Characterization of the Combustion of Light Alcohols in CI Engines: Performance, Combustion Characteristics and Emissions

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, alsoNOX. 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 53 % was achieved by using a high rc=27 piston
and 50 % EGR at 6 bar IMEPG.

The low fuel consumption and high torque output of the diesel engine makes
it a very attractive power source, both in the private sector as well as in the
transport industry. Except for the mentioned advantages, diesel engines emit
very small amounts of carbon monoxide and hydrocarbons.
The disadvantage of this engine type is the emissions of high concentrations
of oxides of nitrogen (NOX) and soot, which are both harmful for health
and the environment. Because of this, the industry as well as various research
institutes have focused on minimizing the emissions of these pollutants while
simultaneously reducing the fuel consumption by investigating different advanced combustion concepts for this engine type.
One of these combustion concepts is partially premixed combustion, PPC.
This concept uses an injection which is timed slightly earlier in the cycle in
comparison to conventional diesel engines, while introducing cooled exhaust
gases together with the fresh air. This causes a higher level of premixing which
increases the local oxygen availability and maintains a lower combustion temperature. This strategy reduces the concentration of soot and NOX in the exhaust gases.
To ease the process of premixing of the air-fuel mixture in this combustion
concept, the use of a gasoline, with a research octane number rating of 70, is
preferred over diesel fuel. Although the soot emissions of gasoline are lower
than those of diesel, the disadvantage of gasoline, similarly to diesel, is that
there is a trade-off between soot and NOX. In this work, methanol and ethanol
are used as fuels - both neat and in blends with other fuel types - to circumvent
this trade-off, due to the soot formation tendency of these alcohols is extremely
small. Moreover, these alcohols can be produced from biomass which in turn
reduce the carbon dioxide emissions. Another advantage with methanol and
ethanol is their cooling effect which further reduces the combustion and thus
also reduces the heat losses to the combustion chamber walls as well as the
NOX emissions.
In this work, the research have contributed to achieving an operating condition
in which all the regulated emissions have been maintained below current
legal restrictions without the use of any exhaust after treatment systems.
Meanwhile, the experiments in this work have contributed to an increase in
efficiency and reduction in emissions in both light- and heavy-duty engines by
using methanol and ethanol