Diesel engines for additized ethanol
One option to use ethanol is the compression ignition engine. The properties of ethanol resemble gasoline, not diesel fuel and if ethanol is going to be used in compression ignition engine, either the engine or the fuel has to be modified, the latter option requiring fuel additives. Ethanol differs significantly from conventional diesel fuel regarding cetane number, heating value, self-ignition temperature, vaporization characteristics, and boiling point. Cetane number of neat ethanol is less than 12, whereas today the requirement for diesel in Europe is higher than or equal to 51 (ethanolproperties). Neat ethanol as such will not ignite in conventional diesel engine. Also the lubricity of ethanol is unacceptable for high-pressure injection pumps in diesel engines. The low boiling point of ethanol increases the risk of cavitation, and the high conductivity the risk of corrosion (Peckham 2001, McCormick 2001).
In the past Detroit Diesel manufactured glow-plug equipped heavy-duty engines for using methanol or ethanol. However, many problems like engine wear, and glow-plug failures and excessive fuel consumption were encountered. Thus the production of these engines was discontinued. In Sweden another approach was selected: adding ignition improver and lubricity additive to ethanol to enable the usage of the conventional diesel combustion process. In some cities in Sweden buses manufactured by Scania run on additive treated ethanol. The first ethanol bus started service in 1985, and in 2000 there were 407 buses running in Sweden. Now more than 600 buses have been supplied by Scania. Today ethanol buses are running also in other countries, for example, in Mexico, Australia, and Denmark.
The modifications of diesel engines for ethanol-use include e.g. increased compression ratio, a special fuel injection system and a special catalyst to control aldehyde emissions. Scania's current 3rd generation ethanol engine is an adaptation of Scania's latest 9-litre diesel engine with air-to-air charge cooling and exhaust gas recirculation, EGR. The ethanol version features, among other things, elevated compression ratio (28:1) to facilitate ignition, higher fuel delivery to compensate lower energy density of the fuel, and special materials for the fuel system. The engine is available with Euro V and EEV emission certification (Scania 2007, Nylund 2011).
Etamax D fuel
The fuel used in ethanol buses in Sweden is called Etamax D. It contains 92 % m/m of hydrated ethanol (grade 95%), 5.0 % m/m ignition improver (poly-ethylene-glycol derivative from Akzo Nobel, Beraid 3540), 2.8 % m/m denaturants (2.3 % m/m MTBE and 0.5% m/m isobutanol) and corrosion inhibitor additive. Etamx DTM fuel contains 6.4 % m/m water. (SEKAB brochure, Westman 2008). In Sweden, standard SS 15 54 37 is specification for alcohols for high-speed diesel engines (Table 1). The standard does not include denaturants, ignition improvers or colorants.
Table 1. Selected properties of SEKAB's specification for Etamax DTM and Swedish SS 15 54 37 standard for ethanol used in high-speed diesel engines. Complete requirements are available from respective organizations.
Earlier generations of Scania's ethanol buses showed exhaust emissions similar to advanced diesel buses equipped with an oxidation catalyst or particle filters, but higher emissions than those of sophisticated CNG buses (NOx). The smoke emission from ethanol buses was almost negligible (likewise buses on gaseous fuels). Acetaldehyde emissions were high for ethanol buses, but the cancer risk index low (similar EPA factors used as for E85). (Ahlvik 2001).
Nylund et al. (2011) reported of tests with Scania's new generation ethanol vehicles: two trucks and one bus; all with the 8.9-litre 270 hp ethanol engine. As reference, three diesel trucks (Euro V), three diesel buses and two CNG buses (stoichiometric and lean-burn) were tested. In general, NOx emissions from ethanol vehicles were average, but PM emissions lower (buses) or significantly lower (trucks) compared to diesel vehicles without particulate filters (even 75% reduction in PM). The stoichiometric CNG bus showed lowest, whereas the lean-burn CNG bus the highest NOx values. PM emissions were higher for ethanol than for CNG average (Figure 1). Energy consumption of the ethanol bus was some 8% higher, and for ethanol trucks marginally lower, than the average diesel. It was noticeable that for the CNG buses energy increase was close to 40%, respectively. The conclusions seems to be quite similar with the new and older generation of ethanol vehicles: exhaust emissions from ethanol vehicles are close to clean diesel vehicles equipped with NOx and PM reducing technologies, whereas the sophisticated stoichiometric CNG vehicles are the cleanest.
Figure 1. NOx vs. PM emissions for buses. The results with ethanol bus is marked with red star. (Nylund et al. 2011).
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