Saving fuel through waste heat recovery

MAHLE has developed a solution to increase the efficiency of the combustion engine in commercial vehicles. This solution opens up further possibilities. More ways to meet statutory emissions –regulations. More ways to improve transport using existing –technologies. MAHLE is the first to succeed in combining all these possibilities in one compact box: using a waste heat recovery (WHR) system. Now is the time to tap into the immense potential that lies in the use of exhaust gas heat from combustion engines. We take a look at a very special technology that is making a major contribution to climate protection.

30 percent

is the reduction in CO2 emissions required by the EU by 2030 for heavy commercial vehicles in the EU vehicle classes 4, 5, 9 and 10.

3.8 million

is the expected number of commercial vehicles to be newly registered worldwide in 2030.

80 percent

of commercial vehicles will still be equipped with a combustion engine in 2030.

HANNES MARLOK; WHR project manager - Working beyond the limits of their individual locations, the Berlin and Stuttgart teams join forces to drive forward the WHR system.

What do diesel engines and MAHLE have in common? They’re both success stories. And they’ve both been established on the market for a long time. MAHLE has been involved in combustion engine technology from the outset. So, if anyone knows that the technology still has untapped potential, it’s MAHLE. While the call for CO2 reductions has sparked a sometimes biased debate about the ­internal combustion engine and brought local zero-­emissions powertrain technologies to the fore, MAHLE has continued its development in all directions. And in doing so, we’ve found a way to ­reduce CO2 right now—using special technology. After all, the combustion engine will continue to be an indispensable part of the diversified drive mix in the ­future. Especially on long-distance routes in the commercial vehicle segment — until well after 2030. In addition to hybrid and purely electric power­train technologies, innovations in the combustion engine are also playing a key part in the overall ­reduction of CO2.

Waste heat recovery (WHR) has the ­potential to reduce CO2 emissions today — not sometime in the distant future.

Voltastraße, Berlin/Germany. The name “MAHLE” is printed in large letters on the truck that’s turning into the yard at our Berlin location on this cold December morning. On this day in particular, the street bearing the name of the electric battery’s inventor —Alessandro Volta — acts like a harbinger. Today, we’re focusing on the combustion engine and talking about electrical energy at the same time. As the truck finishes turning and comes to a stop, a red box gleams from underneath the vehicle frame, directly behind the driver’s cabin. It bears the words “Boost Box.” This box is a source of power, since it houses a waste heat recovery system that converts the heat from the exhaust flow into electrical energy.

“The technology isn’t unknown,” begins Hannes ­Marlok, project manager in the WHR team. “But no one before us has ever managed to make the whole thing ready for series production.” MAHLE has packaged “the whole thing” in a compact box with a width of just 55 centimeters and developed a perfectly coordinated system, thus paving the way for series production. “But there’s even more to come in the future, because the next generation of the Boost Box will be much smaller than this,” ­Marlok says with a grin. The box looks simple. But it’s clear from the expressions on the faces of the WHR team members who’ve gathered in the yard in Berlin that something really special has been achieved here. “The waste heat recovery box cuts fuel consumption by up to 5 percent, thus also ­reducing CO2,” explains Michael Hötger, Managing Director of MAHLE’s Berlin location.

The technology is well-known, yet MAHLE is the first to really get it going. The reason is simple. “No ­manufacturer wants to take on the risk and change the entire vehicle structure, because that costs money and requires time and effort. But we’ve –eliminated that risk with our box. Now the tech­nology is easy for anyone to test without major ­design effort,” ­explains Michael Hötger.

CO2 Calculations

Are vehicles with exhausts bad? And ones without exhausts good? If you want to compare the different powertrain technologies, you need to look at them from various angles to appreciate their potential.

1. From tank to wheel

This analysis only takes into consideration the CO2 emitted by a vehicle during use. The potential of renewable fuels, for example, isn’t covered, nor are any CO2 emissions produced during electricity generation—not tech­nology-­neutral due to the choice of assessment parameters.

2. From well to wheel

In addition to a tank-to-wheel perspective, this calculation also accounts for the CO2 emitted in the production of the fuel or electricity. This approach goes some way toward technological neutrality since it records the potential of renewable fuels as well as any CO2 emitted as part of electricity generation.

3. from cradle to grave

This analysis of CO2 emissions goes a step further than that described under point two. It also includes the vehicle’s production and its disposal at the end of its life cycle. It’s the most complex type of calculation, but due to its holistic approach it’s also the one that offers the highest degree of technological neutrality.

Technological neutrality is important when it comes to clarifying the best technological solutions from both an environmental and an economic point of view. With a technology-­neutral assessment, different powertrains will remain on the market even if CO2 thresholds become stricter, and the customer will be able to choose the technological solution that best suits their primary objective.

We don’t ask: Should it be diesel or not? But: How can we reduce CO2?
Michael Hötger
Managing Director at th eMAHLE location in Berlin

Components of the waste heat recovery (WHR) box

Creating usable energy from exhaust gas heat: how the waste heat recovery system works

  • 1 Exhaust gas aftertreatment
    Fuel is converted into kinetic energy in the engine. However, a significant proportion of the energy held in the fuel is carried away in the form of heat via the exhaust gas. This is where the waste heat recovery system comes in. Instead of releasing unused energy into the environment, it’s now put to efficient use.
  • 2 Evaporator
    The hot exhaust gas is passed through the evaporator after the exhaust gas aftertreatment.
  • 3 Axial piston expander
    Inside the evaporator, the exhaust gases vaporize a working medium, which drives an axial piston expander.
  • 4 Generator
    Kinetic energy is produced in the axial piston expander, which drives a generator – thereby creating electricity.
  • 5 Condenser
    The exhaust steam is then converted into a liquid in the condenser.
  • 6 Feed pump
    A pump is used to pressurize this liquid, which is then fed into the evaporator, heated, and turned into a gas again. This completes the cycle.
  • 7 WHR box
    The WHR box feeds the electricity into the battery.
  • 8 Battery
    Energy recovery: Energyis recovered when driving downhill and when braking. It’s stored temporarily in the battery and is available when needed
  • 9 Electric motor
    The stored energy canbe retrieved whenever an “extra push” is needed. When driving uphill, for example. This reduces fuel consumption and therefore also CO2 emissions.
We’re focusing on precisely what matters for the environment: efficiency.
Michael Bucher
Product expert in the WHR team

Happy to be part of a diverse, lively development unit in which every member of the team can contribute their own strengths.

»We’re focusing on precisely what matters for the environment: efficiency.«

Because even during the predevelopment phase in Berlin, the team of 25 people always had one thing in mind: the needs of the customer. So, the team also knows what matters at the end of the day: “It’s important that the truck is kept on the road. It’s only by staying on the road that it earns money for its owner. The WHR box is relatively simple to integrate into the vehicle structure.”

MAHLE focuses on the customer and the customer’s needs. It’s a simple maxim and yet at the same time a big claim, which constantly requires new thinking. Michael Hötger remembers one such instance at an intermediate stage in the project: “There was a moment when we realized that, yes, we’ve got all the components, but now we require a brand-new approach. We had to think again and leave everything out that we didn’t need. And we packaged all the technology into one box.” As a result, all the indi­vidual MAHLE components were brought together to form an integrated, ideally coordinated system, which functions perfectly inside a box.

Take a truck and a box. And then we can get star­ted. “All we need is waste heat of 150 degrees and over for the WHR system to work. Within just one and a half to two years— a short time frame when you think about the future — the WHR box has already paid for itself. For the manufacturer. For the freight forwarder. And, above all, for the environment.” Hannes Marlok translates this into a tangible illustration:

“The system saves 6 tons of CO2 per truck, per year. That equates to the CO2 output of 0.6 people or just under four passenger cars with an average mileage. If you think about how many miles a vehicle like that covers and how much fuel it consumes, 5 percent is a great deal of potential.”

MAHLE’s WHR box attracted a lot of interest at IAA 2018. “Customer feedback shows that we have a good product — one that the market has been ­waiting for,” says Michael Bucher, a product expert in the WHR team. “One of our customers now even wants to build the box himself. So, he calls us, looking for parts. We ask which components he wants us to supply, he laughs and says: ‘All of them!’”

At the very moment a harsh wind sweeps around the truck in the yard in Berlin, another WHR box is on the road in China for initial testing with a ­customer. When it comes to series production, MAHLE will work as one across business areas, locations, and national borders. All the hard work has paid off, and the team is delighted with the positive feedback the WHR box is receiving. Hannes Marlok adds: “We don’t yet know what the powertrains of the future will look like on our streets. But at MAHLE, we’re ready. And above all, we’re open to the technolo­gical possibilities that will enable a future with CO2-neutral ­mobility.”

And let's not forget...

... all the many other product solutions, projects, and initiatives that we delivered, initiated, and drove forward in 2018. Here are some highlights.


Our oil management module for vehicles with electric drives enables the cooling of multiple components: the electric drive motor, the ransmission or oil-cooled battery, and even the power electronics. Further advantages are achieved by integrating several individual components into one compact module: weight reduction, lower pressure loss, reduction of interfaces and thus assembly effort compared with the individual components, optimal use of installation space, and optimized thermal management.


In order to further optimize the efficiency of the combustion engine in commercial vehicles, MAHLE has developed a Shifting Roller System that allows engine valve timings to be adjusted to suit the requirements of different engine operating conditions. With this system, MAHLE integrates crucial functions such as engine braking into the valve train, thus making highly efficient operating modes like the Miller / Atkinson cycle possible and enabling exhaust gas temperature management. Under certain cir­cumstances, the technology eliminates the need for expensive retarders to be used as service brakes, while also decreasing fuel consumption and optimizing exhaust gas aftertreatment, which in turn results in lower emissions.

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