Mazda patents advanced supercharged two-stroke engine design
Two-stroke engines have fallen out of favor for the past 50 years or so. Although they feature excellent power-to-weight ratios and are often much simpler mechanically than their four-stroke counterparts, they also tend to suffer from high emissions and narrow power bands. However, Mazda still sees some value in the technology and has patented a new supercharged two-stroke engine design, as spotted by members of the New Nissan Z forums.
The design is inspired by Mazda’s Skyactiv X technology and promises “improved fuel economy”, although compared to that is unclear. The company’s existing four-stroke engine uses advanced spark-controlled compression ignition to run the engine with a leaner fuel-air mixture for improved efficiency, while retaining traditional spark ignition for high load, high power operation where compression ignition becomes more unstable. This two-stroke design implements the same idea, aiming to run in compression-ignition mode for more efficiency while switching to spark-ignition under higher engine loads.
The two-stroke engine features a crankshaft-driven Roots compressor that assists in scavenging – the process in which burnt mixture is exhausted and fresh air/fuel mixture enters the engine. As the two-stroke engine design has the exhaust and intake valves open at least partially at the same time, having a compressor forcing cool air into the engine helps this process tremendously.
Variable valve timing is also used on the engine to control the effective compression ratio, allowing the engine to avoid knocking by leaving the intake valve open longer. Normally this would have the effect of pushing the boost air back into the intake, but the positive displacement Roots supercharger helps counter this. In this respect, it is similar to the operation seen in Mazda’s K6-ZEM Miller cycle engine of the 1990s.
It’s yet another effort by Mazda to keep combustion engines alive in an increasingly restrictive environment for efficiency, fuel economy and emissions. At this point it is not known what applications this engine could be used for. The two-stroke design will get a lot of people thinking about motorcycles, even though it’s not a market in which Mazda has a real stake. A range extender for hybrid electric vehicles could be another idea, although the company has already brought such hardware to market with the MX-30.
The optimist might say that this technology is poised to be Mazda’s last hurray for internal combustion and replacing four-stroke designs in its mainstream vehicles. However, ditching four-strokes would be a moon shot at this point. The reader contacted Mazda for comment on any intended future use of this particular engine design.
On the contrary, this patent may simply be a way for Mazda to take advantage of the intellectual property generated by its intensive research efforts, and not something that is necessarily intended for commercialization. Let’s just hope that doesn’t get in the way of the rear-drive rotary sports car we’re all hoping to see come to fruition.
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