Characterization of the Combustion of Light Alcohols in CI Engines : Performance, Combustion Characteristics and Emissions
Abstract: Alternative fuels for combustion engines are becoming increasingly popular as society is pushing to phase out fossil energy to reduce CO2 emissions. The compression ignition (CI) engine has a high overall efficiency which makes it a valuable option for the transport fleet, despite the well known NOx and soot pollutants it emits. These two pollutants are emitted due to a combination of high local combustion temperature and low level of premixing prior to the combustion of diesel fuel. Based on previous work, it is well known that high research octane number (RON) fuels, such as gasoline, can be used in an CI engine to increase the premixing thus reducing the engine-out soot emissions, and to a certain extent, also NOx. Apart from reducing the regulated emissions, the automotive industry is also focusing on developing CI engines that run at a higher efficiency and emit less CO2, which can be achieved by using biomass based fuel, either neat or in blends. Methanol and ethanol are two good examples of such fuels. The idea of using light alcohols to run a CI engine did not arise recently; in Sweden, ethanol has been used in this engine type to run city buses since the mid 1980's. However, it is worth mentioning that the research of their use in CI engines has not been extensive. This work aims to investigate the performance, combustion characteristics and emissions of CI engines running on light alcohols, either neat or in blends with diesel, to study the advantages and drawbacks. The purpose is to better understand how the potential of these fuels can be further exploited while simultaneously finding ways to minimize the drawbacks of their use. The light alcohols, and in particular methanol, have a high heat of vaporization in combination with a low heating value. This contributes to a cooler combustion which also causes an extensive enleanment of the charge. The cooler combustion increases the efficiency by reducing the heat losses. The excessive enleanment, on the other hand, increases the total hydrocarbon (THC) and CO emissions. Moreover, the combustion instability increases. The findings of this work suggests that it is possible to counter these drawbacks, by increasing the intake temperature, Tin. This could be achieved by using a turbocharger without extensive intercooling. The higher Tin reduces the premixing period and improves the stability, resulting in increased oxidation of THC and CO. The drawback of this strategy is, however, an increased formation of NOx. For similar intake conditions, methanol combustion resulted in a 50 % reduction of NOx in comparison to iso-octane due to the its charge cooling effect. A double injection strategy can be used to reduce the required Tin, however, this will come at a cost of lower thermal efficiency due to the longer combustion duration. Another viable option to reduce the required Tin is by using a high compression ratio, rc. The resulting increase in NOx can be countered with EGR. However, if rc is too high, operating flexibility is reduced due to restrictions in structural integrity; for example, high lambda alongside high EGR rates will be limited to lower loads. The light alcohols do not produce black carbon soot when combusted, thus significantly lower particulate matter (PM) emissions, which makes them a good alternative to the heavier diesel fuels. On the other hand, the particle number (PN) emission is generally higher than that of conventional gasoline or diesel. It is worth noting that the emitted PN only consist of particles with a diameter 30 nm as measured with a fast particle analyzer. Furthermore, an observation of the emitted PM under a transmission electron microscope, using energy dispersive X-ray, strongly suggested that the origin of the PM was the lubrication oil rather than the combustion products of the light alcohols. The light alcohols have shown some noteworthy results in terms of efficiency and emissions. In this work, a gross indicated efficiency of approx. 53 % was achieved by using a high rc=27 piston and 50 % EGR at 6 bar IMEPg.
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