Gasoline HCCI (homogenous charge compression ignition) has been of interest to automakers for years, as the low-temperature combustion mode offers significant improvements in thermal efficiency and fuel consumption along with a reduction in NOx emissions compared to conventional spark ignition gasoline engines. However, traditional HCCI combustion has been realized only in a limited operating range. (E.g., Earlier post, earlier post, earlier post.)
With the announcement of its SKYACTIV-X engine (earlier post), Mazda claims to have developed a novel control system for HCCI combustion that extends the HCCI range out to a much larger percentage of the load map. The essence of Mazda’s Spark Controlled Compression Ignition (SPCCI) is the use of the spherical spherical flame front expanded by spark ignition as a second piston (an “air piston”) to further compresses the air-fuel mixture, resulting in improved compression ignition.
Further, to help raise the ratio of compression ignition combustion, Mazda is equipping its new engine with a highly responsive air supply unit.
We have been aiming to attain excellent real-world fuel economy, emissions and responsive driving. The ideal combustion that helps us meet this goal is CCI—controlled compression ignition. There are two key words here. The first is compression ignition, and the second is control. The capability to completely control the compression ignition is Mazda’s original technology.—Kiyoshi Fujiwara, Mazda Director and Senior Managing Executive Officer
In HCCI, gasoline and air are completely mixed and ignited by the temperature resulting from compression. A very lean air-fuel mixture that is too lean to combust via spark ignition can combust by compression ignition cleanly and rapidly.
Lean combustion—by its nature—reduces the amount of fuel burned. Lean combustion has been applied to spark ignition combustion, but has reached the leanest possible level already, Fujiwara said; further increasing the amount of air or gas results in an inability of the flame to propagate.
Compression ignition, on the other hand, enables a super lean burn at twice the ideal air/fuel ratio. Even lean air/fuel mixtures will ignite and burn in many places simultaneously if highly compressed.
Simply put, compression ignition can achieve combustion with half the amount of fuel compared to conventional combustion.—Kiyoshi Fujiwara
|Right after the piston starts moving from TDC, combustion takes places spontaneously and rapidly—enhancing the force to push the piston and doing so for a longer times, improving efficiency. Click to enlarge.
However, as noted above, conventional HCCI works only at a limited range of load and speed.
To overcome this challenge, we needed to offer spark ignition along with HCCI. But in transient time or under a variety of different conditions, it was difficult to ensure stable switchover between spark ignition and HCCI combustion. Our task was to expand the range in which compression ignition combustion works and to deploy technologies to control completely the switchover between different combustion methods.—Kiyoshi Fujiwara
Delivering stable gasoline combustion at all speed and load ranges requires an engine that supports both spark ignition and HCCI; this by definition requires a spark plug as part of the engine design, Fujiwara said. Mazda turned that into an advantage, and uses the spark plug as a control factor to control compression ignition and the switchover between the modes.
The expanding spherical flame resulting from spark ignition serves as a second piston, further compressing the air-fuel mixture in the combustion chamber, thereby facilitating the necessary environment for compression ignition to take place. Controlling the ignition timing allows Mazda to expand the range for compression ignition and enable a smooth switchover.
In short, Fujiwara said, SPCCI works over a wide ranges of speed and loads and enables stable switching between HCCI and SI.
We succeeded in realizing a clean engine that offers high torque and excellent fuel economy thanks to super lean combustion which has never been possible with spark ignition. Spark ignition kick in when the temperature is extremely low, but even in that condition the combustion is the same as SKYACTIV-G we currently sell, proving the cumulative improvements of combustion technology we have made.—Kiyoshi Fujiwara
Performance. The combination of SPCCI and the air assist mechanism results in a 10-30% improvement in torque, along with a 20% improvement in fuel consumption compared to SKYACTIV-G. Low-speed fuel economy can improve up to 30-45%—offering the same of better fuel economy than the latest SKYACTIV-D diesel.
Mazda will continue to upgrade the SKYACTIV-G and SKYACTIV-D engines—as their cost-competitiveness is improving—while introducing the SKYACTIV-X.
US Patent Application 20140283784: Control Device of Spark-Ignition Engine