Studies highlight effects of different gasoline components on fuel economy, combustion and emissions
Researchers at Jilin University (China) have investigated the effect of gasoline components on fuel economy, combustion and emissions in a GDI (gasoline-direct-injection) engine. The team subsequently followed up this first study with a second, exploring the effects in a PFI (port-fuel-injection) engine. The first paper (Han et al., 2018a) is published in the journal Fuel, the second (Han et al., 2018b) in the ACS journal Energy & Fuels.
So far, there are few complete comparative studies about the effect of different components of gasoline on fuel economy. Many studies aimed at new combustion models and alternative fuels. However, various alternative fuels are used as additives to gasoline which still have unsolved practical problems. And a limited understanding of the effects of gasoline properties on fuel economy and modern vehicle emissions impedes the establishment of stricter fuel standards in China.
According to IEA (International Energy Agency), among light-duty vehicles and passenger cars, 70% will be powered by gasoline engines in 2020. By 2050, 58% of passenger cars will still use ICE. Therefore, it is necessary to systematically study the effect of gasoline components on fuel economy, combustion and emissions, especially fuel economy. And in this way, the research results can be used to guide fuel refining process to improve fuel quality.
In this paper, n-pentane, n-heptane, n-decane, MTBE and toluene were respectively blended into isooctane by mass. The study mainly focuses on the effect of short-chain alkanes, ethers and aromatics on fuel economy. Meanwhile, the effect of chain alkanes of different lengths, ethers and aromatics on engine combustion and emissions were compared so as to investigate the effect of these components with low octane numbers on performance of the engine. The main purpose of the work is to find the optimal configuration of gasoline components so that the fuel economy and emission of engines can be simultaneously optimized.—Han et al. (2018a)
The GDI study was conducted on a 4-cylinder, 4-valve, 4- stroke, water-cooled, turbocharged, direct-injection spark-ignition engine. Isooctane was selected as the base fuel since it is one of the main components of the primary reference fuel (PRF) consisting of isooctane and n-heptane. In addition, the team selected n-pentane, n-heptane and n-decane as short-chain, medium-chain and long-chain alkanes. MTBE and toluene were chosen as oxygenated fuels and aromatics, respectively.
To compare the effect of chain-alkanes on engine’s combustion and emissions, 5% of n-pentane and n-heptane and 3% of n-decane were blended with the base fuel by mass, respectively referred to as P5, H5, and D3. Because of the lower octane number of n-decane, the blending ratio of it was lower to avoid serious knock combustion. The content of short-chain alkanes in gasoline is approximately 20% (depending upon the refinery); consequently, the team blended 20% of n-pentane, MTBE and toluene were blended with the base fue, referred to respectively as P20, M20 and T20. Pure isooctane was referred to as O100.
Load characteristics of the engine were tested at 7 speeds: 1000, 1300, 1500, 1800, 2000, 2400 and 2800 rpm—selected according to Chinese standards. Fuel consumption, combustion characteristics and emissions of different fuels were compared at constant speeds and loads.
Among their findings:
Aromatics benefit fuel economy under all operating conditions due to the lower COVIMEP as a result of the steady combustion process and smaller cyclical variation. T20 (20% toluene blended with isooctane) shows a comparatively lower break-specific fuel consumption (BSFC) for its lower cyclical variation while its maximum fuel saving ratio (FSR) could reach 5.42%.
At light and medium loads of low speeds, short-chain alkanes shows great potential in the improvement of fuel economy. P20 (20% n-pentane blended with isooctane) whose maximum FSR could reach 4.64% presents better performance of fuel consumption under these operating conditions. By contrast, M20 (20% methyl tertiary butyl ether blended with isooctane) presents higher BSFC for its higher cyclical variation and lower heat value, with the mean FSR being −2.75%.
Medium-chain and long-chain alkanes are effective in improving combustion velocity, shortening ignition delay, advancing combustion phasing and reducing the maximum in-cylinder temperature due to their better ignition stability as a result of lower octane numbers, which makes for the better in-cylinder combustion process. Compared to n-decane, the effect of n-heptane on combustion is slightly better because of its higher H/C ratio and the minor tendency of deteriorated combustion.
Medium-chain and long-chain alkanes could reduce CO and THC emissions, to some extent. Compared with pure isooctane and T20, lower CO, NOx and THC emissions are observed for M20 and P20.
The second study with the PFI engine was intended to verify the “validity and universality” of the first study with the GDI engine. The PFI engine study used commercial gasoline with research octane number of 92 and three gasolines with different aromatics, short-chain alkanes and oxygen content.
The test engine was a four-cylinder, natural-aspirated, multi-point PFI engine; load characteristics were tested at 9 speeds: 1000, 1300, 1500, 1800, 2000, 2400, 2800, 3200 and 3600 rpm. Shandong Jingbo Petrochemical Co. Ltd. provided the test fuels.
Among the findings:
The gasolines with the higher aromatics content, compared with the baseline fuel, offer fuel consumption savings under all operating conditions. This confirms that aromatics are able to improve fuel economy, the team said. The mean FSR of gasoline F with the highest aromatics content is 3.68%.
The gasoline with the highest short-chain alkanes content showed good fuel economy performance at light and medium loads of medium speeds while the maximum FSR is 3.55%. This also verifies that short-chain alkanes do have great advantages on fuel economy under specific operating conditions.
Higher oxygen content could benefit fuel economy to some extent. In addition, the research octane number plays a more important role in fuel consumption under knock-sensitive operating conditions.
In general, the emissions of the test fuels were not worse than those of the baseline fuel. Gasolines with the higher aromatics content are even capable of reducing CO and THC emissions. Although the NOx emissions of some of the test gasolines deteriorated at medium loads, the three-way catalyst negated that concern—tailpipe emissions would not deteriorate with the improvement of fuel economy.
Yongqiang Han, Shicheng Hu, Manzhi Tan, Yun Xu, Jing Tian, Runzhao Li, Jiahong Chai, Jiahui Liu, Xiangfeng Yu (2018a) “Experimental study of the effect of gasoline components on fuel economy, combustion and emissions in GDI engine,” Fuel, Volume 216, Pages 371-380, doi: 10.1016/j.fuel.2017.12.033.
Yongqiang Han, Shicheng Hu, Yuncai Sun, Xingyu Sun, Manzhi Tan, Yun Xu, Jing Tian, Runzhao Li, and Zhujie Shao (2018b) “Compositional Effect of Gasoline on Fuel Economy and Emissions” Energy & Fuels doi: 10.1021/acs.energyfuels.8b00722