Mazda’s New Spark-Controlled Compression Ignition Revealed

Earlier this year, EngineLabs reported on the announcement of Mazda’s intention of bringing the first production Homogeneous Charge Compression Ignition (HCCI) engine to market, but with a twist. Recently, a full media package was released to the press, explaining, in-depth, their spin on HCCI technology, the hybrid SPark-Controlled Compression Ignition, or SPCCI. Dubbed the SkyActiv-X, this engine represents Mazda’s effort to combine the benefits of a traditional spark-ignition gasoline engine with the benefits of a compression-ignition diesel engine.

The Goals and Challenges

The goal of the hybrid combustion method is to allow a higher-than-stoichiometric air-fuel ratio intake charge to burn more evenly and in a stable fashion. Traditionally, an extremely lean mixture in a standard spark-ignition gasoline engine offers an unstable combustion event. To control such lean gasoline mixtures, compression-combustion needs to be used in those areas of the engine map. However no reliable or practical method of switching between combustion methods on the fly exists.

Mazda’s solution with SPCCI is that instead of trying to switch back and forth between spark-ignition and traditional compression-ignition (based on load and performance conditions), they’ve modified the mechanics of how compression-ignition works. By introducing the use of a spark plug into the traditional compression-ignition process, there were two positive results. The range of the load map where SPCCI is effective was vastly enlarged over that of traditional compression-ignition, and there is now an easy way to switch between traditional spark-ignition and the more efficient compression-ignition on the fly.

This new combustion method has required a number of supporting engine architecture changes, like a new piston design, an all-new super-high-pressure fuel-injection system, a “high-response air supply” (which is Mazda’s fancy way of saying roots-style supercharger) and in-cylinder sensors. Mazda says that one of the reasons for this current technological breakthrough is the availability of the required much higher-precision electronic controls.

The Actual Science

At this point, assuming your eyes aren’t glazed over at trying to comprehend a new combustion method, you are probably wondering what the mechanism for initiating compression-ignition with a spark plug is. In the SPCCI model, the air-fuel charge is compressed to a point just before compression-ignition can occur. Then, when the spark plug ignites the initial flame kernel, that ignition raises the cylinder pressure enough that the remaining mixture in the cylinder ignites via compression-ignition.

Because a SPCCI system can control the spark plug timing, it can better optimize ignition timing events under all load and driving conditions. That is how SPCCI so greatly expands the effective area of the load map over traditional HCCI. Additionally, because there is a constant spark plug in use, the system can seamlessly switch back to a more traditional spark-ignition method in rpm or load ranges in which compression-ignition is either not possible, or less-than-ideal.

In order for this hybrid process to work, two separate air-fuel ratios must be maintained within the cylinder at once. The outer “zone” must be lean enough to allow compression-ignition, while the inner “zone” must be rich enough to allow spark-ignition. Mazda has been able to control this through a split fuel injection process.

Here’s the step-by-step: First, as the air intake charge is entering through the valves, the first fuel injection occurs and is swirled around the cylinder with the fresh intake air, which creates a lean, evenly-distributed (or homogenous) charge. As that is compressed by the piston, near top-dead center, the super-high-pressure fuel injection system adds another quick burst of fuel for the spark plug to ignite. Spark ignition occurs before the second load of fuel has a chance to mix with the first, thereby creating the two different “zones” of fuel enrichment.

What This Means in the Real World

All the science and technology is worthless unless there is a real benefit seen by the end user. Mazda is touting 10-percent more peak-torque than the current SkyActiv-G engine, and up to 30-percent more torque under the curve at certain engine speeds. It also offers the low-end grunt traditionally found in a diesel engine, but with smooth high-rpm performance traditionally found in gasoline engines.

In addition to an increase in performance, Mazda is claiming a 20-percent fuel economy increase when compared to the latest SkyActiv-G engines in mixed driving, and up to 30-percent increases in low-speed driving where more of the super-lean-burn technology is used. Compared to Mazda’s latest diesel offering, they are claiming equal or better fuel economy, which is absolutely astounding out of a gasoline engine.

While the useful range of SPCCI is greatly increased over HCCI, there are still parts of the load map (high-load, and high-rpm areas of the map) where traditional spark-ignition is advantageous.

Another additional benefit to this hybrid combustion method is lower tailpipe emissions. The HCCI method offers a much cleaner burn, as the lower combustion temperatures don’t allow oxides of Nitrogen to form. While there is still a hot ignition point, the overall combustion temperature is lower, and offers a more complete burn than a traditional spark-ignition combustion.

If this technology pans out, it will likely be pursued in the name of efficiency and a reduction in emissions, as the worldwide demand to go green increases. However, we’re gearheads here at EngineLabs, and we would be lying if we said we weren’t interested in the performance applications of this new system. Diesel engines have proven themselves worthy in various forms of motorsport, so we have to wonder if this will eventually make its way to a racetrack, and if so, how the technology will be exploited by those whose only goal in life is to turn gasoline into horsepower.

About the author

Greg Acosta

Greg has spent nineteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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