Poorly advised politicians want to force us into electric cars and hybrids, but Mazda’s revolutionary new technology proves that petrol engines still have a bright future
There’s life in the petrol engine yet. We take a look at Mazda’s groundbreaking new Skyactiv-x engine
Apetrol engine has its fuel ignited by a spark, while a diesel engine’s fuel ignites when the air into which it is injected is compressed so hard that it gets hot enough to set the fuel on fire. But what if you could set petrol alight the same way? And why would you? Would it make the engine better – more powerful, more efficient, cleaner? And if it did, why has no-one done it before?
You’re reading about this in a magazine focused on a particular type of Mazda because the manufacturer of our favourite Japanese sports cars has already blurred the petrol/diesel boundary more than any other carmaker. A modern petrol engine usually runs with a compression ratio around 10:1 with indirect fuel injection, or maybe as high as 12:1 with direct injection in which the squirt of fuel cools the air being compressed in the cylinder enough to avoid the pre-ignition (knocking) that would otherwise occur. Modern turbodiesels typically run at around 17:1.
Mazda’s Skyactiv motors as launched in 2011, though, run at 14:1, be they petrol or diesel. For the Skyactiv-g petrol engine, this means a bigger bang each time the fuel ignites, so more power for less fuel. For the Skyactiv-d diesel, it means the fuel can be injected earlier and can burn for longer, so releasing more power without getting too hot too soon and igniting prematurely.
Mazda’s new Skyactiv-x petrol engine, planned for production during 2019 and initially to be fitted to the Mazda 3, takes the blurring a stage further. One of the reasons a diesel engine is very fuelefficient is that it can run with a very lean fuel/air ratio. The heat generated by compression is enough to ignite the fuel, even if there is an excess of air which doesn’t get used in the burning process. In a petrol engine, though, the air and petrol need to be mixed in a ratio somewhere near the chemically-correct one (14.7:1), the one that ensures all the petrol and all the oxygen are used up, if the mixture is to be successfully ignited by a spark.
But if the petrol can be injected into air in the cylinder that’s being compressed even more than in the Skyactiv-g, by means of both a massive 15:1 compression ratio and a supercharger, then it’s close to being ignitable without a spark. And instead of a flame propagating from a single point (the spark plug), there will be simultaneous combustion right through the petrol/air mix even if it is much leaner – maybe as lean as 30:1 when cruising, to the great benefit of economy.
This sounds great in principle, but petrol burns more quickly than diesel and controlling the combustion in a compression-ignition environment is very tricky. The petrol might not ignite, or it might ignite too soon if there’s too much fuel, causing the ‘pinking’ of preignition. Please meet, then, the compression-ignition engine with spark plugs: Skyactiv-x.
Here lies the Skyactiv-x breakthrough. It uses the expanding fireball around the spark plug, as it ignites the petrol/air mix, as an ‘air piston’ to increase pressure in the rest of the combustion chamber and so trigger compression ignition in the rest of the mix, however lean. This sounds like a recipe for instant pinking, and in a sense it is a very
carefully manipulated version of exactly that. The secret lies in controlling – by means of computer power unthinkable a few years ago – exactly when it happens by altering the timing of both the spark and the incoming squirt of fuel into the fast-compressing air within the combustion chamber.
But as well as this squirt, Mazda’s
SPCCI system (Spark-controlled Compression Ignition) also gives a squirt during the induction stroke. The mixture this creates is too lean to ignite either by compression or spark on its own, but it means that at least some petrol is already distributed through the new slurp of air as it’s compressed. And that means the injectors don’t have to squirt quite as much petrol in their second pulse, on the compression stroke, in the tiny time they have to do it. Pressurewave sensors detect if pre-ignition is starting to occur, and the timings are altered to stop it.
This compression ignition mode covers most of the engine’s running, but the lean mixture doesn’t burn quickly enough for power at high engine speeds. That’s when the ignition and injection timing, and the amounts of fuel in each squirt, are altered to get the mixture combusting sooner, so the energy of its expansion doesn’t disappear down the exhaust pipe. It’s now running as a normal spark-ignition engine which means that, unlike a diesel, the
Skyactiv-x can rev in the way we like petrol engines to rev.
Mazda promises a particularly keen throttle response as well as a ‘drastic improvement’ in fuel economy. Early reports suggest that the 2.0-litre experimental engines so far sampled, in mules based on the current Mazda 3, feel and sound quite ‘normal’ and have impressive low-end torque. Currently they ‘pink’ too often, under sudden loads like a normal engine with over-advanced ignition but also, curiously, when coming off the throttle. All will no doubt be fixed; these are early days.
What we want to know, of course, is when the Skyactiv-x engine will arrive in an MX-5. Mazda rules it out for the current ND generation, but it would surely be a part of the NE offering should the MX-5 continue in a fifth incarnation. Mazda won’t say if there will indeed be a next MX-5, but the company’s ‘Sustainable Zoom-zoom 2030’ manifesto makes it clear that ‘driving pleasure, the fundamental appeal of the automobile’ will involve the internal combution engine for a long time yet. And it’s hard to imagine this Mazda future without some sort of MX-5, especially given the sports car’s prominence in the manifesto’s photographs of people having a good time with cars.
The company’s target is to reduce its corporate average well-to-wheel CO2 emissions to half of 2010 levels by 2030, and by 90 per cent by 2050. Electrification will play a part, but Mazda’s view is that the biggest gains will come from improving the efficiency of internal combustion engines because they will ‘help power the majority of cars worldwide for many years to come’.
Above: Skyactiv-x is the future of Mazda’s petrol engine range compression ratio of 15:1, made possible by the new engine’s ability to ignite a highly compressed fuel and air mixture. Power, economy and emissions all benefit
Below & right: Looks familiar, but this is the first significant development in petrol engine technology for many years. The Skyactiv-x engine has a
Skyactiv-x engine is currently undergoing testing in a fleet of Mazda 3 test mules. While the new engine won’t see service in the current MX-5, it will surely feature in the next generation