Quadrifoglio engine magazine article (long)
A name that evokes Milan but that has in Modena, in a completely anonymous building as it is appropriate for a place where prototypes circulate and the technical solutions of the future are studied, the center of excellence where new products are developed, together with those Maserati. It was there that we went to meet Gianluca Pivetti, the technician who has a long experience in the design of high performance petrol engines at Ferrari and who was catapulted, by direct decision of Sergio Marchionne, to the 'rival' Maserati to coordinate the development of what the Italian-Canadian boss intended to be the jewel in the crown of FCA petrol units.
Objective centered in full, since the V6, coupled first to the Alfa Romeo Giulia and then to the Alfa Romeo Stelvio, both in the Quadrifoglio set-up, was received with great enthusiasm by the specialized press and by the most passionate customers who appreciated this marriage between a chassis of high dynamic performance and an engine capable of bringing it to the limit. A union that presented some risks but that was perfectly successful and that was sealed by the lap record on the Nürburgring track, a 'test bench' on which many are measured and where the echo of a prestigious result is soon world Tour.
"According to our calculations," says Pivetti, with that Emilian accent that fits very well when it comes to engines, "the Nürburgring record would have come with an engine of around 460 hp. Then the car gradually matured, we experienced a different weight distribution and improving the dynamic response we increased the cornering speed and so we moved to 470, then to 480 hp and climbed again. I must say that the structure held up well even higher powers, and so the famous record arrived ... "
The 90 ° V6 is the layout used by the current Formula 1 engines. In practice it is like taking two three-cylinder units (with cranks offset from each other by 120 °) and joining them in a single base with three bench supports and as many crank buttons on which two side-by-side connecting rods are mounted.
Going into the details of the engine, which we recall is a 90 ° biturbo V6 with a displacement of 2,890 cc that on the Stelvio Quadrifoglio expresses 510 HP at 6,500 rpm and a maximum torque of 600 Nm between 2,500 and 5,000 rpm, Pivetti begins to describe it pointing directly to the heart of the most passionate: "We started from the same configuration as the V6 of Formula 1. It is a V of 90 ° with three crank buttons on which three pairs of connecting rods are mounted side by side. In this way, unlike a 6-cylinder in line with equidistant ignitions of 120 ° there is a 'lame' burst order, at 90 ° -150 ° -90 ° and so on, with an order 1 vibration harmonic , 5. This configuration allows first of all to make a V6 shorter, and this is advantageous for the arrangement in the engine compartment, then each cylinder on each bank has regular bursts of 240 °, and this generates exactly equidistant phases of discharge inside the turbine . But this also satisfies another important project requirement, namely the deactivation of a three-cylinder bank to optimize consumption and emissions according to driving needs ".
All the drive shafts are dynamically balanced by removing material from the cleavers. The ignition order is 1-6-3-4-2-5.
The piston has a mantle covered with a tungsten disulphide coating, which acts as a protection during the break-in phase.
The characteristic shape of the sky, designed in Alfa Romeo and the result of Ferrari's experience in direct injection engines with side injector and central spark plug.
This partialization is a fundamental point for a very high performance engine mounted on a much wider vehicle than a super sports car. But precisely because in this segment of cars comfort is an important part of the equipment, the partialisation must be done in an absolutely transparent way towards the driver. "This regularity of operation of each bank allowed us to be able to completely deactivate, under certain map conditions, the right one. And here comes back the way of vibrating of order 1.5, which is the same as a three-cylinder. When we pass by six and three cylinders, since the order of vibration always remains 1.5, the transition does not generate vibrations or discontinuities, so much so that the driver does not notice it. "
The distribution is double shaft with hydraulic tappets and cam profiles that work on rocker arms. The angle included between the valves is 40 °.
This is the collapsible tappet system, which is activated when the right bank is excluded. The mobile part is pulled out to be able to observe the system of springs that when given the command works like a normal valve spring, replacing the hydraulic circuit under pressure.
I must say that Pivetti is absolutely right: in the test we carried out with the red Stelvio Quadrifoglio the passage is really imperceptible both in the 'cut' and in the 'recovery' so as not to understand when the device was operated or not, also because the software adapts the partialization according to the requested torque and therefore also manages a certain 'swinging' between the two configurations. "We even made bets between us in development. I am using one of the first Stelvio Quadrifoglio, still with the manual gearbox, on which to understand when it went to three or six I had a red LED installed ... ". Obviously there is always a flap of the medal: "If on the one hand, to complete the pros under the hood beats a heart that beats like a V6 of Formula 1, on the other the order 1.5 compared to a typical order 3 of a six-cylinder 'normal' requires having technical solutions in terms of NVH inside the car capable of filtering these low-frequency vibrations that begin to perceive already at 600-700 rpm. This is why our competitors use active anti-vibration pads which vary the damping according to the intensity of the vibrations at the various frequencies. I must say that even then we had foreseen the use of this type of media from the start of the project, but in practice there was no need to use them ".
The ducts split in two near the valves. The inner surface is maintained with the same surface roughness obtained from the fusion.
Pivetti points out to us, however, that the cars on which this V6 is mounted have a substantially sporting soul and that if in the future it were thought of using it on a luxury sedan, a flagship, so to speak, the use of variable damping supports. Another interesting feature is that the three deactivated cylinders keep the valves closed and, obviously, they have the injection cut. To do this, special hydraulic tappets have been used on the right side, which through a valve system increase compliance and allow the rocker arm on which the cam profile acts to compress the tappet, leaving the relative valve closed.
The sub-base with the semi-supports of the bench and the one-way lamellar valves that, as explained in the text, limit the 'pumping'.
“In practice, with the valves closed the cylinders compress air, but this compression work is returned, except for friction, in the expansion phase. Each cylinder is basically a sort of air spring ... ". If you continue to keep the suction, compression and discharge phases active, the system is less efficient, since the work spent to suck and expel the gases is not recovered. "With our system we have recovered about 7% of fuel economy", continues Pivetti. "Mind you, it's expensive to make, since there are valves that control the oil flow in the tappets and a boost pump that instantly brings the oil pressure into the tappets to the optimal value, not to mention the software development and the implications on the diagnostics, since in fact we are going to force an operating condition that under normal conditions would be detected as anomalous. It has been a challenge of which little is known, since those who test the car go above all to seek maximum performance, while neglecting an aspect that is technically in my opinion just as interesting. I'm glad that Auto Tecnica talks about it ”.
A totally new project.
In this regard, Pivetti is keen to point out that this engine is not, as has sometimes been written, the V8 Ferrari to which two cylinders have been cut, but that it is a totally new project that has obviously benefited from the experience gained on the engines. Ferrari of the F154 series but only to share some peculiarities that we knew would have worked well: “In reality there are differences, starting with the crankshaft, which has cranks placed at 90 ° on the V8 while here they are at 120 °. So we couldn't just 'cut' a piece ... Clear that having been an active part of the team that designed the V8 Ferrari, when I came here I didn't forget all my past! The starting specifications were shared with Ferrari to take advantage of past experiences and shorten costs and development times ". A completely understandable position from the perspective of a car manufacturer like Alfa Romeo that must make profits. "And then", adds Pivetti, "the 90 ° six-cylinder V-shaped configuration fits perfectly into Maserati history, which had used it on the Merak and then on the Biturbo and the Ghibli, which was also used on the track in the Ghibli Cup around the mid-1990s. And still in the 90s the Alfa Romeo used a V6 at 90 ° in the DTM championship. That said, the base is similar to that of the V8 enough to avoid having to revalidate the pipes and the water passages, exploiting known geometries, technologies and materials. Also because this engine was built in just 2 years: I was the first to be hired in this project, wanted by Sergio Marchionne himself and activated in May 2013. After a few months he joined me Philip Krief, who is now technical director in Ferrari, and with him in July the core of a dozen technicians dedicated to motor and vehicle was created. In 2015 the engine was ready to be produced ".
The base adopts 'wet' steel rods, or with the external surface completely lapped by water. The upper mechanical seal is carried out by the collar and the hydraulic one is lower than three o-ring rings.
The "low" part
The bore and stroke measurements remained the same (86.5 × 82 mm for a unitary displacement of 481.6 cc and a total of 2,889.8 cc), so that the connecting rods and pistons are more or less similar to the V8, even if it is changed the piston supplier and the segmentation is different. The crankshaft consists of a semi-finished product in special steel enriched with vanadium that presents an excellent predisposition to the subsequent nitriding. The billet, which weighs about 9 kilos, is forged and machined by a machine tool, finally the bench supports and the crank pins are nitrided according to a well-established process also on Formula 1 engines.
A 3D drawing with the base and the sub-base joined by 16 prisoners. This guarantees an exceptional rigidity of the whole, essential to always guarantee the perfect alignment of the supports under the heavy loads induced by the torque developed by this propeller.
"In our case, we use gaseous nitriding, enriching and hardening the surface to a depth of a few tenths of a millimeter. This is also a Ferrari experience: competitors normally use induction hardening, which penetrates much less into the material, but experience has shown that nitriding is the most valid solution due to the stresses of our engines. In the crankshaft there is a lot of added value, especially for the processes, which also include all the oil passages and above all the treatment, which requires long times and high process controls. Each shaft is dynamically balanced with false masses, fitting 100% of the centrifugal masses and 50% of the alternate masses on the crank pins. Normally the torque is balanced by removing mass from the outermost cleavers, those that generate greater torque. The connecting rods are forged steel with the machined and non-fractured hat coupling surfaces. The screws that hold the hats are biting in the stem ".
Moving on to the monobloc, you are faced with an extremely rigid and complex structure, made up of two parts in die-cast aluminum in a shell joined together and which at the point of union create the seats for the bearing support bushes. "For me this is the optimal solution for any type of engine," continues Pivetti. "It is not the cheapest but it is without doubt the best. The steel rods are installed ‘wet’ and at the thinnest point they reach just 2.5 millimeters, with a silicon carbide sliding surface coating, according to a typically Ferrari solution. In this way the barrel always remains floating and cylindrical with a limited blow-by effect and reduced oil consumption. The bushes have an aluminum base with a layer of silver and finally one of bismuth, which went to replace the lead, the use of which is forbidden, which had the beautiful characteristic of being stable up to temperatures of the order of 300 °, when in the meatus the oil typically reaches about 180 °. For these components we have resorted to a Japanese supplier specialized in this type of components ”.
As mentioned, the upper base bears the half bearing supports which, after having assembled the shaft, are completed when the integral sub-base joining the structure is combined, creating a set of exceptional structural strength. “This solution is decidedly better than that with the caps which in high-performance engines are not sufficiently rigid and tend to follow the bending deformations of the crankshaft, compromising the correct dimension of the meatus and compromising lubrication. In practice it is almost like having a monolithic whole ”.
In the sub-base there are three unidirectional lamella valves that prevent the pistons from sucking air from the sump in the upward stroke while favoring the discharge of the pressure generated in the descending phase. In this way the part under the piston is constantly in depression to the point that when the piston exceeds the dead point it is "sucked" downwards, generating a positive thrust for useful work. In addition to this, the cup is isolated from the crank chamber. "A motorcycle solution that I had also applied to the V12 in Ferrari ...", specifies Pivetti. The cup, also in die-cast aluminum, closes the base and incorporates a series of anti-flapping bulkheads.
Lubrication is provided by a variable displacement vane pump. In practice, two flow rates are guaranteed, varying the eccentric position of the rotor with respect to the stator.
The oil pump, with blades, is defined as having a variable flow, but in fact it generates two flow rates depending on the position of the rotor with respect to the stator and consequently the displacement of the pump. "The pressure can be 1.5 or 4.1 bar," says Pivetti. “At low pressure we have advantages in terms of fuel economy. The vane pump has an exceptional efficiency ”. At the center of the V, where there is plenty of space, a water-oil plate heat exchanger has been placed. Its chain drive is derived with a 1: 1 ratio from a pinion splined to the crankshaft.
The "high" part
The head is also a completely new component, although obviously there are some links with the Ferrari experience, starting from the angle included between the valves equal to 40 ° (compared to the vertical 19 ° for suction and 21 ° for unloading). The diameter of the fungus is 32.7 mm for the suction and 29.6 mm for the discharge (conventionally measured on the half of the bevel at 45 ° of sealing), while the risers are differentiated for the two rows: for suction of the right 9.48 mm and for the left 9.254 mm; for the discharge of the right 9.644 mm and for the left 9.425 mm.
The seats are in cobalt-based sintered material. The distribution is double shaft with four valves per cylinder, which is driven by a rocker arm that has one end resting on the valve head and the other on a hydraulic tappet. The point of contact between the profile of the cam and the rocker is provided with a ball bearing to reduce friction and wear.
On the right bank, the one that can be excluded, the hydraulic tappet is of the 'collapsible' type or there is a spring system that, when the hydraulic circuit is not under pressure becomes more yielding than the valve spring and therefore causes the the rocker goes to compress it, leaving the valve closed.
The distribution is controlled by two chains, one per bank, of the single-row type of rollers that do not require any maintenance. "The division of the right bank has raised the issue of whether or not to send oil under pressure to the supports of the turbine shaft", continues Pivetti. “In this case, since the exhaust valves are closed, there is no gas flow and the turbine is stationary, or runs very slowly. Since the seals are dynamic there was the danger of oil leakage to the drain, with pollution problems. For this we have added a solenoid valve that intercepts the flow of oil towards the stationary turbine. As you can see the system is rather complex and, going back to what was said before, to call it a V8 Ferrari without two cylinders seems to me a bit restrictive ... ".
The sub-base with the semi-supports of the bench and the one-way lamellar valves that, as explained in the text, limit the 'pumping'.
The right head, with the special hydraulic tappet to allow deactivation of the valve control and to throttle the three cylinders.
The Alfa Romeo V6 has an unmistakable and bewitching sound, for the true enthusiast. The partialisation, since the discharge of the two banks runs separate to the final silencer, could give rise to an asymmetrical sound, coming out only from the left terminals: “In reality we made a continuous compensation inside the terminal silencer, where the two exhaust lines opening, closing septa depending on whether the engine turns to three or six cylinders, so as to balance the outputs. Speaking instead of intensity of the exhaust sound, when positioning the driving mode in Race, valves are activated in the terminal that bypass the gases towards a freer path, avoiding passage through the silenced volume. In practice under normal conditions the exhaust gases come out of the two inner terminals, in Race from the outside ”.
The water flow inside the heads is transversal: the primary flow of water rises from the base after having lapped and cooled the barrels, a part enters the head from the side of the drain, passes through it heading towards the suction and then rejoins the primary flow to return to the exchanger. In this way the same thermal condition of all cylinders is guaranteed. The cooling of the turbines is also via spil water from the primary circuit.
A scheme with the steps of cooling water.
The combustion chamber obviously follows the Ferrari experience, with the piston crown worked both to create space for the movement of the valves and to create a decentralized bag that allows to favor, starting from the injection point, a triggering of the turbulence useful for combustion . The injection pressure is currently 200 bar, but it is expected in the future to increase it to 350 bar.
The ducts are single and divide near the valves. "Even if the dual ignition is one of the typical features of Alfa Romeo engines with this configuration of the combustion chamber with side injector, only one central spark plug is ideal", concludes Pivetti. "As far as turbulence is concerned, the main motion is that of thumble, as Cosworth has taught us, but it must be well calibrated in order not to incur performance losses due to the extinction of the flame core and irregular combustion. It is an equilibrium that is reached experimentally as a function of valves, angles, capacities, dimensions ... ". The hydraulic phase variators are keyed to the end of each camshaft on the chain side. The ECU is a Bosch Motronic MED 17.3.5. The two turbo-compressor units are supplied by IHI, are water-cooled, lubricated with pressurized oil and have the waste-gate valve controlled by vacuum but the use of the electrically controlled waste-gate is being validated. The air is compressed to around 1.4 bar relative and before being introduced to the aspiration it is cooled by intercoolers fed by a secondary water circuit equipped with a dedicated pump. The engine weighs 218 kg.
The characteristic power and torque curves. The latter is almost constant (and substantial) from 2,500 to 5,000 rpm.
We conclude this brief but detailed description of V6 by thanking engineer Pivetti for the motoring 'lesson' and recalling that this V6 represents the most sophisticated engine coming out of the famous project 'Giorgio', born to re-launch the Alfa Romeo brand and that generated the family of Giulia sedans and Stelvio SUVs.
2019 Alfa Romeo Stelvio Quadrifoglio
Vulcano Black Metallic