Design and optimization of injectors for diesel and hydrogen engines
Micro precision not only for injection systems
In 2016, Liebherr opened a completely new plant in Deggendorf as a competence center for injection systems and micro-precision parts. One focus of this Liebherr site is the development, production and assembly of common rail injectors for diesel engines. Simultaneously with the opening of the new plant, the internal use of simulation software in product development was intensified.
From zero to one hundred: simulation since 2019
With Dr.-Ing. Martin Seidl, who worked intensively on combustion engines and rocket combustion chambers during his studies and doctorate in the field of aerospace, the necessary expertise was added. After initially focusing on 1D simulations with Matlab/Simulink, there was an acceleration in FEM calculations as well as in CFD and electromagnetic analyses in 2019 when the company signed a framework agreement with ANSYS and CADFEM. “In 2019, I received support from my new colleague Matthias Smetana in the use of simulation,” reports Martin Seidl. “Matthias mainly deals with structural-mechanics issues. In addition, we use ANSYS software in the fields of fluid dynamics and electromagnetics, for example to prepare student theses.”
Liebherr Components Deggendorf GmbH (abbreviated CCR for Components Common Rail) is part of the components product segment within the Liebherr Group. At its site in Deggendorf, Liebherr develops and produces micro-precision parts and components for injection systems (common rail) that can withstand high loads and extreme environmental conditions. The system solutions, which are designed for pressures of up to 2200 bar, are installed both in the group's internal combustion engines as well as in units from other manufacturers worldwide.
These include applications for heavy-duty off-highway vehicles, distributed energy systems, maritime applications and mining vehicles. Modern manufacturing processes and many years of experience in the development, design, production and remanufacturing of components contribute significantly to the quality and performance of Liebherr products. For decades, Liebherr has been developing and producing diesel engines, which it itself uses as an OEM in the off-highway vehicle sector. This experience guarantees the highest possible performance and reliability of the injection systems over a long service life and provides a solid basis for mastering future requirements as well.
With regard to climate change and environmental regulations, climate-friendly injection solutions for the use of CO2-neutral fuels have top priority at the Deggendorf site. Existing components and system solutions are currently being tested and further developed for the use of methanol-based fuels. In the field of hydrogen technology, experts in Deggendorf are working on new injector concepts for direct hydrogen injection for use in combustion engines.
Accurate prediction of physical phenomena
The system components developed and produced at Liebherr-Components Deggendorf GmbH are based on complex and interacting effects from the fields of mechanics, hydraulics/pneumatics and electromagnetics.
The use of reliable simulation tools is essential for correctly predicting the physical phenomena that occur and the individual interactions in the injection systems and components. That is why Liebherr relies on various commercial and in-house software solutions.
In order to predict the dynamic behavior of injectors, pumps and switching valves, 1D simulations (sometimes called 0D simulations) are performed. The simulations are carried out either with the in-house Matlab code DIESL or the Simscape environment, an extension of Matlab/Simulink with graphical user interface.
Simplified 1D model of a diesel injector
This illustration shows a simplified 1D model of a common rail diesel injector with a two-way valve (inlet and outlet throttle), which is controlled hydraulically via an electromagnet. In simplified terms, the nozzle needle that can only be moved axially is assumed to be a point mass whose position influences the volume of the control chamber, the blind hole and the cross-sectional area of the seat throttle. In turn, not only do mechanical forces from the spring and stop act on the needle, but also hydraulic forces.
1D models often represent highly simplified and abstracted representations of reality and accordingly cannot reproduce or even predict all details of the complex reality. Model calibration and validation often requires data that is not available at all or only available to a limited extent, for example from measurements. The required information can often be obtained with specialized simulation tools and stored, for example, as lookup tables in the 1D model (see red boxes in the preceding illustration).
Questions often arise that cannot be answered using 1D simulation, such as strength verifications or the determination of the magnetic force of the electromagnetic actuator. The software solutions from ANSYS are used for this, which capture 2D and 3D geometries and effects with precise physical models and can solve large and complex systems of equations.
Further development of the diesel injector platform
In the course of ever stricter emissions standards worldwide, the components of common-rail diesel injection systems must also undergo continuous further development. Accordingly, the LI2 platform for the medium- to heavy-duty range was fundamentally improved during the generation change from GEN2 to GEN3.
By increasing the rail pressure from 2200 bar to 2500 bar and significantly reducing the blind hole volume, pollutant emissions of soot and unburned hydrocarbons have been significantly reduced. In addition, the increase in rail pressure has led to increased engine performance and efficiency, which further reduced CO2 emissions.
Predict increased stress with certainty and precision
The implementation of the measures described presented a major challenge for the Liebherr development team. In particular, the increase in the maximum rail pressure from 2200 to 2500 bar - with local pressure peaks in the direction of 3000 bar - has considerable consequences for the functional behavior, wear and service life of the injector. Therefore, the increased stress due to increased mechanical and hydraulic forces must be predicted with certainty and precision. In addition, the increased tendency to cavitation and resulting erosion damage in the nozzle, especially in the spray holes, has been counteracted with a modified design.
The use of 2D and 3D FEM and CFD simulations is essential for this. In combination with 1D simulation, the further development from GEN2 to GEN3 could be significantly advanced. The importance of simulation in the development process at Liebherr in Deggendorf is illustrated here by two examples.
FEM: Reduction of load and wear on the needle seat
When the nozzle closes at the end of injection, the needle strikes the needle seat with great force. This leads to high mechanical loads on the nozzle body, which must be taken into account when selecting materials and designing the geometry. This is the only way to reliably counteract excessive component wear - or even failure - over the entire service life of the injector (100 million - 1 billion switching cycles).
The design of the nozzle and needle is based on coordinated 1D and FEM simulations, which are compared by measurements on existing systems. The 1D simulation provides the input parameters required for the FEM simulation, such as the impact speed of the needle and the compressive forces acting. Based on this, a detailed analysis follows with FEM simulations. Key figures calculated in this way for stiffnesses, and damping parameters are in turn integrated in the 1D simulation to increase the model accuracy.
The figure shows the mechanical load on the nozzle in the seat for the period directly after needle impact. A comparison between a measurement and a 1D and FEM simulation is shown. The maximum load in the needle seat and the downstream mechanical vibration are very well represented by simulations in terms of both amplitude and frequency. In addition, the mutual comparison ensures good orientation for the development of new designs.
A comparison of the mechanical stress in the needle seat between LI2.9 GEN2 and LI2.9 GEN3 is shown here. The agreement of 1D simulations with the measurements is similarly good for both cases.
Dr.-Ing. Martin Seidl
With the 1D and FEM simulations, a significant load reduction of about 50 percent was achieved. This makes the new design even more robust and ready for the new requirements with rail pressures of up to 2500 bar.
CFD: Reduction of cavitation in the nozzle
Due to the extreme pressure difference between injector and combustion chamber and strong flow deflections in the blind hole, there is a considerable acceleration of the fuel in the inlet to the injection holes. As a result, the static pressure drops locally below the vapor pressure and cavitation occurs (i.e., part of the fuel passes from the liquid to the gaseous phase). This can lead to a reduction in flow and thus to a loss of performance. More seriously, however, the resulting cavitation bubbles will re-condense further downstream of their formation - in the spray holes - when local pressure rises above vapor pressure again due to delayed flow.
If the gas bubbles implode near the wall, strong pressure waves and microjets with high velocity are formed during this process. These often lead to erosive damage to the component surface after just a short time. In the medium term, this has a negative impact on the functional behavior of the injector and the emission behavior of the engine, which in the worst case can lead to complete component failure (e.g., Nozzle tip breakage).
A comparison of CFD simulations for the LI2.9 GEN2 and LI2.9 GEN3 injectors: A normalized criterion is shown with which the risk of erosion through cavitation is assessed (0 = low, 10 = high). Using computationally intensive CFD simulations, a design was found that meets the new requirements and significantly reduces the risk of erosion at both the needle and the nozzle.
New development of a hydrogen injector
The technology of diesel injectors is already considered well-developed. Serious improvements in the combustion process in terms of consumption and pollutant emissions in the diesel sector, as well as for gasoline and natural gas combustion, are no longer to be expected. To meet increasingly ambitious emission targets in the medium and long term, other paths must therefore be taken. In addition to technologies like "electromobility" and "fuel cells," which are subjects of heated public debate and are increasingly pushing their way into the market, an adaptation of existing combustion processes to alternative fuels such as hydrogen is also a possibility. Producing these fuels in a climate-neutral manner offers numerous advantages. These include:
“At Liebherr, we recently developed a new hydrogen injector for low-pressure direct injection, or LPDI, for rail pressures up to 60 bar,” explains Dr. Martin Seidl. “Here, numerical simulation served to overcome numerous challenges. We were able to use it to check whether our newly developed concepts would be at all feasible. Using simulations, we conducted many feasibility studies. This resulted in an iterative approach to the requirements.” This is to be examined in more detail here using two exemplary applications.
Electromagnetic design of the actuator for an H2 injector
In conventional diesel injectors, hydraulic forces are traditionally used to switch the nozzle needle via one or more intermediate hydraulic valves. A portion of the fuel, previously compressed to high pressure, is returned to the tank as a leakage flow.
However, this is not possible for the H2 injector, in which hydrogen is not a liquid but a gas at comparatively low pressures. Firstly, forces equivalent to diesel injectors cannot be built up. Secondly, hydrogen is provided via a high-pressure tank (350 bar or 700 bar), so that no backflow into the tank is feasible due to the pressure drop.
Large cross-sectional areas for the flow
If an additional oil circuit for switching the nozzle needle is to be avoided, the needle must be driven directly by an electromagnetic actuator. Thus, needle and armature together are a fixed axially movable assembly. The piezo elements sometimes found in diesel injectors, which are used for actuation, cannot be considered for gas injectors because of the significantly larger needle strokes. Due to the much lower volume-specific calorific value of hydrogen compared to diesel, large cross-sectional areas are also required for the flow.
The force of the spring on the needle required to seal between the injector and the combustion chamber and the pressure force on the sealing surface when the injector is closed must be overcome by the actuator when it opens. The forces required for this purpose considerably exceed the forces customary for electromagnetic actuators in the diesel sector. In addition, a larger stroke is required to unthrottle the flow. The magnetic force decreases exponentially with the distance between the armature and the pole core of the magnet, which represents a clear tightening of the requirements for the magnet.
Based on electromagnetic simulations, geometry and materials were selected and optimized in the development process so that the requirements were fully met. A comparison between measurements and simulations made it clear that both the static level of the magnetic forces as a function of current and width of the air gap between the armature and pole core and the dynamic attraction behavior of the armature were very well predicted with simulations.
Flow analysis of the mixing behavior in the combustion chamber
In contrast to diesel injectors, hydrogen is injected into the combustion chamber in gaseous form. The design of the injection cap, which determines the shape of the injection jet into the combustion chamber, is geometrically very different from classic multi-hole diesel nozzles. The jet angle and mixing with air in the combustion chamber play a key role in the combustion process and must be predicted using CFD simulations.
The figure below shows a comparison between a CFD simulation and a measurement (visualization of the density gradient via Schlieren photography) for a free jet directed centrally into the combustion chamber with a single axial opening. Hydrogen is introduced into the combustion chamber at supersonic speed. Since information is transported in flows at the speed of sound, decoupling of the injector and combustion chamber occurs. Consequently, the flow of hydrogen into the combustion chamber is determined only by the conditions in the injector (for example, by the rail pressure) and is independent of the combustion chamber pressure. The comparison between CFD simulation and measurement shows that the jet breakup of the H2 jet is predicted very well. CFD simulations can be used to find optimized blowing cap geometries for each specific application.
“The examples listed here show that simulation has always supported us in finding the right way for new developments and optimizations,” asserts Dr. Martin Seidl. “When I joined the company, there were two main diesel systems, which were constantly supplemented by others over the years. Nowadays, systems for hydrogen and other alternative fuels are under development. This has significantly increased simulation requirements. By building up know-how and gaining an ever better understanding of systems, we can now act more flexibly and achieve cost and time savings, partly by reducing the number of tests. At the same time, our simulation models are becoming increasingly accurate through comparison with test data. We will continue along this path to further develop new ideas, continuously improve existing products and adapt them to new customer requirements.”
Liebherr-Components Deggendorf GmbH
Dr.-Ing. Martin Seidl
Authors: Dr.-Ing. Martin Seidl, Liebherr-Components Deggendorf GmbH;
Gerhard Friederici, CADFEM
Images: © Liebherr
Published: June 2022