Mahle, the German company behind many combustion-engine design innovations over the past 100 years, has developed a combustion system called Mahle Jet Ignition (MJI). It’s already in use in the engines of a major Formula 1 team and will also form part of a new modular hybrid powertrain that should make the production of petrol-electric hybrids much easier.
MJI works like this. In a traditional petrol engine, fuel is injected into the inlet ports along with the incoming air (port injection) or directly into the combustion chamber (direct injection). MJI comes in two types: active and passive. The first incorporates both types of injection. The port injector works in the normal way, but tips of both the direct injector and the spark plug are positioned in a tiny pre-chamber immediately above the main combustion chamber.
When the induction cycle happens, the port injector sends 95% of the fuel into the inlet port, which rushes into the main combustion chamber and is then compressed by the piston as normal. As that happens, the remaining 5% is injected into the pre-chamber and ignited by the spark. As it ignites, jets of hot partially combusted fuel and air are forced through tiny holes in the bottom of the pre-chamber, igniting the main charge as a whole rather than at one point near the spark plug. The burn is faster and more even, with quicker pressure build-up to generate power more quickly. The whole process is homogeneous (evenly mixed fuel and air) and can run ultra lean (much higher quantity of air compared with fuel) when less power is needed.
The latest, ‘passive’ version (used in F1) does away with the direct injector, with only a spark plug in the pre-chamber. It’s not a new idea – Honda developed the Compound Vortex Controlled Combustion pre-chamber for its first-generation Civic in the early 1970s, but the execution of it was more complicated than that of Mahle’s passive MJI. As the fuel and air mixture is ingested through the inlet ports into the combustion chamber and compressed, a small quantity of the main charge is pushed through the pre-chamber nozzles into the pre-chamber. There, it’s ignited by the spark plug and high-pressure jets of burning fuel and air are forced back through the nozzles into the main charge, igniting it as before.