Car Tech

What is a Mild Hybrid, and Why Should You Care?

The combination of gas and electric motor vehicle motivation isn’t exactly new, but the general interest in hybrids is stronger than it’s ever been.

Both empirical and anecdotal evidence suggests more Canadians than ever before are considering a gas-electric offering for their next automobile, with results of an AutoTrader survey showing about half of shoppers would consider a hybrid for their next purchase.

But not all hybrids are created equal, with conventional – occasionally called self-charging – models that use a gas engine in conjunction with an electric motor or two, as well as plug-ins that, as the name would suggest, can be charged to provide some limited all-electric driving range.

Add to that list the misunderstood “mild hybrid,” which arrived more recently to much less fanfare. While the basics are similar to those of a conventional hybrid, the execution – as well as the results – are different.

What is a Mild Hybrid?

Whereas conventional and plug-in hybrids alike pair high-voltage electrical systems with internal-combustion engines, mild hybrids do so to a much lesser degree. Crucially, it means the electrical components can’t propel the vehicle on their own. Instead, they work in a supporting role to power in-vehicle accessories, reducing the load on the rest of the powertrain in the process.

While there are slightly different executions of mild-hybrid technology – more on those shortly – the fundamentals are the same: a 48-volt architecture, and a small motor-generator unit that’s used to start the gas engine and power various in-vehicle accessories.

What Does a Mild Hybrid Do?

In the case of the former Chrysler Corp.’s most popular V6 and V8 engines that can be had under the hoods of the Jeep Wrangler and Ram 1500, a mild hybrid system replaces the alternator that’s typically bolted to a gas-powered engine with a belt-driven motor-generator. While a 12-volt starter motor is still part of the package, and is used for cold starts, the motor-generator works to quickly and smoothly start the engine – particularly with the ignition stop-start system engaged that shuts the engine off when idle – while sending pulses of electrons to the crankshaft to minimize harshness during gear changes.

The system, which can also recuperate kinetic energy when braking, feeds electrical current to a small battery pack. Meanwhile, a converter switches the current from 48 volts to 12 volts so it can be used to power in-vehicle accessories, as well as to charge the conventional 12-volt battery under the hood. The latter reduces the load on the gas engine itself, which results in a small reduction in fuel consumption.

For a brand like Mercedes-Benz, the approach is slightly different, with a motor-generator unit replacing both the alternator and traditional starter. The location is different, too. Rather than mounted externally, like with those Jeep and Ram engines, the Mercedes system is sandwiched between the gas engine and transmission.

The Mercedes system uses its electrified torque to aid with acceleration, while also allowing the gas engine to shut off when coasting. In such a situation, the mild-hybrid system continues to power internal components like the steering system, while the gas engine can kick back on as soon as it’s needed.

Why Does it Matter?

How much mild-hybrid systems do to reduce harmful tailpipe emissions is negligible, but at least some fuel savings can be realized. Because while most of these powertrains have replaced gas-only ones, in the case of the ever-popular Ram 1500, the same V8 engine is available with and without the 48-volt setup. And according to Natural Resources Canada (NRCan) data, the 5.7L-powered pickup consumes about 1.7 L/100 km more fuel combined without the system along for the ride.

By comparison, the Toyota RAV4 is rated to burn about 2.4 L/100 km more gas in combined driving than its conventional hybrid counterpart, while the plug-in hybrid (PHEV) version does the same while also providing nearly 70 km of dedicated all-electric driving range. Looking at the Ford F-150, the hybrid version burns about 2.0 L/100 km less gas in combined driving than the same truck without the electrical components.

Crucially, however, those vehicles use much larger electric motors and batteries to propel them for periods of emissions-free driving – something a mild hybrid can’t do. That’s especially beneficial in city driving, where the F-150 Hybrid is rated to burn 3.2 L/100 km less than the same gas-only truck.