The efficiency of combustion engines can be significantly improved, when used as part of a hybrid powertrain
In this case, it is possible to operate the engine in a very limited engine map range. The core of IAV’s concept approach presented in this paper is a significant increase in compression ratio combined with charge dilution and an active pre-chamber ignition.
The ambitious target set by the EU Commission to reduce CO2 emissions significantly increases the requirements for future powertrain concepts. The 37.5 % reduction based on the already stipulated value of 95 g CO2/km from 2021 demands clear steps to reduce fuel consumption, particularly in spark-ignition engines. This will require powertrain concepts with low CO2 emissions in all driving scenarios. Combining internal combustion engines with electric drive components produces additional challenges on the one hand, but on the other hand, it also creates opportunities for making combustion engines essentially more effective through targeted optimisation in the clearly restricted map range. Higher efficiency on the motorway and in customer-relevant driving cycles allows not only to avoid the drawbacks from increased vehicle mass with discharged battery but also considerably reduces fuel consumption.
ARCHITECTURES OF HYBRID POWERTRAINS
Hybrid powertrains are classified either in terms of the installed electrical output (micro, mild or full hybrid) or according to the layout of the electric motor (in series, in parallel or power split). Giving due consideration to technical complexity, consumption potential, performance and costs, the parallel hybrid offers an advantageous topology.
Full hybrid architecture is necessary to achieve the lowest CO2 emissions. It permits implementation of extensive hybrid functions such as recuperation and boosting, besides all-electric driving right through to cross-country speeds. The addition of plug-in capability and the use of renewable energy further increase the savings potential. (1) shows the approach taken in the concept using the combustion engine in the highly restricted map range as part of the hybrid powertrain . The concept resolves the trade-off between low-end torque (LET), rated power and part load. Areas at low part load are operated in all-electric mode. When the battery is not fully charged, this area is bypassed by shifting the operating point, thus eliminating the need for dethrottling at part load. Electric boosting also reduces the significance of LET engine speeds with positive impacts on knock and pre-ignition tendencies. As a result, suitable technologies can be optimised with a focus on maximising engine efficiency.
The key aspect of the concept consists in augmenting thermal efficiency by clearly increasing the compression ratio. This is achieved by restricting the LET range. Knock reduction is also necessary and is achieved by using cooled Exhaust Gas Recirculation (EGR) with high EGR rates as well as Miller valve timing, combined with relatively short combustion durations for lowest combustion losses. The latter must be safeguarded with an adequate ignition system, which is always needed for igniting highly diluted mixtures.
Lower caloric losses are another important effect of charge dilution. High wall heat losses caused by maximum compression ratio and optimum centre of heat release must be counteracted by an augmented stroke-to-bore ratio as well as charge dilution. Minimising gas exchange losses necessitates a suitable exhaust gas turbocharger optimisation with a large turbine for lowest exhaust back-pressure, which can only be implemented by reducing the stress at the LET of a hybrid powertrain. The concept is rounded-off by an additional improvement in gas exchange with a port design optimised for high flow rates and a compressor wheel rated to the best point.
DESIGN FOR 45 % EFFICIENCY
A complete redesign of the combustion engine entails optimising a large number of parameters. The partly diametric behaviour of individual loss mechanisms creates a multi-criteria optimisation challenge. To this end, a 1D gas exchange simulation is coupled with an IAV proprietary mathematical optimiser. The engine model used for this purpose incorporates validated sub-models for knock sensitivity, super- charging, wall heat and combustion.
The optimisation leads to a large number of individual hardware settings with discrete characteristic figures for engine speed, load, compression ratio and the resulting engine efficiency, (2). The optimum configuration produces an efficiency level of 45.4 % at 4,400 rpm. With optimisation focussing on the cycle-relevant operating range, efficiency of up to 45 % can be achieved at a speed of 3,000 rpm, as explained below. These settings provide a compression ratio of 17.4, an EGR rate of 42 % and a stroke to bore ratio of 1.25.
(3) shows the thermodynamic effects of the 45 % setting. The base engine – a turbocharged 1.4 l gasoline engine with direct injection – with a compression ratio of 9.6 at the operating point 3,000 rpm and 12.6 bar serves as a reference. Boosting just the compression ratio only results in a slight increase in engine efficiency, (3) (middle). Even ignoring knock combustion, the increase is no more than 2.4 percentage points due to higher wall heat losses. The actual increase in knock tendency results in far later centres of heat release and an even higher increase in the sum of wall heat and combustion losses.
Finally, all hardware components have to be optimised to achieve the 8.5 % increase in thermal efficiency from the augmented compression ratio, (3) (right). Here, the focus is on using a very high EGR rate of 42 %. EGR reduces peak temperature due to higher thermal capacity, while at the same time there is a larger gas mass in the cylinder, with significant reductions in the elevated wall heat losses. A stroke-to-bore ratio of 1.25 brings about further reductions in wall heat. Efficiency is further enhanced by reducing caloric losses and decreasing knock tendency by means of a cooled EGR with an ideal centre of heat release.
Minimising combustion losses necessitates short combustion durations of 20 °CA (10 to 90 % fuel conversion) despite high charge dilution. A suitable ignition and injection system is necessary for ignition of highly charge-diluted mixtures with fast conversion at the same time. The gas exchange is optimised by means of an enlarged turbine and a compressor design optimised to the best point. This leads to constant gas exchange losses despite a higher flow rate and charge pressure.
PRE-CHAMBER IGNITION FOR CHARGE-DILUTED MIXTURES
In addition to known charge dilution with air (lean combustion), efficiency can also be improved by dilution with exhaust gas. There are advantages in preserving the stoichiometric fuel to air ratio with exhaust aftertreatment by means of three-way catalytic converters. But the EGR rate is limited by the potential of the ignition system. IAV has developed an active pre-chamber ignition system to resolve the trade-off between high EGR rate and ignition capability. A smaller gas mass is encapsulated to make it easier to shift the composition of the partial charge into the ignitable range. At the same time, the pre-chamber generates high-energy flares to facilitate the ignition and swift burn-through of highly charge-diluted mixtures. A special injector is needed to achieve ignitable mixtures with high EGR rates to flush the pre-chamber with fuel/air mixture. The EGR rate at the ignition point is thus clearly reduced compared to the main combustion chamber. The test bench measurements shown in (4) illustrate the significant increase in residual gas compatibility of the combustion process.
The desired significant increase in efficiency is generated by the favourable centre of heat release in the range of limited knock as well as the shorter combustion durations [2, 3]. However, the lack of an adequate purge gradient at the test bench (no active purging pump) meant that the EGR rate was limited to 32 % without restricting combustion stability.
SYSTEM STRUCTURE & POTENTIAL
(5) shows how the active pre-chamber is incorporated in the overall engine concept. The single-stage supercharged spark-ignition engine with exhaust turbocharger, direct injection and charge-air cooling makes use of exhaust gas aftertreatment rated for stoichiometric operation. Additional hardware components are needed to implement the ascertained characteristic figures and settings. The pre-chamber ignition system is the central element. The diagram shows a conventional spark plug in the pre-chamber volume as well as the necessary fuel/air injector. The latter needs an additional low-pressure fuel system together with air supplied by an electric pump in combination with a pneumatic pressure reservoir. There must be an adequate purge gradient between the exhaust and intake system to achieve EGR rates above 40 %.
The low-pressure circuit with post-turbine withdrawal and gas injection downstream from the compressor offers several advantages. Regardless of the operating point, there is a natural pressure gradient that just needs minimum boosting from the purge pump . Furthermore, exhaust enthalpy can be fully exploited for supercharging, and the reduced withdrawal temperature clearly reduces the power demand of the EGR cooler. EGR withdrawal downstream of the three-way catalytic converter and particulate filter reduces soiling in the intake system .
Although the existing variabilities in the valve train are not needed for point layout, they can extend the high efficiency level to other map areas. (6) shows the efficiency map ascertained with the selected setting. Besides the 45 % point, the range of efficiencies above 40 % can be extended well into the WLTC-relevant part load range. The fuel-saving potential thus achieved with pure combustion engine operation of 0.6 l/100 km in WLTC or 1 l/100 km at 130 km/h for a C-segment hybrid vehicle is perceptible in terms of fuel consumption in customer operation, regardless of whether the hybrid functions of the powertrain can be used in view of the battery charge status.
This paper presents an engine concept developed by IAV on the basis of active pre-chamber ignition with stoichiometric operation across the entire map. Achieving this clear improvement depends on the possible restriction of the engine map range in the hybrid setting. An actively purged pre-chamber spark plug permits ignition of highly diluted mixtures with EGR rates of up to 42 %. The high EGR rate in combination with long-stroke layout limits wall heat losses with clearly increased compression ratio. The strongly reduced LET and EGR cooling still permit adequately high mean pressures in the full load range. The targeted layout of the turbocharger combined with a low Miller degree prevents higher gas exchange losses. It is thus possible to achieve efficiencies up to 45 % at the best point of the map. Given the close proximity of the combustion engine best point and the operating points for typical long-distance applications, this concept promises a considerable CO2 reduction in customer consumption for cross-country or motorway driving. Furthermore, extending high efficiencies well into the part load range is also beneficial, in terms of the certification cycle.
 Brannys, S.; Gehrke, S.; Hoffmeyer, H.; Hentschel, L.; Blumenröder, K.; Helbing, C.; Dinkelacker, F.: Maximum Efficiency Concept of a 1.5 l TSI evo for Future Hybrid Powertrains. 28th Aachen Colloquium Automobile and Engine Technology, Aachen, 2019
 Sens, M.; Binder, E.; Benz, A.; Krämer, L.; Blumenröder, K.; Schultalbers, M.: Pre-chamber Ignition as a Key Technology for Highly Efficient SI Engines – New Approaches and Operation Strategies. 39th International Vienna Motor Symposium, Vienna, 2018
 Sens, M.; Binder, E.; Reinicke, P.-B.; Riess, M.; Stappenbeck, T.; Wöbke, M.: Pre-chamber Ignition and Promising Complementary Technologies. 27th Aachen Colloquium Automobile and Engine Technology, Aachen, 2018
 Rohrssen, K.; Höffeler, G.: The IAV Active High-EGR Concept. In: MTZworldwide 1/2011, pp. 22-26
 Fischer, M.; Günther, M.; Berger, C.; Tröger, R.; Pasternak, M.; Mauss, F.: Suppressing Knocking by Using Clean EGR – Better Fuel Economy and Lower Raw Emissions Simultaneously. 5th International Conference on Knocking in Gasoline Engines, Berlin, 2017
DIPL.-ING. MARC SENS is Senior Vice President Powertrain Advanced Development at IAV in Berlin (Germany).
DIPL.-ING. MICHAEL GÜNTHER is Head of Department Thermodynamics Gasoline ICE, Powertrain Advanced Development at IAV in Chemnitz (Germany).
DIPL.-ING. MARIO MEDICKE is Team Manager Gas Exchange/Simulation Gasoline ICE, Powertrain Advanced Development at IAV in Chemnitz (Germany).
DR.-ING. ULRICH WALTHER is Team Manager Combustion Development Gasoline ICE, Powertrain Advanced Development at IAV in Chemnitz (Germany).