Mazda's Skyactiv-X Engine - The Shape of Things to Come

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Starting with the 2019 model year, the Skyactiv-G and Skyactiv-D engines will be replaced with the Skyactiv-X engine. As all our forum members know a gasoline engine is a Spark Controlled Ignition engine (SCI) and a diesel engine is a Compression Controlled Ignition engine (CCI). Mazda's new engine is a Spark Controlled Compression Ignition engine (SPCCI), a radical new design. Using regular gasoline and a compression ratio of ~ 16:1, the new engine is essentially a hybrid with operating elements of both spark and compression ignition types utilized in its operation.

On the intake stroke, an ultra-lean A/F mixture of about 37:1 is introduced into the cylinder. Compare this with the normal stoichiometric A/F ratio of 14.7:1 utilized in a gasoline engine. At the end of the compression stroke, a second shot of fuel is injected right next to the sparkplug dropping the A/F ratio to ~ 29:1. Just before the detonation starts to occur the sparkplug fires igniting the less lean mixture around it creating an expanding flame front. The flame spreads from the plug out and down creating a pressure wave moving opposite the rising piston. This increase in the cylinder's effective pressure ignites the primary A/F mixture and initiates the power stroke as occurs in a diesel engine.

Integral to the functioning of this process is the inclusion of a Roots-type blower used as an air pump (not as a supercharger) to lean out the A/F mixture to improve fuel economy at the same time reducing combustion temperatures, lowering NOx emissions and reducing heat loss to the cylinders.

A number of sophisticated controls are required to obtain finer control over the blending between spark and compression ignition by altering the flames pressure wave using injection and spark-timing adjustments. Changes to the pressure wave equate to changes in the effective cylinder pressures making the engine effectively a variable compression type.

A water-cooled EGR system is also utilized to control cylinder temperatures and prevent destructive overheating.

This has been a bare-bones discussion of this radically new engine. For a more detailed analysis consult the sources available on the web for more information.

How this engine compares with the current Skyactiv-G engine and a description of pre-production driving characteristics will be detailed in the next post.

Happy Motoring!

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Comparison

How does Mazda rate the new 2.0 Liter Skyactiv-X engine compared with the current Skyactiv-G engine used in the 2.0 Liter Mazda3? In the new engine, Mazda is predicting about 188 BHP and 170 lb/ft of torque compared with 155 BHP and 150 lb/ft of torque. This puts it above the 184 BHP of the current 2.5 Liter engine. Mazda also expects about 20 to 30% better fuel economy with a range in the 40 to 50 mpg region.

While the G has electronic control variable valve timing on its intake it uses hydraulic control valve timing on its exhaust. The X uses electronic control for both intake and exhaust for more precise control over valve events.

One problem still to be resolved is the audible knocking that occurs during transition from SI to CI ignition or back. Mazda plans to mask the sound possibly by using insulation to make it less audible.

According to Mazda, because the X is more likely to enter CI during lighter load use and can stay in that mode even at high rpms, selecting lower gear ratios has a minimal effect on fuel economy. This leads to the possibility of using lower final drive ratios (higher numerically) in production cars for more acceleration without penalizing fuel consumption.

Mazda believes that fossil fuel powered internal combustion cars will be with us for awhile and is committed to developing the most fuel efficient cars of this type without penalizing performance. How long it will take to produce electric cars with long-range driving capabilities and rapid charge capabilities is anybody's guess. Given the limitations of energy storage based on the laws of physics it may be a long, long time if achievable at all. But that's a tale for another day.

For now, I'll go with the Skyactiv-X. It looks like a real winner.

Happy Motoring!

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very cool! as always a good read! great to see you posting again

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very cool! as always a good read! great to see you posting again

Thank you. Good to be back.

Happy Motoring!

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For those of you who follow these developments it occurred to me that a little history might be of interest. Mazda's new engine did not spring from nowhere but actually is a development of the Homogeneous Charge Compression Ignition engine (HCCI). This is an engine that, if it would actually work, would be the most efficient form of the gasoline internal combustion engine coming as close as possible to the ideal of instantaneous combustion, zero friction and no heat losses. During the compression stroke the piston moves up towards the cylinder head increasing the pressure and temperature of the A/F mixture. As the piston approaches the cylinder head, the vaporized gasoline ignites without a sparkplug causing combustion to occur simultaneously and so quickly that all of the resulting pressure is turned into useful work on the power stroke.

The problem with HCCI is that there is a very narrow range of rpm and load (low engine load and moderately low rpm) at which it will work. Outside this range combustion timing is unpredictable and difficult to control. Also, detonation becomes a factor leading to possible engine destruction. Thus, there is only a restricted range in which HCCI is possible and outside this range spark ignition is required. For this reason a pure HCCI engine would not be possible. The problem becomes finding a way to smoothly switch between SI and CI as required. So far, this has not been possible to achieve.

What Mazda has done is to use a sparkplug to time the ignition process. In the ranges where CI isn't possible the engine runs as a normal gasoline engine. When CI becomes possible, the engine operates similar to a diesel engine with the difference being the use of premixed air and fuel instead of air alone. During the compression stroke the mixture is swirled around in the cylinder causing a homogeneous A/F blend. As the piston approaches TDC, combustion occurs in the manner described in the 1st post.

Although the HCCI engine is a well-known type, no major car manufacturer has been able to build a model suitable for automotive use. That a small company like Mazda has been able to engineer a practical application of this design is a rather incredible feat and a testimony to the competence and creativity of the engineers that the company employs.

Happy Motoring!

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Does this bring us at all close to the theoretical limit of hydro carbon based fuels?
I'm excited to see how tuners are going to exploit what seems to be nanosecond, A/F, adjustments.
As always, very interesting.

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Does this bring us at all close to the theoretical limit of hydro carbon based fuels?
I'm excited to see how tuners are going to exploit what seems to be nanosecond, A/F, adjustments.
As always, very interesting.

Thanks for your kind words, Jon.
Although I can't say for sure, this latest version of the gasoline-powered internal combustion engine may very well represent its ultimate stage of development. At least, it seems to come as close as possible to the HCCI goal of maximum efficiency. I guess only time will tell. As you point out, this could represent a bonanza for tuners who are capable of exploiting the intricacies of this design.

Happy Motoring!