Making the case for mild hybrid systems for heavy-duty vehicles

  • 29-Jan-2015 03:58 EST
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A mild hybrid topology for a heavy-duty truck. Due to the application of components, which can also be shared with the automotive industry and its high production volumes, the additional product cost for such a mild hybrid system is relatively low compared to a full hybrid system.

Significant improvement potentials in conventional powertrains have been achieved over the past few years. Still, hybrid systems are under development by many OEMs, as the technology offers both the highest fuel consumption reduction potential and partly zero emissions possibilities. The central challenge of such hybrid concepts is a sufficient reduction in total cost of ownership due to the high system cost. For most commercial vehicle applications, full hybrid solutions do not yet provide an acceptable return-of-investment period for the first vehicle owner due to high cost components like high capacity batteries, power electronics, and high power e-motors.

A number of full hybrid commercial vehicle products have been put into production mainly for city bus and distribution trucks applications. Due to the specific dynamic driving cycles, these applications offer the highest potential for fuel efficiency improvement. These vehicles are also owned for longer periods by the first vehicle owner, which allows for extended return of investment periods. For long-haul trucks, full hybrid systems offer a significant potential for fuel consumption reduction up to 8%. However, to satisfy the required return-of-investment period of maximum two years, the hybrid component cost would need to be reduced significantly, which is not foreseeable in near future.

Thus, the commercial vehicle industry is hard at work instead on the development of mild hybrid systems. Due to compromises in the hybrid functionality and component cost, a very attractive reduction of the total cost of ownership can be achieved.

Mild hybrid systems for commercial vehicles, operating at a moderate 48-V level, are intended to drive the auxiliaries of the engines on demand, still providing a moderate level of recuperation from braking energy. During combustion engine use, the braking power can be partly converted to electric energy via a motor-generator. The recuperated energy is stored and re-used on demand to operate the individual auxiliaries.

By analyzing auxiliary systems on commercial vehicle engines it is obvious that the power steering pump, for instance, can provide additional potential by electrification in a mild hybrid system.

For investigation of the fuel saving potential of an electrohydraulic power steering (EHPS) system, a load profile for the steering angle was crucial to determine the gain in fuel efficiency. For the route consisting of Stuttgart-Hamburg-Stuttgart, AVL generated a dynamic load profile based on map data. For this route an EHPS simulation showed a fuel saving potential of 0.82%.

Some OEMs have already developed solutions for variable coolant pumps. The fan can be controlled on demand and the air compressor can be decoupled via a clutch to avoid unnecessary idling losses. Simulations on the ACEA long-haul cycle showed a potential of 0.45% by using an air compressor with a controlled clutch in combination with recuperation.

To reduce system cost, 48-V systems can be installed as a dual board net to serve the existing 24-V board net.

The two voltage levels can be integrated using a dc/dc connection, whereas one battery system is used on each voltage level to stabilize the board net and to buffer recuperated energy. With this approach, existing standard components (e-motors for windscreen wiper, radio, lamps. etc.) can be further applied.

High-voltage safety requirements are significantly less stringent for mild hybrid systems operated at a nominal voltage level of 48 V compared to a high voltage full hybrid system operated at 400 V or more.

Compared to full hybrids, the mild hybrid technology offers lower fuel saving potential. However, 2-4% fuel saving—depending on the individual applications—can be expected, especially if the operating strategy is integrated into an advanced and predictive vehicle energy management control system.

For different electrical board net loads the potential of a 24 V/48 V has been simulated on the ACEA long-haul cycle. At 1.5 kW constant load the 24-V/48-V system showed an improvement of approx. 0.7% in fuel saving in comparison to conventional 24-V system.

To utilize the entire potential of such a mild hybrid system in commercial vehicles, an integrated system simulation platform is required. Real-time capable engine models, the vehicle cooling circuit models, transmission and driveline models, and models for the electrical components and board nets need to be integration into the vehicle models.

With this unique modeling approach of a virtual vehicle demonstrator, it is possible to design mild hybrid systems for specific applications, define the operating strategy, and develop control functions very early in the development process. This approach can save development time, as well as development cost, compared to conventional development methodology.

Further functions for electrification and smart control strategies are under examination at AVL. In a next step the impact of electrically assisted charging, (e-charging) on the performance and emissions of commercial vehicle engines will be investigated.

This article was written for SAE Off-Highway Engineering by Gernot Hasenbichler, Helmut Kastler, Arno Huss, and Helmut Theissl, AVL.

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