Gasoline direct injection engines offer better fuel efficiency from their increased compression ratio, lower pumping losses, higher volumetric efficiency and more accurate injection control. GDI engines also reduce pre-ignition and knock tendencies as compression temperatures are lower; an enhancement in thermal efficiency by a reduction of heat losses is thus possible.

On the other hand, GDI engines have reported to increase the concentration of the Particulate Matter (PM) emissions. The main sources of PM formation in GDI engines are identified as fuel piston wetting, injector fuel deposits and inadequate air-fuel mixing. Consequently, the diffusive combustion of rich-in-fuel areas promotes PM formation, and also wall wetting by fuel impingement also produces an increment of unburned hydrocarbons (HCs) and carbon monoxide (CO) due to a significant grade of incomplete combustion. For this reason, emission standards such as Euro 6c, which includes a strict limit of 6 x 10¹¹ particles per kilometer and comes into force in September 2017, are boosting the development of new technologies to reduce emissions in GDI.

A feasible short-to-midterm solution for addressing additional emissions reduction with a decreased in the demand of high quality fossil fuels is to use renewable bio-alcohols fuels such as butanol, which is considered a second generation of renewable transportation fuel. Butanol provides complementary physicochemical properties to gasoline blends for decreasing regulated emissions as well as improving combustion. Amongst these properties, higher octane number and oxygen content extend the knock limit for advanced spark timings and improve combustion efficiency, respectively, leading to further CO and total hydrocarbons (THCs) reductions. Furthermore, butanol’s higher latent heat of vaporization results in further cooling charge effect in GDI engines, which increases the volumetric and thermal efficiency. The higher latent heat of vaporization combined with its lower adiabatic flame temperature can also assist in NO_x reduction.

… The aim of this investigation is to assess and to further the understanding on the effect of the utilization of high butanol fraction blends on combustion characteristics, gaseous and particulate matter emissions in GDI engines.

—Hergueta et al.

For the study, the team used a fou-cylinder, 2.0-liter Ford GDI engine. The engine was operated at stoichiometric conditions where the oxygen concentration was controlled by a Heated Exhaust Gas Oxygen (HEGO) sensor.

Ford’s calibration strategy to reduce pumping losses is the utilization of valve overlapping to increase the residuals. This technique is known as internal EGR, where the intake valve opening (IVO) was set at 11 CAD bTDC (Crank Angle Degree before Top Dead Center), and the exhaust valve closing (EVO) is at 57 CAD aTDC (Crank Angle Degree after Top Dead Center).

The researchers also equipped the engine with a high-pressure external EGR system (designed and implemented by the University of Birmingham). The external EGR valves were controlled using a standalone control unit that requires a pulse-width modulated input signal to specify the desired valve position, provided by a custom LabView application.

The engine was operated at two steady-state conditions from the New European Driving Cycle (NEDC) for a mid-size/large family vehicle with 2 L engine under urban driving operation: a) 35 N·m/2100 rpm (low load) and b) 60 N·m/2100 rpm (medium load). The 33% v/v butanol-gasoline fuel was blended at the University of Birmingham using standard EN228 gasoline with 5% (v/v) ethanol content (B0) and pure n-butanol.

They found that the influence of the butanol depended on engine load. At low loads, butanol’s physical properties (e.g. high viscosity) are more influential on the combustion performance than its chemical properties (e.g. higher flame speed). Consequently, the combustion of B33 was observed to be more unstable due to the deteriorated fuel spray atomization and in-homogeneous air-fuel mixture, also contributing to the marginally increased carbon monoxide emissions.

Performance combustion of B33 combined with EGR with respect to gasoline combustion: blue + positive, red - negative and - - - insignificant effect. Hergueta et al. Click to enlarge.

On the other hand, butanol’s shorter carbon chain and its oxygen content help to reduce the rest of the emissions. As the engine load was increased—and hence the fuel injection pressure—the combustion performance of B33 was improved.

They found that B33 B33 was an effective fuel to reduce most of the legislated emissions in both engine conditions, while maintaining engine brake thermal efficiency (BTE). Adding EGR provided a general improvement of BTE and brake specific fuel consumption (BSFC) for both fuels and was beneficial to B33 since greater engine-out emissions reduction was achieved.

… high percentages of butanol in gasoline blends combined with EGR technology can be a potential solution in GDI engines for reducing legislated emissions while maintaining the BTE compare to gasoline. However, it is anticipated that the calibration and injection systems of the engine would have to be adapted to minimize its limited performance at low loads, since the physical properties can pre- dominate over its advantageous chemical properties.

—Hergueta et al.

Resources

• C. Hergueta, M. Bogarra, A. Tsolakis, K. Essa, J.M. Herreros (2017) “Butanol-gasoline blend and exhaust gas recirculation, impact on GDI engine emissions,” Fuel, Volume 208, Pages 662-672 doi: 10.1016/j.fuel.2017.07.022