Exploitable results
All key components for building high efficient internal combustion engines as well as the necessary know-how have been created.
High-pressure direct injection
The investigations in HYICE evaluated the potential of hydrogen high-pressure direct injection (HP-DI) concerning efficiency and power density. Specific advantages beneath the high power density are opened up by a wide field of possibilities for optimising the hydrogen combustion process. Only with high-pressure injection, the injection timing can be varied within a wide range and thus controlled fuel stratification can be realised.
Within HYICE, extensive experimental work was carried out at TUG, employing a single-cylinder research engine in a modern passenger car configuration and a likewise transparent engine allowing insights into the mixture formation and combustion process. The best possible mixture preparation regarding efficiency could be pointed out with the definition of a perfect stratification. Further activities on improved injection strategies were carried out to approach this idealised benchmark stratification.
By means of new injection strategies, thus controlling the combustion process, a method has been developed to significantly improve the noise behavior which is of utmost importance in a passenger car application. At a certain operating point with multiple injections, pressure rise as well as maximum pressure can be limited.
Low-pressure direct injection
The important advantage of the low-pressure direct injection (LP-DI) is the reduced complexity of the fuel supply and its components, thus making a development to a higher level of maturity possible. As MAN is planning to equip a fleet of hydrogen urban buses within the follow-up project Hyfleet:CUTE with DI-technology, this question was of utmost interest. But compared to the high-pressure concept described before, this technology, of course, cannot offer as many degrees of freedom as the former.
Within the HYICE project, the new 12.8l six-cylinder engine was specifically designed for hydrogen operation. In a combined effort of three-dimensional computational fluid dynamics simulation at MAN and optical measurements at UBW and TUG, several injector nozzles were investigated and a suitable configuration for good homogenisation was found.
By help of a turbocharger, a remarkable maximum power output of 200 kW and an effective efficiency of 42 % were achieved very quickly. The chosen lean burn concept still offers a lot of potential for further optimisation.
Direct injectors
A major task is the design modification of the DI injectors for hydrogen. One primary function is the precise metering of the required fuel mass during a very short injection time of a few milliseconds. The design of all HYICE injectors is based on a proven gas valve concept of HVT.
Throughout several injector generations most challenges could be solved. The solenoid actuator showed impressive performance concerning switching times. A needle bouncing effect during the closing phase has been observed but different approaches for low- and high-pressure direct injectors have already been performed by HVT in follow-up projects in order to solve this problem.
Cryogenic port fuel injection
In contrast to direct injecting hydrogen engines, for the cryogenic port injection (CPI) the low pressure range of a liquid hydrogen tank is sufficient. This concept takes advantage of the external mixture formation of hydrogen and air by making use of the coldness. At the same time the concept of liquid hydrogen storage and CPI forms a technically straightforward system, therefore supporting the attempt to build economic hydrogen vehicles.
Within HYICE detailed experimental investigation in the mixture formation and the combustion process, as well as validation and adaptation work on CFD tools have been combined into the development of a combustion system for CPI.
Specifically designed cryogenic injectors, an anti-icing nozzle design and an optimised injection strategy provide a homogeneous air/hydrogen mixture with maximum cooling effect. The result is an increase in specific power by 25 % compared to hydrogen port injection at ambient temperatures, thereby reaching the level of current gasoline port fuel injection (PFI) engines. Moreover the engine range of good efficiency is substantially expanded.
The combustion process itself shows a very stable behavior. Thus the CPI engine is predestined for operation with higher compression ratios - leading to a further increase in efficiency - and turbo charging. With increased compression ratio an indicated efficiency of 44 % has been demonstrated, further improvements may be expected. Turbocharged operation with extreme specific power output has also been investigated, whereby 100 kW per liter of displacement have been reached.
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