Enhancing Diesel Engine Performance Through Hydrogen Addition

Abstract

This study evaluates the potential of hydrogen as a clean additive to conventional diesel fuel. Experiments were carried out on a single-cylinder, air-cooled diesel engine under half- and full-load conditions, across engine speeds ranging from 1000 to 3000 rpm. Hydrogen, produced on site via a proton exchange membrane electrolyser, was supplied to the engine at a constant flow rate of 0.5 L/min. Compared to pure diesel, the hydrogen–diesel blend reduced specific fuel consumption by 10% and increased brake thermal efficiency by 10% at full load. Emissions of carbon monoxide and carbon dioxide decreased by 13% and 17%, respectively, at half load. Additionally, nitrogen oxide emissions dropped by 17%. These results highlight the potential of hydrogen to improve combustion efficiency while significantly mitigating emissions, offering a viable transitional solution for cleaner power generation using existing diesel infrastructure.

1. Introduction

Currently, a significant portion of the world’s energy demand is met by fossil fuels. The reliance on these non-renewable energy sources has severe environmental consequences, including global warming, air pollution, and ecological disasters such as dense smog and acid rain. These issues not only harm the environment but also pose serious risks to human health [1]. If the current rate of fossil fuel consumption continues, reserves of natural gas and oil could be depleted within the next 40 years. The transportation sector is a significant consumer of fossil fuels, with diesel playing a dominant role due to its high chemical energy content, which enables efficient power generation during the engine’s power stroke [2]. Compression ignition (CI) engines are essential to heavy-duty, railroad, and maritime transportation and commonly considered as the foundation of the world economy. But these engines’ inability to fully burn diesel results in significant emissions, primarily in the form of carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (UHCs) [3]. Consequently, CI engines are identified as major contributors to air pollution, causing irreversible harm to both the environment and human health. At present, the global trend increasingly favors the adoption of battery electric vehicles (BEVs) as a replacement for internal combustion engine vehicles (ICEVs) [4]. However, despite significant advancements, particularly in light-duty applications, BEVs are not yet capable of fully competing with ICEVs in the near future. This is due to several challenges, including the lack of well-developed charging infrastructure on a global scale and the reliance of electricity generation on fossil fuels in many regions. Additionally, technical issues related to battery performance remain unresolved, such as limited driving range, the dependency on scarce raw materials for battery manufacturing, and the sensitivity of battery efficiency to fluctuations in environmental conditions [5]. These obstacles underscore the complexity of transitioning to a fully electric vehicle market. To address these challenges, blending diesel with alternative fuels such as alcohols, methyl esters, oxyhydrogen, and particularly hydrogen has shown potential to improve the performance and emission characteristics of compression ignition (CI) engines [6,7,8].

Blending hydrogen with diesel fuel in CI engines offers potential economic and environmental advantages [9]. Economically, hydrogen blending can reduce diesel consumption, leading to cost savings and lower emissions. However, its feasibility depends on various factors like hydrogen production costs, infrastructure development, and policy support. Strategic investments and supportive regulations are essential to realizing the economic benefits of hydrogen integration in diesel engines [10].

Bakar et al. [11] highlighted hydrogen’s potential as a safe and effective diesel engine fuel substitute. The results show that specific hydrogen flow rates improve engine performance, reduce emissions, and enhance combustion efficiency. The significant increase in brake thermal efficiency (BTE) till 35% obtained at a 21.4 L/min enrichment flow rate, 25 Nm load, and 1500 rpm demonstrates hydrogen’s viability in optimizing fuel usage.

Liu et al. [12] demonstrated that hydrogen–diesel dual direct injection (H2DDI) achieved an indicated efficiency of 47% with a 50% hydrogen substitution ratio. Combustion noise was lowered by 6 dB while NOx emissions remained below 11 g/kWh by adjusting the hydrogen injection timing to 40° CA bTDC. Early hydrogen injection resulted in premixed combustion, whereas later injection timings led to mixing-controlled combustion.

Dimitriou et al. [13] investigated the use of hydrogen in a heavy-duty hydrogen–diesel dual-fuel engine at low and medium loads. In comparison to diesel-only operation, a hydrogen energy contribution of up to 98% was attained at low loads without experiencing any operational problems, resulting in a reduction of more than 90% in CO and NOx exhaust emissions and 85% in soot emissions. However, at moderate loads, elevated NOx emission levels were recorded, brought on by the significant energy content of hydrogen.

Gültekin et al. [14] studied the impact of the hydrogen energy ratio and intake valve lift on hydrogen–diesel dual-fuel engine performance and emissions. To counteract the reduced volumetric efficiency with higher hydrogen use, increased air intake was tested. At a constant speed of 1850 rpm, varying loads, hydrogen ratios, and valve lifts were examined. According to the data, a 10% increase in valve lift and a 7% hydrogen ratio at 9 Nm load decreased CO emissions by 33% and smoke emissions by 40% when compared to pure diesel operation.

Luo et al. [15] analyzed the impact of hydrogen addition on a dual-fuel diesel engine using a computational fluid dynamics model. According to this study, injecting 15% of H₂ at full load boosted braking power by 10.3% and decreased specific fuel consumption (SFC) by 4.6%. Hydrogen improved combustion by enhancing flame spread, reducing the combustion time, and raising the maximum chamber temperature and pressure. While NOx emissions slightly increased, hydrogen effectively reduced unburned gas emissions at low loads.

Gültekin and Ciniviz [16] examined hydrogen–diesel dual-fuel operation in a single-cylinder compression ignition engine. At a constant speed of 1850 rpm, varying loads (3 to 9 Nm) and hydrogen injector opening times (1.6, 1.8, and 2.0 ms) were tested. The study found that at medium and high loads, in-cylinder pressure increased and specific energy consumption decreased at hydrogen ratios below 14%. While NO emissions rose, other emissions were significantly reduced.

Various research studies have indicated that incorporating hydrogen into conventional fuels enhances engine performance and reduces most harmful emissions, except for nitrogen oxides (NOx), which tend to increase with higher hydrogen addition. However, only a limited number of studies have reported a decrease in nitrogen oxide emissions. This study examines the impact of adding a small amount of hydrogen to diesel fuel on both engine performance and emissions. Key performance parameters analyzed include brake power (BP), thermal efficiency (BTE), and specific fuel consumption (SFC). Additionally, particular attention is given to the effect of hydrogen on carbon monoxide (CO), carbon dioxide (CO₂), and specifically nitrogen oxides (NOx), to assess its potential for cleaner and more efficient combustion.

2. Materials and Methods

2.1. Experimental Setup

Figure 1 illustrates the schematic of the experimental setup. Hydrogen is generated by a proton exchange membrane (PEM) electrolyser with 99.999% purity [17]. The electrolyser consumes about 75 W to produce hydrogen at 0.5 L/min, which is injected into the engine through the intake manifold after the air filter, making sure it mixes with air before entering the combustion chamber of a mono-cylindrical diesel engine connected to a swinging-field DC dynamometer. Initially, the dynamometer operates as a starter to crank the engine. Once the engine is running, it functions as an electrical dynamometer, transforming mechanical power into electrical power, which is then released as heat through an external variable rheostat. The rheostat electrical resistance is adjusted to regulate the engine load, creating braking torque on the dynamometer shaft. The dynamometer outer shell, which rotates slightly owing to magnetic drag, has a graded scale on it which is used to measure the mechanical torque. Engine speed is measured using a Lutron VT-8204 optical tachometer (Taipei, Taiwan), whereas a graduated burette attached to the fuel supply system is used to record fuel consumption. A Testo 350 gas analyzer (Black Forest, Germany) is used to assess the composition of exhaust gases, including CO, CO₂, and NOx levels.

Tests were performed under half- and full-load conditions at engine speeds between 1000 and 3000 rpm. The engine was let to run without load for a minimum of fifteen minutes in order to stabilize it before measurements were collected. To guarantee accuracy, every measurement was repeated three times, and the average values were calculated.

2.2. Engine Characteristics

This study focuses on an experimental investigation utilizing a single-cylinder, four-stroke, air-cooled diesel engine of the Lombardini 6LD325N model. The engine has a displacement of 325 cc and operates with a compression ratio of 18:1. It delivers a maximum power output of 4 kW and achieves a peak torque of 14 Nm at 2100 rpm, with a maximum operating speed of 3000 rpm. The engine specifications are summarized in

Table 1. Engine specifications.

Parameter	Specification
Engine model	Lombardini 6LD325N
Power	5 kW
Maximum speed	3500 rpm
Maximum torque	14 Nm at 2100 rpm
Stroke	4
Capacity	325 cm3
Compression ratio	18:1
Injection type	Direct
Cooling type	Forced air-cooled system

.

The engine is coupled to a dynamometer, which serves a dual purpose: initially acting as a starter and subsequently functioning as an electric generator (Figure 2). This setup enables precise measurement of the power generated during the experiments.

2.3. Uncertainty Analysis

The correctness of the experimental findings and the dependability of the data gathered were assessed using an uncertainty analysis [18]. This assessment helps quantify the errors associated with the measuring equipment used during the experimental tests. The total measurement uncertainties for the test engine were calculated using the standard deviation method [19]. The precision of the measurements and the associated uncertainties are detailed in

Table 2. Measurement instrument uncertainties [19].

Measured Parameter	Measurement Instrument	Uncertainty
Engine speed	VT-8204 optical tachometer	±0.3
Diesel flow rate	Graduated burette	±1.0
Torque	Dynamometer	±1.0
CO	Testo 350 gas analyzer	±3.4
CO2	Testo 350 gas analyzer	±0.34
NOx	Testo 350 gas analyzer	±0.1

.

The overall uncertainty (OU) is calculated as follows:

$$OU\left(\mathrm{\%}\right)=\sqrt{({0.3)}^2+{\left(1.0\right)}^2+{\left(1\right)}^2+{\left(3.4\right)}^2+{\left(0.34\right)}^2+{\left(1\right)}^2}=3.71\%,$$

(1)

The overall measurement uncertainty of 3.71% is quite tolerable for experimental tests.

2.4. Definition of Important Parameters

The energy substitution ratio (ESR) is a key metric used to evaluate the efficiency of replacing one energy source with another [20]. In this study, hydrogen serves as the substitute energy source, while diesel is the primary energy source. In each of the selected operating modes (half or full load and from 1000 to 3000 rpm), hydrogen was added with a constant flow rate. As a result, the energy substitution ratio of hydrogen varied with each operating mode due to changes in engine performance and energy demand.

The ESR of hydrogen was calculated using the following Equation (2):

$$ESR={\displaystyle \frac{m_{H_2}\times {LHV}_{H_2}}{m_{H_2}\times {LHV}_{H_2}+m_D\times {LHV}_D}},$$

(2)

For a 0.5 L/min hydrogen flow rate, ESR ranges from 5% to 1% at engine speeds ranging from 1000 to 3000 rpm, respectively.

The brake thermal efficiency (BTE) of hydrogen–diesel blends is defined as follows:

$$BTE={\displaystyle \frac{BP}{m_{H_2}\times {LHV}_{H_2}+m_D\times {LHV}_D}},$$

(3)

BP is the engine brake power in W, defined as follows:

$$BP=BT\times {\displaystyle \frac{2\pi N}{60}},$$

(4)

where BT in N.m and N in rpm are the measured brake torque and engine speed, respectively.

The specific fuel consumption (SFC) serves as a key indicator of fuel efficiency in dual-fuel engines incorporating hydrogen. It measures the amount of diesel fuel consumed per unit of brake power generated [21]. A reduction in SFC reflects an improvement in fuel conversion efficiency, while an increase suggests lower efficiency.

The specific fuel consumption of the mixed hydrogen–diesel blend was calculated as [22]

$$SFC={\displaystyle \frac{1000\times \left(m_D+m_{H_2}\right)\times 3600}{BP}},$$

(5)

where m_H2 and m_D are the mass flow rates in g/s of hydrogen and diesel, respectively. LHV_H2 and LHV_D are the lower heating values in MJ/kg of hydrogen and diesel, respectively. Details on the hydrogen flow rate measurement can be found in Abdelwahed et al. [17].

An overview of the characteristics of diesel and hydrogen is given in

Table 3. Different fuel properties [23].

Properties	Hydrogen	Diesel
Critical pressure (Pa)	1.3 × 106	2.46 × 106
Critical temperature (K)	33.2	569.4
Lower heating value (MJ/kg)	120	43
Stoichiometric air–fuel ratio (kg/kg)	34.3	14.5
Density (g/L)	0.09	830
Autoignition temperature (K)	813	530
Adiabatic flame temperature (K)	2483	1993
Carbon atoms per molecule	0	13.5
Hydrogen atoms per molecule	2	23.6
Oxygen atoms per molecule	0	0

.

3. Results and Discussion

The impact of hydrogen blends (0.5 L/min) on the performance and emission characteristics of a CI engine was investigated. Experimental tests were conducted at varying engine speeds ranging from 1000 to 3000 rpm under medium- and full-load conditions. The ensuing sections provide and explain the results for specific fuel consumption (SFC), brake power (BP), and brake thermal efficiency (BTE) for each load state. Furthermore, CO, CO₂, and NOx emissions are measured and reported.

3.1. Performance Characteristics

3.1.1. Brake Power

Brake power (BP) is the actual power output of an engine measured at the crankshaft or flywheel, after accounting for losses due to friction and other mechanical inefficiencies [24].

The effect of hydrogen addition on BP as a function of engine speed and for both load conditions is shown in Figure 3. The first observation is that BP increases significantly with rising engine speed, regardless of the fuel type. In addition, the BP results show that adding hydrogen causes a modest average increase of 3% under both half-load and full-load circumstances. Numerous reasons, including the increased volumetric heating value of the intake mixture and improved combustion efficiency, are responsible for this improvement. Additionally, a higher hydrogen concentration in the fuel blend shortens the ignition delay and combustion duration, accelerating both flame initiation and propagation. As a result, this leads to increased power output efficiency according to Ghazal [25].

However, hydrogen addition through the intake air does not directly alter the injection duration, but the engine control system adjusts the injection timing and duration in response to the improved combustion efficiency and reduced diesel fuel requirement.

3.1.2. Specific Fuel Consumption

Specific fuel consumption (SFC) is the amount of fuel used to produce one unit of engine power. It is an important metric for evaluating how well the engine transforms fuel energy into mechanical output that is useful. A lower SFC value indicates greater fuel efficiency and improved energy conversion performance [26].

SFC is higher at low engine speeds because of decreased combustion efficiency, which is mostly brought on by the longer residence time and higher relative heat losses to the cylinder walls. This behavior can also be attributed to the poor fuel atomization and mixing due to the shorter injection duration in the presence of hydrogen, leading to incomplete combustion, which increases SFC despite using less fuel per injection event [27].

At high engine speeds, a similar pattern is seen; however, in this instance, the rise in frictional heat losses is responsible for the reduction in the efficiency of combustion. This observation is the first noticeable trend when analyzing Figure 4, which illustrates the impact of hydrogen blending on SFC under half-load and full-load conditions. The average reduction in SFC increases with higher engine load, starting at approximately 9% under half-load conditions and reaching up to 10% at full load compared to pure diesel. In addition, the highest reduction in SFC was observed at full load and 1500 rpm, achieving a 19% decrease compared to pure diesel.

The presence of hydrogen contributes to a reduction in specific fuel consumption, primarily due to its significantly higher lower heating value (LHV), which is approximately 2.5 times greater than that of diesel as reported by Hamdan et al. [28]. This effect becomes particularly noticeable at lower engine speeds, where hydrogen’s superior energy content enhances combustion efficiency and optimizes fuel utilization.

3.1.3. Brake Thermal Efficiency

Brake thermal efficiency (BTE) is an essential parameter for assessing engine performance. It is defined as the ratio of the brake power output to the total energy released per unit time through complete fuel combustion. From an energy perspective, BTE is a crucial metric that shows how well the engine transforms fuel energy into usable work [29].

Figure 5 shows the impact of hydrogen blending on BTE under full- and half-load conditions at various engine speeds. Contrary to SFC, the BTE ratio is higher at medium engine speeds, and the average improvement in BTE rises with increasing engine load, starting at around 9% under half-load conditions and reaching up to 10% at full load compared to pure diesel. Furthermore, the most significant improvement in BTE was recorded at full load and 1500 rpm, resulting in a 22% enhancement compared to pure diesel. However, according to Debnath et al. [30], hydrogen has a higher energy content than traditional diesel fuel; hence, BTE rose with a higher hydrogen substitution ratio and load.

3.2. Emission Characteristics

3.2.1. Carbon Monoxide

Carbon monoxide (CO) emissions pose a significant environmental threat, contributing to ozone layer depletion. These emissions primarily result from incomplete combustion within the engine, where insufficient oxygen prevents the full oxidation of fuel into carbon dioxide (CO₂), leading to CO formation [31]. The integration of hydrogen into internal combustion engines leads to a reduction in CO emissions. Since hydrogen is a carbon-free fuel, increasing its proportion in the fuel mixture minimizes the formation of CO emissions [24,32].

The engine’s CO emission results under half load and full load as a function of engine speed are displayed in Figure 6. When adding hydrogen, the CO emissions were decreased by an average of 13% and 7% under medium load and full load, respectively, compared to pure diesel.

3.2.2. Carbon Dioxide

Carbon dioxide (CO₂) emissions, a major contributor to global warming, result systematically from the combustion of hydrocarbon fuels. It is estimated that internal combustion engines account for nearly half of the world’s total CO₂ emissions [33,34]. The partial substitution of hydrogen for diesel in a hydrogen–diesel mixture is anticipated to significantly lower CO₂ emissions by lowering the quantity of carbon-based fuel used.

The change in CO₂ emissions with engine speed under both half- and full-load circumstances is depicted in Figure 7. CO₂ emissions tend to rise with increasing engine speed for both tested fuel types, likely due to a richer fuel mixture in the combustion chamber at higher speeds. Compared to pure diesel fuel, the average reduction in CO₂ emissions was observed to be 17% under half-load conditions and 7% under full-load conditions. The reduction in CO₂ emissions with hydrogen enrichment can be attributed to the improved combustion efficiency of hydrogen and the fact that hydrogen, unlike conventional fuels, does not contain carbon atoms [35]. Consequently, the oxidation reactions involving carbon are minimized, leading to lower CO₂ formation. Similar results were reported by Jamrozik et al. [36].

3.2.3. Nitrogen Oxides

Nitrogen oxides (NOx) are one of the primary regulated emissions from diesel engines and pose significant risks to both the environment and human health. They encompass nitrogen monoxide (NO) and nitrogen dioxide (NO₂), which are collectively referred to as NOx and are formed during the combustion process [37]. NOx are formed during combustion through three primary mechanisms: fuel NOx, prompt NOx, and thermal NOx. Fuel NOx originates from the oxidation of nitrogen-containing compounds that are chemically bound within the fuel itself. During combustion, especially in solid and liquid fuels like coal and heavy oil, these fuel-bound nitrogen species are released and react with oxygen to form NOx, typically at moderate temperatures. Prompt NOx is generated in the early stages of combustion through a series of fast, intermediate reactions between atmospheric nitrogen (N₂) and hydrocarbon radicals, such as CH, that are produced during the initial breakdown of the fuel. These reactions occur in fuel-rich flames and can form small but significant amounts of NOx even at relatively low temperatures. Thermal NOx, the most temperature-sensitive pathway, forms when atmospheric nitrogen reacts directly with oxygen at high flame temperatures, typically above 1300 °C. This mechanism becomes increasingly dominant in high-temperature, lean-burn conditions. Each of these pathways contributes to the overall NOx emissions, and their relative significance depends on factors such as the type of fuel, combustion temperature, oxygen availability, and flame characteristics. Among these, NO is the predominant component of NOx, primarily formed through the reaction between nitrogen and oxygen during combustion processes [38].

For both pure diesel and hydrogen–diesel blends, Figure 8 shows the NOx emissions produced through the diesel engine under half- and full-load conditions over a 1000–3000 rpm speed range. The first finding is that, regardless of the fuel type, NOx emissions dramatically increase with engine speed. This increase can be attributed to the higher combustion chamber temperatures at elevated speeds [39]. The phenomenon is further intensified by the increased friction within the engine at higher rotational speeds, which contributes to additional heat generation and subsequently higher NOx formation.

In comparison to pure diesel, Figure 8 demonstrates that the addition of hydrogen to the fuel blend results in a significant decrease in NOx emissions over the majority of engine speed ranges. On average, NOx emissions decrease by approximately 17% under half-load conditions and by 13% under full-load conditions. The incorporation of hydrogen into the fuel mixture results in a higher molar fraction of water vapor in the combustion byproducts. This increase in water vapor plays a significant role in moderating the combustion temperature, thereby contributing to a reduction in NOx emission levels as explained by Cernat et al. [40]. The presence of water vapor enhances the thermal capacity of the gas mixture, effectively absorbing excess heat during combustion and suppressing the formation of nitrogen oxides [41]. This mechanism underscores hydrogen’s potential to improve emission profiles by promoting cleaner and more efficient combustion processes.

A comparable trend in NOx emissions has been observed in previous studies, indicating that while NOx levels generally decrease with the introduction of hydrogen, an excessive increase in hydrogen quantity can lead to higher NOx emissions [42,43,44]. This outcome is linked to the rise in combustion temperature and the associated thermal losses. As a result, the amount of hydrogen added plays a crucial role in determining emission levels, with smaller hydrogen quantities being more favorable for minimizing NOx emissions [45].

3.3. Comparison of Emission Levels with Standard Limits

For small off-road diesel engines, the European Stage V Non-Road Emission Standards [46] are generally applied. According to the Stage V standards, for a diesel engine with a power output < 8 kW, CO and NOx emissions should be less than 8 g/kWh and 7.5 g/kWh, respectively.

Based on the data in Figure 6 and Figure 8, the mean CO and NOx emission values were calculated and then converted from ppm into g/kWh using Pilusa et al.’s [47] formulas:

$$CO\left(\mathrm{g}/\mathrm{k}\mathrm{W}\mathrm{h}\right)=3.591\times {10}^{-3}\times CO\left(\mathrm{p}\mathrm{p}\mathrm{m}\right)$$

(6)

$$NOx\left(\mathrm{g}/\mathrm{k}\mathrm{W}\mathrm{h}\right)=6.636\times {10}^{-3}\times NOx\left(\mathrm{p}\mathrm{p}\mathrm{m}\right),$$

(7)

Table 4. Comparison of the engine emission levels with the standard limits.

	CO (g/kWh)	NOx (g/kWh)
Stage V emission limits	<8	<7.5
Engine emissions_diesel fuel	2.17	0.25
Engine emissions_diesel–hydrogen fuel	1.97	0.20

presents the mean CO and NOx emission values for the engine used in the present study, compared to the Stage V limits. Obviously, the engine CO and NOx emissions are below the Stage V limits for both cases: pure diesel fuel and diesel–hydrogen mixture.

4. Conclusions

This experimental study was conducted on a single-cylinder diesel engine to evaluate the effects of blending diesel fuel with 0.5 L/min of hydrogen, produced using a proton exchange membrane electrolyser. Engine speeds varied between 1000 and 3000 rpm during the tests, which were conducted under half- and full-load conditions.

The hydrogen–diesel blend’s performance was compared with pure diesel fuel. Key parameters analyzed included specific fuel consumption, power, and thermal efficiency, as well as exhaust emissions of carbon monoxide, carbon dioxide, and nitrogen oxides.

Hydrogen addition to diesel fuel demonstrated notable improvements in engine performance. The engine power experienced a slight increase of 3% under both half-load and full-load conditions, while the specific fuel consumption decreased, with a maximum reduction of 10% at full load. The thermal efficiency also improved, reaching a 10% increase at full load.

In terms of emissions, hydrogen addition contributed to an important reduction in pollutants. Carbon monoxide emissions decreased by an average of 13% under half-load conditions, while carbon dioxide emissions showed a notable 17% reduction. Furthermore, nitrogen oxide emissions were also reduced by approximately 17% under half-load conditions.

These findings highlight the potential of hydrogen as a promising alternative fuel for improving engine efficiency and reducing environmental impact, making it a viable step toward cleaner and more sustainable energy solutions in internal combustion engines.