Any technology transfer between the racetrack and the road is usually a one-way street, with motor racing leading and humble production machinery way back on the innovative grid. But just occasionally, there is a role reversal and Mahle Powertrain demonstrates it with its pre-chamber Jet Ignition (MJI) system.
Currently used in high-level motorsport engines, MJI is not yet being produced for volume vehicles—but Ben Hibberd, Mahle Principal Development Engineer in the UK, clarified the sequence of events: “Our first motorsport project—using a ‘passive’ pre-chamber technology to improve burn rates and reduce the likelihood of detonation—was about five years after Mahle in the U.S. started work on it as a development project. In essence, you could consider road and track applications were developed in parallel because they share a lot of the technological features; but most of the advantages, which were so beneficial to motorsport, had already been demonstrated for regular road use applications. Nonetheless, a super-aggressive high-speed application gives us an unusually strong credential on which to build confidence in our alternative combustion approach and pre-chamber robustness.”
Pre-chamber technology per se is not new; it has been used for decades in diesel engines and Honda introduced it in the early 1970s for the first-generation Civic’s 4-cyl. spark-ignition gasoline engine, dubbing it CVCC (Compound Vortex Controlled Combustion).
It may look relatively simple, but its complexity and attainment of required refinement should not be underestimated, said Hibberd. However, with advances in direct-injection gasoline systems’ flexibility and careful consideration of pre-chamber geometry and nozzle selection ongoing by Mahle, he believes it is a key to significant powertrain efficiency gains in terms of fuel consumption (easily rivaling diesel figures) together with reduced gaseous emissions.
Lambda 1 and beyond
Mahle’s MJI system comes in two forms: active and passive. Active contains both a small sparkplug and a low-flow DI fuel injector in a pre-chamber capsule. The passive design (used initially by Mahle for motorsport applications) does not use a secondary injector and takes its combustion charge from the main chamber during the compression stroke. The compact active MJI is seen by Mahle as the potential “ultra-high efficiency” application, its secondary injector maintaining an easily ignitable charge in the pre-chamber. This allows the system to exceed what it regards as established flammability limits in the main combustion chamber, reaching beyond Lambda 1.5. Above that level, air dilution is said to generate lower gas temperatures—good for reduced NOx formation.
The main chamber can be operated at Lambda 1.7 to 2.0, close to diesel air-fuel ratios (AFRs), at which point flame temperatures are sufficiently low to mitigate any NOx penalty that might occur with richer operation.
Hibberd added that both active and passive versions of its pre-chamber technology are in general the same with regard to combustion principle: “Both take benefit from an outgoing turbulent radical jet, forced from the pre-chamber to ignite the main chamber charge; this has the effect of amplifying the available ignition energy, delivering stronger and more reliable combustion in the main chamber from multiple ignition sites and fast burn rates. Regarding the passive system (most race rules permit only one injector per cylinder), it has demonstrated a very high EGR tolerance, so as we are pushing towards a Lambda 1 horizon, it allows us to run relatively high levels of EGR at higher loads, helping to reduce knock propensity at heightened compression ratios, in essence extending our headroom on CR potential.”
Although full details of the Mahle system are subject to commercial confidentiality, Hibberd said the company’s boosted 1.45-L 3-cyl. gasoline engine fitted with active MJI has returned brake-specific fuel consumption around 200g/kWh at multiple operating points.
“NOx emissions were below 100 parts per million (ppm) at ultra-lean conditions, one of the many areas on which we are focused to assist with predicted EU-7 targets,” said Hibberd. “A lot of our development work is currently going into broadening the available operating map while maintaining those numbers. With further improvements in friction, available coating technologies and charge management, we’re demonstrating potential of 45% brake thermal efficiency.”
Along with the MJI injectors, patented design intricacy of the nozzle is a particular aspect of pre-chamber development that Mahle keeps under wraps. Certainly there is no “one-size fits all” solution, explained Hibberd: “The delicate compromise OEMs face between cost and performance leaves no room for inefficiency, a decision-making process that is complicated by uncertain future application requirements. The challenge is that the design intent of the ICE is always changing.
“For example, with a series hybrid, we don’t necessarily expect it to have the same broad spectrum of performance capabilities as a prime-mover ICE. It needs to be extremely thermally efficient and very clean, of course, but perhaps in a limited operating range, so we are tailoring the pre-chamber technology to meet the needs of whatever systems will complement the ICE, both in the near term and further ahead. Although we’ll not have the luxury of tolerating relatively poor performance in any operating modes, we still believe the ICE will remain a relevant contributor to low-carbon transport.”
When could pre-chamber engines be fitted to series-production cars? A potential hurdle is fuel- injector design and its downsizing from present applications. As for durability, there are no moving parts; currently, Mahle’s testing has topped 2500 hours in joint U.S. and U.K. programs. Cold-start testing and after-treatment conditioning have shown “comparable performance with contemporary non pre-chamber combustion systems,” stated Hibberd. He agreed the auto industry generally is rightly cautious about new technology, although current legislation is forcing Asia to become less conservative—ahead of Europe and the U.S.—when making strategic decisions concerning the introduction of new technologies.
Asked if use of Mahle’s active pre-chamber MJI system on a gasoline engine could bring hope of fuel consumption approaching a diesel, “We’re already there,” Hibberd flatly stated. “Combining downsizing, MJI technology and a 48V hybrid system would deliver still better results.”Continue reading »