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In order to adhere with future automotive legislation and incentives, the electric range of plug-in hybrids has steadily increased. At the same time, the installed electric power has risen as well leading to future hybrid vehicles with an electric power share of more than half of overall system power and hybrid configurations with at least two electrical machines come into focus. The concept of adding a separate electrical axle to a P2-hybrid - a so called P24-hybrid, is of special interest. The system complexity of a such a system increases significantly as the number of possible system states increases. Thus, this paper analyzes the efficiencies and benefits of the different system states within the fuel-optimal operating strategy derived by global optimization. By varying the electrical power distribution between the two axles, the impact on fuel efficiency and the changes within the operating strategy are investigated.

The electric range of plug-in hybrids as well as the installed electric power has steadily increased. With an electric power share of more than half of the overall system power, concepts of hybrid electric vehicles with at least two electric machines come into focus. Especially the concept of adding an individual electric axle to a state-of-the-art parallel hybrid, such as a P2-hybrid, is promising. However, the system complexity of a so-called P24-hybird increases significantly because the number of possible system states rises. This leads to an increased development and calibration effort for an online energy management. Especially a transfer from an optimized operating strategy to a rule-based energy management is challenging. Thus, a development framework for the calibration of an online energy management system (EMS) which is as fuel efficient as possible is needed.

During the past few years, economy of commercial vehicles has increased considerably due to higher engine outputs a+ lower engine speeds together with enhanced fuel economy. However, the average speed of commercial vehicles is not only determined by the speed attainable on level ground and on uphill gradients, but also to a large extent by the speed attainable on downhill gradients, with the latter depending on the available constant braking power. Since the displacement of commercial vehicle engines has not been increased or has even become smaller, their braking power has increased only slightly ot not at all. In order to enhance the overall economy of commercial vehicles, it was therefore necessary to increase the engine braking performance as well since the wheel brakes cannot be used for constant braking and additional systems for continuous operation are very complex.

The demand for fuel-efficient, low-displacement engines for future passenger car applications led to investigations with small DI diesel engines in the advanced engineering department at Mercedes-Benz. Single-cylinder tests were carried out to compare a 2-valve concept with 241 cm3 displacement with a 422 cm3 4-valve design, both operated with a common rail injection system. Mean effective pressures at full load were about 10 % lower with the smaller displacement. With such engines a specific power of 40 kW/I and a specific torque of about 140 Nm/I should be possible. In the current stage of optimization, penalties in fuel economy could be reduced down to values below 3 %. The “4-cylinder DI diesel engine with 1 liter displacement” is an interesting alternative to small 3 cylinder concepts with higher displacement per cylinder. An introduction into series production will not only depend on the potential for further improvement in fuel economy of such small cylinder units.