Study: splash blended ethanol fuels with higher ethanol percentage enable higher thermal efficiency in SI engine
A team from the University of Birmingham (UK) and Shell Global Solutions has investigated the effect of RON, octane sensitivity and charge cooling in splash-blended ethanol fuels with different volume percentages of ethanol on a single-cylinder direct-injection spark ignition (DISI) research engine.
In a paper published in the journal Fuel, the researchers report that at the knock-limited engine loads, splash-blended ethanol fuels with a higher ethanol percentage enabled higher engine thermal efficiency through allowing more advanced combustion phasing and less fuel enrichment for limiting the exhaust gas temperature under the upper limit of 850 °C, which was due to the synergic effects of higher RON and octane sensitivity, as well as better charge cooling.
A downsized gasoline engine is one of the proven technologies that improves engine thermal efficiency and thus reduces automotive fleet CO2 emissions, by as much as 25%. Downsized engines equipped with turbo- or super-chargers operate at higher engine loads to deliver the same power outputs as larger engines, thus, downsized engines lead to lower pumping losses and higher efficiency at part load operating conditions.
… However, despite the proven advantages of downsized engines, engine knock, caused by the auto-ignition of the end gas, is one of the main challenges that stop downsized engines from achieving their full potential. High octane rating fuels are one of the key solutions for suppressing engine knock. Ethanol, a widely used renewable gasoline alternative, has a much higher octane rating than conventional gasoline fuel. Splash blending ethanol into gasoline improves the octane rating of the resulting fuel mixture.
… Currently, ethanol is largely used in low percentage blend forms such as E5 or E10. Higher octane splash blended ethanol fuels beyond E10 are expected to give better performance in downsized engines, however, their performance in modern downsized DISI engines, and the contributions of RON, octane sensitivity and charge cooling to combustion are not fully understood. In this study, eight fuels were designed and tested, including four splash blended ethanol (10 vol.%, 20 vol.%, 30 vol.% and 85 vol.% ethanol, noted as E10, E20, E30 and E85), one match blended fuel (E0-MB) with zero ethanol content but the same octane rating with those of E30, and three fuels (F1-F3) with different combinations of RON and octane sensitivity.—Wang et al.
The team carried out load and spark timing sweep tests with an engine speed of 1800 rpm, and full load tests for E10-E85 to assess the combustion performance of ethanol blends. To investigate the effect of charge cooling, the load sweep was conducted for E0-MB, and the results were compared to those of E30. Load sweep tests were also carried out for F1-F3, to understand the impacts of RON and octane sensitivity on engine combustion.
Octane sensitivity is defined as the difference between the research octane number (RON) and motor octane number (MON). RON and MON are measured in CFR engines designed 90 years ago. However, modern spark ignition engines, especially turbo-charged downsized engines, tend to operate at relatively lower temperatures than CFR engines. To compensate for the disconnect between the CFR engine and modern engines, an octane index (OI) was proposed as: OI = RON + K ⁄ (RON-MON), where K is a scaling factor depending solely on the in-cylinder thermal and pressure history experienced by the end-gas prior to the onset of auto-ignition.
Based on the testing results, the team concluded:
Splash blended ethanol has better anti-knock properties than base gasoline, enabling a larger knock-free engine load range and more advanced combustion phasing when the engine is knock-limited. Higher ethanol blends led to better engine indicated thermal efficiency, especially at high and full load operating conditions. Compared to E10, E20 led to 2.8–7% higher indicated thermal efficiency at the full load, depending on the engine speed; the improvements for E85 were in the range of 8.3–27%.
Compared to E10, at knock-limited engine load, the combustion of higher percentage ethanol blends were less sensitive to spark timing retard, resulting in less negative impacts on IMEP and indicated thermal efficiency. At 1.6 bar intake pressure, advances in spark timing from knock-limited spark advance (KLSA) caused a more severe knock intensity rise for E10 than for other higher percentage ethanol blends.
For E30, at knock limited operating conditions, the positive effect of charging cooling was reflected in the more advanced combustion phasing, higher engine thermal efficiency, and lower unburned gas temperature at TDC. The high heat of vaporization and low stoichiometric air/fuel ratio of ethanol blends both contributed to a better charge cooling effect. In addition, the faster burning rate of ethanol also contributed to this.
High RON and high octane sensitivity both contributed to improve the fuel’s anti-knock quality, with the impact of RON being more significant than that of octane sensitivity. For ethanol blends, most of the anti-knock quality improvement was from the RON improvement.
Chongming Wang, Andreas Janssen, Arjun Prakash, Roger Cracknell, Hongming Xu (2017) “Splash blended ethanol in a spark ignition engine – Effect of RON, octane sensitivity and charge cooling,” Fuel, Volume 196, 15 May 2017, Pages 21-31 doi: 10.1016/j.fuel.2017.01.075