Soft hybrids in the workshop


Mild Hybrid Electric Vehicle (MHEV) is one of the cheapest and simplest solutions included in the hybrid drive range. In the following text, we will look at the features of this solution and present the practical aspects of servicing cars in a soft hybrid system.

Classification of hybrid vehicles

First, we will present a classification of hybrid cars. A very large number of hybrid systems have been developed and used in various cars. The term hybrid is used to describe a number of cars, significantly different in construction and capabilities.

Micro hybrids

This term is usually used to describe vehicles equipped with a start-stop system. The term hybrid is a stretch in this case, since such cars are powered by an internal combustion engine and do not have an electric motor. Some systems try to recover braking energy slightly, but with traditional 12V lead-acid batteries this concept does not work. The condition for more current to go to the battery is that it is partially discharged, which in the long run leads to sulfation. In addition, the transfer of more power at 12V generates very high currents, which is impractical.

Soft hybrid (mild hybrid)

This is a drive in which the electric motor is not able to fully propel the vehicle, but acts as an auxiliary during driving. The essence of the system is to replace the starter and alternator with a single electrical machine capable of performing both functions (Integrated Starter-Generator – ISG). 48V batteries are used as energy storage, although some solutions (Mazda) use 24V. Energy recovered during braking is used to assist the internal combustion engine. The car stores only a small amount of energy. Compared to the internal combustion engine alone, you can save approx. 10% of fuel under real-world conditions. The term soft hybrid therefore refers to internal combustion cars equipped with an auxiliary electric motor/generator.

Full hybrid

A classic hybrid system in which a vehicle can be powered by both an internal combustion engine and, to a limited extent, an electric motor without the involvement of an internal combustion engine. This requires a suitable design of the transmission system to allow the two types of drive to work independently. During typical driving, the main function of the electric drive is to recover braking energy and use it for starting, which allows significant fuel savings in city traffic (fuel consumption is lower than for mild hybrid solutions). Electricity in a full hybrid system is obtained only from the generators in the vehicle, we do not have the option of charging from the grid. Hybrid cars of this type get a very short range on the electric motor, on the order of 2km.

Plug-in hybrid

This variant is an extension of the full hybrid system with a battery pack to achieve a useful range in electric mode. We can charge plug-in hybrids from the electric grid and theoretically they can be used as electric cars for short trips. Plug-in hybrids are controversial because, by current standards, they get very good CO2 emissions/km thanks to the fact that they can drive a sizable portion of the test route in electric mode, undercutting average combustion as well. Drivers typically operate these cars on long trips using only internal combustion propulsion, which translates into much less favorable emissions performance and much higher fuel consumption than claimed. A plug-in hybrid involves a heavy vehicle that contains a sizable battery bank in addition to a full internal combustion engine, much like electric cars.

Elements of the soft hybrid system

A typical configuration of such a system in a passenger car includes the following components.

  • An electric machine (motor/generator) with a typical power of the order of 10kW. There are several variants of mounting this component, in the simplest one it is connected by a belt to the engine shaft, just like the alternator (P0 architecture – most commercial solutions). Other systems have an electric machine embedded in the crankshaft (P1 architecture, e.g. Honda IMA, Mercedes BlueHybrid). Nowadays, there is an emerging trend to put electric machines on the drive side which allows better performance, but requires an additional starter. In the case of the location at the gearbox input, we are talking about P2 architecture, and behind the gearbox – P3. In this designation system, the P4 architecture stands for drive-axle integration.
  • Battery with a voltage of 48V (or 24V) and an example capacity of 0.4 kWh – 1kWh. The amount of energy stored in such a battery is therefore similar to the energy stored in a typical starter battery. This type of battery is relatively small and lightweight, and is sometimes additionally liquid-cooled.
  • An electronic system that performs battery charging (energy recovery) or controls the electric motor and sometimes the electric turbine while driving.

Electric turbine – some systems use electric support for the turbocharger. The recharge pressure can be obtained by the power taken from the battery. You may find that the electric support of the turbocharger will allow you to get more acceleration at start-up than what a crankshaft-linked electric motor can offer.

Soft hybrid – properties

  1. Virtually any type of internal combustion engine can be converted into a soft hybrid by retrofitting it with off-the-shelf modules. Soft hybrids can be powered by both gasoline and diesel engines. Manufacturers can convert any existing car models into a soft hybrid without much difficulty.
  2. The system does not significantly increase the weight of the car and does not take up much space, and the costs associated with it are much lower than for full hybrid cars. Unfortunately, many manufacturers price the spare parts found in these systems (e.g., batteries) particularly high, which is not always justified by the real prices of the components.
  3. The system operates on a safe voltage of 48 or 24V. This increases the safety of car operation and does not expose mechanics to high voltage when working on the vehicle. At the same time, by raising the voltage, we get the ability to transfer more power, which turns out to be sufficient for practical solutions.
  4. The system can perform the functions of the Start-Stop system much better than traditional systems of this type in cars. The electric machine has a large reserve of power at startup, and thanks to electronic control can favorably shape the starting torque for the engine. In addition, it is permanently connected to the engine. It has no brushes and does not wear out depending on the number of starts made.
  5. The car’s real-world fuel savings may turn out to be less than what manufacturers claim, but there is no doubt that even partial braking energy recovery makes sense, especially in urban driving conditions.
  6. Soft hybrid (mild hybrid) cars receive preferential treatment, their sales are associated with lower excise taxes, similar to full hybrids.

Servicing soft hybrids

Unlike classic hybrids and plug-in hybrids, servicing soft hybrids does not require additional electric credentials. The voltages present in the system are considered safe. Of course, this does not mean that every mechanic will be immediately ready to service such vehicles. As with any new technology, it is worth starting with training and acquiring the necessary knowledge to safely take the first steps in operating hybrids of this type.

What can go wrong?

Among the most fail-safe components of the system will certainly be the battery module, which is subject to natural wear and tear, and which can also suffer as a result of car operation errors. Admittedly, lithium cells do not need to be fully charged like lead-acid cells – a partially charged state is even beneficial for them, as they degrade more slowly in this state. However, beware of discharge – a deeply discharged battery can be damaged. In many cases, the electronics integrated into the battery can prevent a deeply discharged battery from being recharged. Therefore, according to manufacturers’ recommendations, prevent battery drainage by starting the car at least once a month. You should also be wary of batteries present on the secondary market. If they have spent too long in a discharged state (e.g., accident cars), they may be worthless even despite their not inconsiderable age or mileage. When we say battery, we mean not only a battery of lithium cells, but also integrated electronics, usually equipped with an additional connector for communication with the controller.

The second problematic component is the inverter. This is a component loaded with high currents, which is responsible for power transfer in the system. At the same time, there is a need to miniaturize these components while optimizing costs. In many cases, the failure rate of the inverters is probably due to their relatively short service life, and it is expected that the problems will be eliminated in future generations of systems.

The third, least fail-safe component of hybrid cars of the type in question is the electric machine. Modern motors of this type, which lack brushes and commutator, are durable and should not cause problems as long as they are designed and manufactured properly.

Oscilloscope measurements

An example of a measurement that is relatively simple to make to see in practice how soft hybrid cars work is an oscilloscope measurement using current clamps. If we want to measure the voltage at the same time, it is important to provide a device to which we can safely attach 48V. For many devices, we need to use appropriate clamps with a divider. Using the Scope DT workshop oscilloscope, we don’t have to worry about this, as we can connect such a voltage to any of the inputs. In addition, the design of the inputs protects against the possibility of making a short circuit. When selecting clamps, we must use a component with a suitable current range, such as 400A. With the clamps we cover one of the current wires leading to the battery. By performing the test under dynamometer conditions, we are able to see when current flows into the battery, charging it, and when additional power is transferred to the electric motor, which assists the internal combustion engine.

As many manufacturers as many variants

The specific way to service a particular soft hybrid car depends on the version of the system. There is no shortage of these on the market, and manufacturers are developing new generations of chips that differ in technology and capabilities. When considering 48V type systems, always refer to the service procedures of specific cars.


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