Full-on mild

04 November 2016
Full-on mild

The Ricardo-led 48-Volt ADEPT project shows how intelligent electrification can deliver fuel and CO2 savings equivalent to full-hybrid capability. What is more, the concept’s low-cost mild hybrid architecture is adaptable for diesel, gasoline or alternative-fuelled powertrains, as Anthony Smith reports.

In the almost twenty years since the first mass-produced hybrid car – the first-generation Toyota Prius – rolled off the production line, the world has become used to the concept of electrification improving the fuel-efficiency of the combustion engine powertrain. In addition to such ‘full’ hybrids, consumers have also become accustomed to the so-called ‘mild’ hybrid products where a more modest amount of regenerative braking, energy storage and stop-start functionality is typically deployed.

In terms of the voltages employed in the non-plug-in vehicles developed over this period, there has been a broad distinction between at one extreme, full hybrids with electrical systems operating at up to 650 Volts and, at the lower end, the basic implementations of stop-start technology incorporated within the vehicle’s standard 12-volt architecture, sometimes referred to as micro-hybrids. Between these extremes, many mild hybrid products have traditionally been engineered with significantly higher voltages than the 12V baseline, resulting in a significant manufacturing cost premium over conventionally powered models.

The 48V cost advantage

For a fixed power rating, higher voltages bring some clear advantages in terms of electrical efficiency and reduced volume of copper in electrical machines and cables. Above 60 volts, however, safety considerations regarding shock protection require a much more expensive treatment of aspects such as electrical insulation. In addition, the integration of hybrid powertrain electrical architectures with existing on-board electrical systems – which are still generally restricted to 12 or 24 volts, even in full hybrid products – requires the deployment of multiple DC/DC converters, which add costs and bring incremental efficiency losses. The increasing popularity of 48-volt architectures stems both from the simplicity and cost saving they can bring to mild hybridization, and the significant improvements in the quality and performance of available low-voltage power electronics and ancillaries.

With the early consumer anxieties regarding the robustness and longevity of vehicle electrification systems, in particular of batteries, now largely a thing of the past, perhaps the most significant remaining obstacle to increasing the size of the hybrid
vehicle fleet is the issue of cost. While it is almost universally accepted that improving fuel economy and reducing CO2 emissions is a good thing, few customers are likely to be willing to pay the significant premium. Yet, at the same time, regulation is forcing automakers in exactly this direction. With tightening rules for CO2 in Europe and fuel economy in the US, the fundamental objective is essentially the same: significantly increasing the efficiency with which fossil-based fuel is used.

In a nutshell, the automotive industry is facing the challenge of offering customers new vehicles that are able to deliver significantly improved fuel economy and reduced carbon dioxide emissions, while at the same time maintaining performance and affordability, and achieving the latest regulated emissions limits. While battery electric vehicles, plug-in hybrids and full hybrids will all undoubtedly play their part, the significant reduction of carbon dioxide emissions from the bulk of the vehicle fleet requires fresh thinking about cost-effective measures to improve fuel economy.

Intelligent electrification

Ricardo originally demonstrated its concept of ‘intelligent electrification’ in the form of the HyBoost project (see RQ Q3/2012). HyBoost demonstrated how a practical mix of proven or ready-for-market technologies could be applied across the whole engine system: its innovations were focused on more than the simple delivery of torque and the harvesting of energy through regenerative braking. Instead, it also investigated the use of electrically driven ancillaries, exhaust waste energy recovery through technologies such as turbo-compounding, and torque boosting – for example, to offset fuelling for a given level of torque demand or to enable aggressive engine downsizing. These were combined in a pragmatic manner to deliver extremely high levels of efficiency.

To demonstrate the potential of this concept as applied using the latest 48-volt systems, Ricardo teamed up in a research partnership with the Advanced Lead Acid Battery Consortium (ALABC), Controlled Power Technologies (CPT), Faurecia Emissions Control Technologies UK Ltd, Ford Motor Company and the University of Nottingham. In addition to the contributions of the partners, matched funding was provided by the UK Government’s Office for Low Emission Vehicles (OLEV) implemented through the UK innovation agency, Innovate UK.

Rather than apply the technology to a gasoline powertrain as with HyBoost, the ADEPT (advanced diesel-electric powertrain) project sought to demonstrate what could be achieved by applying the technology to a state-of- the-art diesel vehicle. As such, the baseline for the project was a Ford Focus ECOnetic with 1.5-litre TDCi diesel engine and six-speed manual transmission, homologated at a mere 88 g/km CO2.

“The ultimate goal of the ADEPT project,” explains Gareth Milton, Ricardo chief engineer for the ADEPT research project, “was to show that intelligent 48-volt mild hybrid electrification has the potential to deliver full hybrid economy and CO2 emissions – but at a significantly lower production cost. This was an extremely ambitious endeavour, as the start point was an already highly optimized vehicle in terms of its homologated CO2 emissions.”

ADEPT powertrain architecture

Key features of the ADEPT demonstrator vehicle systems include CPT’s water-cooled SpeedStart switched reluctance belt-driven integrated starter generator (B-ISG), capable of delivering in excess of 12 kW of regenerative braking, as well as near-instantaneous and near-continuous torque assist levels of over 7 kW – sufficient to enable significant engine down-speeding in addition to a highly capable start-stop functionality. In most current manual transmission vehicles equipped with stop-start based on a 12-volt electrical architecture, a ‘stop in neutral’ strategy is used. With engine restart triggered as the clutch is depressed and the gear selected, the 12-volt starter has sufficient warning to bring the engine to idle speed before its torque is required. For the baseline Ford Focus ECOnetic 1.5L TDCi, the time to idle speed is 600 milliseconds, but with the B-ISG implemented, which avoids the ‘stop in neutral’ strategy, the ADEPT vehicle achieves this in just 300 milliseconds.

“The benefit of this 50 percent reduction in the time from engine start to idle speed is to enable significantly increased opportunities to realize the fuel-saving benefits of stop/start,” explains Milton. “On the NEDC cycle this represents a full 65 percent increase from 190 to 315 seconds in engine-off time, and in real-world city-based driving, the benefit could be considerably greater.”

In addition to regenerative braking energy recovered through the B-ISG, further energy recovery is achieved from CPT’s exhaust-mounted 48V turbine-integrated exhaust gas energy recovery system known as TIGERS. Instead of connecting a turbine to a compressor, as in a turbocharger, the TIGERS unit integrates a turbine with an electrical generator. Rated at 2.4 kW, TIGERS is capable of capturing further power recuperated from the exhaust downstream of the turbocharger. A key aspect of the ADEPT control system in this respect is to determine when the thermal requirements of the diesel aftertreatment technology are such that this energy can be recovered without detriment to emissions control. This is achieved via two bespoke emissions control valves, which were developed by Faurecia Emissions Controls technologies for the ADEPT project.

Rather than adopting one of the more expensive battery cell chemistries typically employed in commercial full or mild-hybrid products for its energy storage capability, the ADEPT vehicle uses an advanced 48-volt lead-carbon battery pack developed by Provector, a contractor to ALABC. “This incorporates ‘Ultrabattery’ modules, effectively a VRLA lead-acid battery with high rate partial state-of-charge capability which, in addition to their competitive cost, reliability, robustness and end-of-life recyclability, incorporate an ultra-capacitive effect within their lead-carbon electrodes,” continues Milton. “This enables additional short-duration, higher-power performance, which is particularly useful in torque assistance to the vehicle.”

The ADEPT powertrain includes a range of electrical ancillaries powered from the 48V system rather than directly from the engine, including, for example, the vehicle air conditioning compressor. This enables the energy demands of these systems to be managed, avoiding the inefficiencies of direct permanent coupling to engine speed and also allowing loads to be managed for maximum benefit in terms of torque boosting and fuel economy. In the ADEPT vehicle the 12-volt alternator is no longer required due to the presence of the B-ISG, which can provide all lower voltage requirements through a DC-DC convertor. The mechanical air conditioning pump is replaced by a 48V electric compressor, and a packaging and efficiency study was carried out for similar replacement of the mechanical water pump.

The control strategies deployed have been developed based on extensive vehicle systems simulation work. This has enabled the core powertrain and aftertreatment system, as well as the 48V BSG, ancillaries, battery pack and exhaust energy recovery system, to be operated in a seamless manner, while also providing a valuable computer aided engineering (CAE) capability to explore further potential avenues of development and optimization opened up through intelligent 48V electrification.

Test and simulation

The ADEPT concept includes a range of technologies that were applied to the demonstrator vehicle as well as a number of further enhancements for which there was insufficient time or resource within the scope of the project, but where simulation could be carried out to determine their likely fuel economy and CO2 benefit.

Following extensive testing and simulation by the project partners, the key achievements of the ADEPT project are impressive. Starting from an already highly fuel-efficient state-of-the-art diesel powertrain homologated at just 88 g/km CO2, it has been shown that the integration of hybrid and emissions control systems in the manner envisaged by the project can deliver a Euro 6b-compliant package offering significant fuel and CO2 savings.

To provide some insight into the individual contributions of each of the technologies that the ADEPT concept comprises, and also to demonstrate their effectiveness under the new WLTC drive cycle, the results are presented in the form of the CO2 ‘walk’ (see diagram). Starting from the baseline vehicle, the addition of regenerative braking provides a full 2.4 percent reduction in CO2 by supplying the full requirement for the vehicle’s 12-volt systems. The torque assist and engine down-speeding provide respectively a further 2.5 and 1.4 percent saving, with just 0.3 percent arising from extended engine-off time – arguably less than might be expected in real-world urban driving. In total this provides a measured 6.6 percent CO2 reduction over the WLTC cycle. Beyond this, simulation of the e-water pump and associated micro-circuit improvements gives computed further savings of 0.7 and 0.5 percent respectively, while the University of Nottingham’s study of advanced lubrication systems projected a further one percent CO2 saving.

In total the ADEPT technology demonstrator vehicle has been shown to deliver a CO2 reduction of 6.6 percent under the new LTC drive cycle, which is equivalent to 11 percent under the old NEDC drive cycle. In addition, if the electric water pump and advanced lubrication innovations were also to be implemented, the fuel savings are projected to rise to 8.8 percent (WLTC) and 14-15 percent (NEDC) respectively. Beyond this, however, in real-world driving the potential, and in particular for the extended engine-off functionality in urban or city-based driving, could also be considerable.

Cost advantages confirmed

Production implementation costs are extremely encouraging, too: According to an analysis conducted independently by Ricardo of the potential cost of production of an ADEPT concept such as that demonstrated, the system would represent an incremental cost of in the region of €60 per gram/km of CO2 reduction. This is a result that makes the ADEPT powertrain architecture very competitive with other fuel economy solutions such as full hybridization, where costs of implementation can be significantly higher.

Applicability to gasoline and alternative fuelled powertrains
While the focus of ADEPT was the application of intelligent electrification to an already very fuel-efficient diesel, the basic architecture could be applied very successfully – albeit with application-specific fine-tuning – to a gasoline or alternative-fuel powertrain. In many respects, the advantages with a gasoline application may be even greater. Rather than being obliged to switch off the TIGERS exhaust energy recovery system when required to preserve the operation of downstream diesel NOx and PM aftertreatment, on a gasoline vehicle the TIGERS unit might be positioned downstream of the catalyst and thus be usable across a much greater extent of the duty cycle.

Similarly, the torque-boosting capability of the B-ISG could provide low-speed launch assistance that would enable a far greater level of aggressive engine downsizing than might otherwise be possible. For customers, too, perceptions are likely to be enhanced by the potential for improved acceleration and NVH through engine downsizing and down-speeding, through stop-start, and in more extreme implementations, through silent pure-electric take-off.

An expanding market for intelligent electrification
The potential of 48-volt intelligent electrification to significantly expand the market for mild hybridization is clear. “The ADEPT vehicle is a research demonstrator that includes just some of the 48V technologies that could be deployed, and applies them in conjunction with an already highly optimized diesel,” explains Milton. “It is possible that you will see automakers already developing products that include some of the concepts demonstrated in ADEPT. In this project we were focusing on delivering the
lowest possible CO2 emissions without compromising the driveability of the baseline vehicle. But intelligent low voltage electrification is a flexible toolbox for the automaker to calibrate the vehicle for economy or performance depending on the application. I believe that we should expect to see many more examples of 48V technology on the market within the next
two to three years.”

Beyond this, as he predicted in the Viewpoint column in the last issue of RQ, Ricardo hybrid & electronic systems product group head Steve Doyle believes the widespread attractiveness and low cost of this form of electrification could be considerable. In his article, Doyle argued that this approach may well contribute more to overall CO2 emissions reduction in the next five to 15 years than all plug-in vehicles combined. This does not imply a reduction in the market demand for full hybrids, plug-in hybrids or battery electric vehicles. There remain compelling environmental and operational benefits to all of these technologies for different
applications, market segments and consumers. But for the high-volume mid-market segments where the costs of hybridization might otherwise be prohibitive, 48-volt intelligent electrification could just prove to be the perfectly positioned technology that
allows automakers to come that crucial step closer towards their future European fleet average CO2 and US Federal fuel economy targets.

This article is an edited version of a feature published in RQ magazine, Q3, 2016 - click on the magazine cover below to go to the magazine version:

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