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The influence of octane on fuel consumption


This article is based on a study of fuel octane effects on the fuel consumption in light-duty vehicles (LDV) conducted by Ricardo Strategic Consulting on behalf of the Asian Clean Fuels Association (ACFA) and the European Fuel Oxygenates Association (EFOA).

The study focuses on regulatory requirements for fuel economy or greenhouse gas (GHG) emissions, the LDV technology roadmap to meet those requirements and the role of fuel octane in this future challenge.

Regulatory requirements

The influence of octane on fuel consumption
Figure 1. GHG emissions requirements for passenger cars from key markets, normalised for NEDC.

The growth of both regulation and targets for low-carbon vehicles (LCV) sets a major challenge for the road transport sector. GHG emissions targets from key global markets for passenger cars are shown in Figure 1. These targets have been normalised by the International Council on Clean Transportation (ICCT) to the New European Drive Cycle (NEDC), which is currently the drive cycle used for GHG emissions testing in the European Union (EU). As can be seen, the regulatory targets are challenging, and will require a 4–5% improvement each year in GHG emissions.

Regulatory bodies are not only focusing on GHG emissions requirements, though. The EU and the United States are both pressing ahead with increasingly stringent limits for criteria pollutants, such as nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO) and hydrocarbons (HC) or non-methane organic gases (NMOG).

The influence of octane on fuel consumption
Table 1. Criteria pollutant requirements for California (US) LEV III, US EPA Tier 2, and EU Euro 6 using data from Ricardo EMLEG. EPA Tier 3 requirements mirror LEV III, but with stricter PM limits.

Table 1 uses data from Ricardo to show light-duty vehicle criteria pollutants for U.S. and EU regulations.  GHG emissions reduction will be one of the top drivers for light-duty vehicle development over the next decade. GHG regulatory targets are converging across global markets, although some regions are moving faster than others. Regulatory targets for criteria pollutants such as NOx, PM and HC are also expected to tighten in parallel. Therefore, manufacturers are highly motivated to seek solutions that address GHG and criteria pollutants.

Current rules for GHG emissions focus only on the vehicle and powertrain, hence they are on a so-called “tank to wheels” basis. Fuels are currently regulated separately for their GHG benefits where low-carbon standards exist, so there are limited opportunities for vehicles to be designed comprehensively in an LCV framework.

Although the World Wide Fuels Charter provides a recommended global standard for gasoline fuels, these fuels are ultimately regulated on a market-by-market basis. For example, European Commission sets fuel standards for the EU while the U.S. Environmental Protection Agency (EPA) has the main authority for fuels in the U.S. Governmental agencies rely on industry tests and standards as well, especially for performance-based metrics, such as measuring RON and MON.

The influence of octane on fuel consumption
Table 2. Ranked technologies for improving LDV fuel economy.

In the near term, then, Ricardo expects LDV GHG emissions requirements to remain on a “tank to wheels” basis, with only a nominal consideration of the GHG benefits of a lower carbon fuel. As discussed earlier, though, there is a role for fuels to play in reducing fuel consumption and, therefore, GHG emissions.

The cost-benefit overview for leading fuel economy improvements shows a mix of options available to manufacturers. The most cost effective options are for improvements in internal combustion engine design and powertrains, as illustrated in Table 2. Ricardo’s analysis is that hybrid electric vehicle (HEV) systems are considerably more expensive to reduce one gram of CO2 per kilometre.

The positive impact of higher fuel octane

The various technologies listed in Table 2 are already being implemented by key manufacturers, although their benefits are not simply additive —some synergies are possible, as are some conflicts. As marked in the table, higher fuel octane will facilitate changes to engine compression ratio, direct injection fuel systems and higher boost pressures from turbocharging and other advanced boost systems.

In Ricardo’s view, the internal combustion engine has considerable scope for further development, with a thermal efficiency of 50% or more possible in the longer term, with future improvements coming from a combination of better combustion and the implementation of waste heat recovery systems. Ricardo analysis suggests that these future engines incorporate technologies that will benefit from higher-octane fuel.

Fuel octane number effects

The influence of octane on fuel consumption
Figure 2. (a) Effect of compression ratio on engine thermal efficiency and (b) differences in fuel requirement by CR and bore.

The octane number, or octane rating, is a standard measure of the antiknock properties of a motor or aviation fuel. Most markets report the research octane number (RON) at the fuel pump, although in North America RON is averaged with the motor octane number (MON) to make the antiknock index (AKI).

A higher octane number moves the knock limit further from normal operation, and thereby lets the fuel withstand more compression before detonating. This is attractive because an increase of only one ratio number, such as from 9.5:1 to 10.5:1, reduces on-cycle fuel consumption by about 2%, which is a significant potential improvement from a relatively low-cost change in engine design.

Thus, higher-octane fuels allow fully stoichiometric operation at high speeds and high load in the absence of exhaust temperature limits to protect turbines or catalytic converters. The higher octane number is beneficial, then, because increasing the compression ratio (CR) provides a fuel consumption benefit across the whole engine-operating map. The heuristic is that increasing the compression ratio by one cuts on-cycle fuel consumption by approximately 2%. Figure 2 shows the effect of CR on engine thermal efficiency and also shows how CR can be increased for engines that require premium fuel.

Summary of findings

The influence of octane on fuel consumption
Table 3. Summary of octane effects on fuel consumption and performance.

Overall, RON is seen to have the strongest direct influence on fuel economy. The main benefit of uniformly available fuels with higher minimum RON is that it allows LDV manufacturers to design their vehicles for the improved fuel. A four to five point increase in RON would support design changes that would yield a 2% improvement in fuel consumption over a typical regulatory drive cycle. For existing LDV in use, there will be a more modest benefit to fuel consumption and performance, as shown in Table 3. The picture is more complicated for MON, as MON’s effect on engine performance is strongly influenced by the engine operating point, as well as the engine design.

From the roadmap and literature review results, Ricardo drew the following conclusions:

– Internal combustion engines are expected to remain the primary form of propulsion to 2025 and beyond.

– Future GHG emissions and fuel economy targets worldwide will require significant changes in engine technology over the next 10 to 15 years. These changes will include down speeding, boost and direct injection; all of which are expected to become commonplace.

– A higher minimum fuel RON will facilitate the performance of the engine technologies that are being implemented now, and that are expected over the next several decades, including: direct injection, turbocharging and other boost systems, higher compression ratios

Reprinted with permission from the Asian Clean Fuels Association (ACFA).