The diesel community, already reeling from the low 14:1 compression ratio in Mazda’s new Skyactiv-D diesel, were absolutely gob-smacked when Mazda announced that they are developing a new ultra-low compression diesel engine.
The new Skymaster-Z engine uses an 8.5:1 compression ratio, so low that it requires an auxiliary electrical spark system in order to maintain combustion, and requires fuel that is more volatile than standard low-sulfer diesel. Mazda engineers have calibrated the engine to run on ordinary 87-octane gasoline, but this requires retimed single-pulse injectors and a more complicated intake system that premixes the fuel and air in a relatively constant ratio, as opposed to the variable mixture and power-cycle injection used by traditional diesels.
According to Mazda Chief Engineer Ashiro Nakahonda, the Skymister-T engine’s low compression ratio eliminates the need for a turbocharger and allows use of lighter components that can spin faster, raising the redline to 7,000 RPM. The new engine also produces significantly less noise and vibration than a typical diesel and has a markedly different emissions footprint, allowing it to meet 50-state standards with a standard catalytic converter and without the use of an AdBlue-type additive. The trade off is a 75% reduction in torque and higher fuel consumption. The prototype two-liter engine reportedly develops around 140 horsepower but only 135 lb-ft of torque, returning the equivalent of approximately 30 MPG in the EPA city cycle and 40 MPG in the highway cycle when mounted in a C-segment-size car. “While these numbers are not typical of an ordinary turbodiesel engine,” Nakahonda says, “we believe they will be adequate for the American market.”
Diesel nuts who worry about this sort of thing have expressed concern about the lack of upper-cylinder lubrucation due to the engines’ rapid combustion and reduced fuel stratification, but Nakahonda is confident the Skymassive-F engine will achieve a 180,000 mile service life.
Irked at not having anything more technical-sounding to say, Mazda spokesman Jeremiah Burns vamped, “While the Skymatic-P engine does have some trade-offs compared to conventional diesels, we are confident that the lower noise and ease of fuel availability will appeal to American buyers.”
Mazda plans to offer the Skymackerel-Q engine in its popular Mazda3 compact, offsetting its additional cost by making the interior even cheaper and crappier.