The future RDE specifications require an evolutionary leap in internal combustion engines. A new generation of powertrains with a holistic approach and significantly enhanced functionalities is needed to meet both current and future pollutant and CO2 emission limits. In addition to powertrain electrification, further networking is also necessary to realise the “Internal Combustion Engine 4.0” described by AVL in this article.
The Internal Combustion Engine (ICE) is facing the most significant challenge in its more than one hundred years of history. So far, it has been seen primarily as the undisputed enabler of individual mobility. However, especially in Europe, the diesel engine’s emission issue and the increase of emission limits in large cities have had a lasting negative impact on its image. In addition, the uncertainty regarding possible city-access restrictions is increasingly influencing the customer’s purchasing behaviour and thus not only calls into question the future of the diesel engine in passenger cars but also, in the long term, that of the ICE in general.
Quite frequently, the enormous environmentally relevant reductions of pollutant emissions already achieved with the current ICEs designed for Euro 6d are not considered; neither the fact that the emission issue in some cities is almost exclusively caused by the existing fleet of old vehicles, but no longer by new vehicles complying with Euro 6d.
Assessing the challenges of the ICE on a rational technical basis and in a global view, the main challenge is less the pollutant issue, but rather the sustainable reduction of CO2 emissions. Basically, there are two options for meeting future CO2 fleet targets and pollutant emission limits:
:: Focus on electrification with emphasis on the highest possible proportion of electrification – Battery Electric Vehicles (BEV) and Plug-in Hybrid Electric Vehicle (PHEV) and, if necessary, promotion of a required market penetration with price support measures
:: Balanced approach with a “marketable” growth of BEV & PHEV, and a significant advancement of ICE-based powertrains.
Considering the enormous discrepancy between the required reductions of average CO2 fleet targets and their actual progression, there is a need to utilise all the technical possibilities for CO2 reduction. In conjunction with the tightened Real Driving Emissions (RDE) requirements , this also demands a sustainable improvement or new developments of ICE-based drive systems.
A NEW GENERATION OF ICE
Since real-world emissions will come into focus even more, using all the traffic and environmental information available through the networking of the automobile to optimise fuel consumption and pollutant emissions of the ICE is becoming increasingly sensible. Comparing the requirements of such a fully “Connected ICE” with the definition of “Industry 4.0” in Germany, there is a surprisingly large overlap . However, such a designation can also be derived from the history of the ICE, (1).
ICE 1.0 describes the combustion engines of the first mass-produced vehicles. Affordability and reliability were the defining parameters. In the 1960s, the first emission limits for motor vehicles were set in California, USA, defining a decisive milestone, ICE 2.0. In the following decades, the limits for pollutant emissions and fleet consumption targets to be proven on the chassis dynamometer were steadily tightened and required significantly increased variability – the “fully flexible internal combustion engine” – described here as ICE 3.0.
The future requirements regarding real world pollutant emissions as well as CO2 emissions call for significantly enhanced functionalities of the powertrain, which can no longer be allocated exclusively within the ICE itself, but are distributed to the whole powertrain (“fully flexible powertrain”). In addition, the powertrain will be networked with the vehicle environment, enabling all data useful for emission-optimal powertrain operation to be utilised, and will increasingly be powered by CO2-neutral fuels from renewable sources – “ICE 4.0.” Thus, it is perfectly opportune to call a combustion engine designed comprehensively for future needs the “ICE 4.0.”
ZERO IMPACT EMISSION
In the medium and long term, a general sales stop of ICE is increasingly being discussed, but without considering the still untapped potential of future ICE-based powertrain systems. To remain accepted, at least in the European public opinion, it is generally necessary for the ICE to lower its pollutant emissions towards a level that is no longer relevant for the environment. With Euro 6d-Temp including RDE, a decisive step in this direction has already been made. It can be assumed that such vehicles will certainly fall below the emission limit values in today’s emission-critical zones.
Zero impact emission means that the pollutant emissions are so low that they have no negative impact on air quality anymore. Looking at the pollutant emissions of Euro 6d-Temp vehicles, significant emission values only occur in non-operationally warm or highly dynamic engine operation. (2) explains this item based on the NOx emission of Euro 6d-Temp vehicles with Turbocharged Gasoline Direct Injection (TGDI) in two separate phases of the Worldwide Light vehicle Test Procedure (WLTP) test cycle.
With a warmed-up engine, the NOx emissions are in a range of 2 to 5 mg/km and thus one order of magnitude lower than in the cold phase. Based on an emission model calculation for one of the most critical air quality check points in Germany, Stuttgart Neckartor , the contribution of vehicles complying with Euro 6d-Temp to the total pollution is about 0.2 to 0.5 μg/m3 and thus both in view of the emission limit (40 μg/m3) as well as the non-automotive background emission level (about 18 μg/m3) practically negligible. Thus, for Euro 6d-Temp vehicles with gasoline engines and warmed-up condition, one could even speak here of “zero impact emission”. For the diesel engine, the values are a bit higher, but still an order of magnitude below the background emission level generated by non-automotive sources.
Consequently, the main challenge for further emission reductions with gasoline and diesel engines is in the improvement of pollutant emissions at ICE start and in non-warmed-up operating range, as well as in the temperature management of the exhaust gas aftertreatment. In addition, various measures to limit pollutant emissions are required in the upper load range.
For both areas of concern, it makes sense to use synergy effects with mild hybridisation. With a suitably designed 48 V system to manage cold run, primarily electrically, and to condition the exhaust aftertreatment by means of electrically heated catalyst, the cold emissions can be significantly reduced. Since this mild hybridisation also dampens emission-relevant dynamic peaks, even when the engine is warm (limited dynamics of the ICE), it is possible to achieve an emission level that justifies talking about “zero impact emission”.
ELECTRIFICATION OF THE ICE
In the future, the simultaneous reduction of both pollutant and CO2 emissions will require significantly enhanced functionalities of the powertrain, which cannot be represented sufficiently by the ICE alone. Both automated transmissions and electrification can help to efficiently cover these extended functionalities, which are no longer exclusively applied within the ICE itself (“fully flexible internal combustion engine”), but distributed across the whole powertrain (“fully flexible powertrain”), (3).
Clever balancing of the complementary properties of electrification and ICE results in synergetic potential for improvement both in terms of reducing pollutant and CO2 emissions. Thus, in the future the ICE will change its role from a ‘lone wolf’ to a ‘team player’.
The ongoing changes of the RDE legislation, especially the packages 3 and 4, mean a tightening of the required emission reduction by up to a factor of 10 for gasoline engines. Not only statistically relevant driving styles but to some extent non-representative driving manoeuvres such as aggressive drive-off with cold engine are counted for RDE compliance. The dynamic cold drive-off becomes a crucial criterion for safely meeting future RDE requirements.
With the gasoline engine, the strongest influence of temperature and dynamics is with the Particulate Number (PN). The short-term changes in the RDE boundary conditions require a readjustment of the PN raw emissions and/ or the Gasoline Particulate Filter (GPF). However, the too short lead time of the legislative changes makes this extremely difficult to implement in time, and will be reflected in a temporarily reduced number of vehicle variants.
Gas-powered Spark-ignition (SI) engines are largely free from combustion-generated particle emission, which increases the attractiveness of gas engines. However, in general, some measures for robust RDE fulfilment are in a trade-off to CO2 emission – the future development focus with gasoline engines. This increases the motivation to use low-cost electrification to reduce CO2 emissions and to improve existing trade-offs; for example, an optimised turbocharger layout enabled by mild hybridisation.
The current technological mainstream of gasoline engines – Turbocharged Gasoline Direct Injection (TGDI), extended expansion by using Miller and Atkinson cycle, three-way catalyst and GPF – will continue in the foreseeable future, certainly further optimised by improved turbocharging systems and partly supplemented by exhaust gas recirculation systems, (4).
In addition to this mainstream, however, a certain technology diversification remains. Variable compression ratio, spark-initiated Homogeneous Charge Compression Ignition (HCCI) and high-pressure injection are additional development routes. With increasingly adiabatic engine concepts and combined processes, maximum efficiencies of 45 to 50 % can be expected in the long term.
Future emission requirements such as stoichiometric operation in the whole engine map require either a compromise with CO2 emission or respective add-on efforts. A variety of different measures, such as active exhaust gas cooling by means of integrated exhaust manifold, improved gas exchange (variable turbine geometry, series compressor turbocharger with intercooling), cooled exhaust gas recirculation, variable compression ratio and finally water injection are used to lower the exhaust gas temperature so far that enrichment for component protection can be dispensed enabling stoichiometric operation even beyond 170 kW/l .
Using an electrically heated catalyst with an extended catalyst preheating arrangement (“conversion management”) as well as hybridisation and further emission reductions, a “zero impact emission” level can be achieved even in non-warmed-up engine condition; however, with a corresponding additional effort.
Furthermore, synergy effects with hybridisation also permit an even more fuel consumption-oriented design of the ICE, especially of turbocharging and combustion process. Since with such mild hybridisations, the robustness of the emission reduction in real world operation can be improved at the same time, such systems will increasingly form a broad basis for future ICE-based powertrains.
In the future, the ICE will increasingly have to cover different degrees of electrification. Consequently, an extended modularisation will also gain importance within the ICE itself. While in the past, different torque and power levels were usually covered with different geometric displacements, turbocharging opens a wide power and torque spread without changing the displacement. Miller and Atkinson cycle also allow a separation of geometric and effective compression ratio. Thus, a widely spread power graduation can be displayed at the same displacement within one engine family.
For decades, lean concepts have been constantly discussed for the further development of combustion processes. However, due to the current discussions on NOx emissions, there are additional question marks for any kind of lean operation. Thus, at least in mid-term, a focus on stoichiometric concepts is particularly useful in Europe. Above all, an increase of the compression ratio is a central topic for improving ICE efficiency.
Technically, a relatively simple solution allowing higher compression ratio is a monovalent Compressed Natural Gas (CNG) engine. In addition to the chemical CO2 advantage, the high knock resistance of CNG is transformed into further efficiency improvements. This is a highly attractive solution, which also offers the possibility to use both biogas and CO2-neutral synthetic gas produced from renewable energy within a partially existing and easily expandable infrastructure. It is to be hoped that CNG will gain adequate customer acceptance.
A largely “knock-free” gasoline engine would enable a significant paradigm shift: a more adiabatic engine that transfers wall heat losses into exhaust gas enthalpy and utilises the exhaust gas energy in a downstream expansion process. One possibility for improving the knock behaviour, but also the tolerance for charge dilution, is a pre-chamber spark plug, (5). By appropriate alignment of the ignition jets, the combustion can be specifically accelerated towards the direction of knock-critical zones and thus the knock behaviour can be improved.
The most efficient measure to prevent pre-ignition and knocking, however, is to introduce the fuel just at the end of the compression stroke, immediately before ignition. Thus, the residence time of the fuel in knock-critical areas is too low to trigger pre-ignition or knocking. However, with the very late introduction, the fuel distribution and mixture formation require extremely high injection pressures. With injection pressures of more than 800 bar and adequate system design, a sufficient mixture formation quality and homogenisation can be achieved at very late injection. Thus, even with a high effective compression ratio and low engine speeds, the combustion phasing can be set to the respective thermodynamic optimum, (6).
Based on the current state of technology, which achieves emission levels already well below the RDE limit, the diesel engine is well prepared for the next possible steps in emissions legislation even without hybridisation. Within the optimal temperature window of the exhaust aftertreatment and within moderate engine dynamics, the diesel engine is well below the existing limits defined for the future even with the current state of technology. To further reduce the pollutant emissions of diesel beyond Euro 6d-Final level, a highly efficient exhaust aftertreatment system and correspondingly optimised operating strategies are required, (7).
Since the exhaust aftertreatment system achieves optimum efficiency only within a limited temperature range, the respective temperature management gains decisive importance.
Contrary to public discussion, even in a conventional version and under unfavourable boundary conditions, the diesel engine has no difficulty in complying with future Conformity Factors (CF). Based on the current state of the art as well as further detail optimisation of the combustion and the operating strategies, a CF of 1.0 in combination with a fuel consumption reduction in the order of 2.5 % can be achieved even under the “worst case” conditions, see upper curve in (8).
In this example, “worst case” means life-time aged aftertreatment components, a fully loaded Lean NOx Trap (LNT) and a non NH3 pre-loaded Selective Catalytic Reduction (SCR) catalyst – at an ambient temperature of 0 °C and aggressive driving style. The position of the lower curve under standard conditions makes it clear that we should talk of a result space rather than a single value in terms of compliance factors.
Complementing this classic diesel engine layout with a Belt-driven Starter generator (BSG) with 10 kW and an electrically heated e-cat with a heating power of 4 kW, this 48 V mild hybrid approach results in further reduction of NOx emissions combined with a fuel consumption potential of about 6 % . The next evolutionary step is with extended hybridisation, shown in (9) as 48 V P2 module with a 30 kW electric motor .
In addition, (9) also shows a second active SCR catalyst in under-floor position . This allows the emission level to be further stabilised and reduced, especially in case of high load requirements, such as in high-speed cruising or in heavier vehicles up to light commercial vehicles. With such a combination, the emission level moves within a very wide usage profile down towards the order of measurement accuracy.
In summary, it should be noted that the diesel engine is well prepared to meet the requirements after 2020, even with a conventional layout. Based on a mild hybridisation, which is expected to gain high market acceptance due to customer-relevant positive attributes such as driving pleasure and comfort, the diesel engine has further long-term potential and will continue to make a significant contribution to reducing CO2 values.
SUMMARY & CONCLUSIONS
Regarding the improvement of real-world emissions, the transition to RDE proves to be much more effective than lowering the certification values on the chassis dynamometer. By fulfilling the current RDE legislation, NOx pollutant emissions of diesel engines, for example, are improved by a factor greater than 5, in individual cases up to 10, under real driving conditions. Also with the gasoline engine, the RDE-induced widespread introduction of the particulate filter and the significantly tightened limits for cold and high-load emissions result in essential pollutant reductions.
It can be assumed that Euro 6d-Temp vehicles will certainly fall below the emission limits even in today’s emission-critical zones. Since the extended boundary conditions of RDE legislation are no longer limited to statistically relevant driving conditions only, but can cover virtually all possible modes of operation, the short-term fulfilment of these requirements represents an incredible challenge for the automotive industry.
The uncertainty of the markets’ reaction to future requirements calls for the development of modular structures for ICEs, transmissions and electrification components. In the long term, new potentials of the ICE can be identified in terms of both pollutant and CO2 emissions. In particular, new combustion processes as well as synergy effects with electrification and the networking of powertrain control with all vehicle-relevant traffic and environmental information – Internal Combustion Engine 4.0 – enable both further improvements in fuel consumption and an emission behaviour that can be described as zero impact emission.
This means that future combustion engines will not have a negative impact on the pollutant emission situation any more. With the topic of synthetic fuels (e-fuels), the combustion engine also has the potential in the medium and long term to become both pollutant- and CO2-neutral.
 European Parliament; Committee on the Environment; Public Health and Food Safety; Dalli, M.: DRAFT REPORT on the Proposal for a Regulation of the European Parliament and of the Council Setting Emission Performance Standards for new Passenger Cars and for new Light Commercial Vehicles as Part of the Union’s Integrated Approach to Reduce CO2 Emissions from Light-Duty Vehicles and Amending Regulation (EC) No 715/2007 (recast), (COM(2017)0676 – C8-0395/2017 – 2017/0293(COD)), 14.3.2018
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DR GÜNTER FRAIDL is Senior Vice President Powertrain Systems Passenger Cars at AVL List GmbH in Graz (Austria).
DR PAUL KAPUS is Manager Development Gasoline Engines and Concept Cars at AVL List GmbH in Graz (Austria).
DIPL.-ING. (FH) HORST MITTERECKER is Chief Engineer Passenger Car Diesel at AVL List GmbH in Graz (Austria).
DIPL.-ING. MICHAEL WEISSBÄCK is Vice President Powertrain Systems Passenger Car Diesel at AVL List GmbH in Graz (Austria).