Mazda takes a giant step toward HCCI

By Christopher A. Sawyer
The Virtual Driver

(July 9, 2018) HCCI, Homogenous Charge Compression Ignition, is the holy grail of internal combustion in that gasoline engines would run like diesels on compression ignition, at much higher efficiency levels, and with lower pollution. The basic problem with HCCI, however, has been the inability to control the combustion process under changing conditions. Leave it to the boffins at Mazda to take a look at the problem from a less theoretically rigid angle to create a workable solution.


But what else would you expect from the only automaker able to turn the Wankel rotary engine into a reliable, iconic production powerplant?

The impetus behind the design is nothing if not pragmatic. According to Mazda: “With two thirds of global electricity production currently relying on the use of fossil fuels, Mazda believes regulations placing the emissions of an EV (electric vehicle) at zero to be disingenuous. Indeed, when converted to a 'Well-to-Wheel' figure, the average CO2 emissions of an EV can be very close to a conventional vehicle depending on the energy mix the electricity is made from.

Thus recognizing that electric powertrains do not, in fact, currently satisfy society's wish for a drastic reduction in greenhouse gas emissions, Mazda is focusing on bettering the real-world emissions of EVs by maximizing the efficiency of the internal combustion engine.

Accordingly, Mazda has been looking to significantly reduce CO2 emissions from petrol engines based on the analysis of combustion control factors. One of the key control factors is the use of a highly lean mixture of air and fuel: 2-3 times leaner than in today’s conventional engines. This mixture has so little fuel in the air that a normal engine with spark plugs cannot fire it.”

Mazda’s solution to the extremely narrow operating range of an HCCI deign was equally pragmatic: it added a spark plug and a kernel of highly atomized fuel. In the 2.0-liter Skyactiv-X engine, a lean air/fuel mixture is injected into the engine and compressed at up to 16:1, just below the level at which compression ignition would occur. A small amount of highly atomized fuel is injected around the spark plug. This rich kernel is ignited to create a pressure wave that increases the combustion pressure and temperature enough for the ultra-lean primary charge to ignite as the compression stroke continues.

Mazda also adds a second charge that is injected just prior to the power stroke, and ignites it with the spark plug. This creates a flame front that reliably increases the cylinder pressure to compression ignition levels. This expands the situations under which the engine operates on a lean charge, and eliminates unstable combustion while suppressing pre-ignition. A Roots-type blower leans out the primary air/fuel mixture across the rev range, working with the combustion design to allow compression ignition to be used at moderate loads and high engine speeds.

Mazda’s Spark Controlled Compression Ignition (SPCCI) reverts to conventional combustion during very cold starts or at high engine loads. The hybrid combustion can seamlessly transition between spark and compression ignition and, claims Mazda, can run under compression ignition more than 80% of the time, and improve fuel economy by up to 30% at low vehicle speeds. This reduces CO2 emissions by a like amount, and makes the Skyactiv-X more fuel efficient than Mazda’s more expensive Skyactiv-D diesel.

Mazda says the 2.0-liter Skyactiv-X should be good for 188 horsepower and 170 lb.-ft. of torque, which puts it on the same level as company’s the 184-hp 2.5-liter engine.

It’s set to debut in the 2019 Mazda3 before making its way across Mazda’s sedan and crossover lineups.

The Virtual Driver