MAHLE Powertrain collaborates with Liebherr to co-develop active pre-chamber technology for heavy-duty hydrogen-fuelled engines

“The use of hydrogen as a combustion fuel has the potential for rapid de-carbonisation of many heavy-duty and off-road sectors,” says Mike Bunce, Head of Research for MAHLE Powertrain US. “A wealth of research over the past few decades has confirmed the compatibility of hydrogen with internal combustion engines. The challenge has been to get it to run with stable combustion without resorting to reductions in compression ratios to avoid engine knock and pre-ignition. Our common work with Liebherr suggests we have the answer.”

Hydrogen is a carbon-neutral fuel that is gaining renewed attention due to its increasing availability and the fact that it can be produced from renewable energy sources. As a fuel for proven and existing ICE technology in aggressive environments, hydrogen is ideally suited to the high load duty cycles with sudden load steps, heat, dust and vibration of heavy duty and off-road use. This is not the case for battery electric or fuel-cell powertrains.

The challenge is that hydrogen as a combustion fuel is highly prone to abnormal combustion, leading to engine knock and pre-ignition. Typically, this necessitates an inefficient reduction in engine compression ratio. However, a potential alternative is to depress combustion temperatures by using high levels of dilution, which is more conducive to stable running, but would require a high-energy ignition source. Originally developed for gasoline applications, MAHLE’s pre-chamber ignition technology enables the use of highly dilute fuel/ air mixtures, making it ideal for use in hydrogen-fuelled engines. To achieve this, the MJI was adapted, mainly by modifying the pre-chamber in the cylinder head containing a spark plug, which ignites a fuel/ air mixture. The resulting gas plasma is forced through small orifices into the main combustion chamber to ignite the combustion mixture very quickly and uniformly.

Adaptation of the active pre-chamber to fit in Liebherr’s H966 and H964 engines has shown, in a preliminary investigation, that this technology extends the stable dilution limit of the engine well beyond the capability of traditional ignition systems, with much more rapid and complete combustion.

“Together with Liebherr Machines Bulle SA, we have been able to tailor the active pre-chamber technology to the Liebherr heavy-duty engines, which leads to improving the combustion speed, performance and emission. Additionally, the technology will simplify the hardware architecture of the engines,” explains Mike Bunce. “This has enabled us to mitigate the compression ratio de-rating risk associated with hydrogen, as well as reduce the fuel consumption. The expertise of Liebherr’s engineering teams in the areas of heavy-duty combustion and alternative fuels research have both been major assets in this program,” adds Bouzid Seba, Head of Predevelopment at Liebherr Machines Bulle, pointing out Liebherr’s openness to technology and the Group’s pursuit of different approaches to energy conversion as well as suitable fuels.

Some of the collaboration results to date were jointly presented at the 8th International Engine Congress in February 2021. The two companies are continuing their collaboration in order to push towards zero emissions vehicles that can meet the significant demands of the heavy-duty and off-road sectors.