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.

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

Accurate pressure measurement plays a vital role in the early days of automotive fuel cells

Accurate pressure measurement plays a vital role in the early days of automotive fuel cells

While Electric and hybrid EVs are firmly entrenched as maturing technologies much development is still needed in storing the electrical energy in a safe, convenient and cost effective manner. As a potential solution to expensive storage batteries most manufacturers around the world are studying ways of harnessing hydrogen to generate electricity to drive the electric traction motors.

Hydrogen fuel cells employing proton exchange membranes, also known as polymer electrolyte membrane (PEM) fuel cells (PEMFC), have already seen limited series production in vehicles such as Toyota’s Mirai. 

Fuel cells consist of MEAs (membrane electrode assemblies) sandwiched between separators. An MEA is a solid polymer electrolyte membrane with catalyst layers applied to it.

These cells generate electricity through a chemical reaction between hydrogen and oxygen: Hydrogen and ambient air are respectively supplied to the anode (negative electrode) and the cathode (positive electrode) of fuel cells to generate electricity.

Since one cell yields less than one volt, several hundred cells are connected in a series to increase the voltage. This combined body of cells is called a fuel cell stack. 

Furthermore, although small PEM fuel cells are operated at normal air pressure, larger fuel cells, of 10kW or more, are usually operated at higher pressures. The advantages and disadvantages of operating at higher pressure are complex, and the arguments are not at all clear-cut, with many convincing points of view on both sides. 

Supercharging the hydrogen fuel cell 

As with conventional Internal Combustion Engines, the purpose of increasing the pressure in a fuel stack is to increase the specific power, by extracting more power out of the same size cell. Ideally, the extra cost, size, and weight of the compressing equipment will be less than the cost, size, and weight of simply getting the extra power by increasing the size of the stack. 

In the case of ICEs the advantages clearly outweigh the disadvantages. However, with fuel cells the advantage/disadvantage balance is much narrower. Above all, this is because there is little energy in the exit gas of the PEMFC, and any compressor has to be driven largely or wholly using the precious electrical power produced by the fuel cell.

Image 1: Schematic of a fuel cell system
Image source: James Larminie, Andrew Dicks (Fuel Cell Systems Explained)

The simplest type of pressurized PEM fuel cell is that in which the hydrogen gas comes from a high-pressure cylinder. In this design only the air has to be compressed; the hydrogen gas is fed from a pressurized container, and thus it’s compression ‘comes free’. This method of feeding hydrogen to the anode is known as deadening; implying there is no venting or circulation of the gas – it is entirely consumed by the cell.

However, the compressor for the air must be driven by an electric motor, which of course uses up some of the valuable electricity generated by the fuel cell. Typically, for a 100kW system the power consumption will be about 20% of the fuel cell power. As in ICEs, for optimal efficiency the compressed air also needs to be cooled before entry into the PEM cell.

Balancing the pressure to optimize performance

As this is a young evolving technology the reliability and durability of these “supercharged” fuel cells need to be tested and developed if it is to be widely adopted. Hence there is considerable research and development taking place to improve the performance and lifespan.

Testing under controlled conditions is an important step towards the viability and uptake of fuel cells. Detailed measurement data is crucial as input information to create models of the fuel cell operation. Yet, in spite of widespread interest, suitable measurement techniques are still only in the process of being developed.

Typically the PEM fuel cell operates at pressures ranging from near ambient to about 3 bar and at temperatures between 50 and 90°C. High power density is achieved at higher operating pressures but the net system efficiency may be lower on account of the power needed to compress the air. Higher air temperatures also increase power density, but may pose a significant challenge for water and heat management, especially at lower operating pressures.

Therefore selection of operating temperature and pressure of the automotive PEM fuel cell system must be based on (a) high net system efficiency, (b) small component size, and (c) neutral or positive water balance so that the vehicle does not have to carry an on-board reservoir.

The increase in power resulting from operating a PEM fuel cell at higher pressure is mainly the result of the reduction in the cathode activation overvoltage, because the increased pressure raises the exchange current density, which has the apparent effect of lifting the open circuit voltage (OCV), as described by the Nernst equation.

However, as previously mentioned this supercharging comes at the expense of the power that the pressurized fuel cell produces, hence the importance of balancing the pressure to the requirements of the specific fuel cell. As with ICE boost pressures, this can only be done by taking accurate pressure measurements using high quality pressure sensors that are painstakingly calibrated to the environment.

These pressure measurements, recorded with laboratory grade sensors supplied by STS, are then compared to the fuel stack outputs to minimize the parasitic losses while optimizing the gains in electrical output.

As automotive hydrogen fuel cell technology matures and data collected from real-world trials is used to produce predictive models, engineers and researchers will no doubt gain a better understanding of the complex inter-relationship between, temperature, pressure and efficiency: But in the meantime this research will rely heavily on quality sensors recording accurate data.

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