Yanking The Crank At 18,000 rpm

The delectable artifact I’m examining at the top of this page is a piston and connecting rod assembly from last year’s Ferrari Formula 1 racing engine. The design and dimensions of these components reveal much about how today’s Fl engines extract 800 horsepower from just 3.0 liters of displacement.
The diameter of this piston measures 91.5 millimeters. In a V-10 configuration with a maximum allowable displacement of 3000 cubic centimeters, this means that the engine’s stroke is 45.6mm.

A bore twice as large as the stroke is unheard of in a production-car engine, most of which have rather similar bore and stroke dimensions. But the short stroke lets the Fl engines attain dizzying revs, and that’s the source of their enormous power.

That’s because an engine is basically an air pump that burns 14.7 pounds of air with every pound of fuel. For example, the 3.6 liter V-8 in the Ferrari 360 Modena generates 395hp at 8500 rpm. Assuming that it fills its cylinders about 90 percent at that rpm, it consumes 13,700 liters, or about 37 pounds, of air per minute. (Pardon the mixed units, I’m still partly mired in the English system.) In other words, one pound of air per minute produces roughly 10.7 hp.

Following this thinking, an 800-hp Fl engine must suck in more than 70 pounds of air every minute. How can a 3.0-liter engine do that? The easy way would be to bolt on some sort of supercharger. By compressing the air to double its density and making sure the cylinders get 100 percent filled with every intake stroke, a supercharged 3.0-liter engine would inhale 70 pounds of air per minute at a moderate 8700 rpm.

Superchargers, however, were banned from Fl about five years ago, along with some exotic fuels that could extract more power from each pound of air. Therefore, the only way to flow 800hp worth of air through a 3.0-liter engine is through revs — lots of revs, maybe 17,500 rpm. And that’s here the short stroke comes in.

If you tried to spin the Modena V-8 to 17,500 rpm using a powerful electric motor, it would quickly make like a hand grenade — especially the valvetrain and the connecting rods. Renault solved the major high-rpm valvetrain problem about 10 years ago when it replaced steel valve springs with a small piston-and-cylinder assembly filled with air at 150 psi. The air never fatigues and breaks like a metal spring. The connecting rods and the pistons are other matters.

At 18,000 rpm, which is the redline of a ’99 Fl engine, each piston in the engine accelerates from a dead stop at one end of the cylinder to about 100 mph in less than one-thousandth of a second and then comes to a complete stop again less than one-thousandth of a second later. This start/stop cycle is repeated 600 times per second!

This is where the Ferrari Fl engine’s short stroke and longish connecting rod come in. At 18,000 rpm, this combination of its 45.6mm stroke and 110mm connecting rod accelerates the piston at a peak rate of nearly 10,000 g. With the 14.9 ounce combined weight of the piston, rings, and small end of the connecting rod, such acceleration requires more than 9000 pounds of force, which is trying to rip the con rod in two or tear out the piston pin.

That’s brutal enough, mechanically speaking, but if the V-10 engine had been designed with equal bore and stroke dimensions of 72.5mm and had a 130mm con rod, like the proportions in a Mustang GT V-8, then the peak piston acceleration at 18,000 rpm would be almost 17,000g accelerating the piston to a peak speed of about 160 mph. Even accounting for the smaller, lighter piston and rings, such a design would produce forces of about 12,500 pounds. That would tear apart either the piston or the con rod. And making these parts and stronger would just them heavier and increase forces ripping them apart.

That’s why the engineers take every measure to reduce the weight of these components. Look at how little there is to this Ferrari piston. The piston skirts extend less than an inch below the compression ring. And for about two-thirds of the piston circumference (the nonthrust faces), there’s no skirt at all!

You’ll notice that the rings are also rather thin. The compression rings are about 0.028 inch thick, and the oil-ring assembly is only 0.077 inch thick. Conventional rings are more like 0.063 and 0.125 inch thick. These thin dimensions not only reduce reciprocating weight but also help the rings maintain their seal through the pistons’ frenetic gyrations.

The connecting rod and the piston pin are titanium, probably forged, making good use of that material’s excellent strength-to weight ratio. The piston itself appears to be forged from aluminum with some darker coating to resist heat and reduce friction.

Rumor has it that some current Fl engines are using exotic alloys of beryllium and aluminum to further reduce piston weight. That could allow a smaller bore and longer stroke while still keeping stresses within allowable limits.

The reason for pursuing a smaller bore is that the shape of the combustion chamber gets pretty ugly in a large-bore, short-stroke design. In the case of this Ferrari, if we assume a 13.0:1 compression ratio, then the combustion chamber has a minimum volume of 25cc. Were it a regular round shape, it would look like a quarter that has been flattened by a train. In fact, the combustion chamber is even uglier than this because most of the volume exists in the four relief pockets machined into the piston crown to prevent the partly open valves from making contact with the piston during the top of the exhaust stroke.

In fact, these valve reliefs are so tightly machined that engine designers must juggle their camshaft profiles and compression ratios. Opening the valves sooner and farther requires deeper valve-relief cuts, resulting in a lower compression ratio.

Shrinking the bore and increasing the stroke provide more opportunities for favorable compromise, but doing so requires exotic materials if the 18,000-rpm revving ability is to be maintained. The Ilmor-Mercedes engine is thought to employ for this purpose such beryllium alloys in its pistons, and some observers believe this engine’s superior midrange performance is the result. Of course, it might also be using more exotic radial valves, or variable camshaft timing. I’ll reveal all when a Mercedes cylinder head lands in my office.