With concerns about fossil resource price, greenhouse effect and the requirement for increased energy security and diversity, biofuels are receiving increasing attention from the public and research institutions. Renewable feedstock and high octane number rating make ethanol a very promising alternative fuel in CI engine. Due to its superior resistance to auto-ignition (RON = ~111) and volatility compared with gasoline fuel, ethanol is more specifically suited to PPC operation at high load conditions. In addition, the oxygen contained in ethanol molecule (~35% by mass) also provides the potential to reduce soot emissions.

… The research has also underlined that oxygen contributes to keep the combustion efficiency at high levels even with a lot of EGR. The the combination of premixed combustion and oxygenated fuel results in a very low specific fuel consumption of ethanol— 165.3 g/kWh. Besides, owing to its high ratio of hydrogen to carbon (H/C), as well as very short carbon chain with only 2 carbons, ethanol produces less soot as compared to gasoline fuels, e.g., below 0.06 Filter Smoke Number (FSN) versus 1-2 FSN at 1.8 MPa IMEP.

Therefore, it can be concluded that the oxygenated fuel, e.g., a blend of ethanol and gasoline, is more suitable for PPC concept. However, the widespread use of ethanol would not be feasible for a series of issues have been known.… The comparison of fuel properties shows clearly than n-butanol has the potential to overcome the drawbacks of ethanol … n-butanol with four carbons doubles the carbon number of ethanol and contains 25% more energy than ethanol; n-butanol and gasoline can be soluble with each other in any proportion, whereas the maximum solubility of ethanol in gasoline is ~10% without co-solvents; the kinematic viscosity of butanol is significantly higher than that of ethanol, so it is unlikely to cause serious lubricity and potential wear problems in fuel pump in CI engines; and so on. For the given advantages, n-butanol has brighter application prospect as a CI engine fuel than ethanol.

—Yang et al.

Although several research teams have probed engine performance and emissions with n-butanol/diesel blends, there are few reports on the behavior of n-butanol gasoline blends in CI engines so far, the team noted.

For the Fuel study, they evaluated the effect of mixtures of 0%, 10%, 20% and 30% by mass fraction of n-butanol with gasoline fuel on exhaust emissions (regulated and unregulated) under LTC conditions.

Experiments were performed on a single-cylinder CI engine with a high-pressure common rail injection system. An external air compressor simulated boosted conditions. A back pressure valve controlled the amount of exhaust gas flow into the intake pipe.

All tests were carried out at a fixed 1500 rpm and load condition (~0.9 MPa IMEP). EGR sweeps were preformed in 5% intervals from 0-45%. As the EGR rate increased, the start of injection (SOI) was put forward gradually to make the combustion phasing (CA50) fixed.

A single injection strategy was used for all experiments, and a lower injection pressure (40 MPa) was used to keep the Maximum pressure Rise Rate (MPRR) within acceptable limits (1.5 MPa/°CA).

Although low CN fuels such as gasoline and n-butanol can reduce combustion stability, due to the fixed CA50, the coefficient of variation (CoV) of IMEP of each test point was lower than 5%—i.e., the combustion process was stable.

Among their other conclusions were:

• The oxidation of n-butanol slightly decreases the CO and total HC emissions only at high EGR; but NO_x emissions are largely not affected due to the effects of n-butanol’s higher heat of evaporation and octane number exactly canceling each other.

• The NO proportion in total NO_x decreases while NO₂ shows an opposite trend as EGR increases; NO₂ reaches up to 25% with EGR of 45%. n-butanol additive has almost no effect on the proportions of NO and NO₂, indicating that the local equivalence ratio plays a more important role in influencing NO and NO₂ proportions compared with the global oxygen concentration.

• Under LTC conditions, the primary individual hydrocarbons include C₂H₄, C₃H₆, NC₅, IC₅ and AHC, and their proportions in total HC increase with higher n-butanol fractions except AHC’s.

• CH₄ has not been a major issue yet in Gasoline LTC due to less EGR used, while AHC proportions decrease with the increase of n-butanol fraction. HCHO has a good linear correlation with total exhaust HC, and the ratio of MeCHO to total HC exhibits decrease first and then increase with the increasing EGR, while the formation of both HCHO and MeCHO is promoted by the addition of n-butanol in the blends.

Resources

• Binbin Yang, Mingfa Yao, Wai K. Cheng, Zunqing Zheng, Lang Yue (2014) “Regulated and unregulated emissions from a compression ignition engine under low temperature combustion fuelled with gasoline and n-butanol/gasoline blends,” Fuel, Volume 120, Pages 163-170 doi: 10.1016/j.fuel.2013.11.058