This is misleading. The article doesn't claim a specific thermal efficiency anywhere. Not sure why the title of this post says >50%.

Also, the engine itself is not achieving >50% thermal efficiency. The propulsion system is a hybrid electric / ICE unit that is designed inside and out to be able to recover potential energy in a variety of ways, and then turn that back into kinetic energy that propels the car.

So to say that the "engine" technology enables ">50% thermal efficiency" is completely untrue.

Now, to look at the merits of the article. The specific technology that they are claiming to have advanced here is "Rapid Combusion" and "Lean Boost." Rapid combustion refers to the layout of the fuel injectors in relation to the spark plug and the fuel injector timing. In early SPFI (Squential port fuel injection) vehicles and earlier, air was pre-prepared and fully mixed with an adequate amount fuel before it entered the combustion chamber. This mixture enters the combustion chamber and is ignited by the spark plug. The flame front propagates away from the spark plug until the (air within the ) mixture is consumed.

What Honda is doing now is injecting only a small amount of fuel into the combustion chamber up front. This small amount of fuel is ignited by the spark plug, and then the rest of the fuel is injected into the cylinder. This enables Honda to use another technique called "lean boost" that has been around in diesel engine technology for decades.

By initiating combustion with a small amount of fuel, the flame front propagates faster and burns hotter. The added intensity of the burn means that subsequent fuel injected into the cylinder will self-ignite without any further spark from the spark plug. These two flame fronts essentially "meet in the middle" of the combustion chamber. Tech geeks can think of the performance improvement of having two concurrent flame fronts propagating simultaneously as an improvement similar to the data transfering out of a RAID 0 array vs a single drive.

Because the fuel mixture is injected directly into the combustion chamber, (instead of being mixed outside and then pushed in like SPFI) pre-detonation is virtually impossible. Pre-detonation is avoided in passenger cars by injecting more fuel into the combustion chamber than what is required. This additional fuel doesn't carry additional power, but it makes the mixture robust enough to withstand high temperature and pressure long enough for the spark plug to ignite the mixture in a controlled way. In an F1 car, wasting fuel for this purpose is not acceptable. This is where "lean boost" comes into play.

In a passenger car, the fuel mixture is already in the combustion chamber when the piston begins it's compression stroke. So the mixture must be robust enough to withstand being compressed by the piston. In an F1 car, the fuel is absent until the piston has reached the very top of it's compression stroke. So in an F1 car, the fuel mixture isn't exposed to high temperature or pressure until the very moment it is expected to burn. As a result it is possible to inject a lesser amount of fuel than what is ideal. The ideal mixture of air:fuel for petrol is 14.7:1. In a passenger car this is a little bit lighter, say 13.5:1 to avoid pre-detonation. In an F1 car they go the other way say 15.5:1, because it saves fuel and there is no risk of pre-detonation due to other design factors.