The concept of using a second, high octane fuel on demand to augment the performance of a standard gasoline fuel reaches back to the 1940s, with a recent resurgence in interest generated by the need to increase engine efficiency.
In a paper in the journal Fuel, researchers at the Fuel Technology R&D Division, Saudi Aramco Research & Development Center, have presented a comprehensive set of engine data for a regular grade E10 gasoline and a high octane E30 gasoline, and compared these fuels with the Octane-on-Demand dual fuel concept using methanol and ethanol as the high octane fuels in a high compression ratio engine with moderate levels of boosting. The results demonstrate that the Octane-on-Demand concept provides comparable or lower specific CO2 emissions to the E30 gasoline, with up to an almost 10% improvement in specific fuel consumption, given the appropriate calibration.
The ARAMCO team found that the use of a non-traditional engine calibration strategy that maximizes the trade-off between thermal efficiency and fuel energy density also enables the amount of high octane fuel required to suppress knock to be reduced by at least 25%, with methanol offering the greatest benefits.
High octane gasolines containing mid-levels of ethanol or methanol have … been widely promoted as the most effective means of raising the octane floor of regular grade gasoline. Ethanol and methanol have high octane numbers (RON ~109) and latent heats of vaporization (HoV) that are between four and seven times greater than regular gasolines on a stoichiometric basis. These properties generally enhance the practical anti-knock quality of conventional liquid fuels, even in relatively small concentrations. This enables higher thermal efficiencies to be achieved from engines that have been optimized to exploit the superior fuel anti-knock quality. Nevertheless, volumetric fuel economy parity with existing market gasolines has so far been difficult to achieve, due to the lower energy density of fuels with significant methanol or ethanol content.
Rather than directly displacing gasoline with methanol or ethanol, an improved approach would involve leveraging a limited amount of high octane fuel to enable the engine to be more efficient in its use of an oil-derived fuel, which has considerably higher energy density. The oil-derived fuel would be used at low and intermediate loads where energy density is generally more important than octane quality, while the high octane fuel would only be used at higher loads to suppress knock. This so-called Octane-on-Demand concept therefore combines the high energy density and widespread availability of oil-derived fuels with the superior octane quality of methanol or ethanol, while minimizing the negative effects associated with energy density, phase separation and cold engine starting.—Morganti et al.
For the study, the ARAMCO team used a single cylinder engine fitted with a prototype piston designed for a compression ratio (CR) of 12:1. The combustion chamber featured a modern penthouse roof design and four-valve cylinder head. The engine was equipped with independent gasoline direct injection (DI) for the gasoline blendstock and port-fuel injection (PFI) for methanol and ethanol, which were used to deliver the necessary amounts of the low and high octane fuels to the engine. A Lotus Active Valve Train electro–hydraulic system controlled the intake and exhaust camshaft timing.
Fuel stratification effects were minimized by maintaining the start of injection (SOI) for the DI and PFI systems at 280˚ and 360˚ TDC, respectively. The single fuel testing on the E10 and E30 gasolines was completed using only the DI system.
The team prepared the E10 and E30 fuels by splash-blending a RON 90 gasoline blendstock base with ethanol. The same RON 90 blendstock served as the lower octane base fuel for the Octane-on-Demand testing.
The test matrix incorporated a range of load and combustion phasing sweeps at four different engine speeds.
The primary conclusions from the study were:
Octane-on-Demand engines that are calibrated to operate at peak efficiency provide no significant benefits over higher octane gasolines with mid-levels of ethanol, e.g. E30. In particular, the benefits realized under part-load conditions from the higher fuel energy density are more than offset by the excessive amounts of methanol or ethanol required to maintain peak efficiency at higher engine loads.
An improved Octane-on-Demand calibration strategy involves maximizing the trade-off between thermal efficiency and fuel energy density. Small amounts of spark retard reduce the high octane fuel requirement significantly, while only reducing the engine efficiency by a small amount. This enabled the fuel consumption to be reduced by up to 9% when methanol was used as the high octane fuel, and up to 5% when ethanol was used as the high octane fuel. The retarded combustion phasing calibration strategy also enabled the amount of methanol and ethanol required to suppress knock to be reduced by at least 25% across all operating conditions.
The retarded combustion phasing calibration strategy introduced a minor trade-off in the specific CO2 emissions. This was due to the lower amounts of methanol and ethanol required to suppress knock, but was only present at higher engine loads. For engine loads below approximately 8 bar, there was no trade-off since the engine was exclusively operated on the gasoline blendstock (RON 90) irrespective of which Octane-on-Demand calibration strategy was used at higher engine loads.
Methanol appears to offer several key benefits over ethanol in the Octane-on-Demand configuration. In particular, the amount of methanol required to suppress knock at a given operating condition was always less than the amount of ethanol, despite its lower energy density. This would translate into a lower volumetric storage requirement onboard the vehicle, or alternatively a fewer number of refueling events (for the secondary tank) for a given driving distance.
The most promising Octane-on-Demand calibration strategy offered considerable benefits over the high octane E30 gasoline. For all operating conditions, the specific CO2 emissions were similar or marginally lower than the E30 gasoline, while the specific fuel consumption was reduced by up to 10%.
This study suggests that powertrains designed around the Octane-on-Demand concept may provide greater social and environmental benefits than those designed for high octane gasolines with significant methanol or ethanol content. Additionally, targeting improved fuel economy is often a more effective method to reduce the specific CO2 emissions than exclusively moving towards fuels with higher hydrogen-to-carbon ratios.—Morganti et al.
Kai Morganti, Yoann Viollet, Robert Head, Gautam Kalghatgi, Marwan Al-Abdullah, Abdullah Alzubail (2017) “Maximizing the benefits of high octane fuels in spark-ignition engines,” Fuel, Volume 207, Pages 470-487 doi: 10.1016/j.fuel.2017.06.066