Mazda’s SPCCI Engine

Japanese automaker Mazda has introduced a revolutionary IC engine dubbed as SKYACTIV-X. It uses Spark Controlled Compression Ignition (SPCCI) technology. It is similar to HCCI engine, but it has a spark plug. With this, Skyactiv-X is set to become the world’s first commercial gasoline engine to use compression ignition.

While Diesel engines use compression ignition, and gasoline engines use spark plug to ignite, the SPCCI works as a hybrid between the two. It first compresses the air- fuel mixture to the extent of ignition and then ignites it using spark plug. The expanding fireball acts as an ‘air piston’ creating pressure waves which travel down the cylinder, igniting the air fuel mixture, thus rapid and simultaneous combustion happens at multiple points. This pushes the cylinder down with more power and piston travels more distance which ensures efficient combustion. The air- fuel ratio used is so lean that it cannot be ignited by spark only, hence the compression.This results in better fuel efficiency, more power and better acceleration. The SPCCI engine utilizes a more lean fuel than gasoline engines (29.4 :1 ratio). Lean combustion is what makes for efficiency. Less fuel in the same amount of air means less fuel burned. Lean burn also reduces combustion temperatures, which reduces heat transfer and cooling loss. It also increases the amount of air for a given level of torque, reducing loss from throttle closure, called pumping loss.

Thus, SPCCI engine harnesses the undesired effect of pre-ignition (knocking)to its benefit, eliminating the audible effect of knocking, and maximizing the zone of combustion. Mazda calls it a “good knock” which occurs during the power stroke. The spark plug gives the benefit of controlling the timing of combustion and thus controls detonation. While Diesel engines are more efficient, they struggle in the high RPM, because diesel burns slower than gasoline. But the SKYACTIV-X merges the diesel engine’s compression ignition, its efficiency and low-RPM grunt, with the gas engine’s high-RPM responsiveness, giving greater RPM range. Also, SPCCI engine can seamlessly switch between compression and spark ignition as per working conditions.

Mazda’s new engine also has a different use of a supercharger. Usually, a supercharger packs extra air into combustion chambers so the engine can mix it with extra fuel, for the sake of more power. In the SKYACTIV-X, it’s there not to add power but to keep pushing in enough air at high RPM to maintain an ultra-lean fuel-air ratio. While Mazda calls its commitment to reduce CO2 emissions, it has some shortcomings as well.

Challenges
• IC engines have an uncertain future due to depletion of natural resources like oil and fossil fuels. India has already planned to have only EVs sold after 2030.
• Timing issues for combustion may develop overtime.
• It is too expensive.

With this, its initial tests gave a 15% increase in mileage and more responsiveness.

It’s a fact that IC engines are going to be around for a few decades and IC engine technology is not a relic past, but a living developing technology. With companies like Infiniti coming up with variable compression engines, Tesla with electric vehicles and Toyota with Hydrogen fuel cell engine technology, the automobile industry does have a promising and competitive future ahead.

Rajat Shukla
17ME158