F1, Ferrari: turbocharger surging behind power unit difficulties in Mexico City

New details emerge regarding the difficulties experienced by Ferrari in Mexico City Grand Prix. The lack of competitiveness of the F1-75 is linked to several factors, such as the incorrect interpretation of the set-up for the Autodromo Hermanos Rodriguez, including the need to give up some power to preserve its engine reliability at 2200 meters altitude.

The key component which ended up again under the magnifying glass is the turbocharger of the Maranello engines, which already during the season revealed some critical issues, although of a different nature than those that emerged in Mexico. Alberto Antonini (former Ferrari F1 press officer) and currently an F1 expert for FormulaPassion.it explained:

“The problem with the turbo could recur [in Brazil], even if Carlos Sainz were to mount a new power unit. I’ll explain it short: the compressor (not the turbine) of the PU Ferrari suffers from an issue called ‘surge’: in practice – I apologize to the engineers – it is as if part of the compressed air comes back, generating a high frequency vibration that produces a sharp and characteristic whistle”.

The compressor of the Maranello’s power unit therefore suffers from surging, a phenomenon of instability regarding the operation of the turbo-group that can even undermine its structural integrity. The behavior of a compressor is traditionally described by means of a map, where the pressure increase to meet the request crosses with specific flow conditions, to be understood as mass flow, temperature and pressure. At 2200 meters above sea level in Mexico City, the inlet air pressure is about a third lower than that of the sea, which is why flow properties change and more compression work is required to supply the engine with satisfactory levels of pressure and flow, all while also increasing the speed of rotation of the turbo-group.

In summary, in order to compress the air sufficiently not to compromise the performance of the internal combustion engine, a series of parameters are changed that move the operating point of the compressor on the map.

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The criticality is represented by the fact that under certain conditions of pressure, compression and flow rate, the compressor reaches a point of instability beyond which its operation is compromised, as any fluctuations in the flow characteristics no longer self-compensate. The risk is that, in such circumstances, as the inlet air flow decreases, the useful compression work of the turbo-group also decreases, to such an extent that it is no longer able to overcome the resistance given by the greater pressure at the outlet of the compressor. The result is that due to the difference in pressure, the air flows in the opposite direction, from the outlet to the inlet, until those conditions are re-established so that the compressor is able to work again, restarting the cycle. In this way there are repeated bounces and oscillations of the flow, inducing vibrations at critical frequencies for the mechanical parts involved, causing them to break.

The temporary countermeasure adopted by Ferrari was to limit the speed of rotation of the turbo-group by exploiting the braking action of the MGU-H electric generator, reducing the flow rate but also the work of compressing the air, leading to a loss of power from the internal combustion unit.

Ferrari has opted for a different size of the turbo compared to the competition in terms of the diameters of the rotating parts and the dimensions of the blades, seeking an advantage at normal altitudes but getting closer to the condition of instability at 2200 meters in Mexico City. “On the one hand, there is a difficulty of the power unit for our size of the turbo, which at that altitude with that rarefied air penalized us more than the opponents, at least for the type of mapping we used and so on” – Scuderia Ferrari team principal Mattia Binotto explained at the end of the race.

However, the size of the turbo in Maranello would have also been done considering the anomalous critical conditions of Mexico, with the origin of the problem which, according to what was reported by Alberto Antonini, would be found elsewhere: “The problem, specifically, is not related to the materials, but with the energy losses found on some units. Basically, not all turbos mounted, although identical in specification, are also identical in performance”. Due to its particular design, therefore, the Ferrari turbo is approaching critical conditions at high altitudes such as to induce surging, with the possibility, however, that a safety margin has been maintained from the project.

The problem would instead arise from anomalous losses of pressure, temperature and therefore energy in the various ducts and internal parts, moving the operating point again in the compressor map and incurring surging. The origins of the discrepancies between the different engines of the Ferrari engines which are identical in terms of design are not clear at the moment, although the supply and / or production of the components are among the possible causes.

By its nature, surging appears as a phenomenon with a relatively sudden and non-progressive trigger, which is why at the 700 meters of altitude of San Paolo it could be completely absent, without recurring therefore in a lower and proportional measure compared to the 2200 meters of altitude in Mexico City, as pointed out by Alberto Antonini.

Ferrari boss Mattia Binotto has shown optimism about the next round of the 2022 Formula 1 season, also taking into consideration the fact that the power unit did not encounter this issue at 700 meters altitude in Austria, but a concern remains due to the not yet perfect understanding of the origin of the energy difference between the power units, making it difficult to predict its behavior. However, surging represents a separate problem from that of the turbo-group shaft torsions which caused the technical failures of the Ferrari F1-75 car earlier this season on tracks at normal altitudes. A problem on which work continues to find definitive corrections for the 2023 F1 campaign.