Is there still a future for the ICE? Where most developers are focussing on alternative solutions to power their vehicles while complying with laws and regulations, Mazda’s newly developed SkyActiv-X brings the internal combustion engine to a whole new level. Sort of. 

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Mazda found a rather clever way of utilising the properties of a compression ignition engine to extract every bit of stored energy out of petrol. Where as regular compression ignition (CI) engines run on diesel, Mazda squared the circle and enabled it to run on petrol. The result of this is the SkyActiv-X, a petrol engine with all the benefits of a modern diesel engine. The theoretical efficiency of an ICE is proportional to its compression ratio (keep this in mind for the rest of the article). That is one of the reasons why diesel engines deliver so much torque. However, merely increasing the compression ratio is not directly applicable to a petrol engine. 

CI engines as we know them

Perhaps you’re not really into engineering and physics. No worries, this article will get way less interesting then. A regular diesel engine works by compressing the air in the combustion chamber when the piston moves upwards. Thermodynamics show a relation between pressure, volume and temperature. The rather complex diagram in the image below shows the increase in pressure, when the camber’s volume decreases. When the air in the combustion chamber is compressed, the temperature rises. Due to the high compression ratio it is able to ignite the diesel that is injected into the cylinder. The rapid expansion of the combustibles, or explosion, pushes the piston down and delivers you torque at the crankshaft. No additional spark is needed in this process. But, if the CI engine already exists, what makes Mazda’s SkyActiv-X so advanced?

Knock Knock!

The thing is, with petrol you are limited by the knock resistance of the fuel. Engine knock is an unintentional pre-combustion at the wrong time in the cycle. Engine knock stresses certain vital components in a way they are not designed to withstand stress. By increasing the compression ratio, engine knock is more likely to occur. In theory you can run a petrol compression ignition engine, but due to the fine line between controlled combustion and knock, it is very hard to control engine knock over the rev range. Keep in mind that engines also operate under various loads. Higher octane fuels are more resistant to knock, but that high number doesn’t rule out the fact that petrol CI engines operate within such fine tolerances that make them unsuited for a car. In short: the reason why a regular CI engine doesn’t work with petrol is because of engine knock. 

The solution

Mazda’s SkyActiv-X engine is revolutionary because it actually makes use of engine knock. In contrast to a diesel engine, the SkyActiv-X engine has a spark plug. However, that does not operate in the same way as it does in a regular petrol engine.

Here’s the thing that makes the SkyActiv-X engine so clever: by running an unheard of high compression ratio (16:1), the piston compresses the mixture to right before the point at which it ignites. Then, a few degrees before top dead centre, a small amount of fuel is sprayed into the combustion chamber next to the spark plug. The spark plug ignites that fuel and, when exploded, it creates a shockwave throughout the cylinder and ignites the rest of the mixture at once. There is your diesel characteristic. In summary, the SkyActiv-X controls engine knock with a spark plug and makes petrol CI technology suitable for automotive purposes. 

What’s remarkable is that because the operation is based on the principle of knock, the engine performs excellent at low octane fuel, fuel that is less resistant to knock. The high compression ratio at which this engine operates, combined with a supercharger, delivers the power you want. It does so while running a rather lean air-fuel mixture and therefore low fuel consumption. Mazda proves the ICE isn’t dead yet, thank you Mazda for helping us petrolheads out.