Exhaust emissions with ethanol or n-butanol diesel fuel blends during transient operation: A review
Introduction
During the last decades, a substantial effort to develop alternative fuel sources, most notably biofuels, has been in progress worldwide, motivated by both economic and environmental issues. Diminishing petroleum reserves and increasing prices, as well as continuously rising concern over energy security, environmental degradation and global warming have been identified as the most influential environmental ones [1].
As regards the financial aspect, the increasing oil prices impose an obvious burden on the trade balances of the non-oil producing countries. In any case, it has been recently argued that if world oil supply should continue to remain generally flat, a possibility exists that oil consumption in OECD countries will continue to decline, as emerging markets consume a greater share of the total oil that is available. If this should be the case, then it is possible that a continuing financial crisis similar to the 2008–2009 recession period might be experienced combined with significant debt defaults [2].
Apart from the economic issues, the extensive use of fossil fuels is responsible for a long-term environmental threat in the form of climatic changes and the slow (but continuous) increase in the average global temperature. The main contributor to the warming of the climate system is the carbon dioxide (CO2) emitted from various combustion sources. According to the US EPA Inventory of US Greenhouse Gas (GHG) Emissions and Sinks 1990–2009, the transportation sector collectively (including marine and air-transport too) accounted for 27.4% of the total US GHG emissions from end-use fossil fuel combustion in 2009. Passenger cars, light-duty trucks and medium/heavy-duty vehicles alone were responsible for almost 86% of the CO2 emitted from all transportation sources [3]. Then, it appears that biofuels, possessing the critical merit of being renewable and thus showing an inherent benefit in mitigating CO2 emissions, seem particularly suitable as viable alternatives to the current situation of the (almost) exclusive use of fossil fuels in automotive and truck applications [1], [4], [5], [6], [7], [8]. To this aim, the European Parliament passed Directive 2009/28/EC [9] on the promotion of the use of energy from renewable sources, which contains a specific mandate for Member States to include 10% (by energy content) of renewable fuel in the transport sector by 2020. The latter one is expected to be met largely by biofuels. The mandate includes specific sustainability criteria, including a requirement that the fuels meet a 35% GHG saving initially, rising to 60% in 2017, as well as a requirement that biofuels used to meet the target are not produced from land with high carbon stock. In parallel in the US, the Energy Independence and Security Act of 2007 (EISA) increased the original Renewable Fuels Standard (RFS) target of 34 billion liters renewable fuel production in 2008 to 136 billion liters by 2022. Such key mandates are expected to boost the market share of biofuels in the near future. Currently (2009), biofuels account for only 0.6% of the global final energy consumption, in contrast to 81% from fossil fuels and 2.8% from nuclear resources [10].
The term biofuel refers to any fuel that derives from biomass, such as sugars, vegetable oils, animal fats, etc. Biofuels made from agricultural products (oxygenated by nature) reduce the dependence of countries on oil imports, support local agricultural industries and enhance farming incomes [1], [4], [5], [6], [7], [8]. Moreover, they are way more evenly distributed than fossil or nuclear resources. This fact renders biofuels a very attractive tool in the endeavor towards increased energy security and diversity, which are essential factors for the aforementioned economic stability.
There are numerous biofuels that have been produced and researched so far, e.g., a variety of vegetable oils, different methyl and ethyl esters (biodiesels), bio-dimethylether, bio-hydrogen, bio-alcohols etc. At the moment, biodiesel is considered the primary alternative fuel for compression ignition (CI) engines, since it possesses similar properties to diesel fuel and can also be blended with diesel practically at any proportion, without changes in the existing distribution infrastructure. It is true that bio-alcohols, particularly ethanol and n-butanol, were initially considered as fuels for gasoline engines. Nonetheless, they are very promising for CI engines too (blended in smaller proportions with the diesel fuel), since they demonstrate a considerable potential for greenhouse gas emission reduction [1], [6], [7]. In fact, life-cycle analyses have revealed that typical CO2 savings from the use of ethanol ranges from 32% (in the case of wheat feedstock) up to 87% (wheat straw feedstock) [9]. It is not surprising then that ethanol production has boomed in the last years with a 17% growth rate during 2010 [10].
It is also well recognized today that one more significant benefit of adding biofuels in the fuel blend is the reduction of the emitted particulate matter (PM) from diesel engines [5], [6], [7], [8]. Since the alcohol molecule possesses higher oxygen content compared to biodiesel, the respective potential for PM emission reduction is accordingly higher [6], [7], [8]. This is a very promising fact in view of the ever tightening emissions regulations concerning passenger cars and heavy-duty diesel engines.
The diesel engine has for many decades now assumed a leading role in both the medium and medium–large transport sector. Major contributors to this are factors such as its superior fuel efficiency over its spark ignition counterpart, its reliability, as well as its inherent capability to operate turbocharged. Nonetheless, discrepancies in the form of exhaust smokiness and noise radiation delayed its infiltration and wide acceptance in the highly competitive passenger car market. Historically, the majority of the research and published studies on diesel engine operation has focused on the steady-state performance. However, only a very small fraction of a vehicle’s operating pattern is true steady-state. As a matter of fact, the greater part of the daily driving schedules of passenger cars, trucks and non-road engines involves transient operation in the form of changing engine speed and/or loading/fueling conditions.
The fundamental aspect of turbocharged transient conditions lies in their operating discrepancies compared with the respective steady-state ones. Whereas during steady-state operation, engine speed and fueling remain essentially constant, under transient conditions both the engine speed and the fuel supply change continuously. Consequently, the available exhaust gas energy varies, affecting the turbocharger shaft torque balance, and hence the boost pressure and the air-supply to the engine cylinders. However, due to various dynamic, thermal and fluid delays, mainly originating in the turbocharger moment of inertia, combustion air-supply is delayed compared with fueling, thus adversely affecting torque build-up and vehicle driveability. What is equally important is that, as a result of this delay in the response between air-supply and fueling, PM and gaseous emissions peak way beyond their acceptable steady-state values [11]. A typical representation is illustrated in Fig. 1 as regards smoke opacity and nitrogen oxides (NOx) development during an acceleration event of a turbocharged diesel engine. Acknowledging these well established transient emission discrepancies, legislative directives in the EU, the US and Japan, have drawn the attention of manufacturers and researchers to the dynamic operation of diesel engines in the form of transient cycles certification for new engines/vehicles [12], [13].
The target of the present work is to review the literature regarding the impacts of alcohol/diesel blends on the exhaust emissions of compression ignition engines, under the very critical transient conditions encountered in the everyday operation of engines and vehicles, i.e., acceleration, load increase, starting and in the collective form of driving cycles. The biofuels that are considered in the present study are:
- a)
Bio-ethanol (ethanol), and
- b)
Bio-butanol (n-butanol)
The analysis that follows will primarily focus on the two most influential diesel engine pollutants, PM and NOx, but results for carbon monoxide (CO) and unburned hydrocarbons (HC), as well as for unregulated exhaust emissions, CO2 and combustion noise radiation will also be presented. The usual approach when analyzing alternative fuel impacts on exhaust emissions is by discussing the differing physical and chemical properties of the various blends against those of the reference fuel. Consequently, the composition and properties of the ethanol and n-butanol, together with their combustion and emission formation mechanisms, will form the basis for the interpretation of the experimental findings. As of equal importance, emphasis will be placed on the discrepancies encountered during transients too, which may enhance or alleviate the differences observed between the biofuel blends and the neat diesel fuel operation.
Section snippets
Ethanol
Alcohols are defined by the presence of a hydroxyl group (–OH) attached to one of the carbon atoms. Ethanol, in particular, (or ethyl alcohol) is a biomass-based renewable fuel (bio-ethanol), which can be produced, relatively easily and with low cost, by alcoholic fermentation of sugar from vegetable materials, such as corn, sugar cane, sugar beets, barley, and from (non-food) agricultural residues such as straw, feedstock and waste woods [6], [7], [14]. Ethanol is isomeric with dimethylether
Historical overview
Table A in the Appendix provides a list of the published papers in International Journals and well established conferences, as well as of the reports from renowned research centers that all deal with exhaust emissions during (truly) transient conditions, when the engine runs on ethanol or n-butanol/diesel fuel blends [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64] (hence, no steady-state cycles, such
Exhaust emissions
As an opening argument for the exhaust emissions discussion that follows, it can be stated that irrespective of the biofuel tested, spray properties may be altered with respect to normal diesel operation owing to differences in the physical and chemical properties such as molecular structure, cetane number, latent heat of vaporization, viscosity, surface tension, bulk modulus of elasticity, and boiling point. All these, in turn, affect the injection timing, the ignition delay, as well as the
Conclusions
A review was conducted of the literature concerning emissions of diesel engines when running on ethanol or n-butanol/diesel fuel blends during transient conditions. The main mechanisms of transient emissions were identified and discussed for all exhaust pollutants, with many of those mechanisms being interrelated with the inherent discrepancies observed during transients, most notably turbocharger lag. The most important conclusions derived are summarized as follows:
- 1)
With only few exceptions,
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