Synergistic engine-fuel technologies for light-duty vehicles: Fuel economy and Greenhouse Gas Emissions

doi:10.1016/j.apenergy.2017.08.213

Applied Energy

Volume 208, 15 December 2017, Pages 1538-1561

https://doi.org/10.1016/j.apenergy.2017.08.213 Get rights and content

Highlights

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Octane-on-Demand was compared with two gasolines containing ethanol (E10 and E30).
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Vehicle fuel consumption was reduced by up to 10% with respect to the E30 gasoline.
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Well-to-wheel GHG emissions were generally comparable or lower than both gasolines.
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Minimizing the well-to-wheel GHG emissions marginally increased vehicle fuel consumption.
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Land use change uncertainties for bioethanol strongly affected the well-to-wheel GHG emissions.

Abstract

Advanced engine technologies will play a central role in achieving future greenhouse gas (GHG) emissions targets for light-duty vehicles. However, these technologies will place greater emphasis on optimizing the engine and fuel as a synergistic system, since many technologies will require higher octane gasolines to realize their full social and environmental benefits. The most extreme example of a synergistic engine-fuel system is the Octane-on-Demand concept. This technology platform makes use of an oil-derived fuel at low and intermediate loads where the octane requirement of the engine is comparatively low, while a second high octane fuel is introduced at higher loads to suppress knock. This paper presents the first comprehensive study of vehicle fuel economy and well-to-wheel GHG emissions for the Octane-on-Demand concept with respect to a regular grade E10 gasoline (RON 93) and a high octane E30 gasoline (RON 101). Experimental fuel consumption maps are first used to evaluate the drive cycle fuel economy and GHG emissions for a light-duty vehicle equipped with two alternative powertrains. The upstream GHG emissions arising from the production of the fuels are then quantified, with consequent uncertainties assessed using Monte Carlo analysis based on probability distribution functions for critical input parameters. The results demonstrate that the Octane-on-Demand concept used in conjunction with either methanol or ethanol generally provides comparable well-to-wheel GHG emissions to the high octane E30 gasoline, with up to a 10% improvement in the vehicle fuel economy. 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 significantly. This increases the distance that the vehicle can be driven before the secondary tank requires refueling by a considerable margin, but comes at the expense of marginally higher well-to-wheel GHG emissions than could otherwise be achieved. These findings are shown to be largely insensitive to uncertainties in the upstream fuel production GHG emissions, with the exception of the land use change (LUC) for bioethanol. Overall, this study has implications for the design of engine-fuel systems for future light-duty vehicles.

Introduction

Advanced engine technologies will play a central role in achieving future greenhouse gas (GHG) emissions targets for light-duty vehicles. For spark-ignition engines, this may include further downsizing and turbocharging, potentially in conjunction with supercharging to boost the low speed engine torque and transient response. Lean Miller cycle combustion with a high geometric compression ratio ( $⩾$ 14:1) and cooled exhaust gas recirculation (EGR) is another proposed CO₂ emissions reduction pathway, with benefits of up to 20% possible with respect to equivalent engines currently in production [1]. Varying degrees of hybridization will also be necessary to reduce the parasitic losses encountered at low engine loads. However, this is often more costly than other improvements that can still be made to existing engines [2].

The introduction of these and other advanced vehicle technologies will place greater emphasis on optimizing the engine and fuel as a synergistic system. In particular, many advanced technologies will require higher octane gasolines to realize their full social and environmental benefits [3], [4], [5], [6], [7], [8], [9], [10]. Higher octane gasolines enable more optimized combustion phasing in engines with high specific outputs or compression ratios. This can provide both fuel economy and CO₂ emissions benefits [11], [12], [13], [14]. Higher octane gasolines can also improve the conversion efficiency of the aftertreatment system by eliminating the need for high load fuel enrichment [15]. For these reasons, higher octane gasolines have been promoted as one of the most cost-effective means of reducing the environmental impact of light-duty vehicles [16], [17].

The most extreme example of a synergistic engine-fuel system that exploits high octane fuel is the Octane-on-Demand concept [18], [19], [20], [21]. A recent study [22] demonstrated that this technology platform offers many of the benefits of higher octane gasolines with intermediate levels of ethanol, while almost entirely eliminating the negative effects associated with energy density, vapor pressure, phase separation and cold engine starting [23], [24]. Unlike traditional engine-fuel systems, the Octane-on-Demand concept adjusts the fuel properties on-demand to optimize the fuel consumption and CO₂ emissions for a given operating condition. An oil-derived fuel is used at low and intermediate loads where the octane requirement of the engine is comparatively low, while a second high octane fuel is introduced at higher loads to suppress knock. In this way, a limited amount of high octane fuel is leveraged to enable the engine to be more efficient in its use of the oil-derived fuel, which has considerably higher energy density than widely available high octane fuels such as methanol and ethanol.

The cumulative environmental impact of different engine-fuel systems can be quantified using a well-to-wheel assessment [25], [26]. The well-to-wheel assessment is segregated into a tank-to-wheel component and a well-to-tank component. The tank-to-wheel component accounts for the primary GHG emissions from the combustion of the fuel in the vehicle, while the well-to-tank component accounts for the secondary GHG emissions from the extraction, production and distribution of the fuel. The well-to-wheel assessment is generally the preferred measure of environmental impact as it can account for different feedstocks, fuel production methods and changes in engine efficiency [27].

Well-to-wheel assessments have recently been used to quantify the potential environmental benefits of higher octane gasolines. In one of the most comprehensive studies, Han et al. [28] compared three high octane gasolines (E10, E25 and E40) with a regular grade United States gasoline (AKI 87). The well-to-wheel GHG emissions reduced with increasing ethanol content, with a maximum benefit of 18% estimated for the E40 gasoline. These benefits were mainly derived from the improved engine efficiency and higher hydrogen-to-carbon ratio of the fuel. Similar findings were reported in a study by Hirshfeld et al. [29] which considered two different scenarios for raising the fuel octane quality: higher rates of ethanol utilization and increasing the octane quality of the gasoline blendstock at the refinery level. Importantly, this study showed that the increased refinery impact of producing higher octane fuels partially offsets the lower tank-to-wheel GHG emissions obtained from the improved engine efficiency. This demonstrated that an optimal level of ethanol blending is required to minimize the well-to-wheel GHG emissions (Fig. 1). Zhang and Sarathy [30] later showed that this optimal blending level can be particularly sensitive to the feedstock from which the ethanol is derived.

These studies collectively suggest that widespread adoption of higher octane gasolines with intermediate levels of ethanol, e.g. E20 or E30, could be the most effective method to reduce the well-to-wheel GHG emissions of the light-duty vehicle fleet. Such fuels would also overcome the main deficiencies surrounding the current use of ethanol in transport fuels. In particular, several studies have demonstrated that low level ethanol blending, e.g. E5 or E10, provides only limited well-to-wheel GHG emissions reductions with respect to conventional gasolines [32], [33], [34], [35], [36]. While higher levels of ethanol blending, e.g. E85, would ultimately be limited by practical supply constraints along with heightened environmental and ecological impacts in areas such as eutrophication and photochemical ozone depletion [37].

It is less clear how the well-to-wheel GHG emissions of higher octane gasolines with intermediate levels of ethanol compare with the Octane-on-Demand concept. Although the former has been studied extensively, the vehicle fuel economy and well-to-wheel GHG emissions for the Octane-on-Demand concept are yet to have been comprehensively evaluated. Traditional well-to-wheel assessments also provide few insights into the expected performance of the Octane-on-Demand concept. This is because the amount of high octane fuel required to suppress knock varies considerably with both the drive cycle characteristics and the properties of the chosen fuels. As a result, neither the upstream fuel production GHG emissions or the in-service GHG emissions can be evaluated using existing methodologies which assume the fuel properties remain constant during vehicle operation. Developing an improved understanding of the primary and secondary GHG emissions has become an increasingly important and yet complicated area for policymakers, particularly as the range of mobility technologies and fuel sources continues to diversify.

This paper therefore presents the first comprehensive study of vehicle fuel economy and well-to-wheel GHG emissions for the Octane-on-Demand concept with respect to a regular grade E10 gasoline (RON 93) and a high octane E30 gasoline (RON 101). Experimental fuel consumption maps are first derived for each engine-fuel system using actual engine test data. The maps are then used to evaluate the drive cycle fuel economy and well-to-wheel GHG emissions for a light-duty vehicle equipped with two alternative powertrains. Consequent uncertainties in the upstream fuel production GHG emissions are assessed using Monte Carlo analysis based on probability distribution functions for critical input parameters. The Octane-on-Demand concept used in conjunction with either methanol or ethanol is shown to generally provide comparable well-to-wheel GHG emissions to the high octane E30 gasoline, with up to a 10% improvement in the vehicle fuel economy. 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 significantly. This increases the distance that the vehicle can be driven before the secondary tank requires refueling by a considerable margin, but comes at the expense of marginally higher well-to-wheel GHG emissions than could otherwise be achieved. These findings are shown to be largely insensitive to uncertainties in the upstream fuel production GHG emissions, with the exception of the land use change (LUC) for bioethanol. Overall, this has implications for the design of engine-fuel systems for future light-duty vehicles.

Section snippets

Experimental and numerical methods

This study uses experimental fuel consumption maps to simulate vehicle fuel economy and greenhouse gas (GHG) emissions using standardized drive cycles. The vehicle simulation results are then used to estimate the life cycle GHG emissions based on a well-to-wheel (WTW) assessment methodology (Fig. 2). The upstream GHG emissions arising from the extraction, production and distribution of each fuel are computed using the ‘Greenhouse gases, regulated emissions, and energy use in Transportation’

Vehicle drive cycle simulation results

The simulated vehicle fuel economy and tank-to-wheel CO₂ emissions for the six different engine-fuel systems is presented in Table 5. The total fuel consumption is reported in units of liters of fuel consumed per 100 km (L/100 km). The conversion of the fuel mass to volume was performed using the fuel densities reported in Table 1. The tank-to-wheel CO₂ emissions are reported in units of grams per km (g CO₂/km).

Life Cycle Assessment results

This section presents the Life Cycle Assessment for the Octane-on-Demand concept with respect to the baseline E10 and E30 gasolines. The breakdown of the well-to-wheel GHG emissions is first considered for each feedstock and engine calibration strategy. A parametric sensitivity analysis is then used to investigate the effect of uncertainties in the upstream fuel production and LUC GHG emissions on the well-to-wheel GHG emissions. All analysis is presented for the US06 drive cycle. The

Conclusions

This paper presented the first comprehensive study of vehicle fuel economy and well-to-wheel GHG emissions for the Octane-on-Demand concept with respect to a regular grade E10 gasoline (RON 93) and a high octane E30 gasoline (RON 101). Experimental fuel consumption maps were first used to evaluate the drive cycle fuel economy and GHG emissions for a light-duty vehicle equipped with two alternative powertrains. The upstream GHG emissions arising from the production of the fuels were then

Acknowledgments

The authors wish to thank Abdullah Ahajhouje for performing the engine experiments, along with Ahmad Radhwan, Mohammed Almansour, Waleed Bubshait and the Crude Products Characterization Unit for preparing and analyzing the test fuels. This work was supported by the Fuel Technology R&D Division at the Saudi Aramco Research & Development Center.

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