How do you sell a revolutionary engine technology that is so technically complicated, it requires mathematical formulas to understand? It’s a pretty hard question to answer, and one that Mazda Australia will have to grapple with.
The good news is that Mazda’s promise on delivering the world’s first commercially produced, petrol spark-controlled compression ignition engine for a passenger car is on the money.
But what exactly is SkyActiv X and why should you care? To put it in layman’s terms (because anything more technical will require a PhD in mechanical engineering, which we dearly lack), it uses technology from a diesel engine to burn petrol at a much leaner rate. Petrol is compressed and burnt like diesel, but without the negative side effects of diesel’s emissions (like NOx).
The formula for thermal efficiency is relatively complicated, but by improving the compression ratio and specific heat ratio, things begin to move in the right direction for efficiency. Basically, using more air to burn the same amount of fuel.
Compared to the previous-generation Mazda SkyActiv G engine, the new SkyActiv X raises the air to fuel ratio to twice the stoichiometric level, so for the same amount of fuel, twice the air is used for a much leaner burn. That is the key to better fuel economy because it burns cooler and there is less heat loss.
Broken down to simpler terms, the lower combustion temperature reduces the temperature difference between the head, pistons and walls, which further reduces cooling loss.
The problem is, though, that as you raise the air:fuel ratio to the levels discussed here, the flame doesn’t propagate as intended, which is where the spark controlled compression ignition (SPCCI) comes in. Using SPCCI means that the range where compression ignition works (in regards to RPM and load) covers the whole combustion range.
How does it do that? Well, go grab your glasses and a cup of coffee because it gets a little tricky. First, air and fuel are compressed to near compression ignition conditions. Then, the spark plug initiates a small fireball, which then expands to increase temperature and pressure until compression ignition conditions are met, whereby the majority of the air and fuel in the cylinder is combusted through compression ignition. Mazda uses a rather complicated control unit to manage the timing of the spark ignition.
Back to basics, then, think of all the things you want from a diesel engine – better fuel economy, torque lower down the rev range, much less heat loss. Then, put it together with what you love about a petrol engine – much cleaner emissions, higher revving characteristics – and combine them to get the Mazda SkyActiv X.
Now, given the emissions-obsessed world that we live in, you might be wondering that if this is such a good idea, why has no other manufacturer tried it before? It’s a very valid question. The answer is that it has indeed been tried before, by none other than the inventor of the automobile, Mercedes-Benz.
The concept in its inception is called DiesOtto, referring to Rudolf Diesel (inventor of the diesel engine) and Nikolaus Otto (inventor of the petrol engine), both German engineers born in the 18th century.
Keeping in line with the Germanic theme, Mercedes-Benz played around with the DiesAuto engine back in 2007, and went as far as fitting a concept engine into a long-wheelbase S-Class and showing it in the F700 concept car at the Frankfurt motor show of the same year.
In Mercedes’s version from 2007, the 1.8-litre four-cylinder twin-turbo unit claimed a very healthy 177kW and 400Nm of torque while using just 4.4L/100km. It remains officially unclear why the company did not persist with the technology, but it’s likely a case of cost, reliability concerns and market demand.
For Mazda, the new 2.0-litre four-cylinder SkyActiv X is supercharged (though Mazda likes to refer to this as an air supply system rather than a supercharger), and in European spec, which we drove in Frankfurt earlier this month for this review, had 132kW of power and 224Nm (at 3000rpm) of torque. Fuel economy remains to be confirmed, but our hour-long drive showed real-world figures around the 6.5–7.3L/100km mark.
So, what’s it like to drive a car with a DiesOtto engine? Not all that much different to driving a standard engine. Sure, it sounds a little different and it’s a bit more refined, but if you were to do a blind test in a new Mazda 3 SkyActiv X and were not told it has the new engine technology, you would be hard-pressed to tell that there is anything new or revolutionary going on under that bonnet. That’s a good thing because that’s the point of bringing new technology to mass production – it just needs to work, and in Mazda’s case it does just that.
Even so, the Japanese company does provide a nice display on the infotainment system that shows the car’s SPCCI system sensor, so you can see when it's working and when it’s not. We found that between 2000–4000rpm the SPCCI was at its best, but when you push past that rev range and start to really get on the right pedal, the system turns off and the engine goes back to running like a regular petrol system.
Power, torque, and the way it goes about delivering both, are hugely improved over the standard 2.0-litre engine from Mazda. The engine feels smooth and the delivery of torque is very linear, and it even feels like it has a lot more torque than what Mazda says. But the question has to be asked as to what is the point of this engine? The reason it has to be asked is because the SkyActiv X engine sits above not only the regular 2.0-litre, but also the 2.5-litre in terms of price.
To answer that question, we need to go back a little bit and look at Mazda as a car company that has persisted with technology where nobody else has. Like the rotary engine (another German invention), which will soon see a rebirth as part of a range-extender set-up.
Mazda is perhaps the only car company in the world that will publicly continue to say that electric vehicles are still some time away from reality, in the way that we want them to be. One, because the ‘well to wheel’ emissions of EVs is still questionable in countries like Australia, where we don’t have clean power. Two, because the cost is still too high, and the use of rare earth materials for batteries and other components is still not completely understood in a full cycle.
Which is why the company continues to invest in the internal combustion engine, when the rest of the industry goes full steam ahead in to electrification – at least from a publicity perspective. The reality is that the majority of car companies are still investing heavily in to both petrol and diesel engines.
It’s a philosophical argument more than anything else, but as a car enthusiast, you can’t help but sympathise with Mazda’s position of trying to build the most efficient combustion engine while it still has the chance. So that is why the SkyActiv X engine exists, and while it will eventually filter down the range, for now it’ll remain the top-spec engine in Mazda’s range.
Although it remains to be confirmed, Mazda Australia is likely to offer the X engine in the absolute top-spec Mazda 3 models only to start with (followed by CX-30), and it’s unlikely to be a volume seller for the time being.
Mazda engineers admitted that the engine – in terms of material costs – is still less expensive to make than a turbo diesel engine, but given the R&D cost for the technology, expect at least a $1000–$2000 price premium over the more powerful 2.5-litre engine (139kW/252Nm).
So, why pick an X engine over a 2.5? It’s like asking why pay extra for a hybrid powertrain over a regular one? It’s a question we can’t really answer if you’re measuring the benefit as a purely cost issue. So, there needs to be some emotional aspiration, plus a sense of value in better CO2 emissions, and a desire to want the latest and greatest in engine technology from the Japanese brand.
We look forward to having a much longer stint with the SkyActiv X engine when it arrives in Australia towards the end of this year.