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Mazda SPCCI uses spark plug as HCCI control factor; “air piston” to enhance compression

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



So, this concept could also have been coined PTDE: part-time diesel engine (but running on gasoline). The HCCI area is significantly smaller than I had expected. The engine is not as advanced as I had thought (and that previous articles indicated).

If you compare with the Skyactive diesel at full load, the fuel consumption penalty for Skyactive-X is quite dramatic. Why the Skyactive diesel curve ends at a BMEP level of 1000 kPa is beyond my comprehension. The diesel has higher BMEP. Perhaps Mazda simply would not like to show that the gap is even bigger at maximum BMEP for the diesel engine. Confounding! Furthermore, if you would compare to a (competitor) state-of-the-art diesel, you should note that such and engine has maximum BMEP at 2500-3000 kPa. With the downsizing (of the diesel engine) this implies, Skyactive-X would lose by quite some margin. Scaling according to load percentage on the x-axis, instead of showing absolute values, is the most reasonable comparison when BMEP levels differ so much as in this case. Comparing apples-to-apples data presumably would also show that, even at light load when HCCI is implemented, Skyactive-X would not reach diesel level. All-in-all, Skyactive-X still seems to be quite far from reaching the efficiency of a diesel engine.

If the Skyactive-X engine was used in a hybrid system that shift load to higher BMEP levels, its advantage would be smaller compared to conventional gasoline engines. The gain compared to Atkinson/Miller gasoline engines would be far less – if any – in a full hybrid system.

In view of what I mentioned above, I am also surprised by the relatively low maximum BMEP (1300 kPa!). However, this is a classic problem for all gasoline HCCI concepts and it seems as Mazda has not overcome that.

I suppose that high-load aftertreatment will be conventional TWC, potentially supplemented by GPF for Europe later.

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@Peter XX,
How does Skyactive -X compare to Mahle Turbulent Jet Ignition or Ricardo turbocharged spray-guided gasoline direct injection (or any related Formula 1 lean burn turbocharged GDI) or Ricardo Magma “extreme” Miller cycle concept?

Brian P

The various systems used in F1 are only applied at part load (to save fuel during the parts of the lap where the driver can not use full load). And, these engines are not called upon to conform to any emissions regulations.

The chart in the article showing relative BSFC is obviously only illustrative, since the Y axis is not provided with any units. It is obvious that the bottom of the graph is not at zero BSFC.

If we accept the "concept" shown in the BSFC chart, if not the scale or the numbers, this engine has BSFC better than Skyactiv-G under all conditions and pretty much equal to the diesel engine up to moderate load on the engine ... which accounts for the majority of real world driving anyhow. Only in acceleration or hill climbing is it not matching diesel BSFC but it's still better than prior Skyactiv-G under all conditions (that the chart shows) and that in turn was better than their old port-injected conventional gasoline engine under all conditions (that the chart shows).

On the other hand, this hopefully won't need the complex and expensive (and potentially troublesome) exhaust aftertreatment system that the diesel needs. And the hardware is familiar to any mechanic that knows modern gasoline engines.

With regards to BSFC still being better with the diesel engine under heavy load ... the only conclusion that I would draw is that the diesel engine will remain the engine of choice for heavy trucks and buses.

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@Brian P,
I was not referring to the MGU-H or MGU-K Energy Recovery Systems that are used during part load in F1, but the combustion system which according to Ferrari is based on the Mahle TJI system that was originally developed by Cosworth Engineering ( a division of Mahle).


Seems to me Honda tried this some long time ago calling it lean burn; but, they couldn't meet emissions so they moved over to a stoichiometric catalytic converter like everyone else.

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Actually, the MGU-H is used under all load conditions. Mercedes F1 probably uses a combustion system similar to the Ricardo/Petronas T-SGDI lean burn system that has been used on some Mercedes AMG vehicles, e.g. AMG CLA45 (check http://media.daimler.com/marsMediaSite/en/instance/ko/The-new-Mercedes-Benz-GLA-45-AMG-All-Rounder-with-Driving-Performance.xhtml?oid=9903716). So F1 tech with proper emission control equipment could meet air quality regulations.

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The original Honda Lean Burn CVCC system goes back to 1975 and used carburetors. This was a very efficient system though it could not meet more strict emission regulations so Honda then opted for 3-way catalytic converters.
Honda is also investigating spark assisted HCCI and is using Lean Burn in F1.
Lean Burn systems may have some NOx issues like diesels, so they cannot use extreme lean air/gas ratios like in racing. (NOTE: Cummins and GE Jenbacher Natural Gas engines are Lean Burn and have low NOx emissions).


Tnx for the info; I can see NatGas NCCI as a good interim engine until BEVs have cheaper,long-range batteries and are ready for prime time.


Very interesting fully compression engine using original design and very hard coating

Nick Lyons

Looks like a winner to me: low NOx, so simpler emissions control; great part-load efficiency, which is where cars spend almost all their running time. It should make for a very economical highway cruiser (my personal use case).


Good discussion, no criticizing, insulting nor bullying attempts. :)


Looks like D-EGR is still better but hard to tell with Mazda's missing BSFC graph scale:

(see page 24)


Am I right in guessing that the use of high EGR limits combustion temps and NOx, and the "air piston" boosts the effective compression to achieve effective ignition at leaner mixtures/lower BMEPs?

The one issue I can think of that would limit this is the lean limit for ignition, absent a stratified charge.  Perhaps something like corona ignition is required.

This would add a lot of flexibility.  The effective compression ratio can be changed on the fly with spark timing.


@Brian P
As you noted, the graph is illustrative and perhaps, somewhat deceptive. You should realize that a “good” diesel engine can achieve double the BMEP (~2 500 kPa) of the Skyactive-X. In a “normal” driving cycle, average engine load corresponds to a BMEP of approximately 200 kPa. Due to the higher maximum BMEP, this equals 400 kPa for the diesel engine. Thus, the most reasonable comparison of fuel consumption at low load is to compare the level at 200 kPa for Skyactive-X with 400 kPa for a diesel engine (i.e. the same load percentage). If you do this simple math, you will find that a diesel engine is far better than Skyactive-X also at low load.

The diesel cycle is the most efficient thermodynamic cycle for piston engines and the most efficient “practical” heat engine of any kind. (Yes, you could argue that diesel-atkinson cycle, or miller system - whatever you prefer to call it - would be even better but this is just a variation of the basic diesel cycle.) In over 100 years, nobody has actually come up with something that is more efficient; in fact, not even equal.


First, I realize that I have forgotten to give Mazda credit for bringing this concept to full production. I can only recall a few attempts to utilize HCCI in the past in production engines and one familiar example, a Honda motorcycle, had a 2-stroke engine. This would be the first production 4-stroke gasoline engine with HCCI and Mazda should have credit for that.

I think the main achievement here is the switching between HCCI and conventional mode. The “highly responsive air supply unit” might be the key here, albeit that Mazda does not elaborate much on this topic.


They invented the ability to expand ignition of lean fuel with spark ignition. The HCCI operation zone is so limited. If X technology is out of the lean burn zone they have to quickly enrich mix and operate conventional SI, hence the quick reacting air supply. So, diesel like performance within a limited range and the "X" technology can improve efficiency in some operational parameters.

I still go back to E85 engine that Cummings engineers developed for the mid level van. They optimized the engine for E85 fuel. The combustion pressure 2x of diesel. They downsized to 1/2 displacement engine to get similar torque. As you know ethanol exhibits fast flame speed and requires spark ignition at almost any compression ratio. Also, the fuel is naturally lower in production of NOX since the fuel chemically contains oxygen. This E85 engine boost had to be limited per engine strength. Nonetheless the engine easily met CA standards with conventional cat technology. A special high PSI spark plug developed. The engine had great torque and great HP. IOWs the best of both diesel and SI engine technology. Field test bested or met gpm of plain gasoline engine comparison in all but the lowest load requirement. Also, diesel beat mpg of E85 engine, but cheaper fuel leveled the fuel cost per mile. This is good since the E85 engine required no high pressure injection equipment, expensive pollution equipment, and weighed half as much.

Engineers also noted that their testing indicated a jump in performance with 100% ethanol. IOWS if engine strengthened to maximise benefit of utilizing E100 fuel their analysis indicated improve engine efficiency more than linear. Engineers also noted a turbo air cooler was not utilized that would have improved their test results.

So, in general the fuel would fit high torque demands of trucking better than current diesel and solve the dirty tail pipe emissions problem of diesel. My guess this test was very corrosive to business as usual politics and quickly shoved under the mat.


HCCI with E100 could make a good range extender.


Where did electric vehicles go?

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I don't think the Cummins Ethos 2.8L E85 engine went far enough! It had Stoichiometric Combustion with a 3-Way Catalyst and 12:1 compression ratio. Even the Mazda Skyactive-G had 14:1 compression ratio in Japan.
In the Final Report Cummins did discuss Lean Burn as one way to improve efficiency. However, they could have gone further. Rolf Reitz at the University of Wisconsin-Madison achieved 58% efficiency with a dual fuel E85/Diesel with Reactivity Controlled Compression Ignition (RCCI) engine.
Now Oak Ridge National Laboratory is studying many of the different Low Temperature Combustion processes, e.g. RCCI, PCCI, HCCI, Partial Fuel Stratification, Moderate Fuel Stratifucation, etc. Project ID: ACS016 .

(continued at Mercedes-AMG Project One PHEV with F1 technology . . .)

Brian P

Ethanol does not make for a good compression-ignition fuel ... the octane rating is too high! Mazda has stated that this new engine is designed for use with regular gasoline (referred to as 87 octane in US and Canada, 91 RON elsewhere). If premium fuel is used, it operates in spark ignition mode - for the simple reason that the higher octane rating resists self-ignition due to compression. Now, yes, it is still operating at a higher compression ratio than a normal gasoline engine; the slower combustion (relative to partial compression ignition) would probably reduce efficiency a bit. Ethanol has a higher octane rating than premium pump gasoline and will resist compression-ignition even more.

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References on Reactivity Controlled Compression Ignition (RCCI):
1. Reactivity Controlled Compression Ignition (RCCI) for high-efficiency clean IC engines
2. https://energy.gov/sites/prod/files/2017/06/f34/acs016_curran_2017_o_1.pdf
3. Patent US8616177B2 - Engine combustion control via fuel reactivity stratification
4. Fuel reactivity controlled compression ignition (RCCI): a pathway to controlled high
efficiency clean combustion


Ethanol isn't a good compression ignition fuel, but better than diesel or gasoline for high compression engines. The Mazda approach, lean burn, RCCI and the rest may prove to work well with ethanol or ethanol blend? Blended fuel appears to be the future of fuel. Some still like the dual fuel concept.

Electric drive cars will be very attractive in future, but battery power storage for primary energy needs may have a big challenge from hydrogen and fuel. My guess a simple mechanical gas car will be very cost competitive and have low maintenance, high resale, with minimal tail pipe emission if operating on high ethanol blended fuel.

We did read on GCC that interests in investing in alternative power is dwindling. We read no breakout technology for battery power.

Diesel fueled heavy trucking is the concern presently for pollution and cost of EPA compliance. Ethanol fuel and natural gas appears to me to be the common sense alternative.

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Final Comment: Mazda has a brilliant engine design. This engine has 190 hp and 207 lb-ft of torque, lean burn, Atkinson cycle, and high compression (up to 18:1 CR). Make mine a Mazda CX-5 PHEV with a 50 mile electric range.



I think it has been said that the compression ratio is somewhere between 15-16:1.

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