How mild hybrids work
Hybrids work. Toyota has been proving this for what feels like an eternity with the Prius, not quite so long with other models. Beyond Toyota, such full hybrids are rarely found so far. Since 2019, mild hybrids have increasingly prevailed instead. Here, the electric assistance is somewhat weaker than in the full hybrid, mild hybrids cannot drive purely electrically at all. Accordingly, the fuel-saving potential is lower, but the design effort is not as high. Read all about the mild hybrid technology here.
Mild hybrid technology in brief
- Approximately 10 percent less fuel consumption thanks to electric assistance
- Lower currents than full or plug-in hybrids
- Integrated or belt-driven starter generator as an electric motor
- Advanced and more convenient start-stop system
In a full hybrid, the gasoline engine works as often as possible in the range of the highest efficiency. This is around 2,200 rpm for the current Toyota Prius, slightly higher for its predecessor. A continuously variable transmission, which variably adjusts the ratio, is best suited to achieve this. The disadvantage: The background noise often does not match the perceived acceleration. European ears don't always like to hear that. A matter of taste: Nevertheless, Toyota today provides one of the most popular taxi models in cities such as Berlin and Barcelona with the Prius.
German manufacturers preferred to compromise on their full hybrids in order to make the background noise more pleasant. Mostly at the expense of efficiency. And that ultimately cost their hybrid models their raison d'être, they hardly sold.
Nevertheless, hybridization is considered a proven means of reducing consumption. So many manufacturers are now foregoing the possibility of actually driving electrically. They build mild hybrids that provide electric support and relieve the diesel or gasoline engine. This significantly reduces the costs and effort of electrification, and thirst is at least noticeably reduced.
The mild hybrid and its starter generator
The most important means of the mild hybrid is a so-called starter generator, a simple electric motor that helps the combustion engine work. It can contribute some power for a short time, start the engine and generate electricity. In all cases, it takes over the task of the alternator completely, that of the starter at least partially.
A starter generator can be placed at different positions on the car. Audi is taking advantage of the gap left by the missing alternator. From here, the starter generator is connected to the crankshaft via a belt. This design is called a belt starter generator (RSG). The advantage: It can be used with existing engine types. Kia and Hyundai are pursuing a similar tactic, as is BMW. Here, engines with RSG 2020 will initially be launched in diesel models of the 3 Series and in the X3 and X4 SUVs. At Volkswagen, the current Golf 8 is the first as the 1.5 eTSI.
Mercedes also swaps alternators for RSG on some engines. In the large sedans and SUVs, however, more effort is made in Stuttgart. There, the starter generator is located between the engine and transmission directly on the crankshaft. Without belts, better power transmission is achieved. On the other hand, the design effort is much greater. Mercedes has developed the drives specifically for the so-called integrated starter generator (ISG). The S-Class (S 450) will be launched in the summer of 2017, followed by other model series in 2018, including the GLE and GLS SUVs and some AMG models. From 2020, Mercedes will use an ISG in conjunction with a 2.0-liter four-cylinder in the E-Class facelift for the first time.
Electric assistance when starting off
Both variants, RSG and ISG, can start combustion engines more comfortably and quickly than a pinion starter. The driver hardly notices any of this, the starting process feels like the gear change of a nimble automatic. That's why the combustion engine always stops when it is not needed, for example when so-called "coasting" without a load. This saves fuel.
Like a full hybrid, a mild hybrid also helps where a combustion engine works less efficiently. For example, when starting off. A little electric torque relieves petrol or diesel engines and thus saves fuel. Here, the SI joint is better because it can transmit more force.
How much you ultimately save depends on many factors. The driving style is still decisive, as is the type of mild hybrid system. Audi claims a reduction in consumption of 0.5 to 0.7 liters per 100 kilometers in everyday life. Kia expects about 10 percent less thirst than without a starter generator.
48-volt technology in large-scale production
In the rarest of cases, the regular on-board electrical system is sufficient to operate a starter generator efficiently. The power limit for this is about 3 kilowatts (approx. 4 hp) – too little for the desired effect. Many manufacturers are therefore retrofitting a second on-board electrical system with a voltage of 48 volts. Here, the cable cross-sections are thinner, and up to 25 kilowatts (34 hp) can be transmitted.
Hyundai, Kia, BMW and Mercedes currently rely on the 48-volt network, while Mazda works with a 24-volt on-board electrical system. Audi uses 48 volts only for engines with six or more cylinders. Four-cylinder engines get mild hybrids with a voltage of 12 volts. Suzuki offers mild hybrids exclusively with 12 volts. These cars save less fuel in comparison because the electric motor provides support with less power. At least in theory, if all other parameters are identical. In practice, many other factors and concrete implementation play a major role.
Volvo has also had mild hybrids in its range since 2019, but is currently still limited to the Volvo XC90 and Volvo XC60 SUV models. Seat and Skoda are also at the start with the first models, followed by Ford, Subaru, the PSA Group and Renault. The strategy is similar in all cases, the implementation varies.
Full hybrids save more fuel than mild hybrids in comparison. However, they use a higher voltage. In the high-voltage range from 60 volts, additional safety requirements apply. In addition, there is a more elaborate construction. Ultimately, they are therefore significantly more expensive. Mild hybrid systems can be installed in existing models. They compensate for the additional weight of the system without any problems.
Additional extras with 48 volts
A pleasant side effect: the higher voltage of the 48-volt network opens up new possibilities. Audi and Mercedes, for example, use electrically driven compressors. They sit in the intake train and compress the air before the turbochargers have overcome their second of remembrance. The cars respond more spontaneously and thus improve acceleration and exhaust gases.
Some manufacturers also use the 48-volt network for roll compensation. Electric motors are located in the stabilizers, which tension the chassis against the direction of the curve. The body rolls less and stays nice and straight in fast bends. This effect is particularly noticeable in large SUVs.
Overall, mild hybrids are worthwhile for manufacturers and customers. Both are happy about lower consumption. Car manufacturers can add additional extras to the surcharge list. The hybrids will remain optional equipment in small vehicle classes for the time being – they are still too expensive for basic models. But down to the compact class, various models with 48-volt mild hybrid will be added in 2020. Manufacturers can no longer do without the technology in the volume segments because they face high fines if they fail to meet the EU's CO2 targets. Every gram counts.
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