Skyactiv X with Auto Motor und Sport Technology Award

Mazda cutting edge engine wins media founder Paul Pietsch Award

Every year, auto motor und sport media presents the Paul Pitch International Technology Innovation Award. At a time when electric mobility is increasingly seen as a replacement for an internal combustion engine, the Paul Pietsch 2020 award was awarded for just such a heat engine. However, it has an avant-garde character. No other company has achieved such a combination of the formation of a homogeneous mixture, as in a gasoline engine, and self-ignition, as in a diesel engine, with the advantages of both types of engines in a production model. This is an occasion to tell you again about how this device works.

With petrol injection pressure like in a diesel engine, spark plug ignition, self-ignition, "λ" that is constantly changing, Skyactiv X is truly a revolution in the automotive industry.

Mazda's developments in the HCCI engine industry span more than 30 years and are largely based on an extremely in-depth analysis of fuel processes in the development of the Wankel engine. Several generations of engineers are trained on this basis, which creates many headaches and problems, but also brings a lot of experience.

It was in the depths of the rotary engine that the first prototypes of machines with homogeneous mixing and self-ignition were found. The Wankel engine also serves as a platform for the development of various turbo-related technologies - this is the RX-7, which introduces elementary VNT turbochargers, twin-jet turbines and cascade refueling in a gasoline engine that was used only by Porsche.

Secret materials

However, the direct basis of the current Skyactiv X is the already proven new generation of petrol machines Skyactiv G and Skyactiv D. If you look at the solutions presented in these devices, you will inevitably find that they are to some extent “realized”. “In the new SPCCI plant, from experience gained from analyzing combustion chambers to flow turbulence.

According to this hypothesis, the Skyactiv X is more efficient than the 2ZR-FXE gasoline engine powered by the Toyota Prius (using the Atkinson cycle) by 39 percent, but Mazda itself is aware that this high point is not the most important. Most of the time the engine runs at part load and the average efficiency of a gasoline engine drops dramatically. Due to the fact that in most cases Skyactiv X operates with a wide open butterfly valve, pump losses are significantly reduced and average efficiency is increased. This, combined with a high compression ratio, results in a joint increase in efficiency.

A great achievement of Mazda engineers is the fact that their Skyactiv X operates in a uniform mixture and self-ignition mode over a very wide range of speeds and loads. In practice, it combines the processes used not only in diesel and gasoline engines, but also similar processes in gas-diesel engines and gasoline engines with lean mixtures. In the latter, normal and bad sections are also formed, but unlike them, where the process occurs completely with the flash front, in the case of Mazda, the poor mixture spontaneously ignites with the help of a spark plug.

What is happening in Skyactiv X? All experimental engines operating on the basis of the HCCI regime created so far are based on very complex self-ignition control (based on heat and pressure during compression and preliminary chemical reactions between fuels, gases, and air) with unstable operating parameters in a number of modes. to normal engine operation. The Mazda engine always uses a spark plug as a combustion initiator. However, the difference from the normal operation of the gasoline engine is in subsequent events. Thanks to this, the transition to different modes becomes much more balanced, and this method of control in HCCI mode leads to a steady and stable process.

Things in theory

Skyactiv X is based on a four-cylinder, two-liter Skyactiv G with atmospheric filling, which in itself is a good foundation with high efficiency. In addition, it has a working volume of 0,5 liters per cylinder, optimal in terms of the speed of combustion processes. To create conditions for working in HCCI mode, the geometric compression ratio was increased to 16,3: 1. Thus, the mixture thickens to a temperature close to the self-ignition temperature of most fractions in gasoline with an average octane rating of 95H and normal engine operating temperature.

Based on data from several sensors, among which four pressure sensors on each cylinder are key, the computer decides which mode of operation to select. The latter is determined on the basis of several functional zones, depending on the speed and load (in other words, the degree of depression of the accelerator pedal) of the engine. With the help of a special swirl module called SCV (including a special air control valve in one of the intake ports), an intense turbulent flow is created around the cylinder axis. Depending on conditions and based on a comparison of the compression and combustion pressure build-up curves, as well as many other parameters in predefined "maps", the multi-port injector injects fuel at pressures approaching those of the first generations of common rail diesel systems. - from 300 to 1200 bar - in several portions. This is done from one long pulse (in a normal flaring process) to several pulses during the intake and compression stroke (in self-ignition operation). Obviously, the record injection pressure for a gasoline engine is also a key element in the formation of the mixture. However, a logical question arises - how will the entire set of parameters change if and when to switch to lower engine power and turbocharging, with an increase in cylinder pressure, as well as with the need to increase fuel portions ...

Everything happens faster.

Mazda's SPCCI patent is 44 pages long and details that the car runs in spark plug auto-ignition (SPCCI) mode for a significant portion of the time. The control is based on several types of SPCCI self-ignition modes during its operation - one with a mostly poor mixture, a mostly normal mixture, and a slightly rich mixture. In all cases, the injection and swirl configuration creates layers of different composition (stratification) concentrically around the axis, with a richer inner zone (air:fuel ratio about 14,7-20:1) and a leaner outer zone (35). -50:1). The internal has sufficient "flammability", and the external has reached almost the critical temperature for self-ignition near the top dead center of the piston during compression. The spark of the spark plug initiates the ignition of the inner zone, causing the temperature and pressure to rise sharply, and this causes the others to spontaneously ignite at the same time. Since there is no flash front, it occurs at temperatures below the threshold for the formation of nitrogen oxides, which drastically reduces the presence of nitrogen oxides, and a weak homogeneous mixture provides more complete combustion and very low levels of particulate matter, carbon monoxide and hydrocarbons.

Depending on operating conditions – such as medium speed and high load, and in all cases at high speed – the mechanical compressor kicks in to help provide more air and further deplete the mixture. Although its purpose is not to increase power, it contributes to the good dynamic qualities of the car. The patent also mentions that the car could be turbocharged, and logically, the lower exhaust temperature could allow the use of a variable geometry turbine. For now, however, control with a more responsive mechanical compressor has become easier (provided such a definition is compatible with Skyactiv X). According to Mazda engineers, the use of a turbocharger may come at a later stage.

It is important to note that they managed to create something that no one else has been able to do - at least not in serial form. Many sensor parameters are compared to preset behaviors for mode selection, but the fact is that in practice the "SPCCI Mode" sign is shown on the Mazda display most of the time, even at very low and very high rpm ranges - even at very low rpm Mazda3 smoothly moving in sixth gear.

How does this happen in real life?

After such a long theoretical part, the time has finally come to answer the question - what does all this lead to in practice in the end. Like the petrol counterpart, the car picks up speed easily and responds quickly. During tests, including climbs and turns in the Iskar Gorge, normal intercity and highway mode, Mazda 3 Skyactiv X maintains its consumption in the range of about 5,2 l / 100 km. The average test consumption achieved by colleagues in Germany is 6,6 l / 100 km, but this also includes high-speed driving. In an economical driving test, they achieve 5,4 l/100 km, which is 124 g/100 km CO2, which is on par with the Audi A3 2.0 TDI, BMW 118d and Mercedes A 200d. However, it should be noted that despite the complex operation process, this machine does not need complex gas treatment technologies, but, on the other hand, the very high pressure injection system increases its cost. On the other hand, a small mechanical compressor is cheaper than a turbocharger, so it should be positioned as a price between diesel and gasoline engines.

The engine is in harmony with the dynamic character of the Mazda 3 and its good settings for pleasant cornering. The steering is precisely laid out, and the car maintains a neutral demeanor, showing a tendency to turn the rear wheels only at sharp provocations. Added to this is a good mix of assistance systems and equipment, which at Mazda are part of the equipment at various levels. We have already talked enough about the new ergonomic composition of the control. The functions are not controlled by the monitor and are simple and convenient to operate. Overall, the interior has a subtle sense of lightness and quality that was only found in luxury models many years ago. In short - Skyactiv X works - and it really turns you on.