Renewable fuels

Hydrogen engine innovation is on course.

Hydrogen engine innovation is on course

Within the next ten to twenty years, sustainably produced hydrogen will be widely accessible, providing a new future for the internal combustion engine. NPS Driven is fully committed to the energy ecosystem of the 21st century, in which hydrogen will be the primary fuel. This means sustainable use of combustion engines that best match the familiar mechanical engineering concepts and the applications of diesel engines for current customers. The transition to hydrogen-based combustion engines is an exciting and quickly accessible alternative that requires minimal additional training and adaptations. The engines will become commonplace between 2025 and 2030.

Running a diesel engine on gas

The government-subsidized initiative aims to make a traditional diesel engine suitable for hydrogen combustion. NPS Driven is leading this work package that is part of the GTD-H project. Creating the optimal ratio of oxygen and fuel requires more than conventional mechanical technology. Diesel injection and injector technology, which creates high pressure in the combustion chamber by compression, is well-known to motorists and auto mechanics. The modified engine’s valves operate as usual to draw air in by opening and closing. Following the introduction of fuel, a spark starts the combustion.


Figure 1:

The injection duration of a hydrogen combustion engine during three transient loadsteps. Because of hydrogen’s low density, a larger volume of fuel must be injected. This requires a longer injection window.

The proper dosage of hydrogen must be administered in gaseous form for it to burn in an engine. Computer technology is essential for the required precision. Fortunately, CNG has already accomplished this, so hydrogen is an obvious contender for the next advancement. Technically speaking, the transition from CNG to hydrogen is not straightforward because the two fuels have different combustion characteristics and provide unique difficulties.


Figure 2:
The spark map of a hydrogen combustion engine. The spark map defines the crank angle, where the hydrogen/air mixture is ignited. The angle is dependent on the engine speed and fuel flow.

LPG and CNG experience as a basis for hydrogen

NPS Driven, Lumipol, and TNO have been working for a long time to reduce exhaust gas emissions. In another role, NPS Driven employees have contributed to using LPG and CNG as fuel for diesel engines. This successful project not only resulted in insights but also various patents that now ensure a flying start in the development of a hydrogen combustion engine. Experiments with hydrogen injection conducted at the time yielded exciting results. However, the need to use hydrogen was still lacking, and the availability of this fuel was uncertain. The experiences and insights from that time give NPS Driven a head start now that TNO is working on a heavy combustion engine on hydrogen. Hydrogen can reduce emissions from fossil fuels to the level now required.

Essential steps toward efficient combustion

Making a diesel engine suitable for hydrogen requires more space in the combustion chamber. To achieve this, we reduce the compression ratio in the experimental configuration by grinding off a portion of the piston. The shape of the piston is also optimized for hydrogen combustion. To enable efficient combustion, we need to adjust the valve cover, add ignition coils and add a fuel rail. All these steps are essential to facilitate optimal combustion. The end product will be a ready-made conversion system.

Hydrogen has several benefits over LPG and CNG. Hydrogen has wide flammability limits, which means the engine can run very lean or very rich and thus needs special attention. Luyckily Hydrogen allows for a more precise determination of the appropriate fuel-to-air ratio. It can also be adjusted accordingly in the engine. There is a large ignition window when the engine starts; hydrogen can ignite even with little fuel and lots of air, starting the engine. Other advantages are:

  • High flame speed.
    This allows hydrogen engines to come closer to the thermodynamically ideal engine cycle and thus run more efficiently.
  • High diffusivity which means its ability to disperse in air is much greater than that of petrol or diesel. This causes a more homogeneous mixture of fuel and air and it is safer, because it disperses rapidly in the event of a hydrogen leak.


Figure 3:

The air fuel ratio of a hydrogen combustion engine during two transient loadsteps. The stoichiometric air fuel ratio is 34. Because of the wide flammability limits (4% – 74%), it is possible to run the engine very lean or very rich. 

Despite all these benefits, hydrogen’s energy curve differs significantly from that of fossil fuels. The system is operational in the test environments. We know the forces that must be adjusted as well as those released when hydrogen ignites.

Hydrogen in the tank or on-demand production

NPS Driven and TNO are testing engines with pure hydrogen Hydrogen must be stored safely under high pressure. It is not desirable for every customer to drive around with such fuel. NPS Driven has involved DENS in this project to work on storage with less demanding specifications. The subsidized development program works in two phases. First, hydrogen, produced with solar energy or wind power, is converted into hydrozine, better known as formic acid, with added CO2. It can be stored and transported as a liquid at normal temperatures, which is much easier than working with hydrogen. Then, at the destination, a reformer/reactor converts the hydrozine back into hydrogen through a chemical process. For applications in engines, hydrozine can be added to the tank, and the desired power of the engine determines how much hydrogen must be produced immediately. Then, the previously added CO2 is rereleased as an emission. DENS and NPS Driven nevertheless speak of an emission-neutral system because the CO2 released is equal to the previously absorbed CO2 for the production of the hydrozine.

In a similar way, the hydrogen from DENS is also intended for specific electric fuel cells in combination with a battery pack.

Introduction of the H2 engine in 2024

The first hydrogen combustion engines are running in the laboratory as a replacement for the diesel engine for generators. NPS Driven expects to market them in 2024 as the first option to use hydrogen as an alternative to diesel. This application is quickly feasible because generators operate in a limited energy spectrum: the speed and load only experience limited peaks and troughs.

For heavy-duty traction engines, the broader energy spectrum is the first challenge: conquering slopes and varying speeds must work as flexibly as the drivers are accustomed to.

The transport of the fuel is the second task. Bringing enough hydrogen to the next refuelling station in cylinders at 350-700 bar is much more complex than a traditional diesel fuel tank. The other solution is adding equipment to the truck/vessel to store and convert hydrozine to hydrogen during the journey. These kinds of adjustments for the new fuel mean a rigorous intervention in the design of trucks.

These innovations are in the spotlight worldwide, and NPS Driven is determined to remain firmly at the forefront.

Peter van der Heijden, Peter Wezenbeek & Jarne Lotens

H2 ICE is not a project. It’s a mission

“We borrow the earth from our children, so it is our job to come up with solutions for sustainability issues,” says Peter van der Heijden. “NPS Driven is known for distributing diesel engines and developing and constructing emission-reducing catalysts. Together with colleagues from TNO and DAF and our own H2 specialists, we are working on the realization of a clean alternative to the traditional combustion engine.”


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