Could a high-pressure direct-injection hydrogen engine replace the turbodiesel?

Could a high-pressure direct-injection hydrogen engine replace the turbodiesel?

Having fallen from grace, the once iconic diesel power unit appears to have run its course. Even cities, such as Paris, that once incentivized the use of diesel are now calling for OEMs to stop production by 2025. Although this is highly unlikely to happen, it is an expression of the world’s concerns over global warming and air pollution in general.

To meet ever tightening emissions regulations OEMs are studying new and often untried forms of propulsion: Everything from full electrification to hybrids and even hydrogen fuel cells are being tested as possible solutions.

Hydrogen in particular is piquing the interest of researchers around the world – it’s hailed as a clean burning fuel that could very well end up powering the transport of the future.

The difference between hydrogen and conventional hydrocarbons lies in its wide stoichiometric range from 4 to 75 percent by volume hydrogen to air, and under ideal conditions the burning velocity of hydrogen can reach some hundred meters per second. These characteristics make it highly efficient when burning lean mixtures with low NOx emissions.

Forty years of hydrogen injection

Hydrogen injection has been around since the 1970s and works by injecting hydrogen into a modified, internal combustion engine, which allows the engine to burn cleaner with more power and lower emissions.

Earlier low pressure systems, which are still in use today, injected the hydrogen into the air prior to entering the combustion chamber. But with hydrogen burning 10 times faster than diesel and, once mixed with the diesel in the combustion chamber, increasing the burn rate several problems have been experienced. The most significant being:

  • Light-back of the gas in the manifold
  • Preignition and/or autoignition.

The best way to overcome these problems is to fit a high-pressure direct injection system that provides fuel injection late in the compression stroke.

Optimizing the combustion process through accurate pressure measurement

In order to do this the injection needs to be accurately mapped to the engine. This can only be accomplished through gathering test data regarding temperature (manifold, EGT and coolant), pressure (cylinder/ boost, line and injector), the turbulence in the manifold and combustion chamber, and the gas composition.

The mixture formation, the ignition and the burning processes are commonly studied through two different sets of experiments. The aim of the first experiment is to obtain information about the highly transient concentration and distribution of hydrogen during the injection process.

During this test a Laser-Induced Fluorescence (LIF) on tracer molecules is used as the primary measurement technique to study the behavior of the hydrogen under compression and ignition. Using a constant volume combustion chamber (CVCC) with the same dimensions as the actual C.I. engine, implying that the volume in the CVCC equals the volume in the cylinder at the top dead center, pressurized hydrogen is injected into the cold pressurized air through a hydraulically controlled needle valve.

Using high quality pressure sensors, the effect of various injection pressures on the combustion process can be studied. By observing the behavior and volume of unburned gas, the time taken to optimize the injection pressure for a specific number and position of injector nozzle holes and also the injection direction is drastically reduced.

And using unique software the ignition delay, which is dependent on the temperature and the concentration of hydrogen in air at a given pressure can be determined. Once again, it’s important that the pressure readings are accurately recorded, across a range of pressures that vary between 10 to 30 MPa.

Furthermore, this method allows for the definition of areas of the injection jet where self-ignition conditions exist, which is useful for the development of an optimized injection system for engines to be converted from diesel fuel to hydrogen.

In recent tests carried out by a premium brand OEM,the optimized high pressure hydrogen injected engine showed a promising increase in specific power while reducing fuel consumption and achieving 42% efficiency – values that match the best turbodiesel engines.

Based on the findings it would certainly appear as if work carried out on optimizing the pressure of these 30 MPa systems may in fact offer another source of clean energy for future transport.

Hydrogen embrittlement in steel

Hydrogen embrittlement in steel

The sensor chip of piezoresistive pressure transducers is usually surrounded by a steel membrane. For the housings of these measuring instruments, stainless steel is also used in most applications. But should contact with hydrogen occur, this material can become brittle and then crack.

Hydrogen embrittlement affects not only steel, but also other metals. This is why the use of titanium offers no alternative in regard to hydrogen applications.

What is meant by embrittlement?

Hydrogen embrittlement refers to a loss of ductility in the material. Ductility describes the property of materials to plastically deform under stress before they finally fail. Depending on its type, steel can deform by more than 25 percent. Materials that do not have this ability are termed brittle.

But ductile materials can also become brittle, or frail. When this embrittlement of the material is the result of hydrogen absorption, this is then termed hydrogen embrittlement.

Hydrogen embrittlement occurs when atomar hydrogen diffuses into the material. The prerequisite for hydrogen embrittlement itself is usually hydrogen corrosion.

Hydrogen corrosion, also known as acid corrosion, always takes place whenever oxygen deficiency exists and metal comes into contact with water. The end product remaining from this redox reaction is pure hydrogen, which then oxidizes the metal. The metal goes into solution as ions and causes the material to be evenly degraded.

The hydrogen released by this redox reaction diffuses into the steel due to its small atomic size of only about 0.1 nanometer. The hydrogen directly occupies the metal lattice of the material as atomic interstitials. Lattice imperfections arising here then increase the absorption capacity. This leads to a chemical fatigue in the material, which can ultimately cause cracks from the inside to the outside, even at low loads.

Hydrogen and pressure transmitters

Because of its very tiny dimension, hydrogen can not only penetrate the material, but can actually penetrate it completely. For this reason, not only an embrittlement of the material can occur. The metal membranes of piezoresistive pressure sensors are very thin – the thinner they are, the more sensitive and accurate the sensor becomes. If hydrogen diffuses into and through the membrane (permeation), it can then react with the transfer fluid surrounding the sensor chip. As a result, changes in the metrological properties of the measuring bridge occur due to hydrogen adsorption. At the same time, an increase in pressure can also occur as a result of these deposits, with outcomes ranging from a buckling of the sensor membrane through to its complete destruction.

Besides using a thicker but somewhat more inaccurate membrane, this process can be greatly retarded by using a gold alloy and the unit lifespan thus optimized. You can read more about this here.

Lifespan optimization of pressure transmitters in contact with hydrogen

Lifespan optimization of pressure transmitters in contact with hydrogen

Hydrogen atoms are extremely tiny and because of this property can even penetrate solid materials in a process known as permeation. Over time, pressure transmitters become inoperative due to this process. Their lifespan can however be optimized.

In piezoresistive pressure transmitters, the sensor chip is enveloped in a fluid, usually oil. This section is in turn enclosed by a very thin, 15 to 50-μm-thick, steel membrane. Because of the miniscule atomic dimension of hydrogen, the gas can diffuse through the crystal lattice of metals (see infographic). Over time, this penetrating gas leads to a no longer tolerable zero shift in the signal arising and the steel membrane bending outwards. The pressure sensor thus becomes unusable.

Overview of hydrogen properties

Infographic: malachy120///AdobeStock

Pressure sensors come into contact with hydrogen in a wide array of applications, be it in the monitoring of hydrogen tanks themselves, in submarines or in the automotive sector. Especially in the latter case, hydrogen is being increasingly used in the development of alternative drive systems. Many manufacturers have been working for several years on models incorporating fuel cells and some cities have already opted for hydrogen buses in public transport. The advantages are not to be dismissed, since only hydrogen and oxygen are required as source materials. Through a chemical reaction, energy in the form of electricity is produced, with no exhaust gases created at all (the combustion product is water vapor). Furthermore, hydrogen, as opposed to fossil fuels, is available in inexhaustible quantities. Development is already well advanced and there are now models, which consume only 3 liters of hydrogen over 100 kilometers, whilst distances of up to 700 kilometers with one tank filling are, in part, already possible.

In this branch, high-performance, high-precision pressure transmitters are necessary, which monitor the hydrogen tanks of the vehicles. More specifically, the pressure and temperature inside the hydrogen tank of the vehicle must be monitored. Pressures of up to 700 bar can arise here, but a broad temperature range must also be covered. It is imperative, of course, that the pressure transmitters used perform their duty over an extended period of time at the required precision. To optimize the lifespan of sensors in hydrogen applications, several influencing factors must be considered:

  • Pressure range: The gas flow through the sensor membrane is proportional to the square root of the gas pressure. A ten-fold-lower pressure increases the lifespan of the sensor by about 3 times.
  • Temperature: The gas flow through the sensor membrane increases at higher temperatures and depends upon the material constant.
  • Membrane thickness: The gas flow is inversely proportional to the membrane thickness. The use of a 100 μm instead of a 50-μm-thick membrane doubles the lifetime of the sensor.
  • Membrane area: The gas flow is directly proportional to the membrane surface area (the square of the membrane diameter). With a Ø 13 mm instead of an Ø 18.5 mm membrane, the lifespan of the sensor doubles.

Since high pressures and broad temperature fluctuations can occur inside the hydrogen tanks of vehicles, the lifespan of the sensors cannot be influenced by these two factors. The factors of membrane thickness and membrane area also only promise a limited remedy. Although the lifespan can be improved by these factors, it is still not yet optimal.

Gold coating: The most effective solution

The permeability of gold is 10,000 times lower than that of stainless steel. With the gold coating (0.1 to 1 μm) of a 50 μm steel membrane, hydrogen permeation can be suppressed significantly more effectively than by doubling the membrane thickness to 100 μm. In the first scenario, the time for a critical hydrogen gas volume to accumulate in the interior of the pressure sensor can be increased by a factor of 10 to 100, whereas in the second example only by a factor of two. The prerequisite for this is gapless and optimized welding, as well as a largely defect-free coating.

Image 1: Example of a pressure transmitter with gold coating

Because of these properties of gold regarding the permeability of hydrogen, STS uses gold-coated stainless steel membranes as standard in hydrogen applications.

Downlaod our free infographic on the subject:

Image 1: Example of a pressure transmitter with gold coating