At the height of Group B madness, Lancia's efforts in the field of twincharging paid off. With Group S on their sights, Lancia evolved the design and tried to solve some of the issues of the supercharger- turbocharger combo, developed the Triflux cylinder head.
The issues with Forced Induction
Before explaining the advantages of the Triflux engine, we need to have a little background on how engines work. As many of you may know, a greater compression ratio means higher thermal efficiency. Forced induction also helps dramatically the efficiency and provides greater power output.
The issue with these solutions is the increasing the chance of pre-ignition occurring (commonly referred to as engine knock). This spontaneous combustion of the air-fuel mixture occurs due to high pressure and temperatures inside the chamber.
The starting point
The twincharged engine in the Delta S4 was an interesting piece of design. A supercharger provides a low rpm boost until the engine speed produces enough exhaust gas to overcome the rotational inertia of a large turbocharger, then the turbocharger generates boost on the high rpm range. Smart, right?
But there's a caveat the supercharger is powered directly from the crankshaft, which reduces the overall efficiency of the powerplant. From an efficiency standpoint, a setup with two turbochargers would be far more desirable.
How they did it...
It would be easily achievable with today's technology but bear with me. In the 80s how could we install a couple of independent turbos in an inline-4, with high compression ratio, while avoiding engine knock and maintaining a modular design with a minimum power output in the neighbourhood of 600 bhp?
Just rearrange the valve placement in a crossed pattern.
Below we can see the difference in the configuration viewed from the top of the engine.
With this configuration, we have an exhaust manifold on each side of the engine block, allowing to install a turbocharger group per side, with the ability to function independently gives you three air flows, two from exhaust one intake that's the reason why it´s named Triflux.
The setup used a small turbo that can spool up quickly and generate boost at low rpm, on the high rpm range a larger turbo kicks in.
The main thermal advantage is the uniformization of the temperatures in the engine. With a conventional cylinder head, the engine will always have a cold side (intake valves) and a hot side (exhaust valves ) that translates to an uneven temperature along the combustion chamber and the surface of the piston. these let's call them "hot spots", combined with the pressure caused by the compression stroke and the forced induction are the cause of engine knock. Even with high octane race fuels if the conditions are right it will occur. Having an even temperature throughout the combustion chamber, those chances are greatly reduced, which leaves more headroom for a higher compression ratio or boost figure. Another Advantage of this uniform temperature is a more uniform dilation of components, reducing wear due to thermal cycles, especially on areas around gaskets and seals.
To sum up, the new design allowed fully modular turbocharging, had better thermal performance, it was more compact and could deliver more power in an linear fashion.
The Triflux engine found its home in the ECV project, an experimental composite evolution of the Lancia Delta S4.
Sadly the development of the ECV project was halted by the sudden end of Group B and Group S. The five Triflux engines built were Shelved due to its very specific application. Lancia was once truly great, the end of this amazing piece of engineering was unfortunate , perhaps someday, someone will Make Lancia Great Again.