48V Holds the power of life or death over the diesel
Once considered the planet’s savior from greenhouse gas emissions, diesel was delivered a body blow by the American Environmental Protection Agency on the 25th September 2015 when VW was issued with a notice of violation of the Clean Air Act: Not only did this set in motion a series of events that would cost the manufacturer an estimated USD30 Billion, but more importantly, turned the diesel engine into an international villain.
Despite the motor industry’s offer of appeasement, proposing to update and even buy back older diesels, cities from Stuttgart to Mexico came out in favor of banning this once revered form of propulsion.
Image Source: BMW Blog
At this point the issue had already moved from a technical discussion on the pros and cons of diesel power in passenger vehicles, to a political resolution calling for an end to diesel. While proponents of the power source pointed out that diesels emitted about 15% less CO2 when compared to similar gasoline engines, adversaries pointed to the tens of thousands of lives claimed by hazardous emissions.
In response to the uncertainty over the life or death of the Compression Ignition engine, the European market penetration dropped below 50% for the first time in a decade in 2016, with other parts of the world recording similar trends. In India, for instance, market share fell from 52% in 2012 to only 26% last year, after the Indian Supreme Court imposed an additional 1% sales tax on all diesel cars in an attempt to discourage sales.
But what if hazardous exhaust gas emissions, such as NOx, could be reduced? And not just reduced, but reduced at a price that consumers could afford!
48-Volt electrification throws diesel a lifeline
In a demonstration of the benefits that 48V electrification hold for diesel emissions the Advanced Diesel Electric Powertrain (ADEPT) project led by Ricardo combined several low-cost, advanced mild hybrid technologies to reduce current class-leading C-segment CO2 emissions by an additional 15-20%.
Based on a Ford Focus DV5FC ECO Diesel which is homologated at 88 g/km, the project was able to reduce CO2 levels to 75 g/km, with a further drop to 70 g/km at a cost/emissions reduction ratio superior to that of a full-hybrid.
By fitting a 48V switched reluctance SpeedStart 12.5 kW Belt Starter Generator (BSG), configured in a P0 topology and controlled through a 12V-48V DC-DC converter with a Ricardo hybrid supervisory controller, the project was able to:
1) Improve start-stop functionality
• Stop earlier – in-gear, with speed >4 km/h
• Start later – in-gear, just before vehicle launch is enabled by rapid engine start offered by 48V BSG
• Significant increase in engine-off time over NEDC
2) Increase Regenerative Braking performance — substantially improved regen braking recovery through the 12.5kW BSG
3) Optimize Torque Assist
• 10kW available from BSG in torque assist
• Near instantaneous application of torque at the crank – supports engine downspeeding
• 10kW at 1000RPM with 95Nm crank torque from the reapplication of regen energy to achieve overall reduction in fuel consumption and NOx emissions
In a further demonstration of the flexibility of the 48V technology in reducing emissions, Valeo was able to achieve a 25% reduction in fuel consumption during 3rd gear acceleration from 30 to 60kph at 2m/s², in a 1.6L C-Segment MPV (1430kg), fitted with a 6-Speed MT.
Furthermore, the reduction of engine loading through electric assistance during this transient phase resulted in an 80% decrease in brake specific nitrogen oxides (BSNOx), to the point where OEMs could consider doing away with the Selective Catalytic Reduction (SCR) system to reduce costs.
With the engine emitting significantly less NOx, it is possible for the OEM to balance the NOx reduction against CO2 savings.
Image Source: Valeo
While the impact of torque-assist in reducing transient emissions is crucial, it is also important that the manufacturer implements a ‘cold start emissions’ strategy; especially in light of the lengthy engine-off operation encountered during the RDE cycle.
Diesel emissions are significantly impacted by extended engine off periods experienced in real world city traffic. This makes it difficult for them to comply with the American super ultra-low emissions vehicle (SULEV) 30 and PM/PN limits which demand a balance of catalyst light off and minimal particulate formation.
Equipping the vehicle with a 48V electrically heated catalyst allows manufacturers to dramatically reduce the warm-up period of the catalytic converter, thereby improving emissions.
Image Source: Continental
And the additional electrical power doesn’t only have to power the heated catalyst it can also be used to drive a powerful 48V eSupercharger (eSC). Although the fitment of a 48V eSC to luxury vehicles such as the Audi SQ7 is aimed at reducing turbo lag and improving transient response, the additional air flow the unit provides during acceleration can also reduce combustion temperature in a diesel engine by as much as 100°C during this phase, thereby curbing NOx formation.
So, while NOx emissions may be problematic, 48V electrification supports a wide range of technologies which can effectively reduce Green House Gas and other harmful exhaust gas emissions. This reduction of emissions together with improvements in the combustion process may just see diesel engines restored to their former glory.
However, cleaning up the diesel act may already be too late: Cities such as London, Paris, Athens and Mexico City have made it clear that from 2025 diesels will be banned from their streets which could very well see the end of this energy source in passenger transport.
Should this be the case, the 48V MHEV could very well step in to replace the diesel as a fuel efficient, clean and cost effective form of transportation. Coming in at a similar cost to a Micro Hybrid (Stop/ start) diesel, a gasoline powered MHEV can achieve similar fuel consumption but without the attendant harmful emissions.
Replacing the diesel without increasing GHGs
At an on-cost of between USD1200 to USD2400, depending on the topology, a gasoline 48V MHEV can achieve up to 80% of the emissions savings of a Hybrid EV at between 30 to 50% of the cost. In addition, as a P0 topology, the platform requires minimum engineering changes, and can be implemented well within a model cycle.
In tests conducted in Europe, a 48V MHEV achieved an emissions reduction of between 12 to 15%, measured over the New European Driving Cycle (NEDC), and an impressive 20 to 28% over Real Driving Emissions (RDE) testing on the road.
Primarily, this saving is made possible by using the energy harvested during braking or deceleration to enable torque assist when accelerating, thereby reducing engine load, and as a result, fuel consumption at selected points over the test cycle.
Energy recovery and torque assist are even more beneficial over the RDE where extended engine-off coasting phases are often encountered. In an evaluation of a 48V P0 system, on public roads in Germany, Continental found that 22% engine-off coasting could return an impressive 8% fuel saving.
So, whether it’s rescuing the diesel or replacing it, 48V electrification is set to play an important role in the final chapter of the internal combustion engine. So much so, that Navigant Research predict that by 2025 approximately 60% of all new vehicles will be MHEVs.
Reuters; Economic Times Auto; German diesel car sales drop 19% in April, lose market share: KBA; May 2017; http://auto.economictimes.indiatimes.com/news/passengervehicle/cars/german-diesel-car-sales-drop-19-in-april-lose-market-share-kba/58498142
James Ayre; Clean Technica; Volkswagen’s Cumulative Dieselgate Costs Now Total ~$30 Billion; September 2017; https://cleantechnica.com/2017/09/30/volkswagens-cumulative-dieselgate-costs-now-total-30-billion/
Dr. Drury; Ricardo; 48 Volt Systems – Demonstrable benefits and future component opportunities; February 2017; BIS Group, Automotive 48 Volt Power Supply and Electrification Systems Forum.
David Alexander; Navigant Consulting, Inc; Projected low-voltage technology adoption; February 2017; BIS Group, Automotive 48 Volt Power Supply and Electrification Systems Forum.