A year or two ago, word on the street was that Mazda was developing an engine that operated on the principles of HCCI. It’s an acronym that stands for Homogenous Charge Compression Ignition – in simple terms, it’s making a petrol engine function like a diesel engine with compression ignition rather than the use of spark plugs. A novel idea, it promised much better fuel efficiency by running leaner, and much cleaner emissions as a result.
Those rumours were true, to an extent. Mazda’s engineers did attempt a HCCI engine (and they were far from the only people to try), but there were a number of problems. The first was that HCCI could only operate within a certain RPM range and was very sensitive to conditions, beyond which it would need to revert back to conventional spark ignition (SI). The second was that the switch between HCCI and SI proved to be very rough no matter how hard they tried.
So it was back to the drawing board, but Mazda had learned a great deal from their experience with HCCI. The engineers at Mazda wanted to take advantage of compression ignition being able to run extremely lean, but they also needed a way to smoothen the transition to spark ignition. Part of the reason the transition for the HCCI engine was so rough was that the static compression ratio of the engine needed to be very high to achieve compression ignition (which is why diesel engines tend to have higher compression ratios). For those who aren't familiar with the term, the compression ratio of an engine is the ratio of volume between the piston at its lowest point and the piston at its highest points. A higher compression ratio means the engine squeezes the air-fuel mix tighter before ignition.
And then there was a breakthrough. Mazda went for a high (but achievable) compression ratio of 15:1, which wasn’t quite high enough to achieve compression ignition but was still low enough to make spark ignition feasible and smooth. To get the air-fuel mix to ignite by compression, they would need a compression ratio of around 18:1 – and how they did it was nothing short of genius.
Mazda looked at phased combustion and non-homogenous mixtures. With modern direct injection technology, this is even more feasible and can be executed with more precision. The idea was to ignite the spark plug and create a small fireball in the combustion chamber. The air-fuel ratio needed to create this fireball wasn’t quite the same as the rest of the mixture in the cylinder, hence a non-homogenous mixture.
What this fireball did was temporarily raise the compression ratio of the chamber with combusted gases expanding rapidly. When timed right, the engine could achieve compression ignition smoothly with the help of the fireball. It’s important to note that this isn’t the same as spark ignition; the fireball is not the reason the rest of the fuel mixture ignites, and the majority of the mixture still ignites by compression ignition.
They called this method of combustion SPCCI, or SPark Controlled Compression Ignition. In terms of effectiveness, SPCCI could operate under a wider range of loads and up to a higher rpm before requiring the use of conventional SI. The direct injection system would fire multiple times during the course of combustion, but the precision of timing and control of the mixture is what resulted in this very unique method of operation. It’s lean burn, taken to its only inevitable conclusion.
By now, some of you may be wondering what the difference between compression ignition and knock is. The engineers themselves admitted that it is not easy to differentiate between knocking and compression ignition in the cylinder, but with the adjustment of parameters and monitoring by the ECU, it can ensure that the engine operates smoothly by exceeding the conditions required for knock.
When most people look at lean-burn systems, they automatically assume that the engine is going to be fairly gutless and disappointing on the road – but not so with the SKYACTIV-X. That’s not the case, as the SKYACTIV-X produces more torque than the SKYACTIV-G, all the way from idle to redline. It does this regardless of fuel quality, as Mazda has been testing the engine on both RON 95 and RON 91. Did we mention it comes with a supercharger as well? The supercharger was necessary to keep regular power output levels with an incredibly lean-burn mixture, but it also adds a little bit of performance up top when transitioning to SI. While the SKYACTIV-X does not produce as much mid-range torque as the SKYACTIV-D, it makes its torque all the way to redline.
Throttle response is also vastly improved. The engine doesn’t rely as much on throttle position to control engine speed, but rather through injection amount and timing. Therefore the default physical throttle position is open, making for uninterrupted airflow into the chamber. When you put your foot down, the SKYACTIV-X gets up and goes a lot quicker than the conventional SKYACTIV-G – and that’s saying a lot considering Mazda’s current engine line-up is incredibly responsive.
Also one thing to take note of is the fuel consumption does not vary as much whether you’re driving sedately or flat out. Generally with more engine load, fuel consumption tends to increase drastically once you exceed a certain engine speed, which means your fuel consumption goes from impressive to horrible in no time at all. With a flat fuel consumption curve, it means that you can probably get those quoted manufacturer fuel consumption figures in your own daily driving, even with a bit of spirited moments.
The SKYACTIV-X engines are probably coming to market in 2019, most likely being offered with the Mazda 3. It’s impressive to see how far Mazda is willing to take the conventional engine, and it shows that there was still a lot of development room left in it. The Hiroshima-based company is looking at hybrid systems and autonomous driving on the side, preparing for the inevitable, but what they have achieved here and now is just as important.