U. Mich, Ford team studies effect of ethanol in reducing PM from DISI engines; insights into fueling strategies to reduce soot
A team from the University of Michigan and Ford’s Research and Advanced Engineering group in Dearborn has studied the effects of ethanol on reducing particulate emissions from a direct injection spark ignition (DISI) engine by comparing neat anhydrous ethanol with a baseline fuel of reference grade gasoline (indolene).
In a paper published in the ACS journal Energy & Fuels, they reported that ethanol produced over an order of magnitude less soot under all operating conditions compared to indolene; however, ethanol produced measurable soot at cold coolant and early fuel injection timing conditions.
|Results of combustion and soot imaging for SOI = 320° bTDC. The false color images are averages of the intensities of 15 cycles. Every 10th CAD is shown after 10° aTDC. Credit: ACS, Fatouraie et al. Click to enlarge.
They also found that high coolant temperatures significantly decreased in-cylinder soot formation at every fuel injection timing for both fuels, and retarding fuel injection timing decreased in-cylinder soot formation for both fuels at both coolant temperatures. Their results, they said, can provide insight into fueling strategies to reduce soot emissions in DISI engines.
The results of the study confirm the soot mitigating properties of ethanol and indicate that ethanol can be expected to decrease soot emission in DISI engines, at the load conditions studied here, compared to gasoline; however, there are limiting conditions where even 100% ethanol can produce particulates.—Fatouraie et al.
Gasoline direct injection engines—i.e., DISI engines fueled by gasoline—offer a number of features attractive to automakers seeking to meet fuel regulations such as higher thermal efficiency, lower fuel consumption, and lower CO2 emissions. Accordingly, the engines are growing in market share (e.g., Ford’s EcoBoost).
However, one of the other attributes of gasoline direct injected engines is higher particulate emissions. The possibility of higher surface wetting of combustion chamber surfaces by the direct injection fuel spray can result in higher PM emissions than from a port fuel injected engines.
Previous studies of DISI engines have shown that piston and wall wetting by the liquid fuel spray can lead to pool fires that become significant sources of PM. In general, soot formation is a complex function of the fuel properties and the in-cylinder combustion physics which affect the local in-cylinder equivalence ratio and temperature. Further understanding and characterization of the underlying physical and chemical mechanisms of PM formation in DI combustion systems is critical to achieving clean emissions.—Fatouraie et al.
The properties of ethanol differ from the properties of gasoline, impacting engine performance. These, identified by other studies, include:
A higher laminar flame speed compared with iso-octane, resulting in shorter combustion duration and therefore higher thermodynamic efficiency.
The less complex chemical structure of ethanol compared with iso-octane results in lower exergy destruction.
Lower combustion temperatures and a lower boiling point compared with gasoline result in lower NOx and unburned hydrocarbon (UHC) emissions.
Ethanol appears to have a strong ability to suppress the formation of benzene as well as some higher aromatic species, species considered precursors or building blocks for PM.
Studies have also shown that ethanol/ gasoline blends lead to a decrease in sooting. E85 has been shown to reduce PM number density and PM mass. However, the researchers noted, particulate emissions of engines operating on ethanol blends are also highly dependent on the gasoline blendstock. This can lead to a higher propensity of PM formation by ethanol blends if, for example, the blendstock contains higher concentrations of low volatility, aromatic hydrocarbons.
The fuel properties affecting charge preparation, in particular spray breakup, atomization, and vaporization, play important roles on fuel impingement on combustion chamber surfaces and on thermal and compositional charge stratification. The higher kinematic viscosity of ethanol affects the turbulence induced by the spray and spray breakup. The lower heating value of ethanol results in a larger volume of ethanol injected in each cycle compared with gasoline to develop equivalent power. These properties coupled with the significantly higher enthalpy of vaporization and lower boiling point of ethanol affect the spray pattern, spray tip penetration, and mixing which in turn affect the liquid fuel spray impingement on the cylinder wall and piston surfaces, which affect the likelihood of sooting in DISI engines.—Fatouraie et al.
For the study, the team used an optically accessible single cylinder engine with a displacement of 506 cm3. Indolene, a reference grade gasoline (research octane number = 97.2, motor octane number = 89.0) was used as the baseline fuel, and anhydrous ethanol with a purity ≥99.9% was used and designated as E100.
All experiments were conducted at the same load conditions with a net indicated mean effective pressure of IMEPnet ≈ 5.5 bar and an intake manifold absolute pressure of 76 kPa. The engine speed was fixed at 1500 rpm, and the fuel injection duration was controlled to achieve stoichiometric combustion. Spark timing was adjusted to target combustion phasing (CA50) of 8° aTDC.
The team studied the effects of engine coolant temperature and fuel injection timing on fuel spray characteristics and soot formation for each fuel.
A high speed camera recorded crank-angle resolved in-cylinder images of the fuel spray, combustion, and thermal radiation from the soot formed for the engine operating conditions studied. The researchers also used a smoke meter to measure particulate emissions in the engine exhaust gas.
Mohammad Fatouraie, Margaret S. Wooldridge, Benjamin R. Petersen, and Steven T. Wooldridge (2015) “Effects of Ethanol on In-Cylinder and Exhaust Gas Particulate Emissions of a Gasoline Direct Injection Spark Ignition Engine” Energy & Fuels doi: 10.1021/ef502758y