Koenigsegg va plus en détail sur la façon dont Freevalve aide aux démarrages à froid, en particulier lors de l'utilisation de carburants à base d'alcool (un grand argument de vente de cet hybride est son respect de l'environnement, y compris sa capacité à utiliser des «carburants à base d'alcool renouvelables»), qui sont apparemment plus difficiles à vaporiser que l'essence et qui nécessitent généralement de mélanger de l'essence pour obtenir une bonne manivelle froide. Plus précisément, la société mentionne la recirculation interne des gaz d'échappement (ouverture de la soupape d'échappement pour aspirer les gaz d'échappement chauds dans le cylindre pendant la course d'admission), la possibilité de modifier le taux de compression, l'actionnement spécial de la soupape pour faciliter le mélange et un «mode de chauffage» qui démarre le moteur plusieurs fois pour utiliser le piston comme pompe pour augmenter la température de l'air d'admission:
Le système Freevalve surmonte ce problème de démarrage à froid en utilisant un mode de chauffage de pré-démarrage pour le démarrage initial et la première combustion. Une soupape d'admission est utilisée et ouverte tardivement avec une faible levée pour une turbulence et une vaporisation maximales. La désactivation du cylindre est appliquée pour augmenter la quantité d'air et de carburant dans chaque combustion pendant le démarrage et le réchauffement. Internal exhaust gas recirculation (EGR) is activated in the following combustion cycles, using hot residuals from the previous combustion to vaporize the fuel. Lastly, the engine’s variable compression ratio is adjusted by the Miller cycle.
Furthermore, the TFG can be turned multiple times before turning on the ignition, using the piston to pump the air back and forth to the inlet in a “Heating Mode” cycle. The result – the inlet air temperature is increased by 30˚ C in 10 cycles (taking about 2 seconds) and without need to add petrol to the fuel or starting cycle anymore, making the TFG fossil fuel independent in any climate.
It might seem a bit odd that a 1,700 horsepower car is using the Miller cycle and worrying about warming up catalysts, but the reality is that even supercar manufacturers have to pass emissions regulations. Not to mention, it’s 2020, and buyers of high-end cars expect more than just fire-breathing big-block gas engines. Clean is now cool.
Jason Fenske from Engineering explains breaks down Freevalve very nicely with his video, showing the pneumatic method of actuating valves, and how hydraulic oil is used to stabilize a certain valve position and to damp its motion:
Frank Markus from Motor Trend breaks down the mechanical bits simply, writing:
…compressed air opens the valve almost instantly, electronically controlled hydraulic pressure holds it open, a coil spring closes it, and passive hydraulic pressure cushions its “landing.” An electric coil provides highly precise sensing of each valve’s position.
Markus also spoke with Freevalve marketing director Andreas Möller to learn about the power draw of Freevalve, which is a significant drawback:
The second major concern is the one that generally doomed these systems in the past: energy draw. Möller says that although the compressor (featuring standard AC-compressor innards capable of peak pressures near 300 psi) generally consumes more energy than camshafts, this is partially offset by a reduction of the engine oil flow and pressure required to lubricate cams and valves. (Freevalve’s head employs a dedicated hydraulic circuit.) And Freevalve’s high-speed friction penalty is countered by the fact that at idle and lower speeds its parasitic losses undercut those of several VVT systems.
I reached out to some auto engineers to get their opinions on Koenigsegg’s Freevalve technology. One engineer, who used to work for a major supplier on timing drives, admitted that he may have some concerns about reliability and cost, but on the plus side, the system could be more forgiving if timing is off. (Indeed, Motor Trend confirmed that a piston will not break the valve if timing is off.) He also mentioned other benefits of ditching all those valvetrain parts, writing about rotational inertia in his email:
It greatly simplifies engine design since you don’t have account for a timing drive and the associated resonance dynamics it brings with it. I don’t know how it compares weight-wise to a timing drive, but it certainly has a lot less rotating inertia to worry about (no cams, chains, or heavy vct units to worry about, so it can react quicker).
A powertrain R&D engineer at a major OEM who asked not to be named also gave me his thoughts, saying:
There are certainly a lot of benefits to having a flexible valvetrain, like the ability to …retain internal exhaust gas residuals for intake charge heating and dilution (NOx reduction & de-throttling), while still having cam profiles that can trap a lot of air at high loads while keeping the intake open late to give up some compression ratio for high maximum power and knock reduction. Turning off one of the intake valves for increased swirl is also a well-known idea that has been implemented in production as well. So, fundamentally the concept and idea is sound.
None of this is new though; this is the motivation behind adding cam phasers, Honda’s VTEC (yo), BMW’s Valvetronic, FCA’s Multi-Air, and all sorts of switchable cam profiles, slide-cam, switchable roller-finger-followers etc. Of all of these systems, most of them are discrete, having two or three defined positions and profiles instead of a continuously variable range (Cam phasing, Valvetronic and Multi-Air are continuous mechanisms in one dimension – they only affect phasing, not lift profile, or they affect both but in a well-defined relationship). Most manufacturers stick to discrete systems or limit the number of continuous dimensions for three reasons:
First, its cheaper to develop and produce discrete-step mechanisms.
Second, it’s much easier to diagnose discrete-step mechanisms, specifically for OBD-II requirements. For the mechanism to be OBD-compliant, there needs to be some sort of feedback to the computer, either through an internal model or an actual sensor, to determine whether the mechanism is working as-intended since it’s proper operation is necessary to maintain emissions compliance. A continuous-action mechanism generally needs a much more complicated and expensive sensing setup to ensure that it is working as intended.
Third, if you choose the discrete valve profiles and cam phasing angles well, you can get 99% of the same benefits as a fully flexible valvetrain. Even with advanced combustion modes like HCCI and SPCCI that rely heavily on internal residuals and precise valvetrain control, conventional systems are still capable of performing well.
He went on, saying that, while Freevalve should allow for faster opening and closing rates at lower engine speeds than cam-driven valve, he doesn’t think this a énorme advantage over conventional systems. At high RPM, faster closing valves could provide more of a benefit, but “you’re still limited by the valve spring stiffness, valve mass, and seating rates to avoid physically damaging components,” he told me via email.
In addition, he also mentioned losses from the pneumatic pump that actuates the valves, and the complexity of the OBD requirements, which will necessitate significantly more channels to be monitored. He ultimately concluded by saying that the setup is really best used on a pricey car like the Koenigsegg:
So, in summary, it’s a complicated, expensive way of doing something that all major manufacturers are already doing, and the incremental benefits only make sense for a supercar where the additional cost & complexity is inconsequential, especially compared to the bragging rights that could come with it.