Mazda's next-generation SKYACTIV-X gasoline engine technology

Engine with SPCCI combustion technology to be introduced in 2019



Next-generation Mazda SKYACTIV-X engine with SPCCI technology
(Exhibited at Tokyo Motor Show 2017)

In August 2017, Mazda announced its Long-Term Vision for Technology Development. As part of the technology to achieve this vision, the company disclosed plans to introduce its SPCCI (Spark Controlled Compression Ignition) engine for production in 2019. In September and October, Mazda held technology briefings for the media and conducted vehicle test drives, demonstrating their development progress. This report introduces Mazda’s SPCCI engine technology, which includes estimations based on Mazda technical data.


Technology concept of the SPCCI engine

Academically, HCCI (Homogeneous Charge Compression Ignition) combustion is considered to result in ideal combustion in gasoline engines. As such, SPCCI can thus be regarded as the technology concept that makes it possible to achieve HCCI combustion in gasoline engines.

Technically speaking, SPCCI differs from HCCI combustion, but allows an engine to realize the performance advantages of HCCI combustion, thus making SPCCI a technology that enables the intent of HCCI combustion technology. Mazda’s SPCCI technology can be considered a development breakthrough because their technology makes it technologically feasible to achieve stratified combustion for SI by using spark ignition (SI) combustion to control compression ignition (CI) combustion, which Mazda has taken to a technological level where it is ready to be put into a production engine.

This report first describes the characteristics and technical issues associated with HCCI combustion, and then introduces the technological concept of the SPCCI engine.


Related reports:

Tokyo Motor Show 2017: Mazda, Suzuki, Subaru, and Daihatsu's exhibitions (December 2017)
The growth of xEVs and improvements to ICE vehicles (Part 1) (July 2017)
Evolution of the internal combustion engine: Mazda's quest and AVL's roadmap (February 2016)


Characteristics and issues associated with HCCI combustion

Fig. 2.1.1 SI and HCCI Combustion Concept (Source: Mazda)
SI: Spark Ignition, flame propagation by spark plug; air-fuel ratio = normal ratio
CI: Compression Ignition, homogenous charge ignition; air-fuel ratio = lean mixture ratio

As shown in Fig. 2.1.1., HCCI combustion involves compression ignition of a homogeneous super lean air-fuel mixture, whereas normal spark ignition (SI) combustion tends to cause engine operating issues such as misfire due to the difficulty of flame propagation in a super lean air-fuel mixture.

HCCI combustion makes it possible to achieve an ultra-lean engine burn when a super lean air-fuel mixture is ignited by compression ignition (CI) under a sufficiently high compression ratio, which tends to generate subtle knocking.

A super lean air-fuel mixture will have an air-fuel equivalence ratio of at least double stoichiometric levels, i.e. λ > 2.0.

Issues associated with HCCI combustion

Issues associated with HCCI combustion are shown in Fig. 2.1.2 and Fig. 2.1.3.

Fig. 2.1.2 is used here only to describe the basic concept as the values are not precise, but it clearly shows the key quantitative values. In the diagram, the horizontal axis represents the unburned mixture temperature (TuK) at the end of compression, the vertical axis represents the burned gas temperature (TBK), and it shows the validity range of HCCI combustion. The validity range is the light green colored area. HCCI combustion is valid in a very narrow range where the temperature of the unburned mixture (TuK) is about 1000K and the temperature of the burned gas (TBK) is about 1500 to 2000K. Outside of this range, which is required for HCCI combustion in an engine, misfire and knocking intensity increases.

Fig. 2.1.3 shows the range of combustion varies with the octane number. The solid line represents the limit for engine knock, the dashed line represents the limit for misfire, and the area between the two lines represents the validity range of HCCI combustion. The graph shows that combustion characteristics can change significantly depending on differences in the octane number and fuel properties. It also shows that at high engine rpm the combustion of the air fuel mixture burns more quickly because of the higher compression ratio, increasing the range where misfire occurs.

The above suggests that HCCI combustion characteristics can be greatly influenced by factors including engine rpm and load ranges, changes in operating conditions, and differences in the fuel components. In other words, the lack of robustness of HCCI combustion is a significant issue for the practical application of the technology.

The fuel-air equivalence ratio (φ) is the reciprocal of the air-fuel equivalence ratio (λ);
i.e. φ=1/λ
Fig. 2.1.2  Efficiency of HCCI combustion
(Source: HCCI Combustion Efficiency, Mr. Ota
Fig. 2.1.3 HCCI Combustion Validity Range
(Source: Study of Auto-Ignition Characteristics in HCCI combustion, Shibata 2007)

Technology concept of the SPCCI engine

The technology concept of an SPCCI engine is comprised of three main technologies: SPCCI technology, combustion technology, and combustion switching technology, as described below.

SPCCI technology

As shown in Fig. 2.2.1, SPCCI technology first uses spark ignition (SI) by means of a spark plug to create an expanding fireball. When the plug sparks, the expanding fireball (spherical flame front) acts as an air piston to further compress the super lean fuel mixture to a compression level of 1,000 degrees (K) resulting in compression ignition (CI). SPCCI technology aims to achieve HCCI combustion by controlling the spark timing for compression ignition.

Specifically, when CI combustion is difficult due to engine operating conditions (variances in atmospheric pressure that come with elevation changes or weather swings) or the use of high octane fuel, SI combustion takes place earlier in the highly compressed unburned gas. And, when the engine is operated under conditions where CI is more easily achieved, the SPCCI technology controls the combustion so that SI combustion and the compression of the unburned gas is delayed. In other words, SI timing is controlled so as to achieve optimal CI combustion.

Also, control of the SI timing is achieved by feedback control of the in-cylinder pressure sensor (CPS). Details are shown in Fig. 2.2.2. For example, when the intake air temperature rises from 120 degrees Celsius to 156 degrees Celsius, the CI timing required for optimum combustion remains constant, but SI timing requires that the crank angle (CA) before top dead center (BTDC) needs to change from 26 to 16 degrees to achieve optimum combustion. Feedback control of SI timing by the in-cylinder pressure sensor (CPS) allows CI timing to be precisely controlled.

As shown in Fig. 2.2.3, SPCCI technology has greatly expanded the range across which HCCI combustion can take place to realize a more robust combustion, thereby paving the way for commercial application of HCCI combustion technology.

Fig. 2.2.1 SPCCI technology (Source: Mazda)

SPCCI = Spark-Controlled Compression Ignition Combustion utilizes a spark plug
Spark-ignited combustion is used to achieve the required pressure and temperature to bring about instantaneous compression ignition (CI).

Under conditions where CI is difficult to achieve such as low load, high rpm, cold outside air temps, low in-cylinder pressure, and high octane fuels, the spark ignites the air-fuel mixture quickly, creating significant compression. Under easier CI conditions, ignition is delayed and the effective compression ratio is lower.


Fig. 2.2.2 Feedback Control of Ignition Timing (Source: Mazda)

The target combustion state can be controlled by the ignition timing.

Fig. 2.2.2 Feedback Control of Ignition Timing (Source: Mazda)
Graph: (Horizontal axis) Torque, (Vertical axis) In-cylinder Temperature Error

The robustness of combustion control improves significantly when the combustion range is expanded.

SPCCI Engine: Combustion Technology

As can be seen by the combustion pattern in the lower part of the photos in Fig. 2.3.1, with lean spark ignition combustion, the air-fuel ratio of a super lean mixture can have an air-fuel equivalence ratio (λ) of greater than 2 (λ = 1.0 is at stoichiometry) where combustion will not take place, or even if it does take place the flame won’t propagate. As shown in Fig. 2.1.1, the problem is that combustion tends to be unstable with a super lean air-fuel mixture even when the flame propagation is initiated with a spark plug.

Fig. 2.3.1 Spark Ignition (SI) Combustion (Source: Mazda)
Top figure: Ordinary SI combustion @ 750 rpm; air-fuel ratio = 14.7:1
Lower figure: Challenge - Lean SI combustion @ 750 rpm; air-fuel ratio = 29.4:1


Fig. 2.3.2 Conceptual Diagram of a Spray-Guided Direct Injection System
(Source: Toyota Stratified Charge Combustion, Article: December, 1998)

The SPCCI engine achieves stratified combustion to ensure ignition in a super lean air-fuel mixture. As shown in Fig. 2.3.2, stratified charge combustion technology involves positioning the high pressure fuel injector near the center of the combustion chamber with the spark plug angled in from the intake port towards the combustion chamber, where the fuel mixture is injected towards the piston bowl. First, the super lean air-fuel mixture required for compression ignition is distributed throughout the combustion chamber. Next, precision fuel injection and swirl is used to create a zone of a richer air-fuel mixture around the spark plug. By stratifying the fuel density near the ignition plug, SI ensures stable combustion.

This stratified charge combustion concept is referred to as a spray-guided injection method. Daimler demonstrated a similar technology with their Stratified-Charged Gasoline Injection (CGI) engine on the Mercedes-Benz CLS 350 CGI in 2006 and BMW installed such a system on three models in their 5 Series, but both were discontinued. Mazda seems to have solved the issue of transitioning between SI and CI by precise fuel injection and swirl used to create a relatively rich air-fuel mixture around the spark plug for creating the expanding fireball ignited by the plug.

The stratified charge combustion concept is shown in Fig. 2.3.3. In the CI zone, the air-fuel ratio is 30 to 40, and in the SI region it is 25 to 30. To ensure stable combustion, the air-fuel ratio near the area where ignition takes place should be about 20. To ensure a lean burn to suppress the generation of NOx emissions it is necessary to precisely control parameters such as in-cylinder injection flow, ignition timing, and fuel density distribution within the combustion chamber.

Fig. 2.3.3 Stratified Charge Combustion (Source: Mazda)

Fuel density distribution within the combustion chamber is controlled to ensure lean burn while suppressing the generation of NOx. Precise fuel injection and swirl is used to create a zone of richer air-fuel mixture around the spark plug.
CI air-fuel mixture: A lean mixture that does not generate NOx
SI air-fuel mixture: A richer mixture that minimizes the generation of NOx

(Supplementary explanation)

In summary, Mazda's SPCCI technology differs from HCCI combustion technology in that it employs a spark plug to assist combustion events. Starting with the intake stroke, a super lean air/fuel mixture is introduced to the cylinder. The air/fuel ratio varies but is always far greater than stoichiometric (λ) and so lean that it cannot be ignited by spark—hence the need for compression ignition (CI). At the end of the compression stroke, a second squirt of fuel is injected right next to the spark plug where it's held by the swirling mixture and quickly ignited by spark. The flame spreads from the plug out and down, creating a pressure wave moving opposite the rising piston. The increase in the cylinder's effective pressure (not the flame front) combusts the primary air/fuel mixture and initiates the power stroke.

Mazda says that SPCCI overcomes two issues that has impeded commercialization of compression ignition gasoline engines: maximizing the zone in which compression ignition is possible and achieving a seamless transition between compression ignition and spark ignition.

SPCCI Engine: Combustion Switching Technology

As shown in Fig. 2.4.1, to simultaneously achieve good fuel economy and high engine output the area where combustion takes place is divided into and switched between two areas, i.e. the area that is controlled by CI combustion and the area controlled by conventional SI combustion.

The switching technology is a key element of the SPCCI engine technical concept, but further elaboration on this subject is pending upon the release of further technical detail by Mazda at a to be determined date.

Fig. 2.4.1 Switching between CI and SI areas (Source: Mazda)
Graph: (Horizontal axis) Engine speed, (Vertical axis) Engine load



Main specifications and subsystem technologies of SPCCI engine

Next, we introduce the main specifications of the engine and the subsystem technologies used to realize the technical concept of the SPCCI engine described in Section 2.

Main specifications

The main specifications of the engine are shown in the table below. Mazda announced that the SPCCI engine will have an engine displacement of 2.0L, with a compression ratio of 16. If the compression ratio is 16, it shouldn’t be necessary to further increase the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the B × S (cylinder bore diameter x piston stroke length) specifications are likely to be the same as those of SKYACTIV-G 2.0 engine.

And, one distinctive characteristic of the engine room is that it adopts an engine encapsulation structure. There are probably two reasons for this. One is to improve practical fuel consumption by thermal insulation of the engine. The other is to act as sound insulation against the knocking sound that common occurs in CI combustion. Even though the knocking sound of CI combustion is subtle, it is still considered knocking.

Major specifications are estimated based on Mazda information
Fig. 3.1.1 Engine specification (right-side image: Response)

Key subsystem technologies

Next, we will introduce the subsystem technologies that we believe Mazda is using, based upon the photos of the engine shown in Fig. 3.2.1.

  1. High fuel pressure injection system technology
    The high pressure fuel pump is located at the rear end of the engine and is driven by a timing belt. The connection between the pump, the fuel rail and the high fuel pressure injector is a structure with high rigidity. From this, it can be deduced that Mazda has increased the fuel pressure capacity of the SPCCI engine significantly versus the current engine.

  2. Air supply technology
    Supercharging is indispensable to improve power during combustion with super lean air-fuel mixtures. Mazda refers to their supercharging technology as air supply technology. Supercharging of the air supply is carried out using an Eaton three-lobed Roots-type supercharger. To improve knocking, the supercharged air is cooled by a water-cooled intercooler before induction into the intake manifold. Particularly at high rpm loads, supercharging improves the scavenging of residual gas and reduces knocking.

  3. Control technology for intake and exhaust valve timing
    An electric Variable Valve Timing (VVT) system is adopted for the intake and exhaust cams. Timing of the intake and exhaust valves is precisely controlled to reduce pumping loss by the mirror cycle at low loads, improve knocking by controlling the effective compression ratio, and by controlling the residual gas to optimize the unburned (Tu) and burned (Tb) air-fuel mixture.

  4. Cooled EGR technology
    Regular EGR technology circulates a portion of the exhaust gas to the intake air, reducing NOx emissions and improving thermal efficiency. However, if a large amount of EGR gas is directly recirculated to the intake air stream, the intake air temperature will rise, adversely affecting CI combustion and cause knocking. Cooled EGR technology includes a large capacity EGR cooler in the EGR system that cools the recirculated exhaust gas to suppress such problems.

  5. ISG (Integrated Starter Generator) technology
    ISG technology integrates the function of the starter and generator to achieve energy regeneration mainly during idle stop and deceleration (regenerative braking). The capacity of Mazda’s ISG unit is probably around the 2 - 4kW level for a 12V specification system, so it is unlikely the ISG is capable of supplying the engine with additional power (power assist). A dual mode tensioner has been adopted because the belt tension moves in opposite directions when the starter and generator are in operation.

Fig. 3.2.1 Engine Exterior and Key Technologies (Images: Mazda)



Engine performance

The fuel economy achieved in the gasoline SPCCI engine realizes fuel consumption performance similar to the latest diesel engines under development. The fuel economy achieved in the 2.0L SKYACTIV-X SPCCI engine is believed to be superior to that of the current SKYACTIV-D engine. However, like the current engine, fuel economy decreases significantly when the engine is run at very low rpm loads. If this technology is applied to hybrid vehicles with a 48V system that include an integrated starter-generator (ISG) system, further improvements in the fuel economy of hybrid vehicles are likely to be achieved.

By adopting a supercharger, Mazda aims to improve the SPCCI engine output performance by about 20% versus the current engine. Knocking is said to be greatly improved by the adoption of a supercharger and cooled EGR technology.

エンジン性能 エンジン性能
Fig. 4.1 Engine Performance (Source: Mazda)
Left-side chart: (horizontal axis) Engine load, (vertical axis) Fuel consumption
Right-side chart: (horizontal axis) Engine speed, (vertical axis) Torque

Mazda, engine, SKYACTIV, HCCI, SPCCI

<Automotive Industry Portal MarkLines>