Tuesday, 4 June 2019

Technology Focus - Jet Ignition

Mahle is pushing the boundaries of achievable efficiency for spark ignition engines with the use of its Jet Ignition system - which is evolving quickly as a leading technology enabler to reduce emissions even further for next generation of engines. Mahle has already used Jet Ignition combustion concepts successfully in high-performance engines with a different development focus - but impressive results!

Mahle Jet Ignition concept - Injector and spark plug both mounted in a pre-chamber

The available Jet ignition concepts developed by Mahle are classified as passive and active -  Passive using a ‘passive’ pre-chamber technology to improve burn rates and reduce the likelihood of detonation. The passive design does not use a secondary injector and takes its combustion charge from the main chamber during the compression stroke. The passive system has demonstrated in race engine applications, a very high EGR (Exhaust Gas Recirculation) tolerance, so at engine operating points where Lambda 1 if the target AFR (Air-Fuel Ratio) setting, it allows operation at relatively high levels of EGR at high load. This helps to reduce the possibility of engine knock at heightened compression ratios. This allows the possibility to extending the compression ratio and benefit from the increased cycle efficiency.

 The active concept contains both a small spark plug and a low-flow direct-injecting fuel injector in a pre-chamber capsule. The active jet ignition is proposed by Mahle as the potential “ultra-high efficiency” application. The system operates by employing the secondary injector to maintain an easily ignitable charge inside the pre-chamber. This allows the system to exceed the established flammability limits in the main combustion chamber, reaching a Lambda value greater than 1.5. The main chamber is operated at this level which is actually close to diesel air-fuel ratios (AFRs), at which point, flame temperatures are sufficiently low to mitigate any NOx generation that might occur with richer operation.

Detailed view of pre-chamber assembly

Both active and passive versions of this pre-chamber technology are in general the same with regard to combustion principle: as both benefit from an outgoing turbulent radical jet, forced from the pre-chamber, which ignites the main chamber charge; this has the effect of amplifying the available ignition energy, thus delivering stronger and more reliable combustion in the main chamber - from multiple ignition sites and with fast burn rates.

It may seem a relatively simple system - but its complexity and attainment of required refinement should is a considerable task. However, advances in direct-injection gasoline systems’ flexibility and with careful consideration of pre-chamber geometry and nozzle selection - Mahle believes this technology is a key to significant powertrain efficiency gains - in terms of fuel consumption (easily rivaling diesel figures) together with reduced gaseous emissions.

Pre-chamber technology is not new; it has been used for decades in diesel engines (but now old technology - replaced by direct injection, common rail diesel technology) and Honda introduced it in the early 1970s for the first-generation Civic’s 4-cyl. spark-ignition gasoline engine, calling it CVCC (Compound Vortex Controlled Combustion).

Development testing has shown that NOx emissions were below 100 ppm (parts per million) at ultra-lean conditions are achievable in anticipation of Euro 7 targets. Used in conjunction with further technical improvements in friction, available coating technologies and charge management - a potential of 45% brake thermal efficiency has been demonstrated by Mahle. In summary - could this technology allow a gasoline engine to achieve the fuel consumption of a diesel? Mahle stated they have achieved this already - and that by combining downsizing, Jet ignition technology and a 48V hybrid system
could deliver even better results.