Performance of Hydrotreated Vegetable Oil–Diesel Blends: Ignition and Combustion Insights

1. Introduction

In light of growing pressure to cut emissions of greenhouse gases and air pollutants, the transportation sector—particularly road and maritime transport—is facing the challenge of gradually phasing out conventional fossil fuels. One of the most promising solutions in this context is to implement second-generation renewable fuels, such as hydrotreated vegetable oil (HVO) [1,2,3]. HVO is a synthetic biofuel obtained from fats of plant and animal origin, that undergo a hydrotreatment process, yielding hydrocarbons with molecular structures resembling those of conventional diesel [4,5,6].

Compared to conventional biodiesel, HVO exhibits a number of advantageous operational and environmental properties [7,8]. Most notably, it contains no oxygen in its molecular structure, which results in improved oxidative stability and a higher heating value. In addition, the combustion process of HVO is cleaner—reduced output of particulate matter, unburned hydrocarbons and carbon monoxide are typically observed [9,10].

Importantly, HVO may be obtained from various types of waste-derived feedstocks, including used cooking oil, which significantly lowers its carbon intensity and limits overlap with the food industry [11].

Despite its numerous advantages, the full substitution of diesel by HVO in current transport infrastructure is not always technically or economically feasible. Consequently, growing interest is directed toward the application of diesel–HVO mixtures, which support progressive fuel decarbonization while maintaining compatibility with currently operated diesel engines [12,13].

In the context of implementing HVO and HVO–diesel blends in real-world applications, logistical and regulatory considerations also play a significant role. As a paraffinic fuel, HVO complies with the requirements of [14], which allows it to be used in modern vehicles without modifications to the fuel system—provided that it is approved by the engine manufacturer. In the case of HVO–diesel blends, it becomes essential to understand the interactions between fuel components, as these can influence not only the combustion process but also key physicochemical properties such as viscosity, density, and flash point. Properly selected blend ratios can result in a fuel with optimized ignition characteristics and a more favorable environmental balance than conventional diesel, without compromising performance. This approach represents a compromise between the goal of reducing emissions and the need to maintain continuity in existing fuel and engine infrastructure.

A critical parameter of fuel that requires evaluation—especially in the context of diesel engine operation with blended fuels—is its autoignition behavior. A shorter ignition delay period helps limit the volume of fuel that builds up in the combustion chamber during this stage, consequently lowering the pressure rise rate after ignition. As a result, mechanical stress on components of the crank-piston assembly is lowered, contributing positively to engine durability. Typically, this contributes to lowering maximum combustion temperatures, thereby decreasing nitrogen oxides (NOₓ) emissions. A more detailed discussion of the influence of ignition delay on diesel engine operating processes can be found in [15,16,17,18].

In light of the above, a thorough examination of fuel autoignition behavior becomes particularly important from both operational and environmental perspectives. Research in this area can contribute to a deeper understanding of combustion processes in engines fueled with both conventional and renewable fuels, with particular emphasis on HVO, which is currently the focus of numerous studies.

Fuel autoignition behavior is most often evaluated using the cetane number (CN) or its derived form (DCN), as defined by applicable testing protocols [19, 20]. However, measuring CN using a test engine, as described in [21, 22], is complex and costly. As a result, CN data for fuels that are less common than diesel or biodiesel—such as HVO—are less readily available. It is worth noting, however, that HVO does not exhibit issues typically associated with certain alternative fuels, such as reduced lubricity or the risk of damage to injection systems. Given its physicochemical profile, which closely resembles that of diesel, HVO can be safely used within standard testing procedures. The role of CN in shaping engine performance characteristics has been thoroughly addressed in the literature [23,24,25,26,27], highlighting the importance of this parameter in evaluating how a fuel’s autoignition behavior affects engine operation in diesel systems.

Instead of a test engine, the constant-volume combustion chamber (CVCC) approach is now more frequently applied to evaluate the autoignition properties of fuels under controlled laboratory conditions. In this context, the concept of DCN is applied. Studies focused on the DCN utilize correlations between ignition delay time and the cetane index assigned to the fuel [25,28,29,30]. Standardized procedures for determining DCN are described in [31,32,33,34,35]. In some standards, for example [32, 33], DCN is determined solely from the experimentally obtained ignition delay. In contrast, methods outlined in [34,35] also account for what is known as combustion delay, which is discussed in more detail in [36,37]. A distinctive feature of instruments used for standardized DCN determination is their ability to perform measurements under strictly controlled and repeatable conditions. This enables precise analysis of how various fuel delivery variables—including injection pressure [36, 38] and initial chamber temperature [37,38]—affect ignition delay for different types of fuels.

Numerous studies have investigated how HVO affects performance indicators in compression-ignition engines, including [39,40,41,42,43,44,45,46,47,48,49,50]. The cited works primarily focus on powertrain efficiency and the generation of toxic exhaust compounds. These publications cover not only engine operation fueled with neat HVO but also its use in combination with other fuels, including blends with diesel. However, relatively few of the existing studies related to HVO use in compression ignition engines specifically address autoignition-related behavior of HVO and its mixtures with conventional diesel.

Some data on HVO ignition delay can be found in the work of Bjørgen et al. [40]. The authors investigated quantitative combustion-related indicators and soot behavior in spray combustion zones of three fuel categories: HVO, biodiesel (rapeseed methyl ester), and diesel. Experiments took place in a chamber with optical access under autoignition conditions, resembling thermodynamic states typically observed in engine systems. Based on these results presented, HVO exhibited the shortest ignition delay among the tested fuels over the entire range of gas-phase temperatures into which the fuel was injected (approximately 825 to 975 K). This was attributed to the highest cetane number observed for HVO.

Millo et al. [39] conducted a comparative analysis involving diesel and HVO to examine how injection-related parameters affect spray behavior along with the development of combustion. Engine tests were carried out at an engine rotational speed of 2000 rpm and three different levels of brake mean effective pressure: 8, 6, and 2 bar. For each operating point, the authors determined the ignition delay values under three different exhaust gas recirculation rates (19%, 22%, and 25%), three start of injection (SOI) timings (6, 8, and 10 CA deg aTDC), and three injection pressure (pₐᵢₙⱼ) values (640, 840, and 1040 bar). The parameter values given in parentheses refer to the engine operating point corresponding to the highest load condition. The results showed that, regardless of engine operating point and parameter variation, HVO consistently exhibited shorter ignition delays compared to diesel. Moreover, the measurements indicated that injection pressure and SOI timing had the most significant impact on ignition delay at medium as well as high load conditions. Under low-load operation, SOI timing also had a noticeable influence over ignition delay.

Alkhayat et al. [51] investigated the ignition delay characteristics of HVO and its blends with ULSD (ultra-low sulfur diesel) using an ignition quality measurement device. In addition to the base fuels (HVO and ULSD), the study also included blends with HVO content of 10 %, 20 %, 30 %, 40 %, and 50 % by volume (v/v). The tests were conducted according to the procedure described in [31], and the results indicated an increase in DCN with rising HVO content within the blend. According to the authors, a clear drop was noted in ignition delay with increasing initial chamber temperature and a higher volumetric share of HVO. Furthermore, the study [51] highlighted changes in both the chemical and physical phases of ignition delay. A key factor contributing to the decrease in the physical ignition delay phase with increasing HVO content was identified as the significantly lower T90 distillation temperature of HVO compared to ULSD. A shorter chemical ignition delay observed for HVO was linked to the presence of long-chain paraffins and the lack of aromatic compounds.

Driven by the growing demand for renewable fuels used in compression ignition engines—including HVO—and the limited availability of research data in this area, this work is aimed at evaluating key ignition-related characteristics of diesel–HVO blends. The volumetric share of HVO in the blends ranged from 25 % to 75 % (v/v). Tests were also performed on the base fuels: conventional diesel designated as B7, and neat HVO. B7 refers to a standard diesel fuel, as defined by current European directives, which allows a maximum volumetric content of 7 % fatty acid methyl esters (FAME).

The novelty of this study is the systematic and extended characterization of the autoignition behavior of HVO–diesel blends across a wide blending range, conducted under controlled constant-volume chamber conditions at two distinct initial chamber temperatures. Unlike many earlier studies, which focused almost exclusively on ignition delay (ID) or derived cetane number (DCN), the present work integrates a broader set of indicators, encompassing ignition delay (ID), combustion delay (CD), pressure-rise-based parameters—average pressure rise rate (APRR), maximum pressure rise rate (MPRR), and maximum pressure rise (MPR)—as well as the apparent heat release rate (aHRR). This combined evaluation provides a more complete and reliable picture of how blending ratio and initial temperature affect not only the onset of ignition, but also subsequent combustion phasing and in-cylinder pressure development. Importantly, the inclusion of APRR, MPRR, and MPR offers new insights into the mechanical stresses imposed on the crank–piston assembly, which are directly linked to engine durability. For completeness and comparability, the derived cetane number (DCN) of each fuel was determined according to the standardized procedure in [34,35]. The results thus fill a gap in the limited body of data on HVO–diesel mixtures and establish a robust framework for assessing the autoignition quality of renewable fuel blends in compression ignition engines.

The findings of this investigation may offer a meaningful point of reference for further research on optimizing combustion processes in engines fueled with renewable fuel components such as HVO. Since the authors employed the ignition delay determination method described in [34,35], the findings can also provide a valuable complement to the results reported in [51], where the CVCC method was likewise applied, although using an instrument compliant with the standard specified in [31].

2. Methodological Framework

2.1. Composition and Analysis of Tested Fuels

Autoignition property tests were performed using five distinct fuel samples. Among them was a standard, commercially available diesel, which, as noted earlier, is labeled as B7 at gas stations in Europe. This designation indicates that the fuel may contain up to 7% (v/v) FAME. The same designation is used throughout this paper. The second reference fuel was HVO. For both base fuels, the majority of key chemical and physical properties were established by the authors. The results of these measurements are summarized in Table 1, which also includes information on the test equipment and measurement methods used. Distillation profiles for the B7 and HVO fuels evaluated in the present work are shown in Figure 1.

Three additional fuel formulations were prepared by mixing HVO with B7 in volume-based ratios of 25%, 50%, and 75% HVO. Owing to their composition—especially the exclusive presence of paraffinic hydrocarbons in HVO—these blends show very high miscibility. Further details concerning the characteristics of HVO–diesel mixtures at different blending levels are provided in [42]. The samples were prepared under controlled conditions at 21 °C ± 1 °C for both HVO and B7. All samples were kept in hermetically sealed glass containers at the same temperature. The specific sample designations, along with their corresponding volumetric HVO content, are summarized in Table 2.

2.2. Description of the Test Stand

The experimental tests were carried out using a commercially available CID 510 (Petroleum Analyzer Company; Houston, TX, USA) device, which incorporates a constant volume combustion chamber. The primary purpose of this apparatus is to determine the DCN, following the procedures specified in standards [34,35]. Similar equipment has also been used in previous studies [36,52,53,54,55,56,57,58,59]. A simplified layout of the experimental setup is presented in Figure 2, while detailed construction information is provided in [34,35]. Key components of the system—excluding the automation and control modules—include a heated constant volume combustion chamber (internal volume: 473 cm³), a Common Rail fuel injector, a hydraulic system with a pressure intensifier, a fuel supply system, air and exhaust gas management systems, and a cooling circuit for the injector linked to an external thermostatic unit. The combustion chamber is encased in a thermally insulated jacket that ensures a stable temperature of the synthetic air. Temperature sensors are embedded in the chamber walls. Because the chamber remains continuously heated, only a short interval is needed between injections for the system to reach steady-state conditions. The initial temperature of the synthetic air inside the chamber (T_a) is assumed to be equal to the wall temperature (T_ch), i.e., T_a = T_ch. Details on the composition and characteristics of the synthetic air used in this study are provided in Table 3.. A high-frequency pressure transducer is installed at the base of the chamber. In the low-pressure segment of the fuel circuit—extending from the sample tank to the pressure intensifier—nitrogen gas is used to circulate the fuel at approximately 0.6 MPa. A PTFE filter is integrated into this section to remove any impurities from the fuel prior to injection.

The fuel injection pressure, p_inj, was generated by a hydraulic system incorporating a pressure intensifier. A Bosch injector (No. 0445110181) featuring a six-hole nozzle—typical of modern direct-injection engines—was installed at the upper part of the combustion chamber. The chamber’s initial pressure was regulated using a static pressure sensor and a relief valve. A filtration and drainage system was also employed to protect components from contaminants. Prior to every measurement, the system was inspected for tightness, ensuring that any pressure drop did not exceed 0.75 kPa/s. The calibration process used a certified reference fuel comprising a mass-proportional blend of 40% n-hexadecane (minimum purity 99.0%) and 60% isocetane (minimum purity 98.0%). Details regarding the calibration procedure are provided in Table 4. Proper calibration of the testing apparatus is essential for standardized DCN measurement accuracy.

2.3. Research Methodology and Data Acquisition

Each fuel variant underwent a total of 11 combustion cycles—5 initial conditioning cycles followed by 6 main measurement cycles. Throughout each cycle, the in-chamber pressure was captured using a dynamic pressure transducer operating at a sampling rate of 25 kHz. For analytical purposes, only pressure traces from the 6 main cycles were averaged and analyzed.

During these 6 cycles, the following variables were also recorded: the initial pressure in the combustion chamber, p₀, wall temperature, T_ch, fuel injection pressure, p_inj, and the MPR relative to p₀. All pressure values are given as gauge pressures, i.e., referenced to ambient pressure. During the tests, atmospheric pressure was stabilized at 98.7 kPa with fluctuations not exceeding ±0.2 kPa. The temperature of the injector cooling medium, T_co was also monitored and maintained at T_co = 50 °C consistent with calibration settings (see Table 4). The method used for determining ID, CD), and MPR is shown in Figure 3, following the standardized protocol described in [34,35].

Based on the pressure profiles obtained from measurements inside the combustion chamber, the APRR was determined using the formula:

$$APRR={\displaystyle \frac{p_{max}-p_{ID}}{t_{p\left(max\right)}-ID}}[\mathrm{M}\mathrm{P}\mathrm{a}/\mathrm{m}\mathrm{s}]$$

(1)

where:

pₘₐₓ–represents the peak pressure value measured during combustion, in MPa,

p_ID–pressure value recorded at the conclusion of the ignition delay phase, in MPa,

tₚ₍ₘₐₓ₎–denotes the time corresponding to the attainment of pₘₐₓ, in ms,

ID–is the ignition delay duration, in ms.

The instantaneous pressure inside the combustion chamber was determined by numerically differentiating the pressure traces using a finite difference method. This approach enabled the identification and analysis of the MPRR, as illustrated in Figure 4.

To examine how the initial temperature within the combustion chamber affects ignition behavior, measurements were carried out for all fuel samples listed in Table 2 at two synthetic air temperatures: Tₐ = 550 °C and Tₐ = 650 °C. These temperatures are representative of end-of-compression conditions in modern diesel engine cycles. Throughout the tests, parameters such as fuel injection pressure, p_inj, initial chamber pressure, p₀, injector energizing time, t_inj, and injector coolant temperature, T_co, were carefully maintained along with a constant composition of synthetic air (see Table 3), in accordance with calibration conditions (see Table 4).

The apparent heat release rate, aHRR, was estimated using a simplified formulation derived from the first law of thermodynamics:

$${\displaystyle \frac{dQ}{dt}}=\left({\displaystyle \frac{\gamma }{\gamma -1}}\right)p{\displaystyle \frac{dV}{dt}}+\left({\displaystyle \frac{1}{\gamma -1}}\right)V{\displaystyle \frac{dp}{dt}}$$

(2)

where:

${\displaystyle \frac{dQ}{dt}}$ –denotes the aHRR, in MW,

V–is the volume of the combustion chamber, which is equal to 473 cm³ (i.e., 0.000473 m³),

p–indicates the pressure inside the chamber, in Pa,

t–represents the time at which the pressure is recorded, in ms,

γ–refers to the ratio of specific heats at constant pressure and volume for the gas mixture (taken as 1.32).

In this approach, it was assumed that pressure changes result exclusively from combustion, while heat transfer between the gas and chamber walls was neglected. Moreover, the specific heat ratio γ, was treated as time-invariant. Under these conditions, the formula for aHRR is simplified. Given that the tests were carried out in a constant volume combustion chamber, the expression reduces to:

$${\displaystyle \frac{dQ}{dt}}=\left({\displaystyle \frac{1}{\gamma -1}}\right)V{\displaystyle \frac{dp}{dt}}$$

(3)

For the parameters ID, CD, APRR, MPRR, MPR, and DCN, the standard error of the mean (SEM) was calculated based on six combustion cycles. The uncertainty values were indicated by preceding the corresponding parameter symbol with Φ. These measurement uncertainties are summarized in Table 5 and Table 6 (for T_a = 550 °C and 650 °C respectively). Both tables are included at the end of Section 3. The DCN results were obtained under calibration conditions (see Table 4), and in this case, the SEM values for each fuel sample are displayed directly on the DCN graph.

3. Experimental Results and Analysis

Figure 5 illustrates the pressure traces recorded in the combustion chamber for the tested fuels at two initial temperatures: T_a = 550 °C and 650 °C. Based on these profiles, the following parameters were evaluated in subsequent sections: ID, CD, MPR, APRR, MPRR, aHRR, and DCN. In all tests, the fuel injection pressure was maintained at p_inj = 100 MPa. The plots clearly demonstrate how fuel composition influences pressure rise behavior.

At T_a = 550 °C, it was observed that for fuels with a higher HVO content, the combustion process proceeded more rapidly and intensely, which was reflected in the steep slope of the pressure curves. In contrast, for the B7 sample, the pressure rise was more gradual and extended over time, indicating a less intense combustion process. At T_a = 650 °C, variation in combustion intensity among the fuel variants was less pronounced, while all pressure curves exhibited a sharp and consistent rate of increase. This indicated a strong influence of the initial temperature of the surrounding air at the moment of fuel injection on the intensity of pressure rise—higher temperatures promoted faster and more vigorous combustion, regardless of the sample composition.

Figure 6 illustrates the relationship between the HVO volumetric proportion in the diesel blend and the ID, evaluated at two different initial chamber temperatures. The data indicate that, under the applied experimental conditions, a higher HVO fraction in the mixture consistently resulted in a shorter ID at both examined values of T_a. These results align with earlier observations reported in [39,40,51], which explored the autoignition characteristics of HVO and its mixtures with standard diesel. Figure 6 also reveals a notable effect of the initial temperature in the chamber on the autoignition response of the samples. A 100 °C increase in T_a led to an approximate 50% decrease in ID. Moreover, the results confirm that no signs of negative temperature coefficient behavior were observed throughout the studied temperature interval—in other words, ID continued to decrease with increasing T_a [60]. The identified ID patterns—especially the shorter values observed at elevated T_a—were attributed to the distinct chemical characteristics of the fuel, which significantly affected the ignition delay’s chemical initiation stage [51,61,62].

Paraffinic hydrocarbons, which constitute the primary hydrocarbon structure in HVO, exhibit very good ignition properties due to their saturated molecular structure, which results in shorter ID and a more stable combustion process. Olefins, which contain double bonds, are characterized by moderate autoignition reactivity and typically exhibit medium ID values. Aromatic compounds, which are more abundant in B7 fuel, are characterized by a low cetane number and tend to exhibit the longest ignition delay due to their stable ring structure. The presence of aromatics in the fuel negatively affected the autoignition process; however, under elevated temperature conditions, the stability of the aromatic ring structure was reduced, which in turn explains the clearly shortened ID observed for B7 and its HVO blends at the higher Tₐ. It should also be noted that at higher temperatures, the presence of aromatics in the fuel—decreasing with increasing HVO content—became less significant, as the combustion process was increasingly dominated by the enhanced oxidation rates of H, OH, and HO₂ radicals. This explains the relatively small differences in ID values among the tested samples at elevated Tₐ. In summary, the predominance of paraffinic hydrocarbons and the virtual absence of aromatic species in HVO are key contributors to the enhanced ignition behavior observed for HVO and its mixtures with B7, relative to neat B7. Further insight into the ignition performance of different hydrocarbon types is provided in [51,63,64]. Additional interpretation is supported by modeling data discussed in [65,66].

As illustrated in Figure 7, the variation in the CD parameter exhibited a trend comparable to that observed for ID. The influence of HVO concentration on CD was more pronounced at the lower temperature of T_a = 550 °C, with a difference of approximately 2.5 ms between B7 and HVO. At the higher temperature of T_a = 650 °C, this difference decreased to around 0.7 ms. As defined in Figure 3, the CD value was determined not only by the maximum rate of pressure rise after autoignition, but also by the progression of pressure from the start of the recording to the moment when the maximum pressure was achieved.

The CD interval, as defined in the schematic presented in Figure 3, acts as a supplementary indicator of a fuel’s tendency to autoignite and, along with ID, is employed to determine the DCN following the procedure outlined in [34,35]. The ID value corresponds to the initial phase of autoignition, commonly referred to as the cool-flame stage. As noted by Lapuerta et al. [38,59], this stage is particularly relevant in studies utilizing constant volume combustion chambers, such as the one used in this study. The formation of cool flames is mainly linked to the presence of linear and cyclic paraffins in the fuel formulation [59]. The second, main phase of autoignition begins once the surrounding temperature of the fuel droplet becomes high enough to initiate chain-branching reactions. Lapuerta et al. [59] characterized this phase based on CD and pressure evolution, noting that the CD is generally slightly longer than the true duration of the primary autoignition phase.

As shown in Figure 8, under constant volume combustion conditions, how the HVO share in the fuel mixture affects the maximum pressure rise (MPR) was observed only at the lower of the two analyzed initial temperatures, T_a. In this case, the increase in HVO content—characterized by higher chemical reactivity due to the presence of only paraffinic hydrocarbons—resulted in faster ignition, as indicated by the measured ID values. As a consequence of greater fuel accumulation during the premixed combustion phase, a noticeable rise in MPR occurred, particularly in the case of B7-HVO-75 and pure HVO.

For the B7 fuel and the B7-HVO-25 and B7-HVO-50 blends, the ignition and combustion reactions proceeded more slowly due to the higher proportion of hydrocarbons with ring structures in these samples. As a result, lower MPR values were recorded. At the higher analyzed temperature, Tₐ, the chemical part of the ID was significantly shortened, and the physical part was also reduced, as the elevated temperature favored faster fuel droplet evaporation [67]. Under these conditions, the rate of chemical reactions became sufficiently high that differences in fuel reactivity no longer played a significant role. Consequently, regardless of blend composition, the MPR value remained nearly constant, indicating that a threshold had been reached at which an increase in the proportion of chain-structured hydrocarbons no longer affected the rate of fuel oxidation reactions.

Numerous studies—for example, [68,69,70,71]—indicate that reducing ignition delay, for example through pilot fuel delivery or decreasing the timing of fuel injection, lowers the amount of fuel accumulated prior to ignition. As a result, premixed combustion occurs with a lower rate of pressure increase, helping to mitigate combustion noise and NO_X emissions. For this reason, an evaluation of the rate of pressure increase within the combustion chamber was carried out for the tested fuel blends at both T_a levels. The key parameter in this evaluation was the APRR, calculated using Equation 1, based on selected points from the recorded pressure traces (Figure 5). Subsequently, the MPRR was determined. This methodology has also been described and applied in the authors’ previous studies, e.g., [37,58].

As illustrated in Figure 9, the APRR value for blends of HVO and diesel was influenced by the initial chamber temperature, Ta. At the lower T_a, a distinct rise in APRR occurred as the HVO share increased, despite the concurrent reduction in ID (Figure 6).

At the higher temperature, T_a = 650 °C, despite the continued decrease in ID as the HVO fraction in the fuel mixture increased (although the decrease appeared clearly smaller in this case), APRR exhibited the opposite trend—its value decreased as the HVO content increased. At the same time, as shown in Figure 9, a similar pattern was observed for MPRR, which increased with HVO content at the lower T_a, but decreased at the higher T_a. Some confirmation of the APRR results at the higher T_a can be found in the findings presented in [62], where pressure traces recorded in an engine cylinder showed that, for various injection strategies and EGR rates, HVO produced a smoother pressure rise in the phase between the SOI and the peak cylinder pressure.

Figure 10. MPRR for B7, B7-HVO blends, and HVO at 550 °C and 650 °C.

Figure 10. MPRR for B7, B7-HVO blends, and HVO at 550 °C and 650 °C.

As indicated by the pressure traces (Figure 5) and Equation (1), at the lower T_a, given the similar maximum pressure values inside the chamber across the tested samples, the interval between the point of peak pressure and the end of the ID period decreased as the HVO share in the fuel increased. This explains the observed drop in APRR and MPRR readings with rising HVO share at the lower T_a. At the higher T_a, this time difference also decreased slightly with increasing HVO content, which similarly resulted in a decline in the APRR and MPRR parameters.

Under constant volume combustion conditions, the pressure traces are directly reflected in the corresponding aHRR profiles, which were determined using the recorded pressure signal. The aHRR results for the tested fuel samples—presented as a supplement to the data describing autoignition behavior and combustion characteristics—are shown in Figure 11.

As presented in Figure 11, at the lower of the two evaluated temperatures, T_a, despite the notable decrease in ID as the proportion of HVO in the diesel mixture increased, the heat release intensity increased, with the highest peak value recorded for neat HVO. This indicates a dominant effect of the higher reactivity of fuels associated with a greater proportion of paraffinic hydrocarbons. In this case, the reduced quantity of fuel involved during the early stage of premixed combustion, due to shorter ID, did not lead to a decrease in heat release intensity—as is typically observed for conventional fuels, which contain approximately 20–30% aromatic hydrocarbons by mass.

At the higher initial temperature, T_a, the trend in aHRR reversed: the highest heat release intensity was recorded for B7, and it progressively decreased as the proportion of HVO in the diesel blend increased, reaching its minimum value for neat HVO. This effect was associated with the reduced fuel mass participating in the early combustion phase, resulting from the shortened ID values. As previously mentioned, under higher initial chamber temperatures, both the chemical and physical aspects of ignition delay were shortened. Although the combustion conditions within a constant-volume chamber differ significantly from those inside an operating diesel engine, the data presented in Figure 11 for T_a = 650 °C are, to some extent, consistent with engine data reported in [62], where HVO exhibited a lower rate of heat release relative to diesel.

Based on the recorded values of ID and CD, the average DCN values were calculated in accordance with the procedure described in [34,35], and are presented in Figure 12. The DCN measured via the CVCC technique shows equivalence to the CN values obtained from engine-based procedures described in [21,22]. DCN analysis is particularly relevant in cases where the objective involves evaluating the ignition quality of a tested fuel sample against standardized criteria or with the specifications outlined in the Worldwide Fuel Charter (WWFC) [72].

As shown in Figure 12, an increase in the proportion of HVO blended with B7 led to a linear rise in DCN. For the B7 sample, a DCN value of 50 was recorded, which is slightly below the minimum acceptable limit specified in [19] (min. 51). However, in terms of DCN, the B7 sample met the requirements for all diesel fuel categories defined in [20] (min. 40 and 30), as well as for Category 1 in the WWFC [72] (min. 48). Starting from the B7-HVO-25 blend, all tested samples comfortably met most of the requirements set by the referenced standards and the WWFC. Only the B7-HVO-25 sample did not meet the threshold for Categories 4 and 5 as defined in the WWFC (min. 55). It is important to recognize that DCN ratings would vary if different base fuels (i.e., B7 and HVO) were used. A similar linear increase in DCN with rising HVO content in diesel fuel was also reported by the authors of [51], who used the method described in [31] to determine DCN.

The experimental data presented in this section are complemented by an analysis of measurement uncertainty. The standard error of the mean (SEM) was used as the measure of uncertainty and was calculated for each data point based on six repetitions. Due to their small magnitude, the SEM values were not displayed as error bars in the graphs. Instead, they are listed in Table 5 and Table 6, separately for each of the two analyzed temperatures. As noted in Section 2.3, SEM values for the measured parameters are indicated by the symbol Φ.

Table 5. Set and recorded fuel injection parameters and SEM values for the measured parameters at T_a = 550 °C.

	ΦID [ms]	ΦCD [ms]	ΦAPRR [MPa/ms]	ΦMPRR [MPa/ms]	ΦMPR [MPa]	Tch (Ta) [°C]	p0 [MPa]	pinj [MPa]	tinj [ms]
B7	0.0560	0.0689	0.012	0.118	0.003	550.1	2.00	99.4	2.5
B7-HVO-25	0.0383	0.0582	0.023	0.107	0.004	549.8	1.99	99.2	2.5
B7-HVO-50	0.0354	0.0484	0.043	0.269	0.001	550.3	2.00	98.9	2.5
B7-HVO-75	0.0207	0.0305	0.048	0.092	0.180	540.2	2.00	99.5	2.5
HVO	0.0328	0.0304	0.062	0.144	0.021	550.2	1.99	99.3	2.5

Table 5. Set and recorded fuel injection parameters and SEM values for the measured parameters at T_a = 550 °C.

	ΦID [ms]	ΦCD [ms]	ΦAPRR [MPa/ms]	ΦMPRR [MPa/ms]	ΦMPR [MPa]	Tch (Ta) [°C]	p0 [MPa]	pinj [MPa]	tinj [ms]
B7	0.0560	0.0689	0.012	0.118	0.003	550.1	2.00	99.4	2.5
B7-HVO-25	0.0383	0.0582	0.023	0.107	0.004	549.8	1.99	99.2	2.5
B7-HVO-50	0.0354	0.0484	0.043	0.269	0.001	550.3	2.00	98.9	2.5
B7-HVO-75	0.0207	0.0305	0.048	0.092	0.180	540.2	2.00	99.5	2.5
HVO	0.0328	0.0304	0.062	0.144	0.021	550.2	1.99	99.3	2.5

Table 6. Set and recorded fuel injection parameters and SEM values for the measured parameters at T_a = 650 °C.

	ΦID [ms]	ΦCD [ms]	ΦAPRR [MPa/ms]	ΦMPRR [MPa/ms]	ΦMPR [MPa]	Tch (Ta) [ °C]	p0 [MPa]	pinj [MPa]	tinj [ms]
B7	0.0090	0.0109	0.012	0.132	0.005	649.6	1.99	99.6	2.5
B7-HVO-25	0.0129	0.0172	0.013	0.149	0.004	649.6	1.99	100.0	2.5
B7-HVO-50	0.0118	0.0076	0.016	0.126	0.010	649.6	1.99	99.3	2.5
B7-HVO-75	0.0182	0.0149	0.012	0.208	0.017	649.9	2.00	99.3	2.5
HVO	0.0110	0.0184	0.011	0.279	0.002	649.6	2.00	99.3	2.5

Table 6. Set and recorded fuel injection parameters and SEM values for the measured parameters at T_a = 650 °C.

	ΦID [ms]	ΦCD [ms]	ΦAPRR [MPa/ms]	ΦMPRR [MPa/ms]	ΦMPR [MPa]	Tch (Ta) [ °C]	p0 [MPa]	pinj [MPa]	tinj [ms]
B7	0.0090	0.0109	0.012	0.132	0.005	649.6	1.99	99.6	2.5
B7-HVO-25	0.0129	0.0172	0.013	0.149	0.004	649.6	1.99	100.0	2.5
B7-HVO-50	0.0118	0.0076	0.016	0.126	0.010	649.6	1.99	99.3	2.5
B7-HVO-75	0.0182	0.0149	0.012	0.208	0.017	649.9	2.00	99.3	2.5
HVO	0.0110	0.0184	0.011	0.279	0.002	649.6	2.00	99.3	2.5

Table 5 and Table 6 also include the average values of T_ch, p₀, and p_inj, which are set parameters in the adopted measurement methodology. With the exception of T_ch, the values of these parameters corresponded to the calibration settings defined in Table 4. The SEM values for DCN are presented in Figure 12, as they refer to the standardized measurement performed at Tₐ = T_ch = 588 °C.

4. Conclusions

The conducted investigations of autoignition characteristics were performed using a constant-volume combustion chamber. Measurements involved a representative diesel fuel commercially available in Europe under the name B7 (indicating the maximum volumetric content of FAME), for HVO, and for blends of these fuels with HVO volumetric contents of 25%, 50%, and 75%. To enable a more comprehensive assessment of autoignition behavior—particularly for HVO and its blends with diesel—experiments were performed at two different initial chamber air temperatures: 550 °C and 650 °C. Several measurable parameters were used to characterize autoignition properties, including the DCN. The experimental setup used in this study included a high-pressure fuel injection system, representative of those used in current diesel engine injection systems. On the basis of these outcomes, the following insights and observations were established:

• An increase in the proportion of HVO within the diesel mixture resulted in a clear reduction in ID and CD, although these changes were significantly smaller at the higher initial temperature of the intake air into which the fuel was delivered.
• Increasing HVO share in the diesel mixture led to a rise in MPR at lower initial combustion chamber air temperature, with the most pronounced changes observed for the 75% HVO blend and neat HVO. At the higher initial chamber temperature, the HVO content in the blend had virtually no effect on the MPR value.
• The APRR parameter increased with rising HVO content at the lower initial combustion chamber temperature. However, at the higher temperature, APRR values were significantly lower and decreased as the HVO content increased. Under engine-like conditions, it may therefore be beneficial to inject fuel into a hotter combustion environment when operating on neat HVO or diesel–HVO fuel mixtures.
• For the MPRR parameter, the trend was consistent with that observed for APRR; however, the differences in absolute values were smaller when comparing the two initial combustion chamber temperatures.
• At the lower of the two analyzed initial combustion chamber temperatures, despite a clear reduction in ID, as the proportion of HVO in the diesel mixture increased, the heat release intensity rose, with the highest peak value observed for neat HVO.
• At the higher initial combustion chamber temperature, the trend in aHRR was reversed: the highest heat release intensity was recorded for neat diesel fuel and decreased as HVO concentration in the blend rose, reaching its lowest value for neat HVO.
• A higher HVO share in the diesel mixture led to a linear increase of DCN.

The present study expands on the scarce data currently available regarding the quantitative evaluation of autoignition characteristics for neat HVO and its mixtures with regular diesel, as well as their comparison to those of a widely used commercial diesel. Applying a constant-volume combustion chamber and maintaining strict control over initial conditions at the moment of fuel injection enabled an accurate evaluation of autoignition performance in relation to only two key parameters: the initial combustion chamber temperature and the volumetric ratio of HVO to diesel. The results obtained may be useful for optimizing control strategies in fuel injection systems of diesel engines powered by HVO or HVO-diesel blends.