Road #3: Diesel engines, petrol engines, ISG and RSG, 48 V on-board electrical system: High-efficiency internal combustion engines: everything old is new
Mercedes-Benz attaches key importance to the optimisation of modern, internal combustion engines in its road map to sustainable mobility. In particular, the economical, clean and, especially in Europe, highly popular diesel engine makes an important contribution to the further reduction of fleet consumption. 2017 saw the launch of a new range of petrol engines in the S-Class, which once again sets the benchmark in terms of efficiency. Electrification and the 48 V on-board electrical system have made possible the sort of fuel savings that until now were the exclusive domain of high-voltage hybrid technology. An important role in boosting the efficiency of internal combustion engines is played by the EQ Boost integrated starter generator that Mercedes-Benz has now introduced in series production, as the first manufacturer. Furthermore, belt-driven starter alternators will also be introduced in additional engine variants shortly. Meanwhile, something that has long been standard for diesel engines is now also coming being widely used in petrol engines: the particulate filter.
Mercedes-Benz is on the right track. In the two decades since 1995, the average consumption of the passenger car fleet has dropped by nearly half from 9.2 l/100 km (230 g CO2/km) to 5.0 l/100km (123 g CO2/km). The objective of cutting the European Mercedes-Benz Cars new vehicle fleet's CO2 emissions by 2016 to 125 g/km has already been met in 2015. Despite a shift in the sales structure towards the intermediate and luxury vehicle category, the 2016 value remained unchanged compared with the previous year's results, as the CO2 emissions of individual models have been further reduced at individual vehicle level. Taking into account the expected, average vehicle weight, Daimler has specified a value of 100 g CO2/km (NEDC) by 2021 as part of the target programme.
The new range of premium diesel engines by Mercedes-Benz represents drive systems for the future. Following the debut of the OM 654 in the E 220 d (combined fuel consumption: 3.9 l/100 km, combined CO2 emissions: 102 g/km) in spring 2016, this engine range is becoming increasingly prevalent across the full model portfolio of Mercedes-Benz Cars and Vans. There are plans for several output variants as well as longitudinal and transverse installation versions in vehicles with front, rear and all-wheel drive.
The second representative of the premium diesel engine family is the in-line, six-cylinder engine (OM 656), which just premiered in two output categories. The Mercedes-Benz S 350 d produces an output of 210 kW (286 hp) and 600 Nm (combined fuel consumption: 5.1 l/100 km; combined CO2 emissions: 134 g/km). The S 400 d with 250 kW (340 hp) and 700 Nm is the most powerful series production passenger car diesel engine ever offered by Mercedes-Benz (combined fuel consumption: 5.2 l/100 km; combined CO2 emissions: 135 g/km).
Like the four-cylinder variant, the new six-cylinder diesel engine is designed to comply with future emissions legislation (RDE – Real Driving Emissions). All components relevant to efficient emissions reduction have been installed directly on the engine. The integrated technology approach combining the new gradual recess combustion process, dynamic multi-way exhaust-gas recirculation and near-engine exhaust-gas aftertreatment, combined for the first time with variable valve-lift control, makes further reduced consumption with low emissions possible. Thanks to the near-engine insulated configuration, exhaust-gas aftertreatment does not suffer great levels of heat loss and generates extremely favourable operating conditions.
Electrified: the new petrol engines
Modular design, reduction in variants and standardisation of the interfaces between power unit and vehicle – this strategy was also applied as part of the development of the new premium petrol engines from Mercedes-Benz. The first representative, an in-line, six-cylinder engine (M 256) with integrated starter generator (ISG), was launched in summer 2017 to set new benchmarks for efficiency. With effect from 2017, the new family will also include a four-cylinder engine with belt-driven starter alternator.
The new and systematically electrified in-line six-cylinder unit is initially available in two output categories. In the Mercedes-Benz S 450 it delivers 270 kW (367 hp) and 500 Nm of torque (combined fuel consumption: 6.6 l/100 km; combined CO2 emissions: 150 g/km). The S 500 produces an output of 320 kW (435 hp) and 520 Nm (combined fuel consumption: 6.6 l/100 km; combined CO2 emissions: 150 g/km). Over a short period, the integrated starter alternator generates a further 250 Nm of torque and 16 kW of output. Compared with the similarly powerful S 500 predecessor featuring a V8 engine, it has been possible to reduce the CO2 emissions of the engine by around 22 percent.
New, intelligent, forced induction that includes an electric booster compressor, as well as an integrated starter generator (ISG) provide outstanding power delivery without turbo lag. The integrated starter generator (ISG) combines the starter motor and the alternator in a powerful electric motor that is installed between engine and transmission and is also used for cold starts. It replaces both the existing alternator and the starter motor. The integrated electric motor, known here by the name of EQ Boost, assists the combustion engine, for example during acceleration, and supplies the battery with power by means of high-efficiency recuperation. By doing so it makes possible the sort of fuel savings that were previously the exclusive domain of high-voltage hybrid technology. All in all, the new, in-line, 6-cylinder engine delivers the performance of an eight-cylinder engine with significantly lower consumption.
Systematic electrification dispenses with the need for a belt drive for ancillary components at the front of the engine, which reduces its overall length. The slim design, together with the physical separation of intake/exhaust, creates space for near-engine exhaust aftertreatment. The 48 V on-board electrical system is used for intensive consumers such as the water pump and refrigerant compressor as well as for the integrated starter generator.
Belt-driven starter alternator: debut in four-cylinder engine
A four-cylinder unit with a belt-driven starter alternator is set to follow before the end of the year. This is coupled with the internal combustion engine, in the same way as the alternator is today. The combination of starter motor and alternator assists the internal combustion engine when starting, accelerating and recovering energy. The system uses existing alternator mounts and thus does not affect the design of the powertrain. Conversion to a 48 V system is also relatively easy to implement on existing platforms.
48 V on-board electrical system: a host of fuel-economy and convenience advantages
The introduction of the new engine generation once more blurs the lines between petrol and hybrid models. Looking to the future, all Mercedes-Benz passenger cars will be electrified because the company is at the same time systematically advancing the development of the 48 V on-board electrical system. The technology will be gradually introduced in various model series. The 48 V on-board electrical system supplies four times the power of its 12 V predecessor at the same current, but does not require the additional safety architecture of a high-voltage system. Furthermore, this low-voltage system makes fuel savings possible that previously were the exclusive domain of the high-voltage hybrid technology. This comes as a result of the implementation of important "energy recovery", "boost" and "starting off and manoeuvring in electric mode" hybrid functions without high-voltage components for the first time. In addition, such a starter concept is so durable that the engine can be switched off much more often and whenever it is not needed: whether during decelerating or coasting, i.e. when rolling to save fuel at higher speeds, as soon as drivers take their foot off the accelerator.
The integration of a 48 V on-board electrical system also offers advantages for electrical consumers in the vehicle, such as the climate control system, electric heating elements or the extractor fan. At identical power, the electric currents are only a quarter of those in conventional systems. This means that the wiring may be thinner and thus lighter – which indirectly contributes to saving fuel.
Apart from reduced fuel consumption and CO2 emissions, the 48 V on-board electrical system also brings about increased comfort. This comes as a result of the 48 V on-board electrical system being able to crank the internal combustion engine to idle speed very smoothly. This benefits NVH (noise, vibration, harshness) and the transition when the engine starts is even less perceptible: the engine simply restarts as if it had never been off. The 48 V on-board electrical system also paves the way for the further expansion of infotainment and assistance systems.
Particulate filter for petrol models: series-production use at Mercedes-Benz in 2017
What has already become standard for diesel engines is now increasingly also being used in petrol engines at Mercedes-Benz: the particulate filter. Mercedes-Benz is planning the large-scale use of particulate filters for petrol engines as a means of further improving their environmental compatibility. Following more than two years of positive field tests in the S 500, additional variants of the S-Class powered by a petrol engine have been equipped with this new technology since 2017 as part of the model update. The filter will then be gradually introduced in other, new vehicle models, model updates and new engine generations. After that, use of the particulate filter is also planned for the current model series.