Gross carbon emissions from alternative transport ...

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Gross carbon emissions from alternative transport fuels in India Ravi Prakash Department of Mechanical Engineering, Motilal Nehru National Institute of Technology Allahabad (UP) 211 004, India Alex Henham School of Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, UK E-mail: [email protected] Inder Krishan Bhat Department of Applied Mechanics, Motilal Nehru National Institute of Technology, Allahabad (UP) 211 004, India

The objective of the paper is to present a life-cycle assessment of pollution from alternative transport fuels in Indian conditions. The present study is limited to gross carbon emission (as CO2) because of its serious implications for global warming. Indicative values of gross carbon emissions from bioethanol, CNG and electrical energy sources have been evaluated from industrial data and compared with those from oil. The energy and environmental analyses of bioethanol production in India show that bioethanol can significantly reduce global CO2 emissions and, if used as a petrol blend, can help reduce oil imports as well as aromatics pollution from unleaded petrol (now introduced into India). The method used in this research has two main components. The first is an examination of each energy industry in detail, using primary sources of data from power stations, oil refineries and anhydrous ethanol production from molasses. The processes involved in each case are examined, taking into account energy use in any necessary auxiliary activities to evaluate the total carbon emissions. The second component is a detailed examination of one specific form of public transport. This is a three-wheeled 8-seater used in the city of Lucknow in North India. It is chosen because it is available with a petrol or compressed natural gas (CNG) spark-ignition engine (and hence could alternatively be ethanol-fuelled) and in a battery-electric version. Both parts of this data-gathering have been specific to the situation in India. In energy conversion the refinery crude composition and processes, basic resources of biomass and the mix of primary energy for electricity generation are different in each country. The types of vehicle used also vary considerably from region to region. It is observed that while CNG and electric-powered vehicles may have low and zero tailpipe emissions respectively, gross pollution from such vehicles and their associated resource systems may be significant. In the case of electrically-propelled vehicles the gross carbon emission is comparable with that for similar petrol-engine vehicles since about 80 % of electricity production in India is fossil-fuel-based. In comparison, CNG shows a reduction of about a third. Alcohol-fuelled vehicles, by comparison, can show zero net carbon emission. The importance of gross pollution assessments in rational choice of a fuel cannot be overemphasised. 1. Introduction A life-cycle or so called ‘‘well-to-wheel’’ analysis of a fuel draws attention to the fact that CO2 is produced not only in the combustion of a fuel at the point of use but also during extraction, refining and transportation of the fuel. This indirect CO2 production is generally associated with energy inputs in these processes but may also be related to the inherent nature of the processes involved (Figure 1). The sum total of such direct and indirect CO2 emissions may be termed gross CO2 emissions. It should be pointed out here that, apart from CO2, emission of other 10

Energy for Sustainable Development

polluting agents from a fuel such as SOx, NOx, particulates, aldehydes and lead might also be considered. The present study is limited to CO2 emission because of its serious implications for global warming. For a transport fuel, the term ‘‘life-cycle’’ refers to all the events that begin from the source and end at the wheel. In particular it includes stages of feedstock extraction, fuel processing and refining, fuel transport, fuel storage and distribution, and finally combustion in the engine of a transport vehicle to power its wheels. As a practical example, gross CO2 emission has been

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Figure 1. Net energy and gross CO2 emissions G = gross energy produced by combustion of fuel F = total feedback energy in fuel production processes 1, 2 and 3 = F1 + F2 + F3 N = net energy available from the fuel = G - F

evaluated for an important alternative transport fuel, bioethanol produced in Indian conditions, and this has been compared with that for oil, compressed natural gas (CNG) and electricity. A new figure of merit for grading a fuel was proposed by the authors in earlier papers [Prakash et al., 1998; 2000] -- linking net energy and gross pollution from a fuel, where bioethanol was taken as an example. Now this work has been extended and the current paper assesses the gross pollution from various transport fuels on a per kilometre basis, when actually used in similar passenger vehicles for public transport under Indian conditions. 2. Significance of bioethanol as petroleum substitute in India India is one of the largest sugar-cane producers in the world and its sugar industry is the second largest among the Indian process industries, next only to cotton textiles [Gehlawat, 1990]. The estimated annual sugar-cane production in India [MoF, 1997] is 274 million tonnes (Mt) of which about 51 % is processed in sugar mills, 39 % is used in small gur and khandsari (raw and crude sugar) units and 10 % is used as seed material [Ravindranath and Hall, 1995]. The main by-products of the sugar industry are bagasse and molasses. Molasses accounts for about 5 % of the mass of the cane crushed and a yield of 285 litres (l) of ethanol/t of molasses can be achieved [Gehlawat, 1990]. Considering only the molasses availEnergy for Sustainable Development

able from sugar mills, this source can potentially produce two million m3 of ethanol a year. The annual consumption of petrol in road transport in India [TERI, 1997] is about 4.7 million m3. The calorific value of ethanol is 21.1 MJ/l compared with 31.8 MJ/l of petrol [Yacoub et al., 1998], resulting in a potential of petrol substitution by ethanol in road transport of about 28 % (on equivalent energy basis) under Indian conditions. From practical considerations, however, it would be easier to introduce gasohol (petrol containing 10 % anhydrous ethanol by volume) as a transport fuel, since the introduction of this blend would require no engine modifications and vehicle volumetric fuel consumption essentially remains unchanged [SEIS, 1980]. With the introduction of gasohol, the annual petrol saving potential in road transport would be approximately 0.5 million m3 at the current level of petrol consumption in India. Such a substitution should directly reduce petroleum imports and replace octane-boosting lead alkyls in petrol, as have been done successfully in many countries [Hall and House, 1995]. Blending of ethanol with petrol provides additional benefits. The changes in refinery operations that are required to produce fuel of the same octane number without lead reduce the quantity of fuel that can be produced from a barrel of crude oil. This is because reforming lower-octane-rating hydrocarbon components to increase the percentage of more complex octane-boosting molecules alters l

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Table 1. Process energy requirements Process

Fermentation

Energy consumption Energy recovered MJth/l MJth/l 0.95

Distillation

11.88

Dehydration

4.84

Effluent treatment

3.30

Auxiliary equipment

0.21

Total

21.18

11.27 (biogas)

11.27

the chemical constitution of the petrol. This reforming process consumes additional energy in the refining process -- energy directly lost from every barrel processed. The addition of ethanol to petrol effectively gives the required octane boost and the reforming requirement is correspondingly reduced. This means that every barrel of petrol blended with alcohol produced decreases crude oil demand, not only by the quantity of petrol directly replaced by ethanol but also by the crude oil saved through the value of ethanol as an octane enhancer [SEIS, 1980]. Unleaded petrol is now available in India but its use can create its own problems. Fuels containing high proportions of aromatics and olefins produce relatively higher concentrations of hydrocarbon compounds that have a potential to participate in reactions leading to the production of the harmful photochemical smog. In addition, some aromatic compounds are known to be carcinogenic and nerve toxins. For these reasons, the current trend favours the lowering of aromatics content in petrol [Al-Farayedhi et al., 2000]. 3. Gross carbon emission from anhydrous ethanol in India In the case where bioethanol is to be used in India as a petrol blend in road transport without engine modifications, the use of anhydrous ethanol is essential [SEIS, 1980]. Hence it is important to carry out energy and environmental analysis of anhydrous ethanol production from molasses as practised in India. With this objective, energy inputs in ethanol production were obtained from a representative industrial alcohol plant located in the state of Uttar Pradesh (UP), India. The plant, which has a production capacity of 100 m3/day, is operated on a three-shift basis (24 h/day). The production process consists of three stages: fermentation, conventional distillation and dehydration, followed by effluent treatment that is now mandatory for all distilleries. Energy consumption in each of these stages is in the form of process steam and power derived from backpressure steam turbines. These turbines use steam generated at 4.5 MPa (gauge) from bagasse-fired boilers. Bagasse is obtained through backward integration of the distillery with a sugar mill having a cane-crushing capacity of 8000 t/day. The mill-wet bagasse contains about 50 % moisture and has a calorific value [Gehlawat, 1990] 12


of 9.5 MJ/kg. Data recorded from the boiler and the back-pressure turbine used gave the following results: • 1 kg of steam generation requires 0.45 kg of bagasse, i.e., 4.3 MJ of primary energy. • 1 kWh of power generation requires 7 kg of steam, i.e., 30 MJ of primary energy. • About 1400 m3 of spent wash produced per day from 100 m3/day of distillate is treated biologically via anaerobic digestion, generating biogas. Approximately 35 m3 of biogas is generated per m3 of spent wash. This biogas, containing about 60 % methane and having an approximate calorific value 23 MJ/m3, is fed directly into the boilers to save bagasse. The energy consumption recorded during various stages of ethanol manufacture is summarized in Table 1 and more detail may be found in a previous paper by the authors [Prakash et al., 1990]. 4. Carbon emissions and uptake There are significant carbon emissions in the form of CO2 during the production process of ethanol. A large amount of CO2 is released during fermentation, as well as in the burning of biogas and bagasse in the boilers used. CO2 would also be released in transporting ethanol from the distillery to the point of use and, of course, in its eventual combustion. In all of the above processes (except traditional transportation), however, the raw material used (molasses) and energy inputs (bagasse and biogas) are derived from biomass (sugar-cane) from the nearby fields. Therefore, one can safely assume that much of the carbon released is eventually absorbed through photosynthesis in sugar-cane. Hence, in this case, gross carbon emissions minus carbon uptake may be considered to be nil or, at most, very small. 5. Gross carbon emissions from oil and CNG An accurate assessment of gross carbon emissions from a fuel requires a detailed energy analysis of its production process. However, indicative values of carbon release rates (as CO2) for fossil fuel processing and combustion are available [Goldemberg et al., 1988] and are given below: Gross carbon emissions from natural gas

13.5 kg per GJ released in combustion

Gross carbon emissions from petrol

19.9 kg per GJ released in combustion

Specific energy content of natural gas [Baruah, 1993]

46 MJ/kg

Specific energy content of petrol [Yacoub et al., 1998]

42.9 MJ/kg

Hence, gross carbon emissions from natural gas = (0.0135 kg/MJ) × (46 MJ/kg) = 0.62 kg C/kg of fuel and gross carbon emissions from oil = (0.0199 kg/MJ) × (42.9 MJ/kg) = 0.85 kg C/kg of fuel To obtain the feedback energy requirement for CNG, energy data for compression were obtained from the Gas l

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Figure 2. Typical Vikram vehicles: 410P petrol-engined (left) and EV electric-powered (right)

Authority of India Ltd as follows. • In a typical CNG plant, natural gas is compressed from about 40 bar to 250 bar through reciprocating compressors in a two-stage process. • The total electricity consumption in the process (compressor motors, oil pumps, cooling water pumps, valves, etc.) was estimated to be in the range 0.6-0.7 kWhe/kg of natural gas. • The initial compression of natural gas to 40 bar from the lowest pressure of about 3 bar consumes an additional 0.2 kWh e/kg of natural gas. • Hence, the aggregate electricity consumption in compression averages about 0.85 kWhe/kg of natural gas. Carbon emissions (as CO 2) in conventional (coal-based) electricity generation [Brown, 1992] are approximately 0.25 kg C/kWh e. About 80 % of the utility power generation in India [MoF, 2001] is thermal (mainly coalbased) and the remaining 20 % comes from carbon-free (hydro and nuclear) resources. Therefore, 1 kWhe power generation in India is associated with approximately 0.2 kg C emission. Hence, gross carbon emission from 1 kg CNG = 0.62 + 0.85 × 0.2 = 0.79 kg C 6. Gross carbon emission from electric vehicles To estimate gross carbon emissions from electric vehicles, practical data was obtained from Scooters India Limited (SIL) at Lucknow (Uttar Pradesh), India. SIL is involved in the manufacture, running and maintenance of its fleet of 8-seater three-wheelers. These are called Vikram ‘‘tempos’’ [1] and are used for public transport in the city (Figure 2). Each vehicle uses 12 lead-acid traction batteries (6 V, 200 Ah) which run a DC series motor (72 V, 5.5 kW). The average range of the vehicle on one charge is about 100 km and the data recorded from the charging station shows electricity consumption in the range 16-18 Energy for Sustainable Development

kWhe for fully charging a discharged battery bank. Since 1 kWhe power generation in India is associated with approximately 0.2 kg C emission (as in the above paragraph), gross carbon emission from SIL’s electric vehicles is estimated as: [(0.2 kg C/kWhe) × (17 kWh e)]/[(100 km) × (8 passengers)] = 4.3 g C/passenger-km 7. Comparative assessment of gross carbon emissions from various transport fuels Apart from manufacturing electric vehicles, SIL is also involved in the manufacture of petrol- and CNG-driven 8-seater three-wheelers for public transport. These are also known as Vikram tempos as they are similar to the electric vehicles in design, but have an engine of 3.4 kW (200 cm3, 2-stroke) and steel chassis, unlike the fibre-reinforced plastics used for electric vehicles. There is also a diesel vehicle but this is smaller and not directly comparable so has been omitted from this study. Fuel consumption in the petrol and CNG-driven tempos was observed as follows. 1 kg CNG is required for 35 km average run or 1 l petrol for an average run of 18 km. Considering gross carbon emission from petrol and CNG per kg of fuel, gross carbon emission from SIL’s tempos is evaluated as: 4.4 g C/passenger-km for petrol-driven vehicles and 2.8 g C/passenger-km for CNG-driven vehicles. A comparison of the gross carbon emissions from various transport fuels in Indian conditions is shown in Table 2. 8. Conclusions It is concluded that bioethanol, as produced in India, can play a significant role in reducing life-cycle carbon emissions. If used as a petrol blend, it can help reduce oil imports as well as reduce aromatics pollution from unl

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Table 2. Gross carbon emissions from various transport fuels Vehicle fuel

Gross carbon emission g C/passenger-km (released as CO2)

Petrol

4.4

Electricity

4.3

CNG

2.8

Bioethanol

Nil

leaded petrol. Indicative values of gross carbon emissions from various alternative transport fuels have been evaluated and are presented in Table 2. The table shows that gross carbon emissions from electric vehicles are significant and are comparable with those from oil-fuelled vehicles, while CNG is the least polluting among conventional fuels. This shows that although some fuels may be ‘‘clean’’ locally, they can cause considerable pollution on a global basis. The study further demonstrates that gross pollution from a fuel would decrease if clean and renewable energy resources were used in its production process, as in the case of bioethanol manufacture in India. The study needs to be extended, of course, to assess gross emissions of other pollutants from a fuel, e.g., SOx, NOx, particulates, aldehydes and lead, to obtain a comprehensive gradation of fuels, thereby helping in the rational choice of a fuel. A comparison with similar life-cycle assessments for automobile fuel/propulsion system technologies for North America is provided in Appendix A, which further corroborates the conclusions drawn above. Acknowledgements The authors gratefully acknowledge the support provided by Bajaj Hindustan Ltd., India, the Gas Authority of India Ltd. and Scooters India Ltd. in the collection of the industrial data on bioethanol, CNG and electric vehicles respectively. Note

1. The word ‘‘tempo’’ is commonly used throughout India to denote a van, especially three-wheeled, plied for hire or on a fare per passenger basis. However, it originates from the proprietary name of the first and best-known such vehicle, the (Bajaj) Tempo. References Al-Farayedhi, A.A., Al-Dawood, A.M., and Gandhidasan, P., 2000. ‘‘Effects of blending MTBE with unleaded gasoline on exhaust emission of SI engine’’, Journal of Energy Resources Technology, Vol. 122, December, pp. 239-247. Baruah, P.K., 1993. ‘‘CNG as an alternate fuel’’, Proceedings of the Ninth National Convention of Mechanical Engineers, Allied Publishers, New Delhi, pp. 233-237. Brown, L.R., 1992. State of the World, 1992, World Watch Institute Report, Horizon India Books, New Delhi, p. 36. Gehlawat, J.K., 1990. Modernization of Indian Sugar Industry, Arnold Publishers, New Delhi, pp. 256-267. General Motors Corporation, Argonne National Laboratory, BP, Exxon Mobil and Shell (GM et al.), 2001. Vol. 1: Well-to-Wheel Energy Use and Greenhouse Gas Emissions of Advanced Fuel/Vehicle Systems -- North American Analysis, Executive Summary Report, April. Goldemberg, J., Johansson, T.B., Reddy, A.K.N., and Williams, R.H., 1988. Energy for a Sustainable World, Wiley Eastern, New Delhi, p. 438. Hall, D.O., and House, J., 1995. ‘‘Biomass: an environmentally acceptable fuel for the future’’, Proc. I Mech. E, Vol. 209, Part A, pp. 203-213. MacLean, H.L., and Lave, L.B., 2003. ‘‘Evaluating automobile fuel/propulsion system technologies’’, Progress in Energy and Combustion Science, 29, pp. 1-69. Ministry of Finance (MoF), 1997. Economic Survey 1996-97, Ministry of Finance, Economic Division, Government of India, New Delhi, p. 143. Ministry of Finance (MoF), 2001. Economic Survey 2000-2001, Economic Division, Ministry of Finance, Government of India, New Delhi, p. S-29. Prakash, R., Henham, A., and Bhat, I.K., 1998. ‘‘Net energy and gross pollution from bioethanol production in India’’, Fuel, Vol. 77, No. 14, pp. 1629-1633. Prakash, R., Henham, A., and Bhat, I.K., 2000. ‘‘Bioethanol: a sustainable energy option for India’’, Proceedings of the XIII International Symposium on Alcohol Fuels, Stockholm 2000, Part III, Paper 21. Ravindranath, N.H., and Hall, D.O., 1995. Biomass, Energy and Environment, Oxford University Press, New York, pp. 201-203. Solar Energy Information Services (SEIS), 1980. Fuel from Farms: a Guide to Small Scale Ethanol Production, Solar Energy Information Services, San Mateo, California, p. 9. Tata Energy Research Institute (TERI), 1997. TERI Energy Data Directory and Year Book, 1996-97, Tata Energy Research Institute, New Delhi, p. 261. Yacoub, Y., Bata, R., and Gautam, M., 1998. ‘‘The performance and emission characteristics of C1-C5 alcohol-gasoline blends with matched oxygen content in a single cylinder sparkignition engine’’, Proc. I Mech. E, Vol. 212, Part A, pp. 363-379. Weiss, M.A., Heywood, J.B., Drake, E.M., Schafer, A., and AuYeung, F.F., 2000. On the Road in 2020: a Life-cycle Analysis of New Automobile Technologies, Energy Laboratory Report MIT EL 00-003, Energy Laboratory, Massachusetts Institute of Technology, Cambridge, MA, October.

Appendix A. Comparison with similar life-cycle assessments for automobile fuel/propulsion system technologies Comparing fuels and propulsion systems requires a comprehensive, quantitative, life-cycle approach to the analysis. It must be more encompassing than ‘‘well-towheels’’ analysis. Well-to-wheels comprises two components, the ‘‘well-to-tank’’ (all activities involved in producing the fuel) and ‘‘tank-to-wheel’’ (the operation/driving of the vehicle). The analyses must include the extraction of all raw materials, fuel production, infrastructure requirements, component manufacture, vehicle manufacture, use, and end-of-life phases (dismantling, shredding, disposal/recycling) of the vehicle. Focusing on a portion of the system can be misleading. The analysis must be quantitative and include the array of environmental discharges, as well as life-cycle cost information, since each fuel and propulsion system has its comparative advantages. Comparing systems requires knowing how 14


much better each alternative is with respect to some dimensions and how much worse it is with respect to others. Since focusing on a single stage or attribute of a system can be misleading, e.g., only tailpipe emissions, the lifecycle implications of each fuel and propulsion technology need to be explored. MacLean and Lave [2003] have provided a very detailed review of a dozen studies on the life-cycle implications of a wide range of fuels and propulsion systems that could power light-duty vehicles in the US and Canada over the next two to three decades. The studies vary in the fuel/propulsion options they consider, the environmental burdens they report and the assumptions they employ, making it difficult to compare results. All of the studies, however, include the ‘‘well-to-tank’’ and ‘‘tank-towheel’’ activities and the majority of the studies include l

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Table A1. Comparison of life-cycle inventory studies: well-to-tank efficiencies and greenhouse gas emissions Efficiency[1] (%), range

Greenhouse gas emissions [2] (g CO2 equiv./MJ fuel), range

Petrol

80-87

15-26

Diesel

83-90

12-18

Fischer-Tropsch Diesel

52-59

-

CNG

83-91

10-27

Methanol

57-66

22-41

Ethanol (corn): total energy

45-67

-19 to +90

Ethanol (corn): fossil energy

60-67

-

Ethanol (lignocellulosic): total energy

26-56

-85 to +14

Ethanol (lignocellulosic): fossil energy

83-96

-

Hydrogen (various sources)

23-76

76-332

Electricity (various sources)

29-48

127-198

Fuel/source (if applicable)

Notes 1. Efficiency (%) is defined as: (energy in the fuel delivered to consumers/energy inputs to produce and deliver the fuel) × 100, e.g., 100 MJ of energy input results in 80-87 MJ of petrol delivered to the consumer. 2. Negative GHG emission values for ethanol result from carbon sequestration during feedstock growth as well as if a credit is given for selling excess electricity (produced through cogeneration schemes) to the grid and therefore offsetting CO 2 emissions from conventional electricity generation.

Table A2. Comparison of life-cycle inventory studies: well-to-wheel greenhouse gas emissions Fuel/propulsion system

Greenhouse gas emissions (g CO2 equiv./km)

Petrol SIPI[1]

248-333 (342, 157)[2]

Diesel CIDI [3]

211-231 (298, 120)

[4]

169-176 (286, 94)

[5]

152

(242, 81)

Ethanol SIPI: lignocell. [6]

4-161

(112)[7]

Petrol fuel cell

133-201 (224, 161)

Methanol fuel cell

151-161 (199, 120)

Hydrogen fuel cell

70-241

Petrol HEV

Diesel HEV

(186, 107)

Notes 1. Petrol SIPI: petrol-fuelled spark ignition port fuel injection. 2. Table results are generally for near-term mid-size sedan. Results are dependent on vehicle size and efficiency. Since the GM/Argonne [GM et al., 2001] results are for a Silverado pick-up truck and MIT results [Weiss et al., 2000] are for the substantially improved Toyota Camry sedan (high efficiency, low weight), these results are indicated in the parentheses following the range of results of the other studies, e.g., (GM result, MIT result). 3. Diesel CIDI: diesel-fuelled compression ignition direct injection. 4. Petrol HEV: petrol-fuelled hybrid electric vehicle. 5. Diesel HEV: diesel-fuelled hybrid electric vehicle. 6. Ethanol SIPI -- lignocell.: ethanol- (from lignocellulosic feed stocks) fuelled spark ignition port fuel injection. 7. GM result only; MIT did not include ethanol in its study.


a measure of efficiency and greenhouse gas emissions associated with these activities. Comparison has been limited to these activities and measures. Table A1 provides a summary of the ranges of efficiency and greenhouse gas emissions reported in the studies for the well-to-tank portion for the various options. For the well-to-tank portion for the production of electricity, renewable fuels and hydrogen, differing fuel production pathways are most important. Owing to the range of different production options for these fuels (as well as other issues such as study assumptions), results are much more variable. In addition, there is less experience with producing these fuels, resulting in more uncertainty. It is important to distinguish between total and fossil energy required for production when comparing efficiencies among the fuels. Petroleum-based fuels have the highest efficiency for the well-to-tank portion when total energy is considered. However, if only fossil energy is considered, biomass-based fuels such as ethanol become more attractive. The tank-to-wheel portions are more difficult to compare. Each study uses its selected vehicle (e.g., conventional sedans, light-weight sedans, pick-up trucks) and many present assumptions regarding the vehicle efficiencies. The studies, however, do not generally report the range of assumptions or test conditions. The well-to-wheel results (the sum of the well-to-tank and tank-to-wheel activities) of the studies are still more difficult to compare. The baseline vehicle (with a few exceptions) is a current petrol-fuelled ICE port fuel injection vehicle; it combines an efficient well-to-tank portion with a relatively inefficient tank-to-wheel portion. A direct injection diesel vehicle is considerably more efficient and therefore results in lower emissions of carbon dioxide even though the carbon content in the diesel (and hence the well-to-tank portion of the CO2 emissions) is higher than that in petrol. Fuel-cell vehicles have a high theoretical efficiency but generally a low-efficiency well-totank portion, which offsets some of the vehicle efficiency benefits. Table A2 shows the ranges of values reported in the life-cycle studies for the well-to-wheel greenhouse gas emissions. All of the fossil fuel options result in emissions of large amounts of greenhouse gases. Ethanol and hydrogen have the potential to reduce greenhouse gas emissions significantly. This, however, is highly dependent on the pathways for ethanol and hydrogen production, especially the amount of fossil fuel inputs during production. Some of the hydrogen options result in higher greenhouse gas emissions than those of a petrol ICE vehicle. Results for hybrid electric vehicles (HEVs) are dependent on the efficiency improvements over conventional vehicles that are assumed. A numerical comparison of CO 2 emission data presented in Table A2 with those reported in Table 2 should be made with caution. The large differences in numerical values arise from the differing manner in which CO2 emissions have been expressed. In Table 2, emissions are expressed in grams of carbon (only) released as CO2 per l

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passenger-km travelled. In Table A2, emissions are in grams of CO2 equivalent per km travel of the vehicle examined. CO2 equivalent refers to the amount of carbon dioxide by weight emitted into the atmosphere that would produce the same radiative forcing as a given weight of another greenhouse gas, e.g., methane or nitrogen oxides. Carbon dioxide equivalents are the product of the weight of gas being considered and its global warming potential.

Numerical differences notwithstanding, broad conclusions drawn by MacLean and Lave are very similar to what has been obtained under Indian conditions in this article: all of the fossil-fuelled vehicles (including electricity-driven) result in large GHG emissions. The two options that have potential for the largest GHG emission reductions are the ethanol and the hydrogen-fuelled vehicles if the fuels are produced with little or no fossil fuel inputs.

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