The Japanese brand has recently brought the rotary engine back to the market, used in a range extender version on the new CX-60 plug-in hybrid, giving rise to speculation about a possible return of the Wankel on future sports models. Mazda still values too the development of a heat engine with pre-combustion chamberas revealed by six patents recently registered with the United States Patent and Trademark Office. The Japanese company would thus seem to be intending to expand the offer of hybrid and thermal vehicles, considering it still premature to offer an all-electric range.
The engine currently being evaluated is equipped with a passive prechamber system, i.e. without injector in the prechamber and equipped with a direct injection system in the main chamber only. Both combustion chambers of each cylinder are instead equipped with a dedicated spark plug. Pre-chamber engines have the quality of providing an ignition source with greater energy than the traditional spark plug. In fact, at high loads and rotation speeds, the mixture ignited in the pre-chamber propagates radially in the primary chamber, igniting the main mixture in several points simultaneously, giving rise to faster and more efficient combustion. However, passive prechamber systems tend to suffer from misfire (misfire) at low loads, as the pressure difference between the two chambers is not sufficient to guarantee an adequate mixture exchange in the prechamber. “At very low loads, in the pre-chamber there are residual gases in a higher concentration than in the main chamber, which remain trapped and this is where the ignition difficulties arise. At higher loads, this doesn’t happen, because there is a form of washing”explained the engineer Claudio Lombardi during an interview with FormulaPassion.it.
For this reason, Mazda patents state that at low loads and revs the new engine works with hyper-lean mixtures, without resorting to any priming from the prechamber or from the main spark plug. In such contexts, the engine is designed to work with compression ignition combustion, like diesels, using an extremely homogeneous charge. Once a load or rpm threshold has been exceeded, the thermal starts working with less lean mixtures, making use of the double ignition by the main spark plug and the one in the pre-chamber.
In detail, at low revs and high loads, the injection phase is delayed until it occurs during compression. Also, the primary candlestick is checked for prime the main mixture before the one in the prechamber, with the delay increasing as the rpm increases. Mazda engineers have indeed thought of reverse the normal firing order of the two chambers, exploiting the combustion in the main chamber to push the mixture towards the pre-chamber, avoiding washing and misfire problems. The same strategy is applied at high revs and low loads, with the main chamber firing being brought forward before TDC is reached. In conditions of high revs and full load, the engine is instead controlled as a traditional pre-chamber unit, in which the ignition of the secondary mixture precedes that of the main chamber.
Patent registration alone is not sufficient to confirm that Mazda will actually proceed with the development of a passive prechamber engine and especially not in the short term. Already in January 2022 the Japanese company had patented a powertrain with three rotary engines, but a year later the only application of the Wankel remains the CX-60. Nonetheless, the study of a high-efficiency, high-performance pre-chamber motor alone is the clue to how Mazda is pondering new moves in the field of internal combustion engines.
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