A DIESEL WITH GASOLINE
Diesel, named after Rudolph Diesel, is a Compression Ignition (CI) internal combustion concept, not an engine. Thus a fuel is not the process itself, so any CI engine can say it is a diesel.
A long sought-after CI process is called the Homogenous Charge Compression Ignition (HCCI) process. It uses scant amounts of fuel, so is economical. However, the conditions that must occur for HCCI occur are difficult to produce, difficult to sustain, and have only occurred in a very limited range of RPM. Several years ago we drove a test vehicle developed by Volkswagen in Valencia, Spain. It worked, but ran as an HCCI engine only in a very narrow (200-300 rpm) range. We know that GM has, and is, working on HCCI in its Michigan engine development units.
This year Mazda joined other ambitious companies by announcing its own version of the HCCI engine. Mazda alone have said they will deliver an internal combustion engine that uses the compression ignition cycle (versus spark ignition) with gasoline fuel for 2019, the SKYACTIV-X Spark Controlled Compression Ignition engine.
Mazda says their new Spark Controlled Compression Ignition, or SPCCI, combustion method overcomes the problems others have had with HCCI and its limitations in RPM range and other difficulties. The company says they can seamlessly transition between Compression Ignition and Spark Ignition, which is new. When compared to SI gas engines or CI diesels, HCCI engines burn extremely lean. The problem for engineers is when and how to admit fuel so it will burn, as it is too lean to be ignited by a spark, and how to sustain combustion as the air-fuel mix is half, or less, that of normal. In fact it is far outside the stoichiometric air-fuel mix (or Lambda 1) of 14.7:1, as high as 38:1.
In an HCCI engine the combustion takes place after top dead center, after spontaneous combustion ignition. Problems included a very limited HCCI RPM range and difficulty producing a stable transition between CI and SI if a spark plug is included to facilitate ignition. (Spark is still needed under ultra-low temperatures). Mazda says it has conquered these problems.
Mazda says their SKYACTIV-X motor delivers lower fuel consumption than their own 1.5L gas engine while providing better driving performance than their 2.5L engine. CO2 emissions were similar to their clean 2.0L SKYACTIV-D diesel (which we expect to see and test later this year).
Perhaps critical to their breakthrough, Mazda says it uses fuel-air mix ignited by the spark plug to expand (what might be a locally rich mix) to serve as a secondary “air piston.” This, according to the company, further compresses the (lean) fuel-air mixture in the combustion chamber to create conditions where compression ignition can occur. Further, Mazda says their control over ignition timing expands the RPM range of HCCI to approximately 6,000 RPM similar to gas engines.
Are you excited to see a gasoline-fueled diesel? We surely are, and we look forward to getting more information from Mazda relatively soon. A Zoom-zoom diesel sure would be nice, whether it burns gas or diesel fuel! UDBG
Single cylinder test engines with embedded glass view ports show conventional SI combustion, then what a super-lean mix looks like in the same test cylinder. Note the third test with a fuel-air mix of 36.8:1 displays excellent combustion.
Mazda’s breakthrough is using a spark plug “as a control factor to control the compression ignition,” according to Kiyoshi Fujiwara, director with oversight on R&D. Conceptually, the spherical expanding flame front serves as a secondary compression source for the fuel-air mixture, facilitating compression ignition. Mazda calls it an “air piston.”
Highlighted is the “air supply device” for SKYACTIV-X. We expect the motor to be based on existing Mazda blocks, perhaps their 2.0-liter gasoline engine.
According to Mazda’s preliminary figures, acceleration from the SKYACTIV-X engine outpaces their SKYACTIV-D engine, available in Europe and soon North America.
Mazda emphasizes driving and its pleasures, which we applaud. This graphic exposes both fuel economy and torque. More yellow is better.