The Future of Engine Knock and Fuel Octane Numbers in the Era of Biofuels and Vehicle Electrification

Abstract

Engine knock remains a critical limitation in spark-ignition engine design. Future hybrid powertrains employ downsized engines operating on Atkinson cycles, creating different knock conditions compared to modern naturally aspirated or turbocharged engines. At the same time, petroleum-based gasoline is increasingly being replaced by biofuels and electrofuels. This study evaluates knock behavior in projected hybrid engine architectures and examines the chemical composition of emerging fuel blends. The analysis shows that hybrid engines benefit from fuels with lower sensitivity, defined as the difference between the Research and Motor Octane Numbers. This is because the higher end-gas temperatures associated with the Atkinson cycle shift the value of K, which is an interpolation factor used to capture the relationship between fuel sensitivity and anti-knock performance. In conventional engines, K is negative, favoring fuels with higher sensitivity. In hybrid engines, the increased engine temperatures result in K becoming positive, favoring low-sensitivity fuels. Using low-sensitivity fuels allows hybrid engines to operate with higher geometric compression ratios and advanced thermodynamic cycles while reducing knock constraints. Biofuels and electrofuels can meet these requirements by producing paraffinic and naphthenic hydrocarbons with high octane quality and low sensitivity. These findings emphasize the need to align renewable fuel development with hybrid engine requirements to improve thermal efficiency, reduce emissions, and reduce reliance on energy-intensive refinery processes for octane enhancement.

1. Introduction

Since the invention of the automobile, engine knock has been a critical design constraint for spark-ignition engines. Despite significant advancements in engine technology, spark-ignition engines are still knock-limited, where the peak pressures are limited to avoid inducing the chemical reactions that result in knock [1]. Since knock is inherently a chemical reaction, certain fuels are more prone to knock than others. To capture this propensity of a fuel to knock, the Octane Number tests were developed in the 1920s.

Today’s gasoline engines operate under significantly different conditions than the engine conditions in the 1920s, which were used for developing the Octane Number tests. Higher compression ratios, turbocharging, engine downsizing strategies, and aggressive combustion phasing have made engines much more demanding. The vehicle landscape itself is also undergoing further transformation. The increasing adoption of hybrid powertrains alters engine load profiles, reducing the time an engine operates in knock-prone regions and reshaping the relevance of conventional knock metrics. Furthermore, the integration of biofuels, many of which exhibit high sensitivity to temperature and pressure variations, adds another layer of complexity to how knock resistance must be understood and managed.

Given these changes, there is a need to revisit how fuels are formulated and how their knock resistance is characterized. This study systematically examines emerging vehicle and engine technologies, defines the knock-limited operational envelope of future powertrains, and analyzes how future gasoline formulations perform under these conditions. Based on these findings, recommendations are proposed for optimizing engine and fuel formulations to better align with future needs.

2. Background

2.1. Knock and Fuel Octane Numbers

Knock occurs when a portion of the air–fuel mixture in the combustion chamber autoignites due to elevated pressure and temperature [2]. Under normal operation, combustion begins at the spark plug, and the flame front propagates smoothly across the chamber combusting all of the fuel–air mixture, until it reaches the cylinder walls, where it is quenched. As the flame advances, it compresses and heats the remaining unburned mixture ahead of the flame front, known as the end gas. If this end gas reaches a high enough temperature and pressure, it can spontaneously ignite. The sudden release of energy generates sharp pressure waves that resonate within the cylinder, producing the characteristic knocking sound.

The likelihood of knock depends on both the engine operating conditions and the chemical composition of the fuel. With respect to engine operating conditions, knock depends on factors such as compression ratio, engine speed, air to fuel ratio, ambient temperature, and load. High compression engines generate greater pressure and temperature, which accelerate the chemical reactions that lead to autoignition. At lower engine speeds, the end gas remains under high pressure and temperature for a longer period, which increases the chance of autoignition. Prolonged knock can damage the piston crown, cylinder walls, and piston rings due to extreme localized pressures and temperatures [3].

Modern vehicles use knock sensors to detect vibrations in the engine block that indicate low levels of knock and adjust the spark timing to reduce the peak cylinder pressures [4]. Although this helps prevent damage, it forces the engine to operate at a lower performance point and at a lower efficiency. As a result, compression ratios remain limited by knock, which constrains overall engine efficiency.

A fuel’s resistance to autoignition is determined by its chemical composition and is quantified by the Octane Index (OI) [5]. The OI of a fuel compares the knocking tendency of a fuel with that of a Primary Reference Fuel (PRF) composed of iso-octane and n-heptane. Iso-octane resists autoignition, while n heptane ignites easily. The OI of a fuel corresponds to the volume percentage of iso octane in the PRF blend that produces knocking of equal intensity under the same conditions. Higher OI values indicate a lower tendency for knock.

The OI varies with engine operating conditions such as temperature, pressure, and speed [6]. A fuel may behave like PRF96 (96% iso-octane, 4% n-heptane) under one set of conditions but resemble PRF87 (87% iso-octane, 13% n-heptane) under another. To provide a standard basis for comparison, the OI is measured under two controlled tests using a Cooperative Fuel Research (CFR) engine with a variable compression ratio. The Research Octane Number (RON) and Motor Octane Number (MON) were established in 1928 and 1932, respectively, and are defined in ASTM standards D2699 and D2700 [7,8]. In these tests, the engine runs on the sample fuel while the compression ratio is increased until knock is detected by a bouncing pin mechanism. The RON procedure uses an engine speed of 600 rpm and an intake air temperature of 52 °C, while the MON test uses 900 rpm and an intake air temperature of 149 °C.

2.2. Fuel Sensitivity and K Values

Since a fuel’s OI changes with the engine operating conditions, most fuels exhibit a difference between their RON and MON values. This difference is known as fuel sensitivity (S) and is calculated as:

$$S=RON-MON.$$

(1)

Sensitivity varies significantly based on the fuel chemical composition. By definition, PRFs have zero sensitivity. Meanwhile, ethanol exhibits a sensitivity around 18, and some leaded fuels exceed 30 [9]. Gasoline at the fuel pump typically has a sensitivity between 8 and 12 [10]. Most countries have established regulations for the minimum RON and MON, along with the permissible fuel sensitivity values that retail fuels must meet.

The sensitivity of fuels generally stems from a property of PRFs referred to as the “negative temperature coefficient”. In a specific range of temperatures, autoignition chemistry slows down for paraffinic fuels, making them less likely to autoignite. Non-paraffinic fuels generally do not exhibit the “negative temperature coefficient” region, making them more sensitive to temperature changes [11].

With RON and MON serving as reference points for fuel regulation, the OI at any engine operating condition can be estimated using a linear interpolation term, K as seen in the following equation [12]. Note that K is a function of the engine operating conditions:

$$OI=\left(1-K\right)\times RON+K\times MON=RON-K\times S.$$

(2)

When K = 0.5, the OI is simply the average of the RON and MON values, as commonly seen on fuel pumps in the United States. However, modern engines rarely operate under conditions where K equals 0.5. Higher values of K reflect higher in-cylinder temperatures, similar to those in the MON test, but such conditions are less common due to cooling technologies such as direct injection and intercooling. These systems reduce the effective end-gas temperature, often resulting in K values near zero or even negative [13]. A negative K indicates that the OI is extrapolated beyond the RON and MON limits, representing combustion environments cooler than those defined by standard knock tests.

Recent research highlights the importance of mapping K for different engines and fuels. Zhou et al. produced detailed K maps for a turbocharged gasoline direct-injection engine, showing that K varies widely with intake temperature, load, and boost [14]. Szybist et al. and Ratcliff et al. found that octane sensitivity and charge cooling strongly affect knock resistance, with K determined by the end-gas pressure–temperature history rather than being a fixed property of the fuel [15,16]. López-Pintor et al. examined low-temperature combustion concepts and showed that the relative importance of RON and MON shifts as combustion phasing changes [17], while Suijs et al. reported that fuel composition, especially oxygenate and aromatic content, influences how sensitivity translates to knock performance [18].

Figure 1 shows the probability distribution of K for historic carbureted engines, modern naturally aspirated engines, and modern turbocharged/direct-injected (TC/DI) engines based on data in [13]. Historic carbureted engines typically operated at higher temperatures, producing an average K of 0.2 with a large standard deviation, which is lower than the commonly cited value of 0.5. For modern naturally aspirated engines, K averages near zero, meaning anti-knock performance depends almost entirely on RON. In TC/DI engines, higher in-cylinder pressures combine with cooler charge conditions, promoting low-temperature chemistry and further reducing K.

3. Materials and Methodology

3.1. Methodology

The goal of this analysis is to provide a better understanding of anti-knock requirements for future engines and fuels. In particular, it evaluates the change in K values associated with knock-limited engine operating conditions as vehicles transition to hybrid powertrains. It also considers changes in fuel sensitivity due to new fuels that are expected to play a significant role in the future market. Previous work has focused primarily on evaluating K values for standard engines rather than those used in hybrid architectures, and on the sensitivity of modern fuels without accounting for shifts in the fuel market.

This paper applies a standard technology roadmapping approach, assessing changes in engine technology and fuel properties in parallel while considering market trends and policy influences [19]. These advances are integrated through Equation (2), as shown in Figure 2. Changes in engine technology alter the knock-limited operating conditions, which in turn affect K values, while fuel trends modify fuel chemistry, resulting in new octane sensitivities.

The first part of the analysis examines trends in engine technology, with particular attention to continued engine downsizing and the integration of hybrid powertrains. These developments affect combustion chamber conditions, boosting methods, and overall thermal efficiency. Projected trends are modeled in Ricardo WAVE to generate engine maps for current and future engines, allowing identification of knock-limited operating zones. Ricardo WAVE is a one-dimensional engine simulation package that integrates fluid dynamic and thermodynamic models for air, fuel, and combustion products to predict engine operating parameters. The knock-limited operating ranges for each engine were then applied to approximate values of K.

The second part of the analysis focuses on trends in fuel development, particularly the expected chemical composition of future gasoline blends. This includes the increased incorporation of biofuels and other non-fossil-based components such as synthetic hydrocarbons and advanced alcohols. These compositional changes are used to estimate future values of fuel sensitivity as defined in Equation (1).

Finally, the separate roadmaps for fuel and engine development are integrated to assess the anti-knock performance of future fuel formulations in representative future engines. This combined analysis offers a forward-looking perspective on how evolving technologies will interact with knock phenomena and provides insight into how knock considerations may influence the design and optimization of next-generation spark-ignition engines.

3.2. Engine Knock Behaviors

3.2.1. Engine Trends

Engine technology is undergoing a significant shift as electrification becomes a dominant trend in the automotive industry. While full electrification remains a long-term goal, many experts predict that the future of the automobile will be eclectic rather than exclusively electric [20]. In many markets, hybrid powertrains, where engines work alongside electric motors and batteries to generate propulsion, are expected to play a central role [21]. These systems provide a practical balance between reducing emissions, improving fuel efficiency, and addressing range anxiety, while also making use of existing refueling infrastructure. They allow manufacturers to reduce the size of the battery pack, which lessens dependence on scarce materials such as lithium and cobalt.

There are a number of current global efforts supporting hybridization. In China, government incentives and strict fuel-consumption limits have accelerated the adoption of plug-in hybrid electric vehicles, with models such as the BYD Qin Plus DM-i and Li Auto L-series achieving strong sales growth [22]. European manufacturers are also expanding hybrid portfolios to meet stringent carbon dioxide targets and low-emission zone requirements [23]. Brands including Toyota, Renault, and Volkswagen have introduced efficient full-hybrid and plug-in hybrid models that appeal to consumers seeking lower fuel use without relying entirely on charging infrastructure. These efforts align with the global push for electrified vehicles.

Given the growing role of hybrid vehicles, engines will remain a critical component of future powertrains, but their design, purpose, and operating conditions will change significantly in the hybrid era. In particular, the engines will be downsized [24]. Electric motors provide high torque at low speeds, reducing the demand for large displacement engines. Since hybrid powertrains must also house batteries, power electronics, and other components, the available space is limited, so engines will need a smaller physical footprint.

However, the downsizing trend introduces packaging challenges that influence engine design choices. Turbocharging and direct injection, which have been key technologies for increasing power density in modern conventional engines, may not be practical in these compact hybrid applications. Instead, naturally aspirated designs with simplified architectures will likely dominate this space, relying on the electric motor to compensate for any performance gaps [25].

To further enhance efficiency, many small engines in hybrid vehicles employ Atkinson cycle principles rather than a conventional Otto cycle [26]. The Atkinson cycle is compared to the traditional Otto cycle in Figure 3. The Atkinson cycle achieves a higher effective expansion ratio by delaying the closure of the intake valve, which allows a portion of the air–fuel mixture to flow back into the intake manifold during the compression stroke. This results in a lower effective compression ratio while maintaining a longer expansion stroke, improving thermal efficiency and reducing pumping losses. The goal is to extract more useful work from each combustion cycle, improving fuel economy and lowering emissions without requiring significant hardware complexity. However, these benefits are traded-off with decreased engine performance especially high-load or high-speed conditions. As such, the Atkinson cycle is particularly well-suited for hybrid applications, where electric motors provide additional torque to offset the reduced power output at those conditions.

3.2.2. Shift in Knock-Limited Operating Range

As shown in Figure 4, hybrid powertrains generally fall into two main architectures: series and power-split. In a series hybrid, the engine does not directly drive the wheels but instead acts as a generator to charge the battery or power the electric motor, which provides all propulsion. A power-split hybrid, in contrast, allows the engine to either drive the wheels directly, supply power to the generator, or both, providing greater flexibility and efficiency [27]. Power-split hybrids are generally more prevalent than series hybrids especially given that most plug-in hybrids used a power-split architecture.

In a series hybrid, the engine is decoupled from the wheels and operates solely as a generator, allowing it to run within a narrow range near its most efficient speed and load. This configuration is increasingly used in hybrid systems that include a range extender, where a generator set recharges the battery bank when the state of charge falls below a threshold value. This range usually occurs at higher engine speeds, where airflow and turbulence accelerate combustion and reduce the likelihood of knock [28]. Steady-state operation further limits knock risk by maintaining stable pressures and temperatures and avoiding the rapid load changes that occur in conventional engines. As a result, engines in series hybrids are generally not knock-limited.

Power-split hybrids present a different challenge because the engine operates over a broader range of conditions, although this range is managed differently than in a traditional drivetrain [29]. Electric motors provide torque at low speeds, reducing the demand for high engine loads and minimizing knock risk in that region. These engines generally use the Atkinson cycle, which operate at high geometric compression ratios. However, the late intake valve-closing results in a lower effective compression ratio, reducing the peak pressures. Meanwhile, the longer residence time of the intake charge in the cylinder raises the end-gas temperature. Overall, the combination of electric motor assistance and cycle design enables a wider and more efficient operating range with fewer knock constraints compared to a conventional Otto cycle engine.

Table 1. Comparison of knock-related parameters for a current Otto-cycle engine compared to the Atkinson-cycle engines expected to be used in power-split hybrid vehicles.

Parameter	Current	Future
Cycle	Otto	Atkinson
Configuration	Engine is primary source of power	Engine works in power-split hybrid configuration
Intake	Turbocharged (with Direct Injection)	Naturally Aspirated
Compression Ratio	~10	~13
Effective Compression Ratio	-	~9
Temperature at Intake Valve Closing	Lower due to direct injection	Higher due to longer intake duration
Knock Sensitivity	Lower because of cooler charge and faster burn	Higher due to high geometric ratio and hot residuals
Operating Conditions	Low engine speeds	Low engine speeds
Charge Temperature During Combustion	Lower	Higher because of reduced cooling
Likelihood of Knock	Lower in modern turbo DI due to charge cooling	Still significant despite lower effective CR
Ignition Timing Limitation	Somewhat limited by knock at high load	Strongly limited at high load

summarizes the differences in the knock limited range for modern engines versus future engines integrated into hybrid configurations.

3.2.3. Change in Values for K

Since K is a function of engine operating conditions, the value for an engine in a power-split hybrid would be expected to be different from that used in conventional configurations. The value of K scales with the difference between the maximum unburned fuel temperature ($T_{u_{max}}$) for a given engine operating condition and that experienced in the associated RON test [6]:

$$K~\left(T_{u_{max}}-T_{u_{max,RON}}\right).$$

(3)

In Equation (3), the maximum unburned fuel temperature can be estimated as:

$$T_{u_{max}}=T_0{\left({\displaystyle \raisebox{1ex}{$P_{max}$}\!\left/ \!\raisebox{-1ex}{$P_0$}\right.}\right)}^{1-1/\gamma }$$

(4)

where γ is the ratio of specific heats, typically approximated as 1.3.

Engine models were run in Ricardo WAVE to determine the operating conditions for the RON95 test and for a modern naturally aspirated engine. The model is shown in Figure 5. The operating conditions and compression ratio were set to align with the RON95 test in accordance with ASTM 2699 [7]. The model indicated that the maximum pressure in the RON95 test is approximately 38 bar, which aligns with results in literature [30]. With an intake pressure of 1 bar and an intake temperature of 325 K, as set for the RON95 test, the maximum unburned fuel temperature for the RON95 test is about 920 K.

A similar WAVE model, shown in Figure 6, was developed for a standard four-cylinder naturally aspirated engine with a compression ratio of 10. An engine map generated from the model aligned well with maps for production engines [31]. Assuming an intake air temperature of 300 K, the model predicted the knock-limited range of operating conditions based on the pressure and temperature profiles in the engine. These conditions ranged from an intake air pressure of 0.8 bar at 1000 RPM to an intake air pressure of 1 bar at 3000 RPM, which matched the typical knock-limited range.

Across this range, the average peak pressure was 50 bar, and the maximum unburned fuel temperature was estimated at 918 K. These values are very close to those measured in the RON95 test. As a result, K is approximately zero for modern naturally aspirated engines, consistent with values reported in the literature [13].

For the future hybrid engine, the peak pressure will be slightly lower due to the reduced effective compression ratio associated with the Atkinson cycle. However, the initial temperature will likely be higher because the delayed intake valve closing in the Atkinson cycle allows the charge to heat above ambient before valve closure.

The model for the naturally aspirated engine was modified to reflect the increased geometric compression ratio and the late intake valve closing. The engine operating range was set such that at lower speeds and loads, the vehicle relied on the battery-motor over the engine. The models indicate that the air temperature rises to about 330 K prior to intake valve closing. Similar to the standard model, an engine map was generated and validated against those in literature [32]. The WAVE model identified the knock limited range over this engine map. Over the knock-limited range, the average peak pressure was found to be 45 bar and the maximum unburned fuel temperature is about 980 K. Using Equation (3), this suggests that K would be positive, indicating that less sensitive fuels would exhibit better anti-knock performance.

When the maximum unburned temperature approaches the value for the related MON test, K = 1. As such, the model for the RON95 test was modified to the MON test in accordance with ASTM 2700 [8]. For that situation, the maximum unburned temperature was 1050 K. From linear interpolation based on the maximum unburned fuel temperatures, K would be approximately 0.4 for the engine in the Atkinson cycle. The exact values of K will vary based on specific operating conditions, but the higher unburned fuel temperatures indicate that K will primarily be positive. It is important to note that this is different from current engine trends, where K is becoming increasingly negative due to turbocharging and direct injection.

3.3. Fuel Anti-Knock Behavior

As engine technology advances, fuel technology is undergoing similar transformations. Current gasoline blends already incorporate bio-derived components, primarily ethanol, to improve octane quality and reduce carbon intensity. Looking ahead, there is a growing emphasis on expanding the role of sustainable alternatives to further displace petroleum-based gasoline. This section examines current fuel blends and two key categories of renewable fuels: biofuels and electrofuels (e-fuels).

3.3.1. Current Blends

Modern commercial gasoline consists of a blend of hydrocarbons and oxygenates designed to balance performance, efficiency, and emissions requirements. The primary hydrocarbon classes include paraffins, olefins, aromatics, and naphthenes, with ethanol being the most common oxygenate. Paraffins, particularly iso-paraffins, typically make up 30 to 60 percent of the blend and offer high energy density but moderate octane characteristics [33]. Aromatics, such as toluene and xylene, contribute between 20 and 35 percent and provide high octane ratings, although they are associated with higher particulate emissions. Olefins enhance octane ratings but are generally kept below 20 percent due to their reactivity and deposit-forming tendencies Ethanol, often blended at 10 percent by volume in the United States, contributes significantly to octane while also helping to reduce lifecycle greenhouse gas emissions [34].

The specific combination of these components influences both the RON and MON of the final fuel. Ethanol and aromatics tend to raise RON more than MON, leading to increased fuel sensitivity [35]. Meanwhile, paraffins and naphthenes typically contribute to both RON and MON in a more balanced manner, resulting in lower sensitivity [36]. As a result, modern gasoline formulations often exhibit fuel sensitivities in the range of 8 to 12, depending on the blendstock ratios and ethanol content [37]. This range reflects the trade-offs between maximizing anti-knock performance under varied engine loads and maintaining overall fuel stability and emissions compliance.

3.3.2. Biofuels

As the shift toward renewable gasoline alternatives accelerates, biofuels for spark-ignition engines are becoming increasingly advanced. Bio-derived fuels are designed to replicate the chemical composition of gasoline to maintain similar properties and enable direct substitution without major infrastructure changes.

The primary biofuel currently in use is ethanol, typically blended at 10–15% (E10, E15), with higher blends used in flexible-fuel vehicles. Ethanol offers a high RON but a significantly lower MON, which increases the sensitivity of the fuel blend. It also reduces energy density and exhibits water affinity, creating storage challenges [38].

Second-generation biofuels derived from lignocellulosic biomass produce branched alkanes, olefins, and cyclic compounds in the C5–C12 range, matching gasoline’s volatility and combustion properties [39]. Olefins raise RON but require stabilization, while branched alkanes provide excellent knock resistance. These molecules can be produced through catalytic upgrading of olefins or fermentation pathways that generate intermediates such as isobutanol, which can then be converted into hydrocarbons.

Third-generation approaches focus on microbial and algal synthesis of hydrocarbons tailored for gasoline applications. Engineered strains can produce isoprenoids and terpenoids that are hydrogenated into high-octane paraffins, or even short-chain alkanes and olefins. Isobutanol has emerged as a promising candidate because of its higher energy density and lower hygroscopicity compared to ethanol, though it also exhibits high RON and relatively low MON, contributing to increased fuel sensitivity [40].

In addition to potential performance benefits, the use of biofuels can result in a reduction in the lifecycle carbon emissions by substituting renewable carbon for fossil-based carbon in the fuel supply. Studies indicate that bio-based feedstocks, particularly lignocellulosic biomass and algae, can achieve net greenhouse gas reductions of 50–80% over the full production, transport, and combustion cycle compared to conventional gasoline, depending on cultivation and processing methods [40]. Optimizing feedstock selection, conversion pathways, and blending strategies is therefore critical not only for engine efficiency but also for achieving meaningful decarbonization.

Future biofuel formulations will be increasingly optimized for engine performance and emissions reduction. Promising molecules include dimethylfuran (DMF), higher alcohols, cyclic ethers, and alkylated aromatics derived from bio-based feedstocks. The overall objective is to create drop-in, high-octane, low-emission fuels that match or exceed the performance of petroleum gasoline while leveraging renewable carbon sources. There is a significant opportunity to guide these fuel developments to align with the thermodynamic and knock-related requirements of future engines, particularly in hybrid powertrains where less sensitive fuels appear to provide a distinct performance advantage.

3.3.3. E-Fuels

E-fuels, or electrofuels, are synthetic fuels produced by combining renewable hydrogen with captured carbon dioxide. Designed to replicate conventional gasoline, e-fuels typically consist of hydrocarbons in the C5 to C12 range, including alkanes, branched isoparaffins, olefins, and limited aromatics [41]. Their synthesis relies on electricity as the primary energy input, allowing for tight control over molecular composition and enabling compatibility with existing internal combustion engines and fueling infrastructure.

Several pathways are used to produce e-gasoline. The Fischer–Tropsch process converts hydrogen and carbon monoxide into a broad slate of hydrocarbons, which can be refined to emphasize branched alkanes and other gasoline-range molecules [42]. The methanol-to-gasoline route converts CO₂-derived methanol into light olefins and then into gasoline-range hydrocarbons such as isooctane and trimethylpentane [43]. These molecules offer high RON, low sensitivity, and clean combustion characteristics. Advanced catalytic processes can also produce tailored compounds with minimized aromatic content while maintaining energy density and volatility.

The chemical structure of e-fuels can be engineered for specific performance targets [44]. Branched alkanes are emphasized for their knock resistance and stability. Aromatic content is kept low due to environmental concerns, with performance compensated by molecules such as isoparaffins or selected oxygenates. Oxygenates like dimethyl ether and methyl tert-butyl ether have been explored as blending agents, though their use depends on regulatory frameworks.

Similar to bio-fuels, e-fuels offer substantial potential for lifecycle carbon reduction. Because their carbon is derived from captured CO₂ and the hydrogen is produced via renewable electricity, e-fuels can achieve near-zero net greenhouse gas emissions over the full production and combustion cycle. The extent of the reduction depends on the carbon source, the energy mix used for hydrogen production, and the efficiency of synthesis processes. By carefully designing e-fuels with both engine performance and renewable carbon utilization in mind, it is possible to create drop-in fuels that provide a pathway to decarbonizing spark-ignition transportation without major changes to existing infrastructure [45].

3.3.4. Change in Value for S

Given that K is becoming increasingly positive for hybrid engines, fuels with lower sensitivity will provide better anti-knock performance under these operating conditions. Figure 7 illustrates the relative RON and MON values for various hydrocarbon classes and oxygenates [46]. The trend shows that paraffins and naphthenes exhibit low sensitivity, while olefins, diolefins, aromatics, and alcohols tend to have higher sensitivity. For example, branched paraffins such as isooctane have a low sensitivity and thus perform well when K is positive, as in hybrid engines. In contrast, compounds such as toluene (an aromatic) and alcohols like ethanol have higher sensitivity, making them more effective in turbocharged engines, where K is negative. This suggests that future fuel design for hybrids should emphasize low-sensitivity chemistries, including paraffins and naphthenes, rather than high-sensitivity components favored in other applications.

Most biofuels inherently exhibit higher sensitivity because of their molecular structure and oxygen content. Ethanol, for example, has a RON of about 109 and a MON near 90, yielding a sensitivity of nearly 19, which is significantly higher than that of typical gasoline hydrocarbons. Higher alcohols such as isobutanol also demonstrate elevated sensitivity along with greater energy density than ethanol, but they are less compatible with the needs of engines operating in conditions where K is positive. Other biofuel-derived molecules such as ethers (e.g., dimethylfuran) and certain cyclic oxygenates also exhibit high RON and moderate MON, contributing to higher sensitivity values. However, advanced bio-derived hydrocarbons, such as branched paraffins produced via catalytic upgrading of lignocellulosic intermediates, tend to have lower sensitivities and would hence have better anti-knock performance in hybrid engines.

E-fuels offer greater flexibility because their chemical composition can be controlled during synthesis. The Fischer–Tropsch process typically produces a broad range of hydrocarbons dominated by paraffins, which exhibit low sensitivity and are therefore ideal for hybrid engines with positive K values. Advanced catalytic strategies allow for selective production of low-sensitivity fractions such as branched paraffins while minimizing components like olefins and aromatics that increase sensitivity. Methanol-to-gasoline processes can also generate molecules like isooctane and trimethylpentane, which provide excellent knock resistance under these conditions. This molecular tunability positions e-fuels as a strategic pathway for creating renewable fuels optimized for the thermodynamic and knock characteristics of next-generation hybrid powertrains.

4. Conclusions and Recommendations

4.1. Engine and Fuel Recommendations for Future Vehicles

The current transformation of the automotive sector, driven by electrification and sustainability mandates, creates an unprecedented opportunity to align engine design and fuel chemistry. Hybridization fundamentally changes engine operating conditions by narrowing the knock-limited zone and often producing positive values of K, meaning that fuels with lower sensitivity deliver better knock resistance. This shift allows engines to adopt higher geometric compression ratios and advanced cycles such as Atkinson and Miller without compromising durability or efficiency. As a result, the future pairing of engines and fuels should emphasize high-efficiency combustion systems that operate with reduced knock limitations and fuels engineered to exhibit low sensitivity, favoring paraffinic and naphthenic components over highly sensitive oxygenates and aromatics. This co-design approach enables both performance improvements and a transition to renewable fuel chemistries optimized for the hybrid era.

From an environmental perspective, this alignment delivers two key benefits: improved engine efficiency and reduced lifecycle emissions. Knock has historically constrained compression ratio and ignition timing, limiting thermal efficiency in conventional engines. Hybridization alleviates these constraints, allowing engines to operate at higher geometric compression ratios with reduced knock risk, while advanced cycles further improve expansion work extraction. On the fuel side, renewable options such as biofuels and e-fuels inherently support decarbonization goals, but their full lifecycle emissions, including feedstock cultivation, fuel production, distribution, and end use, must be evaluated to ensure net greenhouse gas reductions. Future formulations also need to comply with emerging regulatory targets on carbon intensity and air quality while remaining economically viable within evolving market and policy frameworks.

Aromatics, while useful for octane enhancement, tend to increase particulate number and soot emissions and can challenge aftertreatment durability. Olefins and paraffins generally burn more cleanly, producing fewer particulates and lower levels of unregulated pollutants, though they require attention to oxidative stability and volatility. Paraffinic hydrocarbons, including those produced through catalytic upgrading of bio-derived intermediates or via Fischer–Tropsch synthesis, are particularly well suited because they combine strong knock resistance under positive-K conditions with low sensitivity and a cleaner combustion profile. Integrating these attributes into biofuel and e-fuel development while assessing them against lifecycle, regulatory, and economic criteria will help ensure that strategies to suppress knock support long-term goals for greenhouse gas mitigation, air quality compliance, and cost-effective deployment.

Economic considerations further strengthen the case for low-sensitivity fuels in hybrid engines. Current gasoline formulations rely on costly refinery operations to achieve octane through the production of iso-paraffins and alkylate, but these processes are already aligned with producing low-sensitivity molecules. E-fuels synthesized via Fischer–Tropsch pathways naturally favor paraffinic structures, reducing the need for additional processing. Similarly, biofuels that can be upgraded to paraffinic components, rather than high-sensitivity alcohols or aromatics, provide a cost-effective route to meet both performance and environmental objectives. This cost-efficiency, combined with reduced reliance on sensitivity-boosting components, positions paraffinic biofuels and e-fuels as competitive solutions for the evolving hybrid powertrain landscape.

4.2. Future Works

This study analyzed the interaction between future engine design, fuel formulation, and knock behavior within the context of electrification and sustainability trends. Technology forecasting was used to estimate future values of the knock index (K) for hybrid engines and to assess how evolving fuel compositions influence sensitivity and anti-knock performance. Modeling results indicate that hybridization and advanced thermodynamic cycles, such as the Atkinson cycle, shift knock-limited operating zones and generate positive K values. Under these conditions, fuels with lower sensitivities provide improved knock resistance.

The evaluation of fuel development trends highlights the increasing role of biofuels and e-fuels, which often introduce molecular structures associated with higher sensitivity. While these characteristics have historically benefited turbocharged engines operating under negative K conditions, they present a mismatch for future hybrid engines that require low-sensitivity fuels. Consequently, renewable fuel development should prioritize paraffinic and naphthenic chemistries, which exhibit strong knock resistance and low sensitivity under positive K conditions. Although the automotive industry is transitioning toward full electrification, internal combustion engines and liquid fuels are expected to remain significant in the near and mid-term, particularly in hybrid vehicles. Achieving optimal alignment between future engines and fuels will remain essential for maximizing efficiency and durability, especially regarding knock control.

Future work will focus on validating these findings using real engine and fuel data to better refine and ensure the robustness of the modeling results. In addition, subsequent studies will capture a broader range of vehicles, including diverse hybrid architectures and advanced combustion concepts, to assess how fuel–engine interactions evolve across different platforms. This expanded scope will provide a more comprehensive understanding of knock behavior in an increasingly electrified and sustainable automotive landscape.