Racing Engines

Racing Engines

The simplest way to get a powerful heat engine is just to make it bigger. Some of the race cars in the early 20th century had huge displacements compared to modern engines—some were 1,220 cubic inches (20 L) or more, four times the size of a modern mus- cle car engine! But this size was not necessarily a big advantage. Although the engine is more powerful, it is also much heavier, and

a more massive object requires more force to accelerate it. This is because of Newton’s second law of motion: the acceleration of an object equals the applied force divided by the mass. For any given force, an object with a lot of mass accelerates much less than an object with little mass. Because of the limits of thermodynamics, only a portion of the heat supplied to the engine can be turned into work, so heat engines are already operating at a disadvan- tage. Adding to their mass increases the work they have to do, and engine-designers quickly reach a point where making engines bigger does little good.

A similar situation occurs when more pistons are added. Increasing the number of pistons increases the force that can be applied to the crankshaft, which makes the engine more powerful. But the larger number of pistons also increases the engine’s mass. Another disadvantage is that adding pistons results in an increase in friction losses, since a greater number of pistons moving and rubbing against the cylinder walls means a greater energy loss due to friction.

An important consideration of pistons is not only their num- ber but also the compression ratio—this measures the extent to

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which the fuel-air mixture is compressed in the piston cylinder. The compression ratio is the ratio of two volumes—the volume of the cylinder when the piston is at its low point and the volume of the cylinder when the piston is at its high point, as shown in the figure below. If the compression ratio is large, then the fuel- air mixture occupies a small space immediately before the fuel is ignited. Under this kind of high pressure, the hot gas can expand and do more work, so an engine with a higher compression ratio is in principle capable of more power. In practice, though, the compression ratio cannot be made too large without disrupting the fuel ignition and piston timing.

Other options for getting more power out of an internal com- bustion engine include burning a richer mixture of fuel and air, which increases the amount of hot gases that expand against the piston. More fuel requires more air because the additional fuel requires additional oxygen to support combustion (one of the pri- mary methods of extinguishing a fire is to deprive it of oxygen). The extra air and fuel requires more volume—especially the air,

The compression ratio refers to the amount of volume the piston compresses and is found by comparing the volume of the cylinder when the piston is at the bottom of its motion (A) with the volume when the piston is at the top (B). The volume in (A) is six times the volume in (B), so the compression ratio of this cylinder is 6 to 1.

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which is not dense. But the high compression ratio of powerful engines means that the fuel-air mixture does not have a lot of room. The solution to this problem is to make the air dense at the beginning, and this is the job of an engine device called a super- charger.

Superchargers are compressors. Their job is to get more oxygen into the piston cylinder by squeezing the air into a small space. Superchargers are also called blowers, because they blow high- pressure air into the engine. The higher amount of oxygen sup- ports a higher level of combustion, increasing the available power. The price to pay for this increased power is the energy needed to run the compressor—nothing comes free in physics, particularly when thermodynamics is involved. The supercharger’s compressor usually gets its energy from the rotation of the crankshaft, and this slows down the crankshaft by a small amount. But the increased oxygen in the cylinder more than makes up for the loss.

Another way to increase the amount of air entering the cylinder is to use a turbocharger. The job of a turbocharger is similar to a supercharger, compressing the air before it gets into the cylinder. But turbochargers do not rob the crankshaft of any of its power since they get their energy from the engine’s exhaust gases. When exiting the engine, the exhaust gases turn a fan or a turbine that runs the compressor.

Turbochargers are common in cars and trucks today, for both diesel engines and gasoline-powered cars. German car manufac- turer Porsche is well known for its turbocharged vehicles. But although turbochargers do not rob the crankshaft of power, they do have a disadvantage over superchargers in that they operate with a time lag. When the driver presses the car’s accelerator quickly, the turbocharger responds with a delay—because exhaust is involved,

a turbocharger cannot act immediately. People have used all of these methods—bigger engines, more pistons, turbocharging, and supercharging—to make race-car engines. But today many race cars do not use any of these meth- ods, and professional racing organizations specify the size and properties of the engines that are permitted to compete in the races, making for a fair contest. Race cars known as Formula One

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A NASCAR race car zooms along the track at the Las Vegas Motor Speedway.

(U.S. Air Force/Master Sgt. Robert W. Valenca)

cars must have a V10 engine with a displacement of 3,000 cubic centimeters—183 cubic inches. This is not a big engine, yet the top Formula One race cars can reach speeds of 220 miles per hour (350 km/hr.)! The popular racing organization NASCAR uses eight-cylinder engines with a specified displacement of about 350 cubic inches (5.74 liters or 5,740 cubic centimeters).

The enormous power of today’s race cars come from high compression ratios, rich fuels, and an ability to run at an amaz- ing rpm—the crankshaft of some of the best Formula One racers approach a spin rate of 20,000 rpm (333 revolutions each sec- ond!). These race cars are fast because the engines take in air and burn fuel at an exceptionally high rate. This means that the pistons must be moving remarkably fast, and in some race cars, the pistons experience acceleration forces as great as 9,000 times gravity. This is why the crankshaft turns at such a high rpm, which delivers a huge amount of energy to the wheels.

As mentioned earlier, most ordinary cars operate at a few thou- sand rpm. Twenty thousand rpm generates so much rotational force and so much heat from friction that ordinary passenger cars

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would melt or fall apart. Race cars such as Formula One cars have to be made from special metal alloys and other materials that can withstand the friction and other forces involved with engine speeds of 333 revolutions every second. Even so, most race-car engines must be rebuilt after every race, because many of the parts can- not last long under these extreme conditions. (Some engines do not even manage to last a whole race—engine failure is a com- mon reason that drivers drop out of a race.) Rebuilding an engine is expensive, since they cost about $100,000. These engines are expensive to operate for another reason—a poor fuel mileage of about four miles (6.44 km) per gallon.

Manufacturers of powerful engines also place an emphasis on several other design features, such as the valves for inlets and out- lets in the cylinders, the camshaft placement (the camshaft opens the valves), and the mechanism that pumps fuel into the cylin- ders.

But the fastest land cars in existence today are not piston engines. As of early 2006, the speed record for a car traveling on the ground is 763 miles per hour (1,221 km/hr.), set by Thrust- SSC in October 1997 at the Black Rock Desert in Nevada. (SSC is an abbreviation for Supersonic Car, and the car did manage to achieve supersonic speed—it broke the sound barrier, the first and so far only car to do so.) Speeds of this magnitude require a long, flat surface for a track, such as a salt flat, or a dry lake bed like the Black Rock Desert, one of the flattest surfaces on the Earth. Attaining these speeds also requires much engineering skill,

a brave driver, and a lot of physics. The engines are similar to those used to travel in air or space rather than the piston engines that power modern automobiles.