MAZDA e-SKYACTIV X

* Valid for Mazda3 and CX-30 e-Skyactiv X 2.0. Mazda3 e-Skyactiv X 2.0: WLTP fuel consumption: 6.5-5.0 l/100 km; CO2 emissions: 146-114 g/km. NEDC fuel consumption: 5.5-4.5 l/100 km; CO2 emissions: 124-101 g/km. Mazda CX-30 e-Skyactiv X 2.0: WLTP fuel consumption: 6.6-5.7 l/100 km; CO2 emissions: 149-128 g/km. NEDC fuel consumption: 5.6-4.9 l/100 km; CO2 emissions: 127-111 g/km. Vehicles are homologated in accordance with the type approval procedure WLTP (Regulation (EU) 1151 / 2017; Regulation (EU) 2007/715). To provide comparability the referred values are NEDC – values determined in line with Implementation Regulation (EU) 1153 / 2017.

** Optional on Mazda3 Hatchback and CX-30.

2 - CONCEPT AND AIMS OF SPCCI TECHNOLOGY

Featuring Spark Controlled Compression Ignition (SPCCI), a Mazda unique combustion method, the e-Skyactiv X engine represents the second step in Mazda’s quest to develop a petrol engine with the ideal internal combustion mechanism.

Developing compression ignition for petrol engines has long been a goal of engineers. e-Skyactiv X is a ground-breaking new engine, exclusive to Mazda, in which spark plug ignition is used to control compression ignition, resulting in dramatic improvements across a range of important performance areas.

The benefits of a spark-ignition petrol engine – expansiveness at high rpm and cleaner exhaust emissions – have been combined with those of a compression-ignition diesel engine – superior initial response and fuel economy – to produce a crossover engine that delivers the best of both worlds.

Mazda believes that there is still ample room for further evolution of the internal combustion engine and that this technology has the potential to contribute in a major way to the conservation of our global environment.

Based on Mazda’s corporate vision of protecting our beautiful planet while enriching people’s lives through the 'joy of driving', we plan to continue on our ceaseless quest to develop the ideal combustion engine.

Advantages of lean burn, and associated issues

Mazda has undertaken a fundamental re-examination of the nature of combustion, with the aim of making some major improvements in the efficiency of the internal combustion process.

Mazda’s Skyactiv-G petrol engine boosts combustion efficiency by raising the compression ratio, while cooling losses from the zone of the heat transfer to the chamber wall are reduced through control of cooling water temperatures. Pumping losses and mechanical resistance are reduced through use of the Miller cycle.

In the e-Skyactiv X, the latest Skyactiv engine, we have worked to boost the air-fuel ratio. In order to do this, we had to enable lean burn, in which larger quantities of air are combusted.

The ideal (stoichiometric) air-fuel ratio is 14.7:1. Creating a higher air-to-fuel ratio by more than doubling the amount of air raises the specific heat ratio and lowers the combustion gas temperature. This, in turn, reduces cooling losses. Meanwhile, a design that introduces larger amounts of air reduces the losses from throttle closure, resulting in improved fuel economy.

However, if this kind of lean mixture of air and petrol is burned using the flame propagation-based combustion which occurs when a spark plug is used, combustion tends to become unstable.

To overcome this problem, compression combustion in high-temperature, high-pressure conditions must be employed. This means that such an engine will need to adopt the compression ignition used by diesel engines.

In developing the e-Skyactiv X, Mazda has improved the seven control factors which have a main influence to the effiency of an internal combustion engine.

These include the compression ratio (which needs to be raised in order to realize the required high-temperature, high-pressure conditions), combustion timing near top dead centre (which is found in compression ignition), and a combustion period in which all the fuel burns simultaneously.

Issues associated with homogenous charge compression ignition (HCCI)

One concept underpinning compression ignition in petrol engines is Homogenous Charge Compression Ignition (HCCI).

Conventional gasoline engines use a spark plug for the ignition. The combustion has to spread out from the initial spark, which results in a slow combustion speed. If, in addition to this, a leaner air-fuel mixture with more air is used, the flames created by the spark plug will fail to spread throughout the combustion chamber.

With compression ignition, however, all fuel in the combustion chamber combusts simultaneously, resulting in a far higher combustion speed which, in turn, means that a leaner air-fuel mixture can be burned.

However, HCCI has not yet reached the point where it can be used in commercial applications because it is only used at low rpm and engine load ranges, and even these ranges are apt to change depending on driving conditions.

Furthermore, the very limited range across which HCCI can take place makes it difficult to achieve frequent, stable and seamless switching between spark ignition - required for higher rpm and engine loads - and compression ignition.

Until now, overcoming these issues had required a major increase in the compression ratio, a more complex structure and the addition of high-precision controls.

3 - SPARK CONTROLLED COMPRESSION IGNITION: THE BREAKTHROUGH THAT HAS MADE SKYACTIV-X POSSIBLE

Mazda’s breakthrough has been achieved by questioning the conventional idea that no spark plug is needed for compression ignition and suggesting a different approach instead, in which the spark plug plays a role in both combustion methods. This concept is the basis of Spark Controlled Compression Ignition (SPCCI), Mazda’s unique combustion method.

Using SPCCI means that the engine load and rpm range wherein compression ignition can take place has now dramatically expanded, allowing this technology to be used in almost all driving conditions.

Moreover, because a spark plug is now being used at all times, the engine can switch seamlessly between combustion using compression ignition and combustion using spark ignition.

Key Technological Features of SPCCI

Although SPCCI is an entirely new combustion method, it is based on two existing functions - ignition and injection - which Mazda has further refined and meticulously recombined.

To do this, Mazda has further developed several elementary technologies – a new piston head design, a super-high-pressure fuel injection system to support compression ignition, and a high-response air supply unit which can deliver larger amounts of air – and combined these with an in-cylinder pressure sensor which serves to control the entire combustion process.

Compared with the complicated structures that were previously required in order to utilise the HCCI concept, the hardware for SPCCI is simple and lean, with no unnecessary complexity.

Using Compression Effects Created by A Spark

The SPCCI mechanism can be summarised as a system in which the compression effect of spark-ignited localised combustion is used to achieve the required pressure and temperature to bring about compression ignition.

In other words, the geometric compression ratio is raised to the point where the air-fuel mixture is on the verge of igniting (due to compression) at top dead centre. At this point, an expanding fireball created by spark ignition provides the final push that causes the whole mixture to combust via compression ignition.

The timing and amount of pressure required are in a continual state of flux depending on constantly changing driving conditions. But the SPCCI system is able to control the spark plug ignition timing, meaning that pressure and temperature within the combustion chamber can be optimised at all times.

Because a spark plug is used all the time, the system is able to switch seamlessly to spark ignition combustion in rpm or load ranges where compression ignition would be difficult. In this way, the system ensures that the compression ratio is never raised too high, while enabling a simple design which does not require complicated features such as a geometrical variable compression ratio.

Fuel Density Distribution within the Air-Fuel Mixture

The e-Skyactiv X unit controls the distribution of the air-fuel mixture in order to enable lean burn using the SPCCI mechanism. First, a lean air-fuel mixture for compression ignition is distributed throughout the combustion chamber. Next, precision fuel injection and swirl is used to create a zone of richer air-fuel mixture - rich enough to be ignited with a spark and to minimize nitrous oxide production-around the spark plug. Using these techniques, SPCCI ensures stable combustion.

Controlling the air-fuel mixture to prevent abnormal combustion

Split fuel injection

In order to prevent the abnormal combustion which can occur when rich air-fuel mixtures are compressed for long periods of time (a longstanding issue for HCCI), SPCCI adopts a split fuel injection system, in which part of the fuel is injected during the air intake process and part is injected during the compression process.

First, the low-density lean mixture for the lean burn is injected during the air intake process. Then, during the compression stroke, a separate injection creates the richer air-fuel mixture that is ignited around the spark plug.

This not only distributes the density of the air-fuel mixture so as to allow SPCCI to take place, but also minimises the time lag until the air-fuel mixture ignites under compression, effectively controlling abnormal combustion.

Super-high-pressure fuel injection system

To minimize compression time and make compression ignition as efficient as possible, the fuel must be vaporized and atomized very quickly, and then immediately dispersed around the whole of the cylinder.

e-Skyactiv X therefore features a system capable of injecting fuel at super-high pressure from a multi-hole fuel injector positioned in the centre of the combustion chamber. This causes the fuel to be vaporized and atomized instantly, while powerful turbulence is simultaneously created, greatly improving ignition stability and combustion speed.

Super-high-pressure fuel injection enables SPCCI, which suppresses abnormal combustion even at full throttle/low rpm where traditional petrol engines have to retard ignition and thus sacrifice efficiency and power.

Adoption of the in-cylinder pressure sensor

In addition to these technologies for preventing abnormal combustion, an in-cylinder pressure sensor has also been introduced as a monitoring control. By continually observing whether the above controls are bringing about proper combustion and compensating in real time for any deviations from intended norms, it ensures continuously optimised combustion.

Based on the techniques set out above, SPCCI has expanded the zone of compression ignition right into the full throttle range, and enables smooth switching between SPCCI combustion and spark ignition combustion.

This new combustion method does not merely use spark ignition to assist compression ignition, but delivers an all-encompassing combustion control system which includes control of in-cylinder temperature and pressure, and control of the fuel injection’s air-fuel mixture distribution density and exhaust gas recirculation (EGR).

4 - BENEFITS PROVIDED BY e-SKYACTIV X

e-Skyactiv Xe-Skyactiv X's revolutionary SPCCI technology represents the start of an exciting new stage in our quest to develop the ideal internal combustion engine, fully supporting the Jinba Ittai driving experience Mazda aims to provide.

The main development goal of e-Skyactiv X was to provide a highly efficient petrol engine that supports everyday driving with a combination of excellent response, engaging, free-revving performance, smoothness and superior real world fuel and emissions efficiency.

Excellent response

Like that of a diesel engine, the throttle valve of e-Skyactiv X is almost always open.  In other words, there is lots of air in the cylinders. As a result, the engine generates torque instantly in response to the drivers demand.

The e-Skyactiv X features a highly rigid drivetrain which transfers the driving force smoothly and promptly. By using a high rigid drivetrain, longitudinal vibrations can be avoided.

When pressing on the accelerator, drivers anticipate the car’s response and unconsciously try to balance themselves. e-Skyactiv X provides such an immediate response to the throttle that the driver's balance is stable and controlling the car feels natural and effortless.

Free-revving feel

e-Skyactiv X is designed to deliver a free-revving feel, inspired by the iconic Mazda MX-5. To achieve this, e-Skyactiv X will spin up into higher rpm ranges as smoothly and easily as a typical petrol engine. It delivers a very consistent and linear power output, peaking at 186 PS at 6,000 rpm.

e-Skyactiv Xe-Skyactiv Xe-Skyactiv XThe free-revving feel e-Skyactiv X achieves is based on the sensation of acceleration and a good quality engine sound which matches the acceleration rate. A capsule structure has been adopted for the engine bay, which enhances the sound and matches it to the acceleration rate of the car.

The capsule structure traps also heat and keeps the engine warm which benefits real world fuel economy.

Smoothness

On engine re-start, the Mazda M Hybrid system's belt driven integrated starter generator (ISG) raises the rpm more than a conventional starter would, promptly re-starting the engine and allowing a quiet and responsive re-launch and acceleration.

When engine resistance is small and the rpm do not drop well during manual transmission upshift operations, shift shocks can easily occure. To counter this, the regenerative ISG intervenes to rapidly drop and adjust the engine rpm to match the next gear ratio, giving a shock -free gearshift. Skyactiv-MT 6-speed manual transmission also has close ratios to facilitate a nimble and smooth gear change.

The time taken by the Skyactiv-Drive 6-speed automatic transmission to complete downshifts has been reduced in order to smooth out the gear changes. The rate of acceleration before and after gear changes has also been realigned to match, further smoothing out automatic shifting.

Real world fuel consumption

e-Skyactiv X is able to deliver excellent fuel economy over a broad operating range, despite its 2.0 litre displacement.  The engine delivers notably low fuel consumption in a wide variety of driving scenarios, including both urban and long-distance motorway driving.

By using a lean air/fuel mixture in conjunction with SPCCI, excellent fuel efficiency is expanded over a wide range of engine loads and rpm. The e-Skyactiv X 2.0 litre engine also produces very low levels of CO₂ and NO_X emissions. For exhaust gas treatment, e-Skyactiv X adopts both a three-way-catalyst and a Gasoline Particulate Filter (GPF), cleaning exhaust air for the air supply unit that provides enough air to create a lean combustion at high engine revs.

The Mazda3 and CX-30 e-Skyactiv X meet the Euro 6d emissions rating. The Mazda3 returns WLTP measured combined fuel consumption of 6.5-5.0 l/100 km and generates WLTP CO₂ emissions of 146-114 g/km. The CX-30 achieves WLTP measured combined fuel consumption of 6.6-5.7 l/100 km and WLTP CO₂ emissions of 149-128 g/km.