The world has caught on to the latest trend of killing the combustion engine – electricity is the new hot thing, and why should the world wait? Not Mazda though, who seemingly missed the memo and instead have developed the next iteration of the petrol engine; a system set to offer the benefits of a petrol engine – response, rev-range, and power – with the benefits of a diesel – torque and fuel efficiency. Mazda’s SKYACTIV-X is the next step in the evolution of the combustion engine, but what is it, and how does it work?
What is SKYACTIV-X?
‘SKYACTIV’ is the name given to any of Mazda’s latest engine developments, and ‘X’ is indicative of a crossover between the usual ‘D’ for Diesel and ‘G’ for Gasoline models. The principle of SKYACTIV-X is essentially defined as HCCI, or Homogeneous Charge Compression Ignition.
But what does that mean? Essentially, it combines combustion methods of a regular petrol and diesel engine. To understand more, we need to first understand how each of those fuels is conventionally burnt:
Petrol Engine (HCSI):
Petrol engines utilise Homogeneous Charge Spark Ignition. What happens in a petrol engine is that fuel is introduced into the combustion cylinder during the air intake stroke. The air and fuel mixture is then compressed during the compression stroke, and when the mixture is fully compressed and the piston is at the top of its stroke (something called ‘top dead centre’ or TDC), the spark plug ignites the mixture and causes an explosive force that drives the piston downward.
Diesel Engine (SCCI):
Diesel engines make use of Stratified Charge Compression Ignition. What this means is that air is taken in on the intake stroke, and compressed in the compression stroke, but fuel is yet to be introduced into the system. The compression of the air generates heat within the combustion chamber, and when the cylinder is at TDC, injectors introduce diesel directly into the hot, compressed chamber. The heat and compression generated are enough to ignite the fuel without the presence of a spark – hence this is referred to as compression ignition.
Mazda’s SKYACTIV-X combines these two principles into Homogeneous Charge Compression Ignition. ‘Homogeneous Charge’ refers to the method of fuel introduction and preparation – just like in a petrol engine, SKYACTIV-X introduces fuel during the intake stroke, so the air fuel mixture is compressed together.
However, during compression, there is no spark ignition. Instead, SKYACTIV-X relies on the air fuel mixture being super compressed to a point where it self-ignites, similar to how a diesel engine works. This also means Mazda can increase the compression ratio far beyond a standard petrol engine’s capabilities.
Isn’t self-ignition bad?
You may remember the Tech Talk discussion on fuel octane ratings – where higher octane fuel can sustain higher pressure before self-igniting. Self-ignition, or premature ignition in a HCSI engine is a bad thing, as the explosive force is countering the direction the piston is moving in. But with SKYACTIV-X, and HCCI engines, the ignition due to pressure doesn’t create the same shockwaves that damage engines severely in standard spark-ignition format.
And what’s a ‘compression ratio’?
Current SKYACTIV-G engines run on a compression ratio of 14.0:1 – this means that for every 14 particles of air, there is one particle of fuel. SKYACTIV-X however will attempt to possibly double that, running on a compression ratio of 28.0:1 or even higher.
At this ratio, there is very little fuel in the mixture. In fact, there is so little fuel present, that even a spark wouldn’t be able to ignite the mixture on its own. This means that the pressure inside the combustion chamber will have to be immense when ignition is required. It also means that lots of air will be needed to maintain the correct fuel to air ratio – another problem that needed solving.
Mazda didn’t break the intellectual bank on this one, simply utilising a supercharger to force-feed the engine with enough air to maintain the correct ratio. The supercharger is the simple answer as it runs from idle and generates a consistent amount of pressure based on engine speed, making it easy to judge and control the amount of fuel required.
Don’t think that means SKYACTIV-X will be able to run on garden-grade fuel though. It will still require high octane fuel, as the risk of premature ignition could be incredibly damaging in such a highly compressed setup. Naturally, knock sensors and programming will monitor combustion, and in the event of ping/knock/premature ignition, the air:fuel ratio can be made suddenly much richer, as additional fuel will cool the combustion chamber.
How does this make SKYACTIV-X more efficient?
By running a compression ratio of 28.0:1, SKYACTIV-X is essentially using half the amount of fuel as a regular SKYACTIV-G engine to generate the same power. Less fuel also means it’ll burn quicker, improving response times, and reducing emissions of un-burnt fuel particles.
In addition to this, the fuel also burns cleaner because of the compression ignition. When fuel ignites from a spark, the flame starts at a central point and expands outwards; but in a compression engine, the fuel ignites at multiple points throughout the chamber and burns up quicker. Quicker burn = cleaner burn = quicker response from the engine.
With SKYACTIV-X, Mazda say they will be able to match, or even beat the consumption of their own diesel engines, with increased outputs of about 30%. They give the example that a 2.0-litre engine will be as strong as their current 2.5-litre, but consume less fuel than a 1.5-litre.
Importantly, Mazda claims that SKYACTIV-X is so efficient and offers efficiency at such a broad spectrum of operation, that even the NEDC test results will be achievable in the real world.
What are the benefits of SKYACTIV-X and HCCI?
The benefits are numerous, though the three key aspects are:
- Improved efficiency
- Increased power and torque outputs
- Increased engine response
That last bit also means more fun for people who enjoy driving, for whom turbo-lag is the devil. But in addition to increased response, the increased efficiency at higher rpm will also mean Mazda can shorten gear ratios for more fun driving and punchier acceleration.
Traditionally, manufacturers are forced to put long final drive ratios in their gearboxes to improve fuel consumption, but SKYACTIV-X will negate the need for that, making driving fun in any gear, and improving overtaking and in-gear acceleration.
In Mazda’s example, they say a 2.0-litre SKYACTIV-X equipped model can overtake with the efficiency of a Mazda 2 1.5 diesel, but with the pace and response of an MX-5.
What are the drawbacks of SKYACTIV-X and HCCI?
Though the benefits are many, there are also several drawbacks. Due to the immense pressures in the combustion chamber, SKYACTIV-X engines will only be able to efficiently run on very high octane fuel. Without high octane fuel, HCCI will only be viable at low RPM. It is likely that 95-octane will only just be good enough to reap the benefits of SKYACTIV-X.
In addition, HCCI engines – much like diesel engines – will struggle to operate in very cold conditions where the temperature of the engine won’t be hot enough to ignite the fuel/air mixture. Likewise, if the engine gets too hot, things will ignite far too quickly.
Mazda has a solution – SPCCI:
(SP)ark (C)ontrolled (C)ompression (I)gnition is what will solve the problem, and make the operating range of SKYTACTIV-X engines even broader. Even though SKYACTIV-X engines will rely on compression for ignition, there will be a spark plug present. At low operating temperatures – cold starts etc – the engine will run as a regular petrol engine, utilising a richer air-fuel mixture (more fuel than usual) and igniting via spark until engine temperatures no longer require it.
At that point, the spark won’t stop though. Instead, the air:fuel ratio will be made leaner again; to the point that the spark can’t actually ignite the fuel. The action of the spark will however create a miniature shockwave in the engine that increases the compression, acting as what Mazda calls an ‘air piston’.
Should early detonation be detected and should the engine need to swap back to spark ignition, there would be no delay or obstruction to the changeover as the spark is constantly in operation. All that would be needed would be a change in the air:fuel ratio; something that would be managed by the computer on the fly.
By utilising SPCCI, Mazda will be effectively combining the best aspects of petrol engines, including the control factor of a spark plug, with those of a diesel engine – primarily efficiency.
Whilst the rest of the world looks to be killing the combustion engine entirely, Mazda has given it a new lease on life. Starting from 2019, Mazda will be introducing SKYACTIV-X engines into their models. Though they are still in development, Mazda has high hopes of making the system workable. However, with South Africa’s low quality fuel, it still remains to be seen whether or not this revolutionary new system will work in our local conditions.
Wording by: Roger Biermann
Images courtesy of Mazda
Please note this Tech Talk explainer has been simplified for general understanding. Should you wish to get a further understanding of SKYACTIV-X, and should your Japanese be good enough, you can watch the full press conference from Mazda below.