More and more car makers are adopting 48-volt mild hybrid technology as a way of downsizing engine capacity and improving fuel efficiency.

Car makers and motoring writers might go to great lengths to over-hype an all-electric vehicle future being just around the corner. But the truth is that we’re still a long way off calling time on the internal combustion engine.

Some car makers, like Toyota, made the jump early with the release of vehicles like the Prius and sundry other hybrid systems it’s rolled out since. But that type of drivetrain is expensive and, when you boil it all down, not particularly effective. Then along came plug-in hybrid vehicles as a bridge between something affordable and electric (with the back-up of an internal combustion generator engine) and expensive all-electric vehicles.

But, while plenty of countries around the world have announced that within the next 30 years they’ll ban the sale of internal combustion engine vehicles, or that some have said they’ll electrify their entire ranges within the next five to 10 years, you need to look a little closer at the fine print.

For instance, when Volvo said it would electrify all its vehicles it also said that the first step would be to squeeze a little more from the internal combustion engine. And, to that, brands like Volvo, will turn to 48-volt mild hybrid technology.

This technology will allow car makers to improve the efficiency of their internal combustion engines and, in many instances, get away with downsizing those engines without the expensive jump to a full hybrid system that isn’t any more efficient.

Indeed, according to Bosch which has designed and built a plug-and-play 48-volt battery system for mild-hybrid use by carmakers, there will be 15 million cars on the road by 2025 running 48-volt mild-hybrid systems.

What is a 48-volt electrical system?

This is a combination of things, like a 48-volt lithium-ion battery pack stored in the boot or under the back seats of the vehicle, and a ‘mild-hybrid starter generator’ which replaces the alternator and starter motor (under the bonnet) and a low-voltage DC/DC convertor. But, and here’s where it might get confusing, vehicles fitted with a 48-volt system also still have their conventional 12-volt battery which is linked to the 48-volt system via the DC/DC convertor.

When it’s all wrapped up, it’s referred to as a 48-volt mild-hybrid system. Car makers and components makers like Bosch, Continental and Delphi claim anywhere from 10-15% improvement in fuel economy and CO2 emissions.

Working in conjunction with your car’s 12-volt system, the use of a 48-volt electrical system means there can be more electrical gear onboard, for things like on-board modems and even electric turbocharging or supercharging.

How does a 48-volt electrical system work?

The 48-volt battery pack is used to store energy and supply it to the mild hybrid system. The mild-hybrid starter generator is connected via a belt to the crankshaft of the internal combustion engine and can generate energy via recuperation of up to 12kW and 55Nm of torque generated at the mild-hybrid starter generator shaft. This energy is transferred back to the battery for use later.

What are the advantages of a 48-volt electrical system?

While the system is designed to improve fuel efficiency by reducing drag on the internal combustion engine at certain times, it can also give the internal combustion engine a kick along and helps power things like the air-con when stop/start is activated. In this situation, the 48-volt battery can keep the electrics firing and handle the load of the climate control and facilitate the start-up once the brake pedal is released. Then the mild-hybrid starter generator gives the car a boost too for a smoother and quicker start-up and take-off (start up times can be reudeced by 0.2 seconds). Indeed, at all times, other than initial start-up when, say, your car’s engine oil is still cold and the 12V system is used for starting the car, the 48-volt battery is running the show.

Depending on the battery’s state of charge, the mild-hybrid starter generator acts as a power boost for the internal combustion engine feeding in additional power when needed. If the accelerator pedal is held steady and the system detects the vehicle is no longer being accelerated it will drop the mild-hybrid starter generator into a Neutral mode where it won’t supply any power but instead recuperates energy to recharge the battery. This occurs during periods of braking or overrun.

Quite often, these 48-volt electrical systems will have an extended start-stop period where, when the system detects the vehicle is coasting at a low speed (often up to 30km/h) it will disengage the clutch and switch off the engine.

And then there’s turbochargers and superchargers. These forced induction methods can help make a smaller engine feel like a bigger one but there’s always lag. For instance, with turbocharging you’ve got to wait for the exhaust gases to build and spool up the turbo. But by adding an electric compressor air can be forced through the pipework and the turbo primed before pressure has had a chance to build up in the conventional manner, thus reducing ‘lag’. Audi even has an electric sway bar running off a 48-volt system to eliminate body roll, which while an electric sway bar isn’t anything new, the linking it to a 48-volt electrical system is because it makes it stronger and more controllable.

Volvo also uses a 48-volt system to reduce turbo lag but unlike the VW Group (Audi) doesn’t use a compressor (supercharger) set-up. Volvo calls its system Power Pulse system and this draws in fresh air via the air intake and an electric air compressor which then stores the compressed air in a tank. When the system detects that you’ve hit the throttle the compressed air is forced into the exhaust manifold helping to spool up the turbocharger more quickly and, again, reduce turbo lag.

The benefits of a 48-volt electrical system?

Essentially, it means reduced fuel consumption and emissions and that’s because more stuff can be powered from the bigger, beefier power pack which is being recharged while you’re driving than a traditional 12-volt system. Things like ‘supercharger’ systems, power steering, electric anti-roll bars, pumps, fans and more can all be powered off a 48-volt system and tweaked to user or system demand and not just cause parasitic drag on the engine because their operation is tied directly to the internal combustion engine.

So, we can expect to see more car makers roll out 48-volt systems to stretch the internal combustion engine while infrastructure and development catches up with fully electric vehicles.


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  1. Do not forget that you will have to replace the 48v battery after a few years as well.
    Batteries have a limited charge/discharge cycle number so the more it is used the quicker it fails.
    Then there is the question of recycling all of these failed batteries, and sourcing the raw materials to create new ones, all using the old reliable fossil fuels.

  2. The only difference between 12 volt and 48 volt systems is thinner wires in the 48 volt system and a saving of (marginal) weight.

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