Nerd Alert! Mazda’s Skyactiv-X Engine

by Khiem Dinh


As we are sure many of you have heard, the little car company that could, Mazda, has announced their Skyactiv-X engine with the technology to run Homogeneous Charge Compression Ignition (HCCI). Two major challenges to make HCCI work: timing of the combustion and smooth transition between HCCI and regular spark ignition. The HCCI combustion process has been researched for decades, but the technology to bring an engine using HCCI to market has been elusive until now.

So how did Mazda do it? We are going to take a few guesses, but as with most technologies, it didn’t happen overnight. In fact, we would say Mazda’s first big step was a few decades ago. The final piece of the puzzle is a technology Mazda has called Spark Controlled Compression Ignition (SPCCI), and we will get into how that ties into making HCCI work.

We think Mazda started this whole journey in the 1990s with their Miller Cycle engine that came in the Millenia. Of course we now have Skyactiv-G (gasoline) and Skyactiv-D (diesel), which were required steps before getting to the Skyactiv-X. The new Skyactiv-X engine promises the fuel efficiency of the Skyactiv-D with the emissions and fun Zoom-Zoom DNA of the Skyactiv-G. 


This is a tech roadmap for Mazda’s Skyactiv engine program I found, which shows how all of their engine development has been leading to the Skyactiv-X and beyond. Mazda has been systematically attacking every area of inefficiency in the internal combustion engine.

Before we get into some history on Mazda’s engine development program, we need to gloss over the basics of internal combustion engines and how they get the fuel to burn. Basically every engine currently in production either runs on the Otto cycle using spark ignition or the Diesel cycle. Otto cycle engines use a spark plug to ignite the fuel whereas diesel uses the heat from compression (good ole Ideal Gas Law, chemistry class did come in handy). HCCI by definition can operate on many types of fuel, but we’re going to talk about HCCI with gasoline. HCCI also causes fuel to ignite and combust using the heat of compression, but differs from Diesel cycle due to the timing of the fuel injection. The timing of combustion in HCCI is where the major challenge lies.

In an Otto cycle/spark ignition engine, the fuel ignites when the spark plugs is fired. All those ignition timing tables in modern car ECUs tell the spark plug when to fire relative to crank angle. Old school is the good ole distributor cap. Diesel engines ignite the fuel when the fuel is injected into the cylinder; the air is sucked into the cylinder, compressed to a very high temperature by way of the high compression ratio of diesel engines, and the fuel ignites as soon as it’s sprayed into the cylinder and comes into contact with the hot compressed air. HCCI differs from Diesel cycle in the timing of the fuel injection; fuel is mixed with the air as the air enters the cylinder like Otto cycle and then the whole air/fuel mixture is compressed until it ignites. So basically it does exactly what a standard spark ignition engine tries to avoid, igniting the air/fuel mixture by heat of compression. The challenge is controlling when that ignition happens versus crank angle.


As you can see in this diagram I found online someplace, gas and diesel engines have a flame front that moves through the cylinder whereas HCCI has the mixture igniting everywhere in the cylinder.

Why all the hoopla over HCCI? It should have around the same efficiency as a diesel and the low emissions of a spark ignition gasoline engine. HCCI can have similar efficiency as a diesel because they both can operate with a very lean air:fuel ratio and require high compression ratios to increase the temperature of the mixture enough to ignite. If you recall our article in compression ratio you’ll remember a higher compression ratio leads to greater thermal efficiency. A gasoline engine must run a lower compression ratio to avoid the whole compression ignition phenomenon. Diesel and HCCI engines can also run with a lean air:fuel ratio whereas a gasoline engine needs to stay around stoich to minimize emissions; the 3-way catalyst on gas cars works best when the air:fuel ratio is stoich. To maintain stoich at low loads, gasoline engines limit air sucked into the engine using the throttle plate which creates pumping losses and reduced efficiency. Running lean is great for fuel efficiency but horrible for NOx emissions in diesel and spark ignition gasoline engines.


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Dan DeRosia
Dan DeRosialink
Monday, August 28, 2017 6:17 AM
A lot of guessing as you know, but it seems plausible. And exciting.

Of course, now I want to see productionized versions of exhaust energy recovery systems thrown into the mix too... ;)
Monday, August 28, 2017 8:50 AM
Do you count a turbo as exhaust energy recovery :) I wouldn't hold your breath on waste heat recovery making it into a passenger car. Current systems can only recover about 10% of the energy out of the exhaust. As passenger cars spend most of their time at low load, there's very little energy to recover. It does make sense on semi-trucks or stationary power generation where they spend a lot of time at high power. Even then, it's a lot of cost to only gain a little bit of energy. It doesn't make economic sense unless fuel prices are high. So what's really driving it now is legislation.
Dan DeRosia
Dan DeRosialink
Monday, August 28, 2017 10:05 AM
Well, a turbo would be the most ideal solution... but yeah, you're probably right. Although link motor/generators to the center section of the turbo and, well, stop me if you've heard this one before.

Seriously though, it's really interesting what efficiency gains are still possible.
Monday, August 28, 2017 10:32 AM
I've always been curious as to why pedestrian engines need to rev to 7k rpm. Why not make them more diesel like and only rev to 3k-4k with a much longer stroke for more torque? Or use these same concepts in a hybrid system to drive a generator so the engine is always operating in its optimal speed range.
Mazda seems to be working on making an engine that is both powerful and efficient which is nice, but I think we need to re evaluate what an engine needs to do in a modern vehicle rather than following the same rules we have in place.

And turbo everything :)
Monday, August 28, 2017 11:22 AM
Everything is trending turbo :) Also lower redlines. Like the WRX is only around 6500rpm. Same with the Ford Ecoboost engines. They are designed for max down-low torque which is what everyone wants in everyday driving.

E-booster, electrically driven compressor, is already going into production on some Audi paired I think with a 3.0L turbo diesel. Next step is e-turbo which has the electric motor integrated into the turbo, so current F1 tech. They will probably take a while to get the cost down.

What's making the e-turbo stuff possible is the shift to hybrids which have the high voltage systems needed for the e-turbo stuff. I think all the BMW hybrids use a turbo gas engine, so those are the perfect candidates to go e-turbo once the technology is more mature.
J Finken
J Finkenlink
Monday, August 28, 2017 11:05 PM
Great post spdracerut. With both this new engine from Mazda and the VC-T from Nissan/Infiniti coming up, it will be interesting to see how they compare to conventional engines in normal driving, and if they live up to the claims from the manufacturers.

jeffreyball610: Low revving engines tuned for low end torque and efficiency was actually produced by BMW in the 80s. 525/528e had 2,7 litre engines with 4800 rpm redline and up to 11:1 CR. Pretty low redline considering a modern diesel has around 5000. As far as i know not many have done it since so it probably wasn´t a huge success.
Tuesday, August 29, 2017 8:37 AM
J Finken, thanks! OEMs have been chasing the variable compression ratio engine for decades too with SAAB having a concept back in 2000. I see VC vs. HCCI breaking down like this:
VC is for turbo engines catered to high power/displacement with an improvement in efficiency at part load vs a standard turbo engine.
Mazda’s HCCI engine is catered towards max efficiency at part load (where people spend the vast majority of their time) with a modest increase in torque/power vs a standard naturally aspirated engine.
The thing I like about Mazda’s Skyactiv-X is it’s using all mechanically proven technology: variable valve timing, direct injection, EGR. I’m a bit scared of Nissan’s VC engine though it should have passed their durability testing.
Julian ITR
Julian ITRlink
Thursday, August 31, 2017 6:26 PM
Superb article, I really enjoyed reading it. I wonder how they deal with varying fuel quality and if you can make it not work anymore if you use high octane fuel (102 ROZ)
Monday, September 04, 2017 2:56 PM
Thanks! We try :) Couple thoughts, Mazda has to be able to account for different fuel quality and octane along with different ambient conditions. Cars up to this point use knock sensors and adjust ignition timing as necessary. That might not be good enough for HCCI, so they might have to go with direct cylinder pressure measurement vs crank angle.
Wednesday, April 18, 2018 6:15 AM
CX-9 engine in a CX-5: Ford makes this. They call it the Ecoboost Edge. :)

Handling is definitely better in the (British pronunciation) MAZ-der though.
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