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Tsinghua studies on alcohol-gasoline dual fuel engines show fuel efficiency and particle number benefits

Researchers at Tsinghua University in China are studying the effects of Dual-Fuel Spark Ignition (DFSI) combustion fueled with different alcohols and gasoline. In one paper, published in the journal Fuel, they investigated the use of alcohols–gasoline DFSI Combustion for knock suppression and high fuel efficiency using a gasoline engine with high compression ratio.

In a second paper, also published in Fuel, they systematically compared the stoichiometric alcohol–gasoline and gasoline–alcohol DFSI combustion for engine particle number (PN) reduction (and fuel economy improvement), also using a high compression ratio gasoline engine.

Alcohol–gasoline DFSI uses port-fuel-injection (PFI) of a high-oxygenated, high-latent-heat and high-octane fuel (alcohol) to suppress knock and a direct injection (DI) of a high-energy-density and high-volatility fuel (gasoline) to extend high load. (Alternatively, in the second study, gasoline–alcohol DFSI is based on PFI of gasoline and DI of alcohols.)

For gasoline internal combustion engines (ICEs) in automotive industry, gasoline direct injection (DI) combined with turbocharging, which is named as downsized gasoline engine, occupies the mainstream of gasoline ICE development because of its advantages in fuel economy. However, engine knock with high pressure rise and pressure oscillation caused by end-gas auto-ignition is still the big obstacle for deep improvement of fuel consumption. Researchers are seeking new effective methods for knock suppression and subsequent fuel economy improvement. Dual-injection, which consists of DI and port fuel injection (PFI), combined with alternative fuels could be one promising method.

Alcohols are promising alternative fuels for internal combustion engines. In this paper, methanol, ethanol and E85W15 (15% water and 85% ethanol by volume) were investigated. … Alcohols-gasoline blending combines the advantages of both alcohols and flex-fuel approaches.

… most studies of dual-fuel dual injection combustion modes by using alcohols and gasoline in ICE focus on injecting alcohol directly into cylinder rather than into the intake port, which is different with the approaches in this paper. Furthermore, no research compared the effect of different alcohols combined with gasoline on engine knock suppression.

—Liu et al. (2015a)

The Tsinghua team studied three different alcohols-gasoline combustion modes, including M–G (PFI-Methanol and DI-Gasoline); E–G (PFI-Ethanol and DI-Gasoline); and E85W15–G (PFI-15% water and 85% ethanol and DI-Gasoline); G–G (PFI-Gasoline and DI-Gasoline) DFSI was the reference. They flexibly controlled the PFI to DI fuel ratios. Overall AFR (air-to-fuel) ratio was maintained stoichiometric.

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Schematic of alcohols-gasoline DFSI combustion system. Liu et al. (2015a)

The 2.0L test engine was naturally aspirated, with a compression ratio of 13:1. In each test, the percentage of alcohol was varied from 0-100% as the researchers investigated the effects on knock-limit extension; fuel economy; and combustion characteristics were investigated.

Broadly, the results showed that all alcohol-gasoline DFSI combinations under stoichiometric conditions can extend the knock limit effectively, while fuel economy is improved significantly. M-G DFSI enabled better anti-knock performance and achieved higher fuel efficiency than other combinations.

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Performance characteristics of alcohol-gasoline DFSI combinations. Liu et al. (2015a)

For M-G DFSI, as the mass fraction of PFI-methanol increased from 0 to 40%, BMEP increased at the same time. However, when the fraction rose from 40% to 100%, BMEP dropped.

M-G DFSI BMEP reached peak value of 9.12 bar with 40% PFI-methanol—an extension of 13.2% when compared with the 8.06 bar of the baseline. During that increase in mass fraction to 40%, the higher latent heat of vaporization, octane number and combustion speed of methanol allowed better anti-knock ability and combustion phasing, resulting in the rise in BMEP, the researchers concluded.

The BSFCequivalent of M-G DFSI reached its lowest value of 222 g/kW h with 93% PFI-methanol—an improvement of 24.0% compared with the 292 g/kW h of the baseline.

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Fuel economy performance of alcohol-gasoline DFSI. Liu et al. (2015a)

This work was supported by the National Natural Science Foundation of China and the Ministry of Science and Technology of China.

Particle number study. The second study systematically compares the stoichiometric alcohol–gasoline and gasoline–alcohol Dual-Fuel Spark Ignition (DFSI) combustion for engine particle number (PN) reduction.

As in the earlier study, the team used both methanol and ethanol, with alcohol mass ratio varying from 0-100% for all combustion control strategies.

They found that increasing alcohol mass ratio resulted in significant reductions of the PN —up to more than a 95% reduction of PN compared to the baseline. The magnitudes of the particle number density for the nucleation and the accumulation peaks decreased significantly for all DFSI combustion control strategies.

In this study, they found that gasoline–alcohol DFSI generally showed higher fuel economy improvement and PN reduction compared to the alcohol–gasoline DFSI.

Resources

  • Hui Liu, Zhi Wang, Yan Long, Jianxin Wang (2015a) “Dual-Fuel Spark Ignition (DFSI) combustion fuelled with different alcohols and gasoline for fuel efficiency,” Fuel, Volume 157, Pages 255-260 doi: 10.1016/j.fuel.2015.04.042

  • Hui Liu, Zhi Wang, Yan Long, Shouzhi Xiang, Jianxin Wang, Mohammad Fatouraie (2015b) “Comparative study on alcohol–gasoline and gasoline–alcohol Dual-Fuel Spark Ignition (DFSI) combustion for engine particle number (PN) reduction,” Fuel, Volume 159, Pages 250-258 doi: 10.1016/j.fuel.2015.06.059

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