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Pistons | Rods - Pistons | Rods

Pistons

If you don't have a Mazda Rotary engine in your hood, chances are your engine has good old-fashioned pistons and connecting rods. The piston is essentially a thick metal plate that compresses gas inside the cylinders. The slugs of aluminum inside your engine are living in a fiery hell. At full throttle and 6000 rpm, the piston in the gasoline engine is subjected to almost 10 tons of force every 0.02 second as repeated explosions heat the metal to more than 600 degrees Fahrenheit. 

As automakers chase higher efficiency, piston manufacturers are preparing for a future in which the most powerful naturally aspirated gasoline engines produce 175 horsepower per liter, up from 130 today. Even tougher conditions come with turbocharging and increased outputs. Over the last decade, piston operating temperatures have risen to 120 degrees, while peak cylinder pressures have increased from 1500 psi to 2200.

Pistons account for at least 60% of the engine's friction, and improvements here directly impact fuel consumption. Friction-reducing, graphite-impregnated, screen-printed resin patches on the skirt are now almost universal. Piston supplier Federal-Mogul experiments with an oil ring tapered face that allows the ring voltage to be reduced without increasing oil consumption. Lower ring friction can release as much as 0.15 horsepower per cylinder.

Connecting Rods

The piston is connected to the crankshaft through a connecting rod, often shortened to the rod or conrod. Together, these parts are known as the piston assembly. The connecting rod's two ends are free to pivot: the part of the connecting rod that connects to the piston is called the small end, and the end that attaches around the crankshaft is called the big end. The large end will have bearing inserts that minimize friction and maintain a proper oil clearance with the rod journal on the crankshaft. The connecting rod is divided into a rod cap that clamped around the big end bearing and the crankshaft.

As the workload for pistons increases, so do the requirements for connecting rods. Higher combustion pressures result in more significant stress on the sticks that link the pistons to the crank. With the rare exception of exotic titanium parts, connecting rods are typically either made of powdered steel, compressed and heated in a mold, or forged from high-performance steel stock. The major technological shift is cracked large-end caps for both powder-metal and forged connecting rods. Previously, the rod and the crank-pin end cap were made as separate pieces. Rods with cracked caps come out of the mold as a single, box-shaped piece. The crank-pin end is etched and then snapped in two with a press. The resulting irregular surface improves alignment, results in a more secure connection of the cap to the rod, and allows for a more slender, lighter connection.

The connecting rod carries the force from the piston to the crankshaft. It is continuously subjected to stretching, squashing, and bending forces as it acts as an intermediary in this push-pull relationship. The connecting rod needs to be structurally strong, and it is no coincidence that it takes the form of a miniature I-beam steel, similar to its larger brothers holding skyscrapers and bridges. The I-beam profile gives maximum structural strength at a minimum weight cost, and, like the piston, we want to keep the weight of the connecting rod as low as possible. 

The strength demanded by the connecting rod means that it is made of forged steel or powdered steel. Exotic engines may have titanium rods on them. Cast iron is not used because of its weight.

High-Performance Rods and Pistons

Installing a set of stronger, lighter-duty rods and pistons will create a stronger engine. This may be essential for the turbocharging or supercharging of the engine. Moving from forged rods to titanium or powdered (sintered) steel will lead to a more potent engine.

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