One of the most significant developments in internal combustion engine (ICE) technology for decades, Mazda’s innovative Skyactiv-X Spark Controlled Compression Ignition (SpCCI) combustion system is slated for production in 2018. It has the potential to extend the practical life of gasoline engines, which are increasingly under threat from both global emissions legislation and the accelerating development of electric vehicles (EVs).

According to Kiyoshi Fujiwara, a company director and Senior Managing Executive Officer, SpCCI will form the foundation of Mazda’s gasoline-engine strategy until mid-century. Fujiwara told Automotive Engineering that SpCCI is designed to embrace larger-displacement power units that eventually will run on micro-algae bio-fuels to deliver “zero tailpipe emissions” (see sidebar).

The SpCCI system is the culmination of more than eight years of intensive development by Mazda to design a gasoline engine that embraces the frugality and torque of a diesel with the high-revving capacity of a twin-cam gasoline unit, while delivering sub-60 g CO2/km emissions. Adding to the attraction, SpCCI requires relatively minimal investment in the engine bill of materials—electronic controls and a revised cylinder head comprise the major changes.

Creating an ICE with Otto and Diesel attributes is an engineering target discussed in scores of SAE Technical papers and vigorously pursued by Mercedes-Benz with its DiesOtto (see http://papers.sae.org/2009-01-2701/), General Motors with its Homogenous Charge Compression Ignition (HCCI; http://articles.sae.org/6635/) Honda with its similar investigations (http://books.sae.org/b-hon-016/) and Hyundai, among others. But while these larger, better-financed OEMs have focused significant R&D on HCCI, they have thus far stopped short of committing the combustion regime to production.

Mazda’s new technology partner Toyota also is said to be interested in SpCCI, including potential applications for hybrid-electric vehicles (HEVs), long a Toyota specialty.

Diesel-like torque

At a recent media technical background event at Mazda’s European technical center in Oberursel, Germany, the author test-drove two prototype Mazda 3s powered by a 2.0-L SpCCI engine—one fitted with a manual transmission, the other automatic—over a 30-mile (48-km) test loop. The experience seemed to confirm that the longstanding challenge of smooth transition from spark-ignition mode to compression-ignition had been overcome. The prototype SpCCI engines did display some low-rpm harshness, but final calibrations and engine production still are about a year away, engineers said.

What can be confirmed: in everyday driving, the transitions from SI to CI are barely noticeable. On a route that included high-speed autobahn and country and urban roads, the all-new SpCCI unit pulled strongly to its approximately 6000-rpm redline, accompanied by a healthy—if not outright sporty—exhaust note. The engine’s diesel-like torque curve was amply demonstrated by its willingness to pull without fuss from as low as 1200 rpm in sixth gear.

This wider spread of torque has allowed Mazda to revise the development cars’ gearing to further improve fuel economy and reduce emissions.

Frustratingly, specifications such as bore and stroke, rated torque, power and other technical details remain under wraps as this article was published. Although Mazda is claiming a 20-to-30% efficiency gain over its current Skyactiv-G gasoline engine, the test results seen by Automotive Engineering were less demonstrative. They included a 13.3% improvement in fuel efficiency for the manual over the standard Mazda 3 SKY-G—34.0 U.S. mpg vs. 29.4 (6.9 L/100 km vs. 8.0 L/100 km)—with the engine operating in SpCCI mode more than 95% of the time.

It was a similar tale for the automatic-transmission car: a 14.75% improvement (29.9 U.S. mpg vs. 35.1; 29.9 L/100 km vs. 35.1 L/100 km), while the automatic-backed SpCCI engine spent even more time operating in SpCCI mode.

However, this can be attributed to the exaggerated testing regime, aimed more at assessing the engine’s attributes such as low-speed and in-gear acceleration in high(er) ratios and trying to detect the SI-to-CI switchover points than would typically occur in “normal” driving.

From 18:1 to 15:1 CR

SpCCI is a progression of Mazda’s comprehensive Skyactiv efficiency-improvement initiative unveiled in 2011, which debuted gasoline and diesel engines with a common 14:1 compression ratio (CR). The new Skyactiv-X engine operates at a 15.0:1 CR, according to company engineers.

“We selected 15:1 compression ratio as it is close to compression-ignition conditions in normal ambient temperatures,” explained Powertrain Executive Officer Ichiro Hirose. “The spark creates an expanding fireball that acts like a secondary 'air spring' to create additional compression. Because the spark plug creates this fireball, it effectively controls the switch between spark ignition and compression ignition,” he noted.

Hirose added that achieving a “super-high compression ratio was a key breakthrough in realizing combustion with a lean fuel-air mix. Secondly, the leaner you make the air-fuel ratio, the more the specific heat ratio increases. To make the big step forward we needed to double stoichiometric levels from 14.7:1 to 30.0:1, at the very minimum.”

As AE reported previously, during Skyactiv-X’s development through G1, G2, and G3 program stages, engineers targeted an 18:1 compression ratio at lambda 2.5—a 40% improvement in thermal efficiency by setting the ideal pressure and temperature for homogeneous-charge compression ignition.

A key to Mazda’s achievement for the production engine is precise control of the combustion process (see SAE Technical Paper http://papers.sae.org/2015-01-1803/). The SpCCI engine uses pressure sensors in each cylinder to enable real-time temperature and pressure monitoring, in addition to other engine parameters. The engine-management system controls the twin electrically-variable camshafts, the new split-injection strategy that operates at 500 bar (7252 psi) and the air pump. The latter is a unique Roots-type device engineered by Eaton Corp. for the Mazda SpCCI application, Automotive Engineering has learned.

Supercharging and exhaust gas recirculation are known to be effective for operating an HCCI engine in CI mode at high loads. But as of 2013, Mazda R&D was aiming to achieve successful lean-burn HCCI within a broad load range using normal aspiration, according to Takahisa Sori, then Managing Executive Officer for R&D. Sori's engineers were concerned about an air pump compromising real-world fuel economy.

Speaking with Automotive Engineering, Sori also was bullish on the potential of mating HCCI gas engines with hybrid-electric drivetrains that let the engine run in its most efficient operating range, with e-motor assist as needed. In this de-emphasized role, the electric motor and battery can be downsized, reducing their cost. Such an arrangement would seem to be ideal for collaborative work with Toyota hybrid systems engineers.

SpCCI’s air-fuel mixture is created by two-phase, split injection on the intake and compression strokes. A strong swirl is created in the combustion chamber to create an intentionally uneven distribution of fuel density, with a lean mixture around the periphery for CI and a relatively rich air-fuel mixture around the spark plug in the center—conducive to creating the fireball.

Spark ignition is used to start the engine and under heavy-load conditions, but the switchover to CI is not at any predetermined point. When the right intake-charge boundary conditions are achieved, the expanding fireball in the combustion chamber is created, with SI providing additional compression to the geometric compression ratio of approximately 15-16:1. This reaction induces CI, Hirose explained.

He noted that manufacturing costs for the SpCCI engine fall between those of a diesel and a gasoline engine.

Automotive Engineering will report more details on Mazda’s SpCCI development in future issues and online at SAE.org.