Full Length ArticleThe significance of octane numbers to drive cycle fuel efficiency
Introduction
The global automotive industry is facing increasingly stringent regulations for fuel economy and greenhouse gas emissions. For light-duty vehicles, Japan aims to improve the fleet-averaged fuel economy by 32% (to 25.4 km/L or 3.94 L/100 km) in 2030 [1], and the US mandated a 1.5% annual increase in average fuel economy for new cars through 2021–2026 [2] with more challenging objectives from certain states. Comparably, China targets a 20% reduction in fleet-averaged CO2 emissions (to under 100 g/km) for 2025, and the European Union aims to reduce fleet-averaged CO2 emissions by almost 40% (to 59 g/km) by 2030. Achieving these targets requires more efficient engines and may entail powertrain hybridization. The former is achieved by using technologies such as engine downsizing, high compression ratio (CR), Atkinson cycle, cylinder deactivation, etc., whereas the latter utilizes regenerative braking and electric drive to shift engine operation towards the most efficient speed-load regimes.
These engine and powertrain technologies generally affect the in-cylinder conditions for combustion and often make the engine more prone to knock [3]. For example, downsized engines must operate with higher intake pressures for a given load, and higher compression ratios increase both the temperature and pressure of the unburned air–fuel mixture. Further, engines in hybrid powertrains typically operate within a narrow range of high-load conditions where the efficiency is high [4]. Common measures for knock mitigation, including retarding spark timing and enriching the air–fuel mixture, generally lead to a reduction in fuel efficiency. The actual benefit of using these efficiency-improving technologies therefore can be compromised due to knock mitigation.
Fuel octane quality plays a primary role in determining knock resistance and can greatly affect fuel efficiency [5]. The octane quality is conventionally characterized by the Research Octane Number (RON) and the Motor Octane Number (MON), which are measured by comparing the knock resistance of the test fuel to that of Primary Reference Fuels (PRFs) at standardized operating conditions in a Cooperative Fuel Research (CFR) engine [6], [7]. Fuels with higher RON and MON are generally regarded as more knock resistant. However, with in-cylinder conditions of modern engines increasingly deviating from those in the octane number tests, questions arise regarding how to relate the RON and MON of fuels to their actual knock resistance in modern engines [8].
To help address this issue, the Octane Index (OI) model was proposed by Kalghatgi to characterize the actual anti-knock quality of a fuel using its RON and MON [9]. As given by Eq. (1), OI is a linear function of RON and MON, and K is a weighting factor of RON and MON characterizing their relative contributions to the actual knock resistance. K is generally believed to be primarily dependent on the engine design and operation parameters. Another expression for OI is given by Eq. (2), where S is the octane sensitivity defined as the numerical difference between RON and MON (S = RON – MON). Nearly all practical fuels have a positive octane sensitivity, i.e. RON > MON.
Various studies have found that K is negative in modern SI engines when operating at high-load, low-speed conditions. A negative K means that the actual anti-knock quality (as characterized by the Octane Index) is positively correlated with RON and negatively correlated with MON. That is, fuels with a higher RON and lower MON are more knock resistant under these conditions [9], [10], [11], [12], [13] and these fuels provide superior vehicle acceleration performance [14], [15], [16], [17]. However, real-world driving mostly occurs at lower loads than tested in these studies, and recent work found that K can be positive at intermediate-load, knock-limited conditions [18], [19], [20]. This suggests that the relative importance of RON and MON, as indicated by the value of K, could vary significantly over real-world driving conditions, which warrants investigations using drive cycles that attempt to simulate a range of practical driving conditions. Despite many relevant studies already in the literature [9], [12], [15], [21], [22], [23], [24], a comprehensive, quantitative analysis on this topic has not been reported.
This study therefore investigates the significance of RON and MON to the drive-cycle fuel efficiency of a mid-sized sports utility vehicle (SUV) with a modern gasoline turbocharged direct-injection (GTDI) engine. The OI model and the measured K-maps of a 2.0-L GTDI engine are used to determine the K-distributions in several standard drive cycles. The fuel efficiency loss induced by knock mitigation, termed “knock-limited fuel efficiency loss” (KLFEL), is quantified, along with the impact of RON and MON on such losses.
Section snippets
Methodology
The drive cycle analysis uses the operating data of a 2.0-L Ford GTDI (EcoBoostTM) engine measured in a vehicle over several drive cycle tests. Engine operating conditions in these drive cycles are first converted to K values using the K-map of the engine. K-distributions based on the fuel consumption under knock-limited conditions (KLFC) and on the KLFEL are then calculated. The effects of RON and MON on the drive cycle KLFEL are also evaluated. The K-maps, the vehicle test data, and the
UDDS and HWFET cycles
In the urban (UDDS) and highway (HWFET) drive cycles, used to determine US Corporate Average Fuel Economy, the engine operates mostly at low speeds and low loads. This is demonstrated in Fig. 5 where the engine operating conditions from the vehicle test data are superimposed on the K-map at normal temperature conditions. In both cycles, most operating conditions are below the red line with the knock limited portion (above the red line), i.e., KLFC, accounting for 19.5% of total fuel consumption
Discussion
The constraint of engine knock on vehicle fuel economy varies with driving conditions. Fig. 14 summarizes the results of this work and shows that more fuel is consumed under knock-limited conditions as the drive cycles become increasingly aggressive. The penalty on fuel economy, in L/100 km, also increases correspondingly, despite a reduction in the KLFEL for the Davis Dam towing tests. It is also noted that not all knock-limited conditions produce the same fuel efficiency loss; the operations
Conclusions
This work investigated the significance of fuel RON and MON to the drive cycle fuel efficiency of vehicles powered by a 2.0-L, 4-cylinder GTDI (gasoline turbocharged direct injection) engine, using the Octane Index approach. The analysis only accounted for the direct effects of RON and MON on an existing engine, not for opportunities to optimize engine design e.g. by changing compression ratio. Using vehicle test data and engine K-maps, the fuel efficiency loss caused by measures to avoid
CRediT authorship contribution statement
Zhenbiao Zhou: Methodology, Investigation, Formal analysis, Writing - original draft. Tanmay Kar: Investigation. Yi Yang: Conceptualization, Methodology, Supervision, Writing - review & editing, Funding acquisition. Michael Brear: Conceptualization, Supervision, Writing - review & editing, Funding acquisition. Thomas G. Leone: Conceptualization, Resources, Writing - review & editing, Funding acquisition, Project administration. James E. Anderson: Conceptualization, Resources, Writing - review &
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research was supported by the Ford Motor Company. The lead author also thanks the support from the Australian Automotive Engineering Graduate Program.
References (29)
- et al.
What fuel properties enable higher thermal efficiency in spark-ignited engines?
Prog Energy Combust Sci
(2021) - et al.
Significance of RON and MON to a modern DISI engine
Fuel
(2017) - et al.
Mapping K factor variations and its causes in a modern, spark-ignition engine
Fuel
(2021) - Joshi A. Review of Vehicle Engine Efficiency and Emissions. SAE Tech Pap 2020;2020-April....
- NHTSA, US EPA. The safer affordable fuel-efficient (SAFE) vehicles rule. Corp Aver Fuel Econ 2020;2020....
- et al.
The Effect of Compression Ratio, Fuel Octane Rating, and Ethanol Content on Spark-Ignition Engine Efficiency
Environ Sci Technol
(2015) - et al.
Development of the hybrid/battery ECU for the toyota hybrid system
SAE Tech Pap
(1998) ASTM D2699–19 Standard Test Method for Research Octane Number of Spark-Ignition Engine Fuel 1
ASTM
(2019)- ASTM. ASTM D2700-19 Standard Test Method for Motor Octane Number of Spark-Ignition Engine Fuel. Astm 2019:1–60....
- et al.
The shift in relevance of fuel RON and MON to knock onset in modern SI engines over the last 70 years
SAE Int J Engines
(2009)
Fuel anti-knock quality-part I
Engine studies. SAE Tech Pap
Octane appetite studies in direct injection spark ignition (DISI) engines
SAE Tech Pap
The relevance of fuel RON and MON to knock onset in modern SI engines
SAE Tech Pap
Fuel effects in a boosted DISI engine
SAE Tech Pap
Cited by (6)
The significance of octane numbers to hybrid electric vehicles with turbocharged direct injection engines
2023, FuelCitation Excerpt :In each case, the KLFEL and KKLFEL are calculated and compared for the three powertrains over the drive cycles investigated. Details for calculating these parameters can be found in Ref. [16]. The uncertainty of the reported KLFEL is shown in Appendix A.
Minimum ignition temperature of gas–liquid two-phase cloud
2024, Journal of Thermal Analysis and Calorimetry