Mild Hybrid Audi and VW: How Does It Work?

You buy a new Audi or VW, see MHEV or eTSI on the badge and think you've got a hybrid that can run on electricity. But then you put your foot down and the normal engine does all the work. So what exactly is a mild hybrid Audi or VW, and why bother fitting one? I get asked this quite often, so I want to explain what the system actually does, how it helps and what can go wrong. Because there's one big difference between a mild hybrid and a hybrid that can drive on electricity alone.

What is a mild hybrid, and how is it different from a full hybrid?

Imagine taking a normal engine and adding a more powerful starter-generator and an extra battery to give it a helping hand when needed. That's a mild hybrid. The starter-generator is an electrical unit that can start the engine and recover some energy to put back into the battery. But here's the important bit! Unlike a full hybrid, this system can't drive the car on its own. The petrol or diesel engine still does the main job.

The idea is simple. Rather than keeping the engine running all the time, even when you don't need it, the system can switch it off more often and then restart it quickly and smoothly. When you slow down, it captures some of the energy that would otherwise be lost through braking and stores it in the battery. That energy can then be used again. Just don't expect to drive for miles on electricity alone. That's not what a mild hybrid is designed to do.

Which Audi and VW models have mild hybrid systems?

Not all mild hybrids are the same. Audi and VW use different versions, mainly depending on the voltage of the electrical system and how it helps the engine. That's why, when someone tells me their car is a mild hybrid, the first thing I ask is which model and engine we're talking about. Here are the main versions you'll come across.

• 48 V on the Audi A6, A7, A8, Q7 and Q8 with V6 and V8 engines. Here, the starter-generator works with an additional 48-volt electrical system. If you'd like to know more, read how the 48 V starter-generator works.
• 12 V on some four-cylinder Audi models. The principle is similar, but the system runs on a 12-volt supply.
• 48 V eTSI on the VW Golf 8 and other models with DSG gearboxes. This is VW's version, where the electrical system gives the petrol engine a helping hand.

The important thing is not to treat all these cars as if they're identical. The mild hybrid badge tells you how the system works in principle, but it doesn't mean every version uses the same parts. That difference really matters when you're diagnosing a fault or carrying out a repair. Otherwise, it's very easy to start looking for the problem in completely the wrong place.

What does a mild hybrid actually do while I'm driving?

The interesting thing is that, most of the time, you won't even notice the system working. It's not supposed to make your car feel like an electric vehicle. It's there to help the engine when it makes sense to do so. And it does that in a few different ways.

• It switches the engine off while you're coasting, when conditions allow. The car keeps moving without the engine having to run the whole time.
• It restarts the engine quickly and smoothly. The starter-generator makes restarting much gentler, so you don't feel an unpleasant jerk.
• It recovers energy when you brake. Instead of losing all that energy, the system turns some of it into electricity and stores it in the battery.
• On eTSI models, it also gives the engine a little help when pulling away. The electrical system assists the engine, but it doesn't drive the car on its own.

It might sound complicated, but from behind the wheel, everything should feel completely natural. You lift off the accelerator, slow down or pull away, and the electronics decide when to use the energy they've stored. There's no special button to press, and you don't have to choose when the mild hybrid system works. You just drive. The system takes care of the rest.

What are the common weak points of a mild hybrid?

Now we're getting to the part that interests me most as a mechanic. An extra electrical system means extra components that need to work properly. When a mild hybrid starts playing up, simply plugging in a diagnostic tool, reading the fault codes and clearing them isn't enough. You need to find out which component is actually causing the trouble. Otherwise, you'll end up with exactly the same fault.

• The starter-generator and its belt. The starter-generator has an important job starting the engine and recovering energy, while the belt is part of the system that drives it. Both need checking.
• The 48-volt battery. This stores the energy the system needs to operate. If something goes wrong with it, the mild hybrid system may no longer work properly.
• The normal 12-volt battery. Yes, that still matters, even though the car has an additional electrical system. A replacement battery must be the correct type and properly coded to the car. I've explained why a new battery needs coding.

That's why, when I'm diagnosing one of these cars, I don't just look to see whether there's a warning light on the dashboard. We check the battery readings and how the starter-generator is working to understand what's really happening. A fault code can tell you where to start looking, but it doesn't always tell you which part needs replacing. And changing parts on a guess has never been a good way to diagnose a car.

How should I service my mild hybrid Audi or VW?

The first thing I want you to remember is that having a mild hybrid doesn't mean you can skip normal engine servicing. Just because your car has an extra battery and a starter-generator doesn't mean you can forget about the engine oil, filters and other checks. Quite the opposite. I want you to look after both the normal engine and the additional electrical system.

• Change the engine oil and ALL filters every 10,000 km or once a year, whichever comes first. That means the oil filter, fuel filter, air filter and cabin filter, all changed together. For cars driven in Bulgaria, this is the interval I recommend regardless of which engine oil you use or whether the car has LongLife servicing.
• Check and service the starter-generator belt and tensioner as specified. Don't ignore them just because they aren't showing any obvious signs of trouble.
• Check both batteries at every service. That way, you can catch a problem early, before you're left wondering why the system has stopped working.

Tip from Kris Mladenov: If Start/Stop stops working, don't immediately blame the starter-generator or the 48-volt battery. Check the condition of the normal 12-volt battery first. That's the sensible place to start before replacing any parts. If you'd like to understand why, read how Start/Stop works.

I hope you've found this article useful! If you have, I'd really appreciate you sharing it with a friend who drives an Audi or VW with a mild hybrid system. I've always believed that if I know something useful, more people should know it too. Because the better you understand your car, the less likely you are to end up paying for unnecessary repairs out of your own pocket.

Read more

• Electrical faults and control modules
• Diagnostics
• Audi 48V Mild Hybrid Problems: A6, A7, A8 and Q7
• VW Battery Coding: Why Start/Stop Stops in Winter
• Car Start Stop System: How Does It Work?

Frequently asked questions

What is a mild hybrid?

A mild hybrid is a system with a more powerful starter-generator and an additional battery. It helps the engine start and stop and recovers energy when you brake.

Can a mild hybrid drive on electricity alone?

No. Unlike a full hybrid, a mild hybrid can't drive the car using electricity alone. The petrol or diesel engine remains the main source of power.

What is eTSI on a VW?

eTSI is a type of mild hybrid with a 48-volt system, found on the Golf 8 and other models with DSG gearboxes. It helps the engine when pulling away, makes restarting smoother and recovers energy when braking.

Why isn't Start/Stop working on my mild hybrid?

One of the first things to check is the condition of the 12-volt battery. Don't replace the starter-generator or any other components until the system has been properly diagnosed.