Diesel Engines and Fuels: a Wide Range of Evolutions to Come – General Context and Research Themes

Les biocarburants, éthanol et ETBE ou biodiesel (de première génération ou de synthèse) peuvent être incorporés dans les carburants classiques à des taux d’introduction variables : en mélange banalisé à 5 % volume pour l’éthanol et le biodiesel aujourd’hui en Europe, avec une probable extension à 5,75 % en 2010 (7 % en France) puis 10 % en 2020 (projet de directives européennes), 10 % d’éthanol aux États-Unis, 22-25 % au Brésil, répondant à une volonté politique d’accroître la part des carburants ex-biomasse. Ces biocarburants peuvent être utilisés à des taux d’introduction plus élevés : en France, on citera des autobus ou des véhicules de flottes urbaines alimentés par un carburant contenant 70 % de gazole et 30 % de biodiesel, fonctionnant sous régime dérogatoire. Parallèlement à cette solution qui est mise en oeuvre sans aucune modification, il existe des moteurs Diesel adaptés pour fonctionner avec du biodiesel pur, moyennant quelques aménagements, concernant notamment les matériaux et la calibration du système d’injection. De même, il existe ce que l’on appelle communément les véhicules « FlexFuel », développés pour utiliser indifféremment plusieurs types de carburants à base d’essence et d’éthanol, comme l’E85 promu en Suède et plus récemment en France. Enfin signalons qu’outre ces performances en termes de CO 2 , les biocarburants utilisés sur des moteurs conventionnels permettent des réductions de 10 à 20 % des émissions de monoxyde de carbone (CO), hydrocarbures imbrûlés (HC), particules, Abstract – Diesel Engines and Fuels: a Wide Range of Evolutions to Come – General Context and Research Themes – The first article of this special OGST issue “Diesel Engines: a wide range of evolutions to come” is an introduction to throw light on the global context both from the point of view of energy production (resources: oil and its alternatives) and from the point of view of energy consumption (transport and thermal engines). It then describes the current position of the “Diesel fuel/engine” pair in this context and provides an update on the research themes discussed throughout this issue.


A GLOBAL ENERGY CONTEXT CONTROLLING THE FUTURE…
Before focusing on the main theme of this issue, the "Diesel fuel/engine" pair, it is worthwhile first mentioning the overall energy context controlling the future. One of the main concerns of our century is the depletion of fossil fuels and their replacement in order to guarantee the economic development of both the industrialised countries and that of every country across the globe, even if the economic model of this development has still probably not yet been invented. Behind this question lie two major challenges, for which sustainable solutions acceptable for our economies must be found: firstly the climatic change, consequence of the continued increased in greenhouse gases, especially carbon dioxide (CO 2 ), and secondly the guarantee of our energy supplies. Ultimately, it is the chain of successive efficiencies of all the conversions which will define the efficiency of the global energy system, where less than 40% of the primary energy consumed finally corresponds to useful energy, the rest being largely dissipated or lost as heat in the atmosphere, in particular. Most of this "loss" can be attributed to two sectors: -electricity generation, from fuels with efficiencies from 30% to 55% for the most efficient installations (natural gas combined cycle); -transport, where the efficiencies of internal combustion engines do not generally exceed 20%. Development in this sector remains very strong, due in particular to the high requirement in goods transport caused by generalisation of production according to the just-in-time principle and globalisation of activities, with production and storage delocalised from the sale or consumption sites.

TRANSPORT AND OIL… ...a Long History in Common...
In the past, if we concentrate on the transport sector, oil (and its derived products, mainly gasoline, jet fuel and Diesel fuel) appeared, in one sense, as an alternative energy to coal and wood, offering the possibility of new low-cost technological developments. Its rapid growth led to this unprecedented boom in transport. Oil currently holds a virtual monopoly on the transport market, representing over 95% of the energy requirements in this sector, distributed between four main modes: road, with a share of about 1.6 billion tonnes (more than 75% of the total), rail, air and sea (Source: Oil and Gas information, 2005 data -2007 edition -IEA International Energy Agency/OECD Organisation for Economic Cooperation and Development).
In this key field of road transport, the overwhelming majority of thermal engines (spark or compression ignition) are fuelled by hydrocarbon fuels, a situation which is likely to continue: for the time being, and probably in the short and medium term, there is no real challenge to this type of energy converter. This situation, which has prevailed for more than a century, is the result of continuous technological evolutions in numerous fields ranging from the mutual adaptations between engine and fuel to depollution of exhaust gases, in addition to combustion of course. We therefore observe the emergence of a landscape in which the "oil/thermal engine" pair plays a leading role. One of the challenges for the years and decades to come will be to find alternatives to this pair, or to at least one of its two protagonists, i.e. oil.

Alternatives to Oil...
A first alternative tends to replace oil and its derivatives by biofuels which offer a globally favourable well-to-wheel CO 2 balance, even though some there still remain some difficulties in these evaluations (emissions of N 2 O nitrous oxide, modification in land use, methodology used to allocate greenhouse gases to coproducts, etc.). Currently, biofuels correspond firstly to ethanol (from sugar plants: sugar beet, sugar cane or starch-rich cereals such as wheat, maize) and its derivative ETBE ethyl tertio butyl ether, obtained by reaction of ethanol with isobutene (produced in refinery or in petrochemistry), and secondly biodiesel which is the result of transesterification of vegetable oils, mainly rape oil in Europe. These two families are known as first-generation biofuels. Analysis of the life cycle of this family of biofuels reveals gains in well-to-wheel CO 2 emissions, compared with a traditional fuel, ranging from 30% to 87% for gasolines and 39% to 64% for Diesel fuel for products used pure (Source: "Well-to-Wheels Analysis of Future Automotive Fuels and Powertrains in the European Context", WTW Report, Version 2c, March 2007). In the future, we will see the development of biofuels obtained using new processes and new raw materials: wood, vegetable waste and more generally lignocellulosic material... known as second-generation biofuels, offering an even more favourable well-to-wheel CO 2 balance, with a reduction of up to 90% and more (same source). Several methods are considered: the lowtemperature biochemical or enzyme pathways or thermal treatment of biomass (wood, straw, waste, etc.). The synthesis gases obtained may lead to the production of ethanol or synthetic biodiesel using, for example, the Fischer-Tropsch synthesis. Another technique can be used to produce biodiesel: deep hydrotreatment of vegetable (or animal) oils followed by isomerisation, resulting in a high-potential paraffin (currently used to produce the "second-generation biofuel" called NexBtl). The biofuels, ethanol and ETBE or biodiesel (first generation or synthetic) can be incorporated in varying proportions in the traditional fuels: in standard blend of 5% by volume for ethanol and biodiesel currently in Europe, with a probable extension to 5.75% in 2010 (7% in France) then 10% in 2020 (European directives project), 10% ethanol in the United States, 22-25% in Brazil, in response to a political objective to increase the share of fuels ex-biomass. These biofuels can be added in higher blend rates: in France, this is the case with buses or urban vehicle fleets running on fuel containing 70% Diesel fuel and 30% biodiesel, operating under a tax exoneration scheme. Along with this solution, implemented without any modifications, some Diesel engines have been adapted to operate with pure biodiesel, given several improvements concerning in particular the materials and calibration of the injection system. Other "FlexFuel" vehicles have also been developed to run on a number of different types of gasoline-and ethanol-based fuels, such as E85 promoted in Sweden and more recently in France. Lastly, note that in addition to this performance in terms of CO 2 , biofuels used on conventional engines offer 10% to 20% reductions in emissions of carbon monoxide (CO), unburnt hydrocarbons (HC), particulates, with no change in efficiency. Greater attention must nevertheless be paid to aldehyde emissions from vehicles running on ethanol. The media and the political authorities are currently paying close attention to the development of these liquid biofuels, ethanol and biodiesel, which in practice are the only ones that can progressively replace oil in the transport sector while allowing full use of the current transport and distribution logistics: in 2006 globally the share of transport in the use of renewable energies is still virtually negligible, but the number of projects has multiplied over the last few months in attempts to reduce both the dependence on oil and the CO 2 emissions from road transport.
Another key stake is the evaluation of the resources truly available in the medium and long term in the various regions of the world, currently estimated at about 20% of world consumption. The fact that crude oil and natural gas prices are likely to remain high in the long term is obviously a favourable factor which is an essential condition to support this trend. However, this market is still closely related to partial or total exemptions of taxes levied on petroleum fuels or to mechanisms imposing their incorporation (replacement of MTBE methyl tertio butyl ether in the United States, General Tax on Polluting Activities -TGAP -in France, etc.), but also highly sensitive to tensions on the prices of raw agricultural materials, which reflect the balance at world scale between supply (climatic hazards) and demand, whether in terms of food or energy.

Alternatives and Reinforcements for the Traditional Thermal Engine...
The second alternative is replacement of the thermal engine. The non-conventional engines which have reached a sufficiently advanced stage of development to be either already on the market or considered for industrial production before 2020 for large-scale distribution in the future decades include electric vehicles, hybrid vehicles and vehicles with internal combustion engines dedicated to natural gas. Electric vehicles have always generated considerable interest due to their intrinsic advantages: no local emission of pollutants, very low noise emissions, high starting torque making urban driving extremely pleasant. They have been under development for more than a century and have benefited from considerable state incentives, in particular recently with the zero emission vehicles (ZEVs) in California. Although large scale demonstration operations have been carried out, this type of vehicle has never obtained the success expected and distribution has remained very low. The main problem concerns the limited performance and poor autonomy of these vehicles, typically 100 to 200 km in real use. This situation is largely due to the still insufficient performance of the batteries used to store electrical energy on board the vehicle. Despite the implementation of new technologies and the progress already made or expected, we cannot realistically expect to see a significant increase in the energy density of the batteries. The improvements expected between 2005 and 2020 suggest that the energy density of a highperformance battery, typically 120 to 150 Wh/kg, will remain well below that of a liquid fuel, which is about 12 500 Wh/kg. Prerequisites for widespread use of this engine will therefore be insufficient or prohibitively expensive hydrocarbon fuel, as well as the availability of electricity not produced by the combustion of fossil fuel. This latter point is still far from being a reality on a global scale, even though some countries, such as France with nuclear energy or Iceland with geothermal energy, have already made significant progress. In contrast, the hybrid vehicle equipped with mixed thermal/electric propulsion partially fills this gap. It is equipped with two energy storage systems, a fuel tank and a battery. It also has two types of propulsion, a thermal engine and an electric motor. In the most flexible configuration, all combinations are theoretically possible, the thermal engine can be used to recharge the batteries and drive the vehicle, the electric motor can be used to propel the vehicle and also to recover its braking energy. Hybridisation therefore offers numerous ways of optimising the use of energy on board the vehicle. Hybrid vehicles considerably reduce pollutant emissions (all-electrical operation is even possible in town, for example) as well as consumption (30% to 40% reduction is possible).
We must nevertheless remember that hybrid vehicles must carry two separate drive systems as well as an electrical energy storage system (batteries or super-capacitors) and the power electronics. This leads to a substantial extra cost and a non-negligible increase in vehicle weight. However, hybridisation offers the advantage of being highly modular, opening the way to a complete range of possibilities between light hybridisation, with moderate cost and performance, up to total hybridisation, with high cost and performance. With the so-called "flexfuel" application, in particular, it also offers the possibility of incorporating a greater or lesser proportion of alternative fuels.
Lastly, the engine dedicated to natural gas is also considered to be an excellent candidate, in particular due to the special qualities of this fuel. Concerning pollutants, emissions are potentially lower than those of conventional engines due to the gas properties, and toxicity and reactivity in the atmosphere are lower due to the gas composition. In addition, car manufacturers can take advantage of the fact that natural gas has a fairly high octane number (about 130) to increase the engine efficiency. Lastly, since methane, the main component of natural gas, has a low carbon-tohydrogen ratio, CO 2 emissions are considerably reduced compared with petroleum-based fuels (approximately -23% for equivalent energy production). An engine optimised to run on natural gas can claim a reduction in CO 2 emissions of about 5% to 10% compared with a Diesel engine. As with all gas fuels, however, autonomy may be a problem. Based on a well-to-wheel balance analysis, the use of natural gas in hybrid engines is potentially one of the most efficient solutions after use of biofuel regarding CO 2 emissions. From the technological aspect, natural gas engines are often produced by converting existing Diesel or gasoline engines, since the relatively narrow market for this type of engine does not encourage specific large-scale developments. When optimising engines to run on natural gas, two approaches can be considered. The first is based on a deep downsizing approach, in other words reduction of engine displacement with same performance, associated with turbo-supercharging; the next step, resulting in even better performance, consists in integrating this type of engine in a hybrid vehicle. We can expect to see mass production of these advanced engines both for light vehicle applications as well as for buses and urban vehicles, as part of the European Union incentives to contribute to the Kyoto objectives. The European Commission has in fact published a directive aimed at progressive substitution of conventional fuels by natural gas (2% in 2010, 5% in 2015, 10% in 2020).
Summing up, although there are serious challengers to this "oil/thermal engine" pair, in the short term these alternatives will be of little use in helping to achieve an energy transition in a sector where oil still holds a virtual monopoly. In fact, considering the need to modify the industrial tools and the associated time scales, it would be quite unrealistic to set an early deadline for massive introduction of these new approaches.
In a key sector like road transport, the "oil/thermal engine" pair is therefore a necessity, at least for the time being.

Fuel/Engine Pairs…
The "oil/thermal engine" pair is available in two possible "fuel/engine" combinations.
Premium grade gasoline (including ordinary gasoline) is associated with the spark-ignition engine. This "lead-free" fuel, found virtually throughout the world, is almost exclusively dedicated to private vehicles, a few utility vehicles and motorcycles. Global consumption is about 950 Mt per year (Source: IFP estimations according to IEA International Energy Agency and PEL/KBC data). It still is the reference fuel for cars.
In Europe in general, however, and especially in France, we observe a steady decline in consumption: in France, it has dropped from 15.7 Mt in 1995 to 10.3 Mt in 2006 (Source: CPDP -Professional Committee for Petroleum -2006).
Its production in the refinery is complex, requiring several successive or parallel processes after atmospheric distillation of crude oil to meet both quality (isomerisation, reforming, alkylation, hydrodesulphurisation) and quantity (conversion such as catalytic cracking) requirements.
The "Diesel fuel/engine" pair is also used for private vehicles (mainly in Europe and more especially in France), much more extensively for utility vehicles and almost exclusively for heavy goods vehicles, coaches and urban buses, as well as some marine applications. Global consumption of Diesel fuel represents about 650 Mt (Source: IFP estimations according to IEA and PEL/KBC data) and is also increasing.
In France, consumption has increased from 22.9 Mt in 1995 to 31.9 Mt in 2006, with 42% for cars, 21% light utility vehicles and 34% heavy goods vehicles (Source: CPDP-2006). Development concerns all three sectors, but with a very strong increase over the last few years in the private vehicle market (more than two thirds of the growth) with more than 71%  For many years, this Diesel fuel was easier to produce than premium grade gasoline, only requiring desulphurisation in most cases. Current quantity and quality regulations call for much deeper desulphurisation, but above all the use of catalytic hydrocracking (a large hydrogen consumer).
Since the early seventies in the OECD countries and still up to today, in particular with the rapid development of urban pollution in countries like India and China, the main stake for the public authorities has been to improve the fuel quality to allow a progressive reduction in pollutant emissions (mainly carbon monoxide, unburnt hydrocarbons, nitrogen oxides, particulates and sulphur oxides). In Europe, this led to the creation of directives, amended a number of times, with a new version in the pipeline.
These changes have been, and still are, the subject of heated debates between the automotive industry using these fuels and the petroleum industry responsible for their production. Since the start of the decade, Europe has focused on the sulphur content of these fuels, which will be reduced to below 10 ppm from 2009.
The qualities of these two fuels, Diesel fuel and premium grade gasoline, will nevertheless vary significantly from one part of the world to another in terms of key characteristics, such as the octane and cetane numbers, the sulphur and aromatic contents. Consequently, the compositions and the refining processes implemented in the refineries will also vary considerably.

Predominant Pair: "Diesel Fuel/Engine"
This special issue is dedicated to the "Diesel fuel/engine" pair which, following this analysis, stands out as the key player in this essential field of transport.
The Diesel engine has now become a vital component of the transport sector, in view of its performance in terms of efficiency and therefore CO 2 emissions some 25% less than a traditional gasoline engine of equivalent performance. This property is even more interesting since further progress can still be expected via downsizing. The leading European car manufacturers have commercialised families of Diesel engines which, although having small displacement (1.2 l to 1.5 l), offer very high specific performance (specific torque 150-180 Nm per litre, specific power 50-60 kW per litre). Compared with the larger engines, they are replacing, they offer a further 5%-10% reduction in consumption. This improvement has been achieved by the development of two key technologies: direct high pressure injection and turbo-supercharging, in particular variable geometry turbo-supercharging. The performance offered by Diesel engines is now as good as or even better than that of gasoline engines. This evolution is such that most manufacturers now stress the excellent compromise it offers between performance (specific power and torque respectively 75 kW/l and 200 Nm/l for the most advanced engines) and its low CO 2 emissions. The value given for most mid-range Diesel vehicles is in fact less than 140 g/km, and B-segment Diesel vehicles ("city" or "subcompact" cars) are now appearing on the market with values of around 100 g/km, a figure which until now was reserved for the small A-segment Diesel vehicles (minis).
Consequently, the real stake for the future of the Diesel engine is not related to the level of efficiency it will have to reach, since it is already excellent, and the evolutions expected from injection and turbo-supercharging will bring further progress. The real stake for the Diesel engine lies more in its ability to comply with the future standards on pollutant emissions.
In Europe, standards 2005 (EURO IV) and the future standards to be expected by 2008-2010 (EURO V) are different for Diesel-and gasoline-engined vehicles. Since combustion takes place in conditions of excess air, the Diesel engine cannot benefit from three-way catalysis to reduce nitrogen oxide (NO X ) emissions. This explains why the statutory limit for NO x emissions for Diesel engines is less strict than that for gasoline engines, to promote the development of this type of engine which offers the best efficiency level. Treatment of NO x emissions nevertheless remains a central point and actions through optimisation of combustion and complex deNO x type depollution systems (already used in Europe on heavy goods vehicles since 2006) will be required to move towards "fuel neutral" standards, i.e. harmonisation of emission standards between Diesel and gasoline engines. This is already the case in the United States for the Tier 2 standard applicable between 2004 and 2009. The soot levels required by the Euro V standard in Europe should impose the systematic installation of particle filter technology to meet increasingly stringent regulations.
The progressive introduction of new "low temperature" combustion modes on a less and less restricted operating range of the engine now seems necessary to meet the future standards (EURO V and VI). The principle of these new combustion processes is to make the air-fuel mixture much more homogeneous in the combustion chamber. By lowering the combustion temperature and avoiding zones too rich in fuel, the formation of NO x and soot is significantly reduced. While in a traditional Diesel engine combustion is controlled by the amount of fuel injected, in this case it is mainly the auto-ignition process which must be controlled. Unfortunately, this type of approach tends to increase emissions of noise, carbon monoxide and unburnt hydrocarbons. These problems are solved by the use of multiple injection strategies adapted to control noise and the use of an oxidation catalyst or specific technologies such as variable valve actuation (VVA) to eliminate pollutants (HC and CO), or higher supercharging. A combination between new combustion processes reducing raw emissions together with advanced post-treatment systems should therefore enable the Diesel engine to comply with the future anti-pollution regulations, despite their increasing severity, whilst preserving its leadership in terms of consumption and CO 2 emission.
In Europe, the main stakes which will concern the "Diesel fuel/engine" pair over the next few years can be summarised as follows: -Possible need to adapt fuel characteristics to the new Diesel combustion modes (HCCI -Homogeneous Charge Compression Ignition, LTC -Low Temperature Combustion, etc.): what cetane number, what volatility, etc.? The fuel will play a major role in combustion control for these new technologies. It is clear that a fuel formulated to offer perfect control of vaporisation, the auto-ignition phase and the combustion process will obviously go a long way towards maximising energy recovery. We might therefore expect development of these new combustion modes to be accompanied by more radical changes in the fuels, with in particular more emphasis placed on the chemical phenomena occurring during combustion, which could lead to a revision of key parameters such as the distillation curve and the cetane number, and to the emergence of new criteria better suited to representing the combustion process. A move towards a "more technological" fuel could be considered.
-The possible capacity for the European refining industry to produce additional quantities of Diesel fuel in a climate which is relatively uncertain, both in terms of supply and demand.
There are numerous questions concerning the trend in the actual demand: will the percentage of private vehicles running on Diesel fuel continue to increase? Will there remain a residual demand in domestic fuel oil? Will there be a shift in consumption for bunker from "Low Sulphur Content (LSC) or High Sulphur Content (HSC) fuel oils" to marine fuel oil? Will taxation change?
Uncertainties also remain regarding the supply: quantity of biodiesel (1st and 2nd generation) or synthetic Diesel fuel available, especially via the Gas-To-Liquids (GTL) and Biomass-To-Liquids (BTL) processes? Investments in conversion capacities in a context where CO 2 emissions from European refineries are subject to national quotas? Lastly, this uncertainty is increased by the European issue concerning the continued rise in excess gasoline which currently finds a "natural" market mainly in the United States and which could be further increased with the development of ethanol incorporation and growing shortage of middle distillates (jet fuel, Diesel and/or domestic fuel oil), currently compensated by imports from Russia. Globally, the major uncertainty concerns a significant breakthrough of Diesel engines in the automotive industry of countries such as China and the United States, which would potentially significantly disturb the current refining equilibria.
In conclusion, historically marked by questions and even accusations regarding the impact on public health due to particulate emissions, the Diesel engine could take advantage of developments in depollution technologies (particle filter, SCR -Selective Catalytic Reduction), new combustion modes (LTC, HCCI) and the need to reduce CO 2 emissions to further increase its supremacy.

Main Evolutions to Come for Diesel, Associated Research Themes
The aim of this special issue is to provide an update on the evolutions to come for the "Diesel fuel/Engine" pair through the various research axes developed at IFP: -Limitation of global pollution (CO 2 ) due to the fuel through the use of new biofuels (especially through the conversion of glycerine by In particular, numerous studies concern Global airpath control for a Turbocharged Diesel HCCI Engine. Lastly, engine calibration is an essential feature used to fix the settings of the engine in its various operating zones. Comparison of Engine Calibration Methods Based on DoE provides an update on this specific theme.

Final manuscript received in April 2008
Published online in July 2008