All the details on Mazda's groundbreaking new engine
Friday, 18 September 2020
A compressed air-fuel mixture and ignition are the basic operating principles of internal combustion engines.
In a petrol engine a spark plug initiates ignition and in a diesel it’s the heat and pressure of compression that prompts spontaneous combustion.
Often an engine technology pioneer, Mazda is now blurring the lines between petrol and diesel with new SkyActiv-X engines being introduced to the New Zealand market in premium grade Mazda3 and CX-30 models.
In its first application, SkyActiv-X is a 2.0-litre four-cylinder petrol engine that delivers diesel-like attributes of lower consumption and higher torque along with the throttle response and higher-revving characteristics of a petrol.
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The Mazda engineers have chased down the theoretical benefits of homogeneous charge compression ignition (HCCI), in which an air-fuel mixture is spontaneously ignited by the heat and pressure of compression.
However, HCCI has been difficult to perfect for real world operation, mainly due to cold start challenges while variances in temperature, atmospheric pressure and fuel quality have potentially catastrophic implications similar to detonation in a spark ignition engine.
Blending recent SkyActiv experience from its existing G petrol engines and D diesels, the X designates a crossover technology.
The modern advances in direct injection petrol engines have created more opportunity to stratify the incoming air-fuel mixture to run at - or very close to - the ideal 14.7:1 air-fuel ratio. This is referred to as “stoichiometric” and is where all fuel is burned without excess air.
Mazda’s Spark Plug Controlled Compression Ignition (SPCCI) SkyActiv-X engine can operate at ultra-lean air-fuel ratios in excess of 30:1 - which is so lean that a spark plug wouldn’t be able to initiate combustion.
As SPCCI suggests SkyActiv-X engines still have a spark plug within each combustion chamber. As the lean air-fuel mixture is being compressed a very small amount of atomised fuel is also injected close to the spark plug. When ignited the fire ball – Mazda also calls this an air piston - rapidly increases heat and pressure in the cylinder to ignite the lean charge.
The key piece of technology that enables this is a world-first application of in-cylinder pressure sensors. It’s the feedback from these sensors which precisely controls the size and timing of the fireball.
At cold start and under high loads there is a larger fireball and the engine operates with more of the characteristics typical of a spark ignition engine.
At reduced load conditions the fireball is much smaller and compression ignition becomes the dominant characteristic of combustion. Mazda estimates the SkyActiv-X engine can operate in SPCCI mode in about 90 per cent of driving situations.
By burning leaner mixtures there is reduced consumption – and therefore emissions. And combustion temperature is also reduced so there are fewer nitrogen oxide pollutants produced.
The new SkyActiv-X engine is a 2.0-litre four-cylinder of 1998cc capacity. It shares much of its block and bore/stroke architecture with the SkyActiv-G engines while adopting a new cylinder head design and a significantly higher 16.3:1 compression ratio.
Power output is 132kW at 6000rpm (the current 2.0-litre SkyActiv-G produces 114kW) with 224Nm of torque at 3000rpm. Drivers will notice the peak torque arrives 1000rpm earlier than it does in current 2.0-litre SkyActiv-G engines.
There are several other technologies in use by the SkyActiv-X engine to support the new combustion process.
To rapidly deliver the increased volume of air required there is a small displacement belt-driven Roots-type supercharger attached to the engine.
And Mazda engineers have sought additional efficiency, improved auto stop-start refinement and short-term torque boost benefits by introducing a 24-volt starter-generator mild hybrid system.
Mazda says electric contribution is 4.8kW of power at 1000rpm with a significant 60.5Nm of additional torque at just 200rpm.
Drivers can monitor how the engine is operating from the MZD Connect central display with a small Combustion Status icon lighting up when the engine is in SPCCI mode.
The transition between modes is seamless and based on an initial drive in Germany more than a year ago the SkyActiv-X engine has a little more power and a useful amount of additional torque than you’d expect from a modern 2.0-litre petrol.
Some of the responsive character of the new engine comes from adopting a shorter final drive ratio – without the usual fuel consumption penalty because the engine runs so lean in the light load cruising phase.
Mazda’s back-to-back testing suggest SkyActiv-X achieves a 15-17 per cent fuel consumption reduction in urban driving compared to the 2.0-litre SkyActiv-G unit in vehicles such as the Mazda3 and CX-30.
For the New Zealand market, the SkyActiv-X introduction is being led by new premium grade Takami versions of the Mazda3 hatch and CX-30 compact SUV which are priced at $51,995 and $54,990 respectively.
The premium over Limited grade models is $3200 for the Mazda3 Takami and $4000 on the CX-30 Takami.
Putting a price-tag on the actual engine is a little involved as there’s also some additional equipment to consider in the price equation of these models.
The Mazda3 Takami gains a gloss black finish alloy wheel, 360-degree camera and a heated steering wheel while the CX-30 gains high gloss 18-inch alloy wheels, 360-degree camera and a powered tailgate.
Both SkyActiv-X equipped Takami models go on sale mid-October.