48-volt mild hybrids: A possible cure for diesel emissions cheats?

It's clear that we need to make our passenger vehicles a lot more efficient if we want to avoid some of the very worst effects of climate change. And it's also becoming increasingly clear that diesel—which was once looked at in places like Europe as a panacea for this problem—might not be quite so groovy, what with rampant emissions cheating in the auto industry. Delphi, a major vehicle component supplier, thinks it has a real solution to help us with this, in the form of 48V "mild hybrids."

Climate change is such a big problem that even Donald Trump (who says he doesn't believe in it, publicly) is spending money to defend his properties from sea level rise. Although passenger vehicle emissions are only part of the carbon emission problem, in the US, Europe, and China regulators are taking the problem seriously, with increasingly strict fuel efficiency targets for all new cars. Here in the US, car makers have until 2025 to double their average fuel economy to 54.5mpg, but things are even tighter abroad. China has set 2020 for its deadline, by which time manufacturer averages have to be down to 117 grams of CO₂ per km driven, and the following year the EU requires fleet averages of just 95g/km. And along with those targets come hefty financial penalties for missing them.

Several years ago, we took a deep dive into some of the technologies that automakers are looking at to get themselves out of this bind. These features included variable valve timing, small capacity turbocharged engines, gasoline and diesel direct injection, cylinder deactivation, and stop-start functions. But all of those features are being widely deployed across new vehicle fleets, and it's clear that they won't be enough. Of course, there's also the wide world of electrification, like plug-in hybrid electric vehicles and battery EVs, but adoption of EVs of all stripes remains insufficient to really move the needle—even accounting for Tesla's gigantic Model 3 presales. That's where the 48V mild hybrid comes in.

Normally when we think of hybrids—whether that's a car like the Toyota Prius or plug-in EVs like the Chevrolet Volt—it's a high-voltage system. That means over 60V, identifiable in the car's engine bay by bright orange cables (that being the color that the industry has standardized on). Mild hybrids work at 48V, and while they contribute a lot less power and torque than a high-voltage hybrid system, they're also significantly cheaper, and most importantly, they can be added to traditional powertrains relatively easily by manufacturers.

The concept is not entirely new—Dodge showed off a 42V mild hybrid with its ESX3 concept car in 2000, and more recently Chevrolet has used the idea for some Silverado trucks, as have others. It works as follows: you replace the car's starter with an electric motor/generator unit (MGU), which draws power from a lithium-ion battery located in the trunk. These are both much smaller than the MGU and batteries of a regular hybrid—Delphi says between 9kW and 12kW for the MGU and under 1kWh for the battery. The MGU works in combination with the car's stop-start system and provides a handy slug of torque when moving away from a stop, and it also recovers energy (and therefore charges the battery) under braking.

Mary Gustanski, Delphi's VP of engineering and project management, told Ars that mild hybrids can offer about 70 percent of the benefit of a more traditional hybrid (with regard to fuel efficiency and CO₂ reduction) but at 30 percent of the cost. The company has been developing the system with a 1.6L turbodiesel Honda Civic, which also has an electric turbocharger for added efficiency. On average, the car is about 10 percent more efficient than a conventional Civic turbodiesel, although we're told that the gains can be as much as 35 percent at some parts of the rev range.

The 48V electrical system doesn't replace the car's normal 12V electrical system, which is connected to the 48V system via a DC-DC converter. That's because all the other accessories in our cars have been designed to run on 12V—even power hungry ones like AC compressors. [UPDATE: to clarify, this comment was in response to my asking Gustanski why OEMs don't just ditch 12V and go to 48V entirely. As several commenters point out, currently AC compressors are actually driven from a belt from the engine. Gustankski did say that 48V engine cooling fans and pumps are available though.]

While mild hybridization works with both gasoline and diesel engines, the latter might well benefit more from this tech. As we've seen over the past few months, car makers have been repeatedly caught cheating on their emissions tests, altering control software to meet the test conditions at the cost of reduced power output. Adding a mild hybrid system ought to compensate for the reduced power of a diesel engine that actually meets those test regs (which are getting stricter as regulators move to real-world test conditions).

Interest in such systems appears to be strong. Audi is believed to be planning a mild hybrid version of its Q7 (although that may not come to the US), and industry analysts IHS predict that by 2025 global sales of mild hybrids will top 12 million vehicles, with the bulk in China and Europe as a consequence of those CO₂-per-km regulations.