It's no secret that we're fans of using the racetrack to improve road car technology here at Ars. It's also no secret that we believe the discipline of endurance racing (Le Mans and the like) to have far more relevance to making our road cars better than Formula 1. But it would be incorrect to say that no such tech transfer happens within the ultra-specialized world of F1. And a perfect example of that is a clever engine development being used by Mercedes-Benz and Ferrari that's pushing the envelope of energy efficiency. It's called turbulent jet ignition (TJI), and not only does it do wonders for fuel efficiency, it also results in a cleaner exhaust.

As you probably know, gasoline engines combust fuel with air within each cylinder, and that combustion moves the pistons—and therefore the crankshaft, powering the car. But most of the energy released during combustion is wasted as heat. In fact, the average road car engine wastes between 70 to 75 percent, meaning its thermal efficiency is around 25 to 30 percent. That comes down to the way that the fuel combusts after it's injected into the cylinder, which normally happens around the center of the cylinder by the spark plug (the bit that ignites the mixture). If you can control ignition so that it happens more homogeneously throughout the cylinder, with more air per given amount of fuel (i.e. a leaner burn), less energy is wasted as heat and more of it is converted to work.

But this process can be improved. Take Toyota's latest generation of Prius hybrids, for example. These cars use what's known as an Atkinson-cycle (most engines work via the Otto-cycle). The current Prius engine is supposed to have a thermal efficiency of 40 percent, which is quite an achievement. But there are other options, too, like a technology that's already used in some road cars called direct injection. Rather than traditional fuel injection, which squirts fuel into the engine upstream of the cylinder in the intake port (the bit that the air gets sucked through on its way from the outside of the car into the engine), direct injection uses a high-pressure system to add the fuel into the cylinder itself. This makes it possible to more accurately control the fuel-air mix, whether that's to achieve a stoichiometric air-fuel ratio (14.7:1) or even an ultra-lean mix (useful when cruising with the engine under a light load).

Beyond direct injection and Atkinson-cycle engines, there are some other neat ways of wasting less fuel to heat—homogenous charge compression ignition (HCCI), for one. Several years ago, we delved into this tech in depth, which does away with the spark plug altogether, instead relying on the compression of the fuel-air mix inside the cylinder to cause combustion, similar to the way things happen inside a diesel engine. But HCCI has a few problems, notably the lack of control over ignition at higher engine temperatures.

Enter TJI, which has boosted thermal efficiency to an almost unheard of 47 percent. As far as we know, TJI was first adopted by Mercedes-Benz's F1 team in 2014, when the sport moved from naturally aspirated V8s to turbocharged V6s. At the time, journalists were scratching their heads over Mercedes-Benz's secret, wondering whether HCCI was part of the team's massive advantage over its rivals. (HCCI was denied by Mercedes-Benz at the time.) But one team's advantage rarely remains exclusive, and a mid-season upgrade to Ferrari's engines in 2015 can now be explained as that team's adoption of TJI, a fact that was confirmed recently by Mahle (which supplies Ferrari with its pistons and which also used to supply Mercedes-Benz).

Mahle actually started work on TJI back in 2010, according to the company's latest annual report, which quotes head of Mahle Motorsport Fred Türk:

"The Mahle Motorsports engineers have found the right solution for Scuderia Ferrari. More than five years of development preceded that crucial phone call to Maranello with the proposal: ‘We’ve found an interesting new solution for you.’ Mahle Jet Ignition is the name of the innovation that gives the engines from Maranello a boost. Within a few weeks, in spring 2015, we adapted our solution to the Formula 1 requirements, allowing Ferrari to compete in Canada with this solution for the first time."

The traditional spark plug is replaced with a jet ignition chamber. Most of the fuel (97 percent) is injected as normal into the cylinder when the piston is about 60 degrees away from top-dead-center. (Top-dead-center is when the piston is at the very top of its travel within the cylinder). But within the jet ignition device, there's a small pre-chamber that includes a spark plug, and it's into this pre-chamber that the remaining three percent of the fuel is added and then ignited, when the piston is at 22 degrees from top-dead-center. At this point it becomes a high pressure jet of hot gases that enter the cylinder via a series of tiny holes, igniting the rest of the (now well-mixed) fuel and air.

This means you can have a fuel-rich mixture in the pre-chamber but a lean mix in the cylinder, and the leaner the main combustion event, the more efficient the engine. Or, as Mahle describes in its annual report:

"this results in the formation of plasma jets that reach the piston primarily at the outer edge and ignite the remainder of the mixture. While ignition normally takes place in the centre of the cylinder, with Mahle Jet Ignition it essentially takes place from the outside toward the inside. This allows significantly better combustion of the fuel mixture. The result: more power with considerably less residue."

Clever stuff, indeed, and something that we expect to start appearing in road car engines before too long.