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With a global output of around 1.6million vehicles per year, it’s a long way down the automotive pecking order, and its independence means it lacks the economies of scale present in companies such as the Volkswagen Group, Toyota, Hyundai and Kia, or General Motors.
Yet this independence also gives Mazda relatively free rein to forge its own development path. No other company persisted with Wankel rotary engines for as long as Hiroshima’s engineers (not necessarily a wise investment, but one indicative of Mazda’s focus on finding engineering solutions to problems), and as the rest of the industry turned to turbocharging and downsizing to reduce its environmental impact, Mazda implemented what it calls ‘rightsizing’: refining conventional combustion engines with improved technology and making them appropriately sized for their application.
The company’s upcoming combustion engine series, dubbed Skyactiv-X, takes the concept of constant technological refinement to its next logical stage and introduces a technology several companies have tried, and failed, to implement in the past with petrol engines: compression ignition.
“Skyactiv-X takes the concept of constant technological refinement to its next logical stage”
The idea in itself is nothing new. Diesel engines already operate on the principle of compression ignition and several companies have experimented with it in petrol cars at the prototype stage. Infiniti’s recently introduced variable-compression engine, while not running on compression ignition as yet, features technology that could make it possible with petrol vehicles in the future. But Mazda has found a way to do it today, and with much less complication than previous experiments with the technology.
To explain how it works, one must first understand the basic principles of homogenous charge compression ignition (HCCI). HCCI engines operate by the fuel and air mixture spontaneously igniting at the end of the compression stroke in a four-cycle combustion engine. The ignition is caused by the increase in temperature of the mixture as a result of its compression – to a higher ratio than would be used by a spark-ignition engine. The process is more efficient than spark ignition, partly because the charge ignites throughout the entire combustion chamber at once, rather than propagating from around a spark plug, and partly because the engine uses a leaner mixture. As Mazda puts it, faster combustion gets more work out of the same energy.
However, HCCI is also difficult to control and works effectively in only a limited window of operation: at relatively low engine speeds and at relatively low loads. Introducing a spark to the process and switching between spark ignition and HCCI is a solution to that problem, but a complicated and expensive one, requiring different conditions in the cylinder depending on the mode of operation.
Mazda’s solution is called SPCCI, or spark-controlled compression ignition. Yes, it uses a spark, and the engine can operate as a conventional spark-ignition engine when required, but the spark is also used to initiate compression ignition without the need to mechanically increase the compression ratio of the engine, and therefore complexity and expense are reduced. So how does SPCCI work? In its regular state, the Skyactiv-X engine runs a high compression ratio of 16:1 – but that’s not high enough to cause compression ignition on its own. During the induction stroke, air is pushed into the cylinder with a belt-driven supercharger and fuel from a high-pressure injector. The shapes of the combustion chamber and piston crown promote swirl and even distribution of the lean mixture.
During the compression stroke, the same injector then fires again, delivering a slightly richer mixture to the area around the spark plug. When the spark plug fires at the start of the expansion stroke, it ignites the richer mixture and creates what Mazda describes as an ‘air piston’: a rapidly expanding flame front, raising compression in the chamber, which in turn spontaneously ignites the remaining, leaner charge. The compression ignition phase is constantly controlled so that it happens just after top dead centre, where it can do the most efficient work. The split injection process also helps prevent uncontrolled compression ignition, or ‘knock’, as the initial mixture is too lean to pre-ignite before the final, richer mixture is injected.
A pressure sensor in each cylinder allows the engine’s computer to constantly control spark and fuel injection timing depending on load or engine speed, and if required the system can transition seamlessly from SPCCI to conventional spark ignition. To achieve this the injector gradually moves from its split cycle to a single, more typical charge that is ignited purely by the spark. This happens when the driver demands higher loads or higher engine speeds. At the other end of the scale, during very light loads and low engine speeds, an even leaner mixture can be used for greater fuel efficiency, with SPCCI able to prevent the typical lean-burn issue of incomplete ignition.
“The clatter normally associated with compression ignition engines is absent, because the engine’s base 16:1 compression ratio isn’t actually that high in comparison with many diesel engines”
The Skyactiv-X engine works on the Miller combustion cycle, which is essentially a supercharged version of the Atkinson cycle used in Mazda’s current Skyactiv petrol engines. The Atkinson cycle delays the closing of the intake valves during the compression stroke, reducing the compression ratio and the quantity of fuel/air mixture that is ignited. Meanwhile, the power stroke is effectively longer than the compression stroke – it extends past the point where compression began – so the amount of force generated by the combustion is maximised, meaning greater efficiency has been achieved. In a Miller engine, supercharging ensures enough air pressure at low engine speeds to mitigate the power loss from having a lower effective compression ratio.
The Roots-type supercharger is belt-driven from the engine, but parasitic load is reduced with a magnetic clutch, meaning the supercharger only runs when it’s required to supply suitable volumes of air for the required lean mixtures. So different is its purpose from the performance-biased forced induction we’re more familiar with that Mazda prefers to refer to it as an ‘air pump’, rather than describing it as a supercharger.
For instance, a problem with lean-burn engines has always been high levels of NOx pollution – a result of the higher combustion temperatures involved when running lean. This usually requires a NOx trap in the exhaust system to meet emissions regulations, but because the Skyactiv-X engine can precisely control combustion, it runs cooler than previous lean-burn engines, so produces lower levels of NOx. The complication and expense of a NOx trap is, therefore, unnecessary. Nor is noise a problem. The clatter normally associated with compression ignition engines is absent, because the engine’s base 16:1 compression ratio isn’t actually that high in comparison with many diesel engines. It’s only a little higher than Mazda’s existing Skyactiv-G unit, in fact, so the engine’s construction, while all-new, isn’t far removed from the company’s current petrol engines. Extra sound-damping material around the engine muffles any difference you might otherwise hear, while the precise ignition control means vibration isn’t an issue, either.
“The engine has also been designed to work with mild hybrid electrification”
Variable valve timing is used, allowing the engine to switch between its leaner and richer mixtures as required, but Mazda has avoided the complexity of variable valve lift, the engine’s air and fuel demands instead controlled by the supercharger and with the high-pressure injectors. The only significant change has been to design an all-new computer to handle the calculations – Mazda initially tried its Skyactiv-G ECU, but the calculations needed to carry out SPCCI quickly overloaded it. The engine has also been designed to work with mild hybrid electrification – think an integrated starter-generator – but it’s not strictly necessary for the operation of the Skyactiv-X unit.
Firstly, it works just as well with a manual transmission as it does with an automatic, so Mazda has no plans to ditch its snappy six-speed stick-shifts just yet. Better still, because the range over which the engine operates efficiently is now so wide – Mazda’s graphs suggest it can run as efficiently at 5000rpm as the existing Skyactiv-G does at 3000rpm – engineers have discovered that they can use a shorter final drive for punchier acceleration, without any fuel economy penalty.
Torque is also up by around 30 per cent compared with Mazda’s existing 2-litre petrol engines, and fuel economy is 20 to 30 per cent better, even at this early stage. Not just on paper or in a laboratory either, but in the real world, where it actually matters. Mazda says its Skyactiv-X engine actually betters its current diesel units in some types of driving, without the downsides of diesel combustion. The next-generation Mazda 3, due in 2019, will be the first production Mazda to combine the technology with the company’s latest evolution of its Kodo design philosophy, with the 3-sized Kai concept from 2017’s Tokyo motor show giving a clear preview as to the model’s new look.
And Skyactiv is more than just an engine philosophy – it also applies to the company’s next-generation vehicle architecture, with a more torsionally rigid platform, revised suspension and tyre kinematics, improved NVH characteristics and redesigned seats all contributing to a more comfortable drive, without reducing the kind of involving driving experience Mazda is keen to promote through all of its models. A drive of a Mazda 3 Skyactiv-X 2-litre prototype suggests the company is on the right track. The engine takes a few moments to turn over, before firing up to a steady, quiet hum. Any preconceptions that there might be some diesel-style rattle – at idle, the Skyactiv-X operates in compression ignition mode – are quickly dispelled, and if anything the forced induction, and Mazda’s efforts to clothe the unit in more sound-damping material, mean it’s even more hushed than the Skyactiv-G unit.
Driven back-to-back with the current car, the gearshift feels a little chunkier, and there’s a little more weight to the clutch pedal, but otherwise Mazda’s current, linear controls seem present and correct. Low-rev, small-throttle-opening response seems ever so slightly behind the naturally aspirated Skyactiv-G, but the difference really is negligible. The engine responds predictably when rolling on and off the throttle, and refuses to labour even from tickover at higher loads, pulling with a turbodiesel-style insistence, but without the boosty delivery associated with such engines. A tablet computer attached to the dash indicates when the engine is running in spark ignition, SPCCI and lean-burn modes. It switches between the first two frequently depending on load and engine speed – though predominantly runs in SPCCI, just as Mazda claims – and changes to the last mode during low-revs, low-load driving. Without the lights flashing away it’s impossible to tell how the engine is operating from the driving experience alone.
At this early stage the Skyactiv-X unit also creates less vibration and buzz than Mazda’s current 2-litre offering and there are lower levels of noise from behind the wheel, and while the engine doesn’t develop a particularly sporty tone, nor is it unpleasant – some modern acoustic exhaust tuning and generated sound could easily inject some character in sportier applications. Refinement is enhanced further by Mazda’s new, more rigid body-in-white. Road noise seems lower than in the current Mazda 3, and the kind of drumming resonance you get with certain road surfaces is absent. In combination with the Skyactiv programme’s redesigned seats, the prototype’s refinement, solidity and tight construction bode very well indeed for Mazda’s next-generation cars, from 3s to MX-5s, and perhaps even an RX-8 successor.
While the rest of the industry moves towards downsizing and electrification, the relatively small car maker from Hiroshima might one day end up being the last accessible bastion of the technology that has defined transportation for over 100 years.
