Accurate pressure measurement is crucial to developing an electric oil pump

Accurate pressure measurement is crucial to developing an electric oil pump

Driven by escalating global emissions targets, OEMs are increasingly turning to electrification to reduce fuel consumption and Greenhouse Gas emissions. A popular choice in this regard is the hybrid electric vehicle, often powered by a severely downsized engine.

The problem with these downsized engines is that power-sapping auxiliary systems severely impair drivability and performance. Fortunately these parasitic losses can be significantly reduced by replacing traditionally mechanical components with electrically driven units. Because of this, electrically powered pumps are rapidly finding their way into series production; particularly driving oil and water pumps.

Image 1: Example of an electric oil pump
Image Source: Rheinmetall Automotive

But while the benefits are obvious, electrifying, in particular the oil pump, is technically complex: Engineers, not only wish to circulate the oil at a particular flow rate and pressure, but would like to intelligently match these to the engine requirements.

In order to optimize the performance it’s important that friction and pumping losses are minimized through careful control of the oil flow into different branches of the oil circuit while ensuring the correct pressure is available at all times.

Simulation relies on accurate testbed oil pressure and flow rate information

An electrically powered oil pump is made up of three subsystems – the pump, motor and electronic controller. Therefore the primary challenge of any new application development is the efficient integration of these modules so as to reduce the overall size and weight as well as the number of components, whilst optimizing performance.

The main function of the oil pump is to deliver a specified oil flow at an optimal pressure. For this reason, its design, which is an iterative process, starts with the ‘pumping gears’. For most applications the pump is required to deliver pressures in excess of 1 to 2 bar, often going as high as 10 bar.

As in most engine developments, a combination of simulation and real world testing is used to speed up the design. 

The design loop begins with the preliminary assessment of the volumetric efficiency based on experimental results collected on similar pumps and applications. These include pump speed, oil temperature, pressure and flow rate. 

It’s important that the information used for the estimation is accurate, therefore the data collection must be carried out using highly dependable, precise measuring equipment that can deliver accurate readings under the extreme conditions encountered in and around the engine.

To ensure accuracy and repeatability it’s important that only the best quality sensors are used when measuring the pressure. Not only must these pressure sensors provide reliable readings across a wide range of pressures and temperatures, but they must also withstand vibration.

Over many years STS have developed pressure sensors that meet OEM, first tier and specialist engine designers’ requirements in engine development. 

Developing an electric oil pump that outperforms the mechanical unit 

Armed with the information gathered on the hydraulic requirements at various flow rates, delivery pressures and oil temperatures, a preliminary design of the gears is finalized. Using Matlab’s  Simulink software, the information regarding the behavior of the physical system can be rationalized into an one-dimensional code. 

At this stage it’s important to note that to generate the required flow at a specified pressure, a rotational speed should be selected that facilitates the best packaging of motor and pump without creating cavitation or noise issues: Thus a typical speed range for continuous operation is usually between 1500 and 3500 rpm. 

In the next step, several designs can be generated using LMS Imagine. Lab’s Amesim software that optimizes the design parameters – for example the number of teeth and eccentricity, while satisfying all pressure, flow and temperature boundary conditions. 

After implementing the geometrical features of the calculated hydraulics and the interim design has been finalized the total torque required to drive the pump at critical working points can be calculated as follows: 

Mtot = MH + MCL + Mμ 

Where:

  • MH is the hydraulic torque due to the generation of required pressure and flow
  • MCL is the coulombian contribution generated where there are dry or lubricated contacts between sliding parts
  • Mμ is the viscous contribution due to the fluid movement inside clearances.

Once the design is completed, engineering prototypes are constructed for real world evaluation on an engine testbed. 

Once again oil pressure, flow rate and temperature are measured at various engine and pump speeds to validate the results obtained through simulation. If the results meet the specifications the development program is finalized and the project enters the industrialization phase. 

For optimal performance and durability it’s obvious that all measurements be accurately recorded, but the weight given to information generated by the pressure sensor possibly outweighs all others – insufficient pressure at any point can lead to a catastrophic failure; while excessive pressure wastes energy and could lead to problems with the oil seals.

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 is critical to safe, cost-effective, motor vehicle development

Accurate pressure measurement is critical to safe, cost-effective, motor vehicle development

The principle of hydraulic power to carry out work has been around since ancient Egyptian times, but as systems have evolved, so too have the tools required to design and develop these sophisticated, often critical circuits.

From the earliest manometer invented by Evangelista Torricelli in the 1600’s to the mechanical Bourdon gauge and finally today, the piezoresistive pressure transducer, developers have always sought the best equipment to measure pressures and optimize the design. And in recent times automotive engineers, in particular, have come to rely on these high-quality, accurate pressure sensors when carrying out vehicle testing and development.

These current pressure transducers are typically capable of recording full-scale deflections from about 350 mbar to 700 bar under sustained temperatures ranging from -40OC to 150°C; and best of all, quality sensors such as those produced by STS, are capable of a hysteresis and repeatability of typically around 0.001%!

Image 1: High precision pressure transmitter ATM.1ST with accuracy of up to 0.05% FS

High-quality pressure sensors are used in the development of key automotive systems.

This level of repeatability is critical in the design and development of cooling and fuel delivery systems, amongst others. During development, designers rely on stable pressure measuring equipment to accurately record information so that the effect of even the smallest of design changes can be documented without concerns that the sensor is incapable of repeatable results.

In a recent redesign of an engine cooling system to take advantage of the reduced parasitic losses made possible through electrification, the engineering team at a luxury OEM was initially faced with a pressure drop across the pump of around 250kPa. Before a redesign of the new electric pump was possible, accurate pressure measurements had to be recorded allowing engineers the opportunity to identify the problem. After studying the results logged by the array of pressure sensors the design was modified, reducing the drop to less than 100kPa and cutting the parasitic losses by 500W.

And although electrification and electronic controls are playing increasingly significant roles in vehicle systems, hydraulic pressure is still relied upon to guarantee smooth operation of many critical circuits.

By way of example, during the development of an automatic transmission, port line pressures have to be measured in real time and then compared to design norms to confirm that design parameters are being met. At the same time, shift times and quality are measured and subjectively evaluated to ensure drivability and performance meet customer requirements.

Notwithstanding the value of high-quality pressure sensors in recording valuable data during testing and development, in industrializing future technologies these tools can also significantly reduce design costs.

Pressure sensors make sure future technologies measure up to expectations.

In an attempt to improve the performance of severely downsized engines, manufacturers are taking advantage of the additional power 48V electrification offers, by replacing the turbocharger with an electrical supercharger.

Being a maturing technology, not much research and testing data are available to engineers wishing to optimize eCharge superchargers. Although fluid dynamics and electrical engineering provide a sound platform from which to build, it’s still vital that theories are validated under real-world test conditions.

To achieve this, manifold pressures must be mapped to optimize engine performance while maximizing the energy recovered from the exhaust gas. For this, extremely accurate pressure sensors that provide precise readings over a wide range of manifold boost pressures and temperatures are required. These sensors must also be resistant to vibration and chemical degradation.

And while manufacturers around the world continue to carry out research into electric vehicles, several groups are considering ways to harness hydrogen to generate electricity instead of relying on storage batteries.

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.

Although small PEM fuel cells commonly operate at normal air pressure, higher powered fuel cells, of 10kW or more, usually run at elevated pressures. 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.

Typically the PEM fuel cell operates at pressures ranging from near atmospheric to about 3Bar, and at temperatures between 50 and 90°C. While higher power densities made possible by increasing the operating pressure, the net system efficiency may be lower due to the power needed to compress the air; hence the importance of balancing the pressure to the requirements of the particular fuel cell.

As with ICE boost pressures, this can only be done by taking accurate pressure measurements using high-quality pressure sensors. These measurements are then compared to the fuel stack outputs to minimize the parasitic losses while optimizing the gains in electrical output.

So, irrespective of the course the automotive industry chooses for future technologies, accurate pressure sensors will remain key to the development of safe and efficient vehicles.