Unlike the true-hybrid configurations that use the engine and/or electric motor to propel the vehicle, the 48V mild hybrid system is employed in a typical stop-start vehicle with the integration of a 48 volt electric motor/generator assembly to supplement the engine, improving acceleration and enhancing fuel economy.
In addition, electrifying components such as the air conditioning compressor, power steering pump and engine oil pump further reduces the engine load and increases fuel efficiency.
1 Electric motor/generator assembly.
2 AC/DC inverter.
3 48 volt battery.
4 DC/DC converter.
5 48 volt power distribution unit.
6 12 volt battery.
7 12 volt power distribution unit.
Electrifying vehicles with voltage higher than 12 volts is not new: the automotive industry contemplated using a 42 volt system in the 1990s. This was later dismissed due to concerns about the cost, as well as practical matters such as switches and relays prematurely failing. However, driven by environmental concerns, modern electronics make use of transistors, diodes and microswitches which are more robust, making this a more viable option.
So why stop at 48 volts? Current regulations state anything over 60 volts officially becomes ‘high voltage’. This adds extra cost due to the addition of expensive shielding, connectors and conduits like the orange ones commonly seen on many hybrid and electric vehicles. Yet, this doesn’t mean the electrical architecture of the entire vehicle will move to 48 volts. The conventional 12 volt supply is still used to power many of the standard circuits such as lights, door locks, electric windows and infotainment system.
The common 48 volt mild hybrid consists of only a small number of additional components: electric motor/generator assembly, AC/DC inverter, DC/DC converter, a 48 volt battery and an e-charger.
Electric motor/generator assembly: The water cooled, belt driven electric motor/generator replaces the regular alternator and functions to restart the engine after a stop-start event, while the conventional 12 volt starter motor is used for normal starting via the ignition key.
At times, the electric motor/generator can support the engine to improve acceleration and reduce load strategically to maximise fuel economy. In addition, the electric motor/generator, in generator mode, recharges both batteries, similar to a conventional alternator, but also when the vehicle is coasting or braking.
AC/DC inverter: The AC/DC inverter can either be integrated or non-integrated into the 48 volt electric motor/generator and exists to perform two functions.
Firstly, to convert the direct current (DC) from the 48 volt battery to alternating current (AC), which then powers the electric motor/generator in motor mode.
Secondly, to convert the AC generated by the electric motor/generator whilst in generator mode to DC, as a result recharging the 12 and 48 volt batteries.
DC/DC converter: As this vehicle encompasses both 12 volt and 48 volt systems, a DC/DC converter is installed to reduce the electrical voltage from 48 volts to 12 volts.
48 volt battery: The lithium-ion 48 volt battery is generally located in the rear of the vehicle, and just like the electric motor/generator it can use the cooling system to dissipate heat.
E-charger: The conventional turbocharger is superseded with an electrified version and is better known as an e-charger. Instead of waiting for the exhaust gases to spin the impeller up to speed, an electric motor is used to drive the impeller, instantly providing the necessary boost without the familiar delay that is usually experienced with turbocharged engines. Alternatively, superchargers can also be electrified to provide equivalent results as an electric motor driven turbocharger.
The 48 volt mild hybrid system is always evolving to fulfil exhaust emission regulations, improve fuel economy and increase acceleration. For this reason, vehicle manufacturers are already developing other intelligent enhancements to compliment the 48 volt system and the following are just some examples:
Dynamic skip fire (DSF) technology: Integrates cylinder deactivation with the 48 volt mild hybrid system. The DSF system isolates a cylinder by disconnecting the camshaft followers. This locks the inlet and exhaust valves in the closed position when less power is required, resulting in better fuel economy
Extended stop-start technology: Unlike the conventional stop-start technology of switching the engine off when the vehicle comes to a complete stop, the extended stop-start system will also switch off the engine when approaching a stop or while the vehicle is cruising at a constant speed.
Electrically heated catalytic converter: In order to reduce the amount of harmful emissions, the catalytic converter must reach operating temperature as quickly as possible. Hybrid systems exacerbate this due to frequent stop-start events or coasting with engine off however, this can be easily solved by heating the catalytic converter electrically using the 48 volt system.
Electrifying engine driven ancillaries: The water pump and air conditioning compressor are two examples of components that can be electrified. This reduces parasitic engine drag and more notably, allows the components to set their own duty cycle based on vehicle and driver demands.
So, while the current 12 volt system struggles, compared alongside the other more expensive true hybrids, the 48 volt mild hybrid technology offers a cost effective solution in satisfying emission regulations and future increases in energy hungry electrical components.
The use of 48 volt technology is set to grow even further, especially as stricter CO2 emission targets are being set and the decline of the internal combustion engine continues.
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