Mahle, Liebherr Develop Active Pre-Chamber for Hydrogen ICE
The collaboration attempts to resolve issues with heavy-duty, hydrogen-fueled engines, namely avoiding pre-ignition and knock without reduced compression ratios.
While there is limited scope for internal-combustion engines (ICE) operating on conventional diesel, gasoline and natural gas in a “Net Zero” world, there could be a place for hydrogen-fueled ICEs, either as an interim power source while hydrogen fuel cells are fully developed, or as a power source where it would be difficult to electrify in the given location.
Hydrogen, however, has properties that make it challenging as a straight replacement for diesel or gasoline. The specialist engineering consultancy Mahle has been working with Liebherr Machines Bulle SA on resolving the issues surrounding hydrogen-fueled engines. “The challenge has been to get it to run with stable combustion without resorting to reductions in compression ratios to avoid engine knock and pre-ignition. Our common work with Liebherr suggests we have the answer,” Mike Bunce, head of research for Mahle Powertrain US, told SAE Media.
The key appears to be Mahle’s active pre-chamber technology, Mahle Jet Ignition (MJI), which was originally developed for gasoline applications. Bunce explained the principle: “There is still a conventional spark plug, but here initiating combustion in a very small volume that for our applications is typically 5 cm3 or less – the size of a thimble. Inside the pre-chamber is a very small mixture of fuel and air for ignition, so you create very small combustion events inside that pre-chamber.
“The pre-chamber has a nozzle with a few orifices in it. The products that were formed as a result of that very small combustion event will shoot out very quickly from the nozzle orifices. If you have a six-orifice nozzle, for instance, there will be six jets coming out of it. These jets are very hot and very reactive, because they have combusted, so they then initiate combustion inside the main cylinder.”
Instead of just the single point ignition around the spark plug, there may be six distinct ignition sites around the main combustion chamber, assuming the pre-chamber is fitted with a six-hole nozzle. “Each flame that is formed in that ignition site doesn’t have to travel as far to consume fuel, resulting in very, very fast burning in the main chamber,” Bunce said.
Hydrogen has a tendency to pre-ignite, causing the knocking that is familiar in a gasoline engine with over-advanced ignition timing or too lean a mixture. By directing jets from the nozzle to sites where knocking is likely to occur, the fuel will effectively be burnt there before a knocking event can occur. “This allows the ignition of fuel/air mixtures that are typically not very ignitable,” Bunce explained. “So, you can ignite mixtures that have really heavy dilution – standard dilution or EGR [exhaust gas recirculation] dilution beyond what a typical spark plug would be able to handle.
“You can also ignite fuels that are problematic. Some fuels don’t ignite very well, generally. Some fuels maybe ignite too well, and you lack a degree of controllability. Here a pre-chamber jet igniter gives you that controllability back and allows you to ignite these mixtures that are typically not very ignitable.”
Using the MJI system enables the dilute hydrogen fuel/air mixture that brings the hydrogen pre-ignition under control. Bunce said that dilute fuel/air mixtures usually lead to substantial increases in thermal efficiency. This can be in the 15 to 25% region for gasoline or natural gas engines but has yet to be quantified for hydrogen-fueled engines.
In fact, improving thermal efficiency is not the objective of using MJI with hydrogen. “Lowering the compression ratio is of course going to lower your ability to achieve the target power and torque,” Bunce said. “So, the use of the active pre-chamber here is meant to try to get that compression ratio back up and allow you to get the power and the torque that you want.”
“In terms of development, the good thing about pre-chambers, both active and passive, is that the hardware is very simple,” Bunce continued. “If you think about an active pre-chamber, components include an off-the-shelf spark plug and we use a direct fuel injector for the pre-chamber. That’s a bit of a custom job, but there’s nothing exciting about the underlying technology – it’s just a solenoid DI injector.”
The rapid combustion triggered by the MJI generates very high cylinder pressures, which are similar to those found in a diesel engine. This ensures that diesels are particularly suitable for conversion to this type of hydrogen combustion.
Overcoming unique challenges
Bunce sounds a note of caution regarding hydrogen and steel. “Hydrogen is not particularly compatible with some steels; it tends to cause embrittlement because it’s such a small molecule and can really dig itself into surface-level marks,” he explained. “So, you have to pay close attention to material compatibility for everything the hydrogen is going to touch.”
Fuel/air mixture “blow-by,” where the mixture is forced past the piston rings and into the crankcase, is a common issue with piston engines but can pose a particular problem with hydrogen engines. Because of the tendency to pre-ignite, it could lead to ignition in the crankcase with devastating effects. The Mahle solution is to design the piston rings to prevent blow-by and introduce an electric pumping system to evacuate the crankcase very quickly, routing the gases back to the intake side of the engine.
The tendency for pre-ignition also means that hotspots in the combustion chamber need to be eliminated. “You need to take great care to make sure that you get really homogenous temperatures across the piston crown, so you don’t have individual hotspots,” Bunce said. “That’s what our component group really focuses on, and we’re developing really good solutions in that respect.”
Hydrogen can combust with high levels of air dilution even with a conventional spark plug. “What we found in our work is that even operating with that really high level of air dilution, it’s possibly not lean enough to be able to significantly reduce the pre-ignition problem,” Bunce said. “In order to solve pre-ignition, it’s going to need reduced combustion temperatures and that will reduce the surface temperatures. Air dilution is the best way to do that, but we think that even with a regular spark plug, it won’t be dilute enough to be able to significantly cool those temperatures. So, the active pre-chamber allows much leaner operation than that, to the point that combustion and surface temperatures are cool enough to significantly reduce the likelihood of pre-ignition.”
Having reduced the combustion temperatures so significantly, there are other benefits. First is that it is possible to raise the compression ratio, which in turn improves efficiency. The low dilution levels also help to reduce the levels of engine-out oxides of nitrogen (NOx). Bunce noted that the levels are low enough to consider eliminating NOx aftertreatment.
“You may or may not want to do that, but at the very least you could take whatever is the base aftertreatment solution for NOx and scale it down pretty significantly,” he said. “We’re seeing extremely low NOx levels, essentially close to where a NOx analyzer would not even be able to measure it.”