At CONEXPO 2023, Liebherr is showcasing a hydrogen direct injection solution for heavy-duty combustion engines. With this technology, it aims to achieve maximum power density equal to the one of a conventional combustion engine. Hydrogen-based powertrains are an important part of Liebherr’s technology-open approach to alternative powertrain concepts.
The aim of matching the performance of an H2-DI engine to that of a diesel engine requires the system to be capable of ensuring high flow rates. Due to the low density of hydrogen gas, the injector needs to be a high flow design. To enable precise control of even the smallest quantities, the system pressure must be regulated with pinpoint accuracy. In Liebherr’s H2 injection system, this is achieved by means of a gas volume control valve. Additionally, it is important to ensure that the injector has zero leakage and is gas-tight.
“To achieve the same drivability with an H2 system as with a diesel, the hydrogen injection system must be optimally aligned with the engine’s torque and power,” explains Richard Pirkl, Managing Director of Technology and Development at Liebherr-Components Deggendorf GmbH. “This means that during the transition from idle to full load, the necessary amount of fuel and the corresponding system pressure must be made available as quickly as possible.”
Liebherr’s H2 injection system is designed to provide extremely fast and accurate pressure control, regardless of fuel tank position, machine size, layout or engine installation. This design provides a two-stage pressure control. While the first stage initially stabilises the variable pressure from the fuel tank, the second stage fine-tunes said pressure. The injection pressure is controlled by activating the gas-metering valve via the electronic control unit (ECU). The ECU controls the gas-metering valve by a closed-loop feed-forward controller. Custom-developed hydrogen-specific software modules can be integrated into third-party application software and/ or control units.
“The H2-DI system is designed to operate without an electronic pressure release valve. The idea behind it is to keep the system as simple as possible, whilst avoiding the release of any hydrogen gas into the atmosphere during operation”, Pirkl summarises.
“The injector is the most sophisticated and, at the same time, performance-determining component of the hydrogen fuel system”, explains Pirkl. The overall dimensions of Liebherr’s H2 LPDI injector are similar to those of diesel injectors for heavy-duty commercial vehicle engines. In particular, the critical maximum outer diameter is within the same range as for diesel injectors.
It says hydrogen direct injection is particularly well suited for applications with high demands on dynamics and power density within a limited installation space and is a feasible concept, which Liebherr in Deggendorf is further developing. “Product validation will be one of the most important tasks in the development of hydrogen fuel systems in the future”, says Pirkl. “The next steps are to optimise the low-pressure DI system in terms of injection stability and dynamic performance. Based on the low-pressure DI system for heavy-duty engines, we are also developing and testing a system with higher flow rates for larger engines”, concludes Pirkl.
In parallel, Liebherr is working on H2 PFI injectors. The system approaches for port fuel injection and direct injection use a common, scalable injector platform as the basis. With this extensive product portfolio, Liebherr meets a wide range of engine requirements and enables a broad spectrum of applications for medium and heavy-duty engines, as well as large engines.
In addition, at Conexpo this week, Liebherr is presenting a prototype of its hydrogen engine for construction sites of the future. It employs the described direct hydrogen injection technology to power the engine. Prototype engines have been tested since 2020. Meanwhile, the prototypes have shown encouraging results in terms of performance and emissions, both on test benches and in the field. Both port fuel injection (PFI) and direct injection (DI), have also been assessed in the process. The first prototype construction machines equipped with these engines have been running since 2021.
Initial efforts in the development of a hydrogen engine have considered PFI as the first suitable technology. The first machine running with a 100% hydrogen-fuelled Liebherr ICE is the Liebherr R 9XX H2 crawler excavator, which won a Bauma Innovation Prize in October of 2022.
Encouraged by the results achieved with the PFI technology, Liebherr is pursuing its research and development activities in the field of DI. The 4-cylinder engine prototype H964, exhibited at Conexpo 2023, is equipped with this technology. In this case, hydrogen is injected directly into the combustion chamber, whereas with the PFI solution it is blown into the air intake port. The DI offers increased potential in terms of combustion efficiency and power density, making hydrogen engines an attractive alternative to diesel engines when it comes to more demanding applications.
The components segment expects to kick off series production of hydrogen engines by 2025. In the meantime, the company continues its research activities in fuel injection to further optimise combustion and to ensure maximum power density.
In addition to 100% hydrogen-fueled engines, several research endeavours in the area of alternative fuels are currently in progress. One example is a dual fuel engine that can run on hydrogen ignited by HVO injection or fully on HVO. This technology will allow for more flexibility in vehicle operation with different configurations.