The boxer engine continues to work: Subaru’s patent ignition before the chamber

Some motor enthusiasts may still remember the injection of the diesel engine before the dining room or at least the throaty sound of the old diesel engines from Mercedes-Benz. It seems like this technology has been celebrating a bit of a renaissance in petrol engines in recent years – albeit in a completely different way than in the past. This is also shown by Subaru’s latest patent application with the US Patent and Trademark Office.

Basically, bedroom injection can increase efficiency, cleanliness and durability. To do this, a small chamber in the combustion chamber near the spark is “separated” from the rest of the compression volume with a perforated cover. The fuel-air mixture is ignited in this chamber, where the flames spread across a wide front to the main combustion chamber and ignite the mixture there evenly and in different areas. In gasoline engines, this results in faster burnout and reduced knocking behavior.

Magermotoren to save fuel

Because this allows compression and efficiency to increase significantly, the principle is particularly suitable for engines that operate in combustion mode – that is, with less fuel and more air. Miller or Atkinson engines also benefit from the pre-chamber ignition principle. These days, such engines are often included with a hybrid car. The primary disadvantage of lean combustion is slow ignition and increased production of nitrogen oxides (NOx). A complex, pre-calculated chamber burner can overcome these losses and even burn mixtures with an air ratio of more than 2 cleanly.

Pre-chamber injection and ignition are often achieved with secondary fuel injection per cylinder. Additional spark ignition is often required. Subaru takes a different approach with what it calls a “combustion chamber.” The Japanese go with one spark plug and one injector per cylinder.

Benefits of a cold start

Subaru’s technology is said to save fuel even when starting from cold. After all, hybrid drives in particular often operate in this unusual temperature window. When the combustion engine is cold, the ignition timing is often pushed back to activate the catalytic converters in the exhaust system. The stratified charge is intended to better distribute the air-fuel mixture and reduce nitrogen oxides. However, a lot of oil is sprayed here and a large part is wasted. Excess fuel also collects on the walls of the cooling cylinders, which leads to poor combustion and more polluting emissions. The Subaru code also takes this into account. The patent shows that the walls of the combustion chamber are protected by air rods previously injected through an air nozzle.

Problem: The mixture carries more air – so it becomes too lean, making ignition more difficult. But that is intentional. Only shortly after the piston reaches top dead center is a small amount of additional fuel injected into the main combustion chamber – much less than a normal cold start sequence. The combustion in the antechamber should be able to handle the thin mixture with the best efficiency. Once the walls of the combustion chamber are warm, protection with an air layer is no longer needed. The air injector and the supply of additional fuel can be turned off.

Possibility of continuous use

Subaru wants to save fuel and reduce pollution in the exhaust gases through the pre-chamber principle and also through the additional air injection and cold start benefits. It is also conceivable that higher compression would result in a significant increase in performance in internal combustion engines. The Japanese have yet to reveal what Subaru has in store. Such systems have already achieved a good saving of eight percent in the WLTP cycle.

Because no additional spark or additional fuel injection is required, the system can be easily integrated into an existing boxer unit. In the patent, the Japanese clearly mention that other types of fuel and engine configurations can also benefit from pre-chamber and air injection technology. So there can be intense speculation about where we will see the technology on the road for the first time – perhaps already in the announced hybrid version of the American Forester (see photo gallery).