There are diesel engines and there are gasoline engines. Everyone knows. Two different types of fuel, different technology, different advantages. Even if you add CNG and LPG, they are only subgroups of the “gasoline engine”, the spark ignition engine. The diesel, on the other hand, is a compression-ignition engine. From a purely technical point of view, it has always been an engineer's dream to combine the best results from both worlds. But many manufacturers have already failed because of this. A company like Mercedes-Benz had also tried the “diesotto” and stopped the experiments. But in 2019 the time has come - a small Japanese automobile brand will present the first “gasoline-powered diesel” in series.

Skyactiv-X - Mazda's answer to burning questions

Test drive in the prototype

The prototype stands inconspicuously in matt black paint under the Portuguese sun. It looks like a Mazda 3. And the sheet metal dress is also borrowed from the current Mazda3. Under the sheet, however, there is already a large portion of the successor, which will be launched on the market at the beginning of 2019. And especially important, there are 5 of the 6 existing prototypes with Skyactiv-X technology under the hood that have been lined up here to give journalists an impression of the new technology. A technology that Mazda promises no less than 20-30% more efficiency - thus easily on par with Mazda's current diesel engines - plus driving fun over a wide speed range and, particularly important in the current times, as clean as hardly any other engine. The Skyactiv-X engines will undercut the upcoming EU limit values ​​by a factor of 10. And not just on the test bench, but in the so-called RDE test, i.e. on the road. To achieve this goal, Mazda has opened a whole new chapter on the engine side.

Autoignition by spark ignition

The way the engineers went at Mazda is remarkable. But first, a brief digression into fuel-burning technology should help sort out the progress Mazda has made. In order to burn petrol, the molecules of the fuel and the intake air have to interact with each other. The relationship between gasoline and oxygen is called combustion air ratio or lambda. The ideal ratio for all gasoline atoms to be burned is called Lambda 1.0

Lambda 1 corresponds to 1 kg of petrol and (1.34 liters) and 14.7 kg of breathing air. This ratio, lambda 1, is also known as the stoichiometric combustion mixture and means that the fuel / oxygen mixture is completely burned. That is the ideal case. Theoretically. However, modern engines also use a rich mixture during combustion to increase output or to cool the combustion chamber - in the ideal case, however, an excess of air would be desirable. When more oxygen is available than fuel, it is called a lean mixture. Conversely, if you achieve the same performance with a lean mixture as with a lambda 1 ratio, then this means that you need less fuel for the same work. So the car is more economical. However, a lean mixture actually also has the effect of reduced engine performance. On the other hand, in the case of lean combustion, there is the lower combustion chamber temperature, which is associated with higher efficiency and significantly reduces the development of NOx. The secret of clean combustion lies in the air mixture.

The lean burn, however, has other problems. As soon as too many oxygen molecules are present, there is a risk that not all of the mixture can be ignited at the desired time. In a self-ignition by high compression, this problem could be avoided. Because then the momentum of combustion no longer starts from the centrally located spark plug.

So what is more obvious than burning a lean mixture by auto-ignition?

And that's exactly what all others have failed to do so far. The new approach of the Skyactiv-X technology combines compression ignition with external ignition, enables an air-fuel mixture well above lambda 1 and still prevents uncontrolled compression ignition (knocking) of gasoline. This success is made possible by two factors: on the one hand, modern engine electronics are only now able to control the combustion for each combustion chamber precisely and live, and on the other hand, this is the Mazda trick, by using a spark plug one bypasses the effect a hardly controllable ignition point. That may be confusing at first. A “self-igniter” with a spark plug?

SPCCI - A breakthrough in engine technology

The extremely lean gasoline-air mixture is compressed in a ratio of 16 to 1 and is about to auto-ignite. The combustion process is initiated with a small additional injection process directly in front of the spark plug and ignition of the plug. However, the effect is not that the ignition “spills over” to the fuel molecules, as would be the case. Rather, through the combustion of the small portioning in front of the spark plug and the resulting expansion in the combustion chamber, the compression pressure is increased precisely and the remaining lean mixture in the engine compartment ignites itself. BÄMM. 

What needs several hundred words to explain this happens in the engine of the new Skyactiv-X generation within milliseconds. From the entire effort the driver gets nothing. Nothing at all. Mazda promises a gasoline engine that achieves a whole new level of efficiency. Due to the complex engine control, the new Skayctiv-X engine is able to drive in an extremely wide usable speed range with a Lambdagemisch larger 1. A lean burn, clean, leads to an efficiency on the level of modern diesel engines. Without NOx problems. Without CO2 problems. But with the revving and the agility of a modern gasoline engine.

Drives agile

Unlike current engines with lean stratified charge, keyword Atkinson cycle (Miller cycle), the Skyactiv-X engine is not tied to a narrow speed range in order to be as efficient as possible. This leads to a completely new level of agility when driving. The wide speed range, in which, thanks to SPCCI technology, can be driven with a lean mixture, allows the Mazda3 prototype to weave nimbly and revvingly over country roads. With the targeted 190 PS and 230 Nm, the Skyactiv-X motor is superior to the current Skyactiv-G generation in terms of performance - but still not impressively massive. The compressor charging of the Skyactiv-X does not serve to increase performance, but only to enable a maximum excess of air. The compressor is therefore not active in the low speed range. One could probably have operated with an e-turbo, but the first step was to use the proven compressor.

The wide usable speed range makes the increasingly complex gears superfluous. An engine that is just as efficient at 1.900 revs as it is at 3.500 revs can safely be sent on a journey with six gears - without having to accept any disadvantages in consumption.

The driver does not feel anything from the “externally controlled” compression ignition. Only a faint ringing from a not fully controlled combustion at very low engine speeds occasionally penetrates the ear during the 1.5 hour test drive. Otherwise, the Skyactiv-X and its new chassis platform impress with massively increased noise comfort, clean, attractive damping and sensitive steering.

The new Mazda3 could therefore, when it comes to 2019 on the market, with Skyactiv-X technology, agility and driving pleasure still a real bright spot for the supporters of internal combustion engines.

Consciously different

With the Skyactiv-X technology, Mazda is deliberately taking a different path than the rest of the automotive industry. You may think this is a mistake when you see the efforts other manufacturers are making to convert their fleets to plug-in hybrid technology. But Mazda does not yet believe in the end of the internal combustion engine and is even at the forefront of engine development with the new technology. There are many good arguments why Mazda could be right about this. A classic combustion engine could take the lead in the well-to-wheel balance if its fuel were CO2 neutral. This is not classic gasoline. But Mazda is also working with universities in Japan on an algae gasoline technology. So artificial gasoline. And of course you are preparing an electric car for 2019, because you simply cannot do without it. Even if only for political reasons.