The innovators: the first compression ignition petrol engine

First car with a compression ignition petrol engine The cars we drive today were influenced by these pioneers

Conversations about modern automotive powertrain technologies frequently lean towards the latest developments in electric motors; a hybrid system or hydrogen fuel cells may get a mention. General consensus is that the future of motoring no longer lies with the internal combustion engine (ICE). Despite such innovations as forced induction, fuel injection and improvements in the fields of thermal efficiency, the ICE has essentially remained a metal box of explosive cylinders and dancing pistons since the late-1800s. Many manufacturers may perceive the petrol engine as a developmental dead-end but that’s not to say the latest innovations are consigned to the mists of time.

The most recent gasoline engine “first” can be traced to Mazda’s remarkable Skyactiv-x unit, which made its production debut on the current Mazda3 as recently as 2019. This engine merges gasoline and diesel technologies to harness the benefits of HCCI (homogenous charge compression ignition) where the engine’s air-fuel mixture is ignited by the piston’s compression stroke.

Diesel engines use HCCI, compressing an atomised airfuel mixture into the combustion chamber to induce ignition, as opposed to the spark plug-actuated ignition used by petrol engines. This capitalises on the piston’s full range of motion by igniting the mixture at the top of the cylinder’s travel in the chamber.

Skyactiv-x builds on this by running at a compression ratio of 16:1, allowing it to operate on a leaner fuel ratio than a standard petrol engine. Although diesels have eliminated the need for spark plugs, the Skyactiv-x unit’s process is supplemented by an intelligent spark plug system called Spark Controlled Compression Ignition (SCCI) that operates on a sometimes-on, sometimes-off principle.

It uses a small Roots-type supercharger to draw in an ultra-lean air-fuel mixture and compresses it to the point of near-spontaneous ignition just before a second injector adds another charge of fuel directly to the spark plug. This ignition raises the pressure in the cylinder and the remainder of the air-fuel mixture combusts. The intelligent combustion management system allows the Skyactiv-x unit to take advantage of HCCI when conditions are optimal and standard ignition when they aren’t. It also lessens the impact of detonation or knock – the erratic combustion of fuel within the chamber that’s often a byproduct of high compression – that can potentially damage the block, spark plugs and head gaskets.

The upshot of this lean-burn technology is a 20 to 30% improvement in fuel efficiency and fewer harmful emissions, ensuring the engine retains the top-end power advantage over its diesel equivalents.