, and volumetric efficiency were not significantly affected by the hydrogen mixture. However, adding hydrogen allowed for an increase in the operating limit using a poor mixture, which helped decrease NOx emissions without compromising potency or thermal efficiency.

Catapano et al. [17] experimentally investigated the emissions and performance of a single-cylinder engine optically accessible with indirect injection/port fuel injection (PFI). The engine operated on gasoline, ethanol, methane, and a mixture of hydrogen in methane. The optical measures were performed to analyze the combustion process with high spatial and temporal resolution. The tests were conducted in an engine with a constant speed of 2000 rpm and stoichiometric combustion conditions. A gas analyzer measured CO, CO₂, HC, O₂, and NOx in the exhaust. It was verified that the ethanol flame propagation speed is similar to that of gasoline. Methanol exhibits a slow flame propagation speed, and higher values were observed when 20% hydrogen (v/v) was added, thereby accelerating the combustion process. Spectroscopic measurements revealed that the emission of radical OH• increases significantly when hydrogen is added to methane, thereby enhancing the laminar flame speed. The energy consumption data showed that ethanol combustion was more efficient than gasoline combustion due to its higher oxygen content. CO and CO₂ emissions were also lower, highlighting the more efficient combustion achieved with alternative fuels. On the other hand, NOx emissions were higher for ethanol compared to gasoline due to its higher oxygen content.

Ren et al. [18] employed a poor combustion process, with the potential to reach the highest thermal efficiency for internal combustion engines. The authors used a 1.6 L spark-ignited engine. Before the experiment, four hydrogen injectors were set in the engine. During the experiment, the hydrogen injection pressure was kept at 3 bar. The hydrogen employed in this test was 99.999% pure. A hybrid ECU was developed to control the hydrogen and ethanol injection pulses, as well as the injection advance. In this test, the engine was operated at 1400 rpm with wide-open throttle (WOT). The hydrogen volumetric fractions at the entrance were 0% and 3%. For a certain level of hydrogen mixture, the amount of injected ethanol was reduced to enable the engine to operate under poor conditions. The results showed that thermal efficiency was improved with the mixture of hydrogen. The highest thermal efficiency was improved up to 6.07% after the addition of 3% of hydrogen in the admission air. The addition of hydrogen could increase the engine torque in lean conditions. The exhaustion losses decreased after hydrogen enrichment. The addition of hydrogen contributed to increasing the limit of poor combustion. The HC and CO emissions decreased, while NOx emissions increased after the introduction of the hydrogen mixture.

Hanyuyang et al. [19] also simulated and numerically analyzed the combustion process in a spark-ignited engine fueled with ammonia to investigate the effects of adding a hydrogen-enriched reforming gas on the combustion characteristics and emissions of the engine. The tridimensional geometric model was designed for a four-times spark-ignited natural gas engine. The simulations were performed for only one engine speed (1362 rpm) and lambda values of 0.8, 0.9, 1.0, 1.1, and 1.2. The cylinder pressure, combustion duration, potency, indicated thermal efficiency, and emissions were evaluated. The CDF tridimensional model engine, powered by ammonia, was established through CONVERGE^®. The increase in hydrogen enrichment advanced the onset of combustion, as well as the pressure peak value in the cylinder and the rate of heat release. It was observed that the limits of inflammability of the engines powered with ammonia were extended as a function of the addition of reforming gas rich in hydrogen. Regarding emissions, there was a global decrease in gas emissions, particularly in NOx emissions for lean combustion.

Ayad et al. [20] considered ethanol to be particularly interesting as an option for renewable fuel in internal combustion engines, despite its disadvantages compared to gasoline. The addition of hydrogen to ethanol could mitigate the disadvantages associated with ethanol, while retaining the benefits of using renewable fuels. The research presented an experimental and numerical study of a turbocharged ethanol engine, enriched with hydrogen in stoichiometric conditions. The internal combustion engine used was 2.0 L, four-cylinder, spark-ignited, and turbocharged. A conventional ECU, a flow meter coupled to the system, and a computer were used. The tests were performed at 3500 rpm. It was used 25, 50, 75, and 100% of the nominal potency of the 72 kW engine. The hydrogen flow was controlled to maintain stoichiometric combustion. Regarding the simulations, the AVL Boost^® was used to simulate the engine cycle in this study. In this study, it is assumed that the working fluid acts as an ideal gas. This article used an experimental configuration and operational conditions as an entry point. In addition, the results were used to validate the model. At the end of the data comparisons, it was concluded that hydrogen-enabled spark-ignited engines achieved lower specific fuel consumption, improved performance, and reduced emissions. After calibration, the model simulation was created and proved to be an adequate tool for predicting the ignition engine’s performance when operating on a hydrogen-enriched fuel, thereby reducing the total number of experimental tests required to align the engines that operate with this fuel mixture.

Prasad et al. [21] demonstrated that the combination of methanol and hydrogen, considered to meet current economic and environmental needs, with hydrogen possessing the best combustion characteristics, compensates for the disadvantages of methanol as a fuel. The experimental tests were developed by modifying a single-cylinder stationary combustion engine, refrigerated with water, and naturally aspirated. The injection was fumigation. In this study, the enrichment of hydrogen in methanol resulted in a significant improvement in performance and combustion, with global emissions substantially decreased. The increase in hydrogen enrichment resulted in improved thermal efficiency and potency, as well as a reduction in fuel consumption. The percentage increase in thermal efficiency varied between 20 and 30%, and it was verified that hydrogen added beyond 12.5% affects the volumetric efficiency, thus decreasing the engine’s performance. The CO, HC, and CO₂ emissions decreased by 30–40%; however, there was an increase in cylinder temperature due to slightly faster combustion when hydrogen was added.

Guven et al. [22] reported the behavior of the combustion process in a spark-ignited engine through the addition of biofuels (liquid hydrogen, methane, butane, and propane) to primary fuels (gasoline, ethanol, methanol, isooctane, benzene, toluene, and hexane). A computational model of two zones is proposed for simulating binary combustion. Parameters such as indicated mean effective pressure (IMEP) and thermal efficiency of the engine were obtained. The IMEP of the mixtures increased with the increase in the proportion of butane and propane added to the primary fuel. Nevertheless, it decreased with the increase in the proportion of hydrogen and methane for mixtures with ethanol and methanol. There are optimal proportions for adding additives in engines with gasoline, benzene, hexane, isooctane, and toluene. The exit potency and IMEP increased at a specific proportion of hydrogen and methane and then started to decrease with the addition of increasing proportions of biofuels. The maximum observed increase rate was 82.59% for the combustion of the toluene–hydrogen mixture. The maximum reduction rate of exit potency and IMEP was 10.84% for the combustion of the mixture of methanol–hydrogen. The thermal efficiency of the gasoline, benzene, isooctane, toluene, ethanol, and mixtures of methanol and butane, methane and propane-powered engine increased with increasing proportions of biofuels added. On the other side, the thermal efficiency of the engine powered with hexane mixtures increased until a certain proportion of propane and methane and started to decrease with increasing proportions of the additive. The thermal efficiency of the engine powered with gasoline, hexane, isooctane, ethanol, and methanol decreased with the increase in the proportion of hydrogen. The maximum rate of increase in thermal efficiency is 26.75% when burning a mixture of benzene and hydrogen. The maximum rate of reduction was 29.71% for the combustion of the mixture of methanol–hydrogen.

Among some recent works, the development of Ayad et al. [23] is highlighted. In it, the authors applied a computational simulation to investigate the combined approach of hydrogen with the usage of poor mixtures to increase the ethanol’s potential while using clean fuel in spark-ignited engines. The simulations were conducted based on an ethanol turbo charged engine operating at 3500 rpm and with hydrogen concentrations of 0%, 3%, and 6% (v/v). The characteristics and performance of the engine combustion were satisfactorily reproduced or improved under various operating conditions, especially when using 6% (v/v) hydrogen gas added to the turbocharger. It was observed that the addition of 2% hydrogen (v/v), combined with mixture leaning and turbocharging, simultaneously improved fuel conversion efficiency, promoting the reduction in carbon emissions while maintaining stable NOx emissions. Considering kinetic simulations for the combustion process, there are several good references available.

Amaral et al. [24] evaluated the effects of the addition of a hydrogen-rich synthetic gaseous mixture (HGM) to two primary fuels in a SCRE. Ethanol and gasoline were used as primary fuels by direct injection (DI), and the HGM was added by the fumigating system (FS). The experimental analysis was conducted in a four-stroke spark-ignited engine (SI) with four valves and a displacement of 0.45 L. The results showed that adding reforming gas accelerated the combustion processes of both ethanol and gasoline. The results were significant when using ethanol. There was a decrease in the specific fuel consumption for ethanol, with the combustion centralized, which did not occur with gasoline.

Based on the previous work developed by Amaral et al. [24], the present study conducts an experimental analysis of indicated specific fuel consumption (ISFC) and combustion development, the last quantified by the MFB [25]. ISFC provides an indication of how efficiently the engine converts fuel into useful power. It is an important parameter used in engine testing and performance analysis to optimize fuel efficiency and evaluate engine improvements. The aim is to evaluate the effect of adding a new hydrogen-enriched synthetic gas (HGM’) on combustion behavior and fuel-conversion efficiency in a single-cylinder research engine operating with ethanol and gasoline as primary fuels. The HGM’ is a biohydrogen-rich mixture formulated to reproduce the gas composition obtained from the catalytic reforming of ethanol vapor, a process to be carried out by the Instituto Nacional de Tecnologia (INT). The addition of HGM’ is intended to enhance the conversion efficiency of ethanol and/or gasoline in commercial flex-fuel engines. In this investigation, the engine’s volumetric compression ratio was set to 11.5:1, less than that used in earlier ethanol-fueled tests on the same engine [24,26]. In addition to reflecting the operating characteristics of the current Brazilian flex-fuel vehicle fleet, the volumetric compression ratio was decreased to evaluate gasoline behavior, while with the primary fuel, ethanol shows better results at higher volumetric compression ratios due to its higher octane rating compared to gasoline.

2. Materials and Methods

2.1. Experimental Setup

An AVL 5405 single-cylinder naturally aspirated, four-stroke, with a displacement of 0.45 L, equipped with a fuel direct injection system, spark-ignited engine was used to conduct the experimental tests. Figure 1 displays the schematic of the engine test room and the associated systems for controlling.

Figure 1. SCRE general vision and HGM’ intake from the catalytic reforming process.

The bench-active dynamometer used was the AVL DynoDur 160 model (Graz, Austria), with a maximum capacity of 160 kW of power, 400 Nm of torque, and 10,000 rpm. The software AVL PUMA Open (Graz, Austria) monitors and acquires data of the dynamometer, engine, fuel level, mass flow, coolant and oil temperatures, and air mass flow. The AVL 577 system keeps the engine coolant and lubricating oil at 90 °C. The liquid fuel temperature controlling unit (AVL 753) and mass ratio measuring unit (AVL 733) condition and measure the liquid’s fuel. The Sensyflow FMT700-P measures the intake of air flow. The admission and exhaust temperatures are measured by the resistance thermometer model AVL LPD11DA05, and the admission and exhaust pressures are measured by the sensor, AVL GU21C. The barometric pressure, relative humidity, and room temperature were measured by the humidity and temperature transmitter Vaisala HMT330. An encoder measured the engine speed, AVL 365 C/365 X. The electronic control of the engine, ignition, and fuel injection was performed by the temporization unit (ETU) AVL 427. A transducer, AVL GU22C, measured the pressure in the combustion chamber. The software AVL Indicom monitors and calculates the combustion temperature, heat release, and burned mass fraction instantaneously, as well as fuel injection and ignition signals. IndiModul 662 acquires data from the cylinder pressure through the average of 200 cycles. More details on the experimental apparatus can be seen in Amaral et al. [24].

The compression volumetric ratio used was 11.5:1. Its main characteristics are presented in Table 2.

Table 2. Engine specifications.

Item	Specification
Bore × Stroke	82.0 × 86.0 mm
Connecting rod length	144.0 mm
Displaced volume	454.16 cm3
Compression ratio	11.5:1
Number of valves	4
Intake valve diameter	34 mm
Exhaust valve diameter	28 mm

Figure 2 displays the HGM’s flow rate. The flowmeter allows the intake control of the gaseous mixture into the SCRE through the fumigation system. Associated with the meter, a gas line was installed, connected to the cylinders supplied by the HGM, housed in a safe area outside the test bench.

Figure 2. (a) Gas intake control system. (b) Flow meter.

The maximum uncertainties associated with the measuring system and experimental data are presented in Table 3.

Table 3. Uncertainties associated with the measurement systems.

Property	Uncertainty
Displaced volume [cm3]	1.28 × 10−6
Indicated mean effective pressure [bar]	0.08
Indicated power [kW]	0.03
Indicated torque [Nm]	0.12
Indicated fuel specific consumption [g/kWh]	3.08
Fuel conversion efficiency	0.01

The HGM’ was manufactured and made available with its certification by a Brazilian industrial gases company. Each gas percentage within the mixture was supplied by the Instituto Nacional de Tecnologia (INT) according to Table 4, and is similar to the reforming gas that will be obtained after the development of a prototype for the ethanol-vapor reforming process, proposed by the INT. The mixture fraction refers to the primary fuel volume (in a percentage) which is substituted by the hydrogen-enriched gaseous mixture (also in volume).

Table 4. Chemical composition of the HGM’.

Chemical Entity	Composition [%v/v]
H2	51.6
CO	6.0
CH4	24.3
CO2	18.10

In this procedure, ethanol and gasoline were directly injected into the engine’s cylinder, while a hydrogen-rich synthetic gaseous mixture (HGM’) was introduced by fumigation (FS) in the intake port. The HGM’ was used in varying proportions from 5 to 20% (v/v) and introduced by FS, alternating between ethanol and gasoline (through direct injection), gradually increasing the volumetric concentration from zero (only ethanol and gasoline injection) until reaching a value corresponding to the start of the abnormal combustion (value limited to 20% v/v).

The control of the mixture amount with hydrogen injection was achieved using a flow meter DPC17S-V0L6-4NC-440-SS model, supplied by Aalborg Instruments (see Figure 2). This flow meter was installed between the exit valve of the HGM’ and the fumigation system. The data collected during the tests were essential for validating the benefits of the gaseous mixture produced by ethanol reforming, allowing for a detailed analysis of its potential in improving ethanol combustion. During the experiments, the operating parameters monitored and calculated included engine speed, spark timing, ignition delay, start of combustion (SOC), and duration of this combustion, evaluated in different mass fraction burned intervals: SOC-MFB10, MFB10-50, MFB50-90, and MFB50.

Table 5 presents the test conditions evaluated in the study on a single-cylinder engine using the HGM’.

Table 5. Tested conditions on the SCRE.

Engine Speed [rpm]	IMEP [bar]
1900	3 and 5
2500	3

2.2. Engine Calibration Procedure

The flowchart presented in Figure 3 illustrates the standard calibration procedure used for direct injection (DI) systems. The dwell time map of the ignition system is determined for the engine operating in motoring at 1000 rpm until 6000 rpm (step 1000 rpm) and 0.30 bar until 0.90 bar (step 0.30 bar) of intake manifold pressure. A current probe connected to the IndiCom measures the current in the coil primary, determining the dwell time required to reach coil charging saturation (2.7 ms by default for the coil used).

Figure 3. Detailed flowchart of the SCRE calibration process.

Due to difficulties in stabilizing the engine in the required charge, a 1% tolerance was adopted in the IMEPg. The injection moment was varied during the tests, aiming for an optimal value, in this case we adopted base values such as −360°, −270°, and −180° APMS, a refinement could be performed close to the region of better efficiency and small CoV-IMEPg (initial step of 30° of the crankshaft).

The ignition advance value was set to parameter MFB50 equal to 8°, and a sweep around this point was made (using +5°, +2°, +1°, −1°, −2°, and −5°). The final value is chosen due to the highest torque, considering the limits of the combustion process.

Additionally, a high-frequency filter is applied to the pressure signal inside the cylinder. The obtained signal is normalized (squared), then it is integrated (KP_INT). A 20-cycle window is used to evaluate the smallest integral value (reference cycle without detonation).

It compares the current cycle with the reference cycle without detonation obtained from the previous step; if the intensity ratio value is higher than the reference value (2:1), then the cycle is marked as the detonating cycle (KP_EV). The detonation frequency is obtained for a window of 100 cycles (KP_FRQ). In this way, if fewer than 5% of the detonating cycles are evaluated, then the operating condition is validated for LDI, provided there is still a torque gain under the condition with fewer detonating cycles.

During the calibration process, conditions with detonating cycle frequencies exceeding 5% were not permitted.

2.3. Statistical Analysis

The statistical analysis was performed to compare different datasets, through the Fisher LSD (Least Squares Difference) test, based on the t-Student distribution.

The INDICOM system stores information cycle by cycle, with the time base referenced to the crankshaft angle, and the acquisition is carried out over 200 cycles. The PUMA system receives information from a dataset containing a 200-cycle moving average, stored in INDICOM.

3. Results and Discussion

Herein, all the results obtained from the tests conducted using the gaseous synthetic mixture (HGM’) will be presented. The HMG’ was used with ethanol and gasoline as primary fuels. For all the assayed parameters, as the reforming gas was added, there was a decrease in the total mass flow due to the expansion of the intake gas, which presents different air densities and occupies the volume of the intake collector, decreasing the mass flow rate.

In all graphics in this section, the pure fuel was tested and is shown at the 0.00 mixture percentage for comparison. The mixture fraction refers to the primary fuel volume (in percentage), which is substituted by the hydrogen-enriched gaseous mixture (also in volume).

3.1. Ethanol as Primary Fuel

Regarding the ISFC, the results in Figure 4, Figure 5 and Figure 6 showed a decrease in this parameter as the mixture flow rate increases, with emphasis on the substitution of 5% of the mixture, which shows a significant decrease on the indicated specific fuel consumption, proving that there is no need for high flow rates of HGM’ to reach positive results, as observed by Ren et al. [18]. The ISFC decrease continues as the flow rate increases in all tested conditions.

Figure 4. Indicated specific fuel consumption for ethanol. Engine speed: 2500 rpm. IMEP: 3 bar.

Figure 5. Indicated specific fuel consumption for ethanol. Engine speed: 1900 rpm. IMEP: 3 bar.

Figure 6. Indicated specific fuel consumption for ethanol. Engine speed: 1900 rpm. IMEP: 5 bar.

It is worth noting that the decrease in ISFC indicates that the gaseous mixture (as is pure hydrogen) shows low activation energy for the combustion process, high flame speed, and increase in the total available energy, as the hydrogen gas predominates in the mixture and has the highest low heat value (LHV) compared to ethanol.

Using the HGM’ instead of pure hydrogen gas is justified by the possibility of obtaining it through the reforming process onboard in the vehicle, which spares the need to supply and store the pure gas, representing a high risk of accidents. Adding the mixture demonstrates benefits for the engine operations in stoichiometric conditions, contrary to what was demonstrated by other works in the literature [16,20,23]. These studies demonstrate the need for lean mixtures and pure hydrogen, highlighting the mixture’s ability to extend the limit of ethanol inflammability and stabilize the combustion process [16]. It was not necessary to use the turbocharger to compensate for the power losses associated with excess air regimes, contrary to Ayad et al. [23].

Beyond security, it is highlighted that all the tests, as shown in Table 5, presented a decrease of 21, 9, and 16,5%, respectively, for indicated specific fuel consumption when 5% (v/v) of the HGM’ was applied.

The running point with 2500 rpm and pressure of 3 bar is more significant concerning the urban cycle (FTP-75) and reinforces the potential of using HGMs in commercial internal combustion engines.

The specific fuel consumption indicates how efficiently the engine uses the fuel supplied for work production [27].

A combustion analysis (air–fuel stoichiometric ratio) was performed using the MFB, from the spark timing until 90% combustion completion. The combustion process evolution aims to evaluate the engine’s performance. Figure 7, Figure 8 and Figure 9 illustrate this process.

Figure 7. Ethanol combustion analysis. Engine speed: 2500 rpm. IMEP: 3 bar.

Figure 8. Ethanol combustion analysis. Engine speed: 1900 rpm. IMEP: 3 bar.

Figure 9. Ethanol combustion analysis. Engine speed: 1900 rpm. IMEP: 5 bar.

Considering 0 °CA as Top Dead Center in the compression process, integrating the heat release rate within the window of −30 to 90 °CA determines the total apparent net heat released during the combustion.

This energy alters the pressure and mean temperature of the gas in the cylinder, allowing for the definition of MFB in spark-ignited combustion engines. The analysis of the MFB curve allows for the definition of parameters of the combustion process described as follows:

• Ignition Delay: Crankshaft angular range given by the difference between the ignition time (spark) and the SOC. The SOC is the point in the engine cycle at which combustion begins, often coinciding with the rapid increase in cylinder pressure caused by the ignition of the air–fuel mixture. It marks the transition from ignition delay to the combustion phase, where the fuel is actively burning and producing pressure that drives the piston.
• MFB10: Crankshaft angular position for burn at 10% of the mixture of air–fuel, determined by when the ordinate of the curve reaches 10%.
• MFB50: Crankshaft angular position for burn at 50% of the mixture of air–fuel, determined by when the ordinate of the curve reaches 50%.
• MFB90: Crankshaft angular position for burn at 90% of the mixture of air–fuel, determined by when the ordinate of the curve reaches 90%.

The engine’s experimental calibration involved adjusting the spark timing to achieve the MBT as much as possible, since there was no evidence of abnormal combustion processes, such as detonation.

For this reason, it is necessary to decrease the spark timing ignition to achieve optimal combustion phasing, as MFB50 cannot be positioned around 8 °CA. Ayala et al. [28] recommend centralizing the combustion process (MFB50) at 8 °CA to allow for greater energy conversion into useful work, and consequently, higher efficiency in fuel conversion.

Due to low activation energy for hydrogen [19], the HGM’ added to the fuel promoted a decrease in the ignition delay but did not reduce the combustion duration, as the volumetric compression flow rate used was 11.5:1. This fact disadvantages ethanol, since this fuel shows higher resistance to detonation, and the compression is not sufficient to fully utilize ethanol’s thermal potential, which negatively affects the heat release rate. Amaral et al. [24] conducted a similar study that showed a significant decrease in combustion duration with a volumetric compression flow rate of 14.2:1, differing from the results of this work.

3.2. Gasoline as Primary Fuel

As observed for ethanol as the primary fuel, the results showed a decrease in the ISFC as the mixture flow rate increased, with emphasis on the substitution of up to 5% mixture, which represents a drop in indicated specific fuel consumption. The decrease in IFSC is less significant when the HGM’s flow rate is higher than 5% (v/v) [15].

It is essential to highlight that gasoline combined with 5% (v/v) HGM’, tested at volumetric compression flow rates of 11.5:1 and 2500 rpm/3 bar, decreased by almost 21% of ISFC. Engine speed of 2500 rpm combined with a pressure of 3 bar (Figure 10) represents an essential point in urban roading, as the cycle FTP-75, corroborating the viability for implementing the usage of HGMs combined with flex-fuel engines, which are highly present in the Brazilian vehicle fleet. When the same conditions were applied to more minor engine speeds of 1900 rpm (3 and 5 bar) (Figure 11 and Figure 12), it was observed that the ISFC also decreases, even with increasing pressure.

Figure 10. Indicated specific fuel consumption for gasoline. Engine speed: 2500 rpm. IMEP: 3 bar.

Figure 11. Indicated specific fuel consumption for gasoline. Engine speed: 1900 rpm. IMEP: 3 bar.

Figure 12. Indicated specific fuel consumption for gasoline. Engine speed: 1900 rpm. IMEP: 5 bar.

As was performed for ethanol as the primary fuel, the combustion (approximately a stoichiometric mixture of air and fuel) was analyzed through the MFB, from the spark-ignition point (spark timing) until 90% of combustion was achieved. The combustion process evolution aims to evaluate the engine’s performance.

For the tests conducted with gasoline, the engine’s experimental calibration also involved adjusting the spark timing to achieve an MBT as high as possible, since there was no evidence of abnormal combustion processes, such as detonation.

Due to hydrogen’s low activation energy, the HGM’ added to the fuel promoted a decrease in the ignition delay, and differing from ethanol, decreased the combustion duration, with the volumetric compression flow rate used as 11.5:1. This favors the usage of gasoline, once this fuel shows less resistance to detonation, and its low volumetric compression flow rate is enough to explore its thermal potential, increasing the heat release rate of this fuel. Amaral et al. [24] and Guven et al. [22] conducted a similar study; however, there was no decrease in combustion observed at a volumetric compression flow rate of 14.2:1, which differs from the findings of this study.

Through adjustments in the spark timing, it was possible to conclude that there was an acceleration in combustion [20] once the time between spark ignition and the SOC decreased. Figure 13, Figure 14 and Figure 15 illustrate this process.

Figure 13. Gasoline combustion analysis. Engine speed: 2500 rpm. IMEP: 3 bar.

Figure 14. Gasoline combustion analysis. Engine speed: 1900 rpm. IMEP: 3 bar.

Figure 15. Gasoline combustion analysis. Engine speed: 1900 rpm. IMEP: 5 bar.

This demonstrates once again that the combustion process can and is favored by the addition of the HGM’ at low flow rates to the detriment of the use of pure hydrogen, differing from Mohamad et al. [12] and Yasin et al. [13]. The results obtained are relevant for stoichiometric mixtures, disregarding the leaning of the mixture [14]. Furthermore, it is essential to note that supercharging was not utilized, although it is gaining popularity in the automotive market, it is still not prevalent in the fleet. Studies conducted by Ayad et al. [20] demonstrated the viability of using hydrogen gas in conjunction with this strategy.

It is essential to note the relevance of the tested points, which predominates in urban roading cycles of flex-fuel vehicles. Engine speeds at 1900 and 2500 rpm, which are common in the daily lives of drivers in both big and small urban centers, are present. The decrease in specific fuel consumption, resulting from improvements in the combustion process, would considerably benefit the Brazilian population. For this reason, these points were prioritized in the tests, differing from publications that reported expressive results but were tested on only one engine speed [15].

High-purity hydrogen gas and its mixtures serve as highly reactive fuels in combustion processes due to their low activation energy and high diffusivity. Thus, the flame burns quickly, leading to rapid flame propagation. The presence of hydrogen gas increases the flame propagation speed due to its high reactivity, low molecular mass, and fast diffusion in the combustion chamber [29].

4. Conclusions

This study demonstrated that enriching ethanol and/or gasoline with a hydrogen-rich synthetic gaseous mixture can significantly enhance combustion efficiency and reduce the ISFC of a single-cylinder research engine operated under urban driving conditions. Introducing only 5% (v/v) of an HGM’ was sufficient to achieve notable improvements, including ISFC reductions of up to 21% for gasoline, without requiring the use of a turbocharger, lean-burn strategies, or expensive technologies.

The analysis confirms that the engine’s volumetric compression ratio plays a decisive role in fuel behavior. At the 11.5:1 ratio, representative of Brazilian flex-fuel engines, gasoline benefited from shortened ignition delay and combustion duration, while ethanol, despite its higher knock resistance, exhibited a substantial decrease in ISFC, even without showing a reduction in combustion duration. These findings highlight the compatibility of HGMs as a cost-effective alternative for improving the efficiency of existing flex-fuel engines, as well as their ability to enhance performance across different primary fuels.

Regarding the works presented in this research, specifically those developed by Amaral et al. [24], this study presents results for a new synthetic gaseous mixture containing an additional 0.5% hydrogen gas, compared with that previously employed by Amaral. In addition, two new combinations of rotation and mean indicated effective pressure (1900 rpm/3 bar and 1900 rpm/5 bar) were added to the tests to include more conditions in the urban driving cycle.

Importantly, the HGM’ can be generated on board through catalytic ethanol reforming, providing a safer and more practical alternative to storing or distributing pure hydrogen. This approach aligns with the global transition toward low-carbon energy by improving the efficiency of conventional propulsion systems, while leveraging renewable fuels already integrated into the Brazilian energy matrix.

Overall, the results highlight the potential of an HGM’ as a viable pathway to increase the fuel-conversion efficiency of commercial flex-fuel vehicles, supporting both near-term sustainability goals and the longer-term evolution of ICE technology.

Due to the lack of data acquisition systems that provide proper calibration and reliability, we opted not to perform the emissions analysis. This methodology should be included in future work, as well as the acquisition of data to analyze the production of HGMs using a reformer onboard a vehicle.