A team of engineers from GM Powertrain, Ford and FCA have published a detailed review of how to estimate the engine efficiency benefits of higher octane fuel—e.g., fuel with higher ethanol content—for part- and full-load operation for different engine types and fuel assumptions. Their paper is published in the ACS journal Environmental Science & Technology.
Engine compression ratio plays a fundamental role in engine efficiency; a higher compression ratio improves efficiency, but also causes higher temperatures and pressures of the unburned air-fuel mixture which can lead to knock at high loads. Compression ratio is thus limited to avoid knock. The compression ratio selected for a particular engine depends, the authors note, on the expected duty cycle and fuel octane. A higher compression ratio can be used if an engine will operate primarily at light loads, such that degraded efficiency at high loads is more than offset by improved efficiency at light loads.
A higher compression ratio can also be used if an engine will operate on fuel with a higher octane rating. A fuel’s octane rating is a measure of its resistance to knock. Knock is funda- mentally a chemical process initiated by preflame reactions leading to autoignition. The preflame reactions are a strong function of temperature, so evaporative cooling from the fuel can also play a significant role, which is particularly important for fuels containing alcohol.
Knock and fuel octane rating are becoming increasingly important due to many powertrain design trends including downsizing, downspeeding, cylinder deactivation, and hybridization. … To achieve continued improvement in engine efficiency and CO2 emissions, it is critical to quantify the roles of fuel octane rating and alcohol content. Higher octane fuel improves the efficiency of today’s engines through reduced spark retard (from optimum) at high loads, and could enable even higher efficiency if engines were optimized for higher-octane fuel. Alcohol and gasoline-alcohol blends also offer efficiency benefits independent of their octane value.—Leone et al.
Two major fuel factors contributing to knock resistance are octane ratings and fuel evaporative cooling.
Most regular-grade gasoline in the US is sold with a minimum octane rating of 87 AKI, corresponding to approximately 91−92 RON. Practically, however, the range of regular-grade gasoline in the US is 89 to 96 RON; vehicles need to be designed to accommodate fuel with the lowest expected octane rating. Premium-grade gasoline is listed as 91−93 AKI minimum, corresponding to 96−98 RON. The vast majority (87%) of gasoline sold in the US in 2013 was regular grade, with 10% premium and 3% midgrade.
The concept of an “effective octane number” calculates overall knock resistance of fuel in modern engines, including the effects of charge cooling. The effective octane number (ONeff) is defined as the sum of RON and a charge cooling term (ONcool) which depends on ethanol content and engine type:
For a given fuel, the knock resistance of a given engine depends on a number of design and operating factors including compression ratio, fuel injection hardware and strategy, boosting, bore size, charge mixing, combustion speed, in-cylinder heat transfer, engine cooling system design, exhaust backpressure, spark plug location, combustion chamber design, and coolant and oil temperatures among others. The knock resistance can also be influenced by the engine calibration as well as ambient conditions such as temperature, barometric pressure, and humidity. In practice, these are developed as a system to provide the best combination of vehicle FE, performance, and emissions considering both the applicable certification fuel(s) and available market fuels under the wide range of operating conditions presented by certification testing (drive cycles) and actual on-road operation by customers.
Of these factors, the compression ratio is the predominant design factor that would be adjusted in future engines in response to a significant change in fuel octane ratings. In all engines, including the existing on-road fleet, fuel with higher octane ratings can enable more advanced spark timing and combustion phasing under knock-limited conditions.—Leone et al.
|Higher octane fuels and legacy vehicles
|Leone et al. note that if the minimum octane rating of the fuel available to customers was increased, it may be technically feasible to update (or “reflash”) the engine calibrations on existing vehicles to extract the most benefit from the improved fuel properties.
There have been many studies that have investigated the potential change in CR enabled by a given increase in fuel octane rating while maintaining similar knock characteristics—these, Leone et al.note, have resulted in a wide range of reported values—from approximately 2.5 to 9 ON increase required per CR increase (ON/CR)—due to a number of complications with test methodologies. Better controlled studies using modern engine technology and consistent design practices for each CR level support utilizing a factor of approximately 3 ON/CR for estimation of the benefits of increased fuel octane rating, the authors concluded.
In their study, Leone et al. considered a number of factors, including engine efficiency increase from a higher compression ratio; engine efficiency increase from downsizing (enabled by higher compression ratio); and other fuel-related efficiency considerations. These include charge cooling impact on heat transfer; adiabatic flame temperature impact on heat transfer; heat of vaporization impact on the measured heating value; charge cooling impact on pumping work; combustion efficiency (i.e., chemical energy remaining in the products of combustion); charge cooling and chemistry impacts on combustion stability and dilution tolerance; burn rate differences influence on combustion time losses; and specific heat ratio effects on the unburned and burned charge.
Higher ethanol content is one available option for increasing the octane ratings of gasoline and would provide additional engine efficiency benefits for part and full load operation. This paper provides a detailed review of how such benefits can be estimated for different engine types and fuel assumptions, calculations that are critical for well-to-wheels analysis of increased gasoline octane ratings. Considering vehicles and fuels as a system, such fuel changes would be supported if the net effect of light-duty vehicle efficiency improvements and fuel production impacts yielded reductions in fuel consumption, emissions, resource consumption, and/or cost.
For example, a recent study showed that a transition to higher-RON (98 RON) gasoline in the US with LDVs optimized for the fuel could yield significant net reductions in GHG emissions and a net cost savings. In addition to higher CR, future light-duty vehicle trends will also involve greater use of downsizing, turbocharging, down-speeding, and/or hybridization to improve efficiency. As with higher CR, each of these trends involve greater operation of engines under higher-load, knock-limited conditions and as such, also benefit from higher-octane fuel by expanding the engine map that is not subject to retarded combustion phasing to avoid knock. Higher octane fuel is a key enabler for improved efficiency based on current engine/vehicle design trends.—Leone et al.
Thomas G. Leone, James E. Anderson, Richard S. Davis, Asim Iqbal, Ronald A. Reese, II, Michael H. Shelby, and William M. Studzinski (2015) “The Effect of Compression Ratio, Fuel Octane Rating, and Ethanol Content on Spark-Ignition Engine Efficiency” Environmental Science & Technology doi: 10.1021/acs.est.5b01420