By popular demand: a Q&A on supercharging (and turbocharging).
Q: What’s a supercharger?
A: An internal combustion engine works by drawing a mixture of air and fuel (the intake charge) into its cylinders, compressing that mixture, and then burning it. The more air/fuel mixture that can be crammed into the cylinders to burn, the more power the engine produces. You can increase power in three basic ways: you can improve the engine’s ability to draw more air and fuel into the cylinders and expel its burned exhaust gases (its volumetric efficiency, or ‘breathing’); you can increase the swept volume of the cylinders (the engine’s displacement), so you can fit more air and fuel into each cylinder; or you can force the intake charge into the cylinders under high pressure, squeezing more air and fuel into the available volume. Forcing air into the engine at higher than atmospheric pressure is called supercharging. A supercharger is a mechanical air compressor that pressurizes the air going into the engine. There are several types of compressor used for car and truck engines, most commonly Roots-type, centrifugal, and Lysholm compressors; each has pros and cons, but they have the same basic function.
Q: So, what’s a turbocharger, then?
A: As we said, a supercharger is an air compressor, and it requires a source of power to operate the compressor mechanism. Most automotive superchargers are run by a drive belt (or occasionally a train of gears) operated by the engine, much like a power steering pump or air conditioning compressor. An alternative is to run the supercharger with a turbine wheel placed in the engine’s exhaust manifold, turned by the flow of burned exhaust gases rushing of the engine. An exhaust-driven supercharger is called a turbocharger. (Years ago, they were often called turbo-superchargers, but that term has fallen out of common use, although it is occasionally applied to combinations of engine-driven and exhaust-driven superchargers.)
Q: What’s the advantage of supercharging (or turbocharging)?
A: More power! The more you increase the pressure of the intake air above the local atmospheric pressure (boost), the more power the engine produces. Automotive superchargers for street use typically produce a maximum boost pressures between 5 and 15 psi (0.33 to 1.0 bar), providing a proportionate increase in power. This is particularly useful at high altitudes: thinner air has less oxygen, reducing power by around 3% per 1,000 feet above sea level, but a supercharger can compensate for that loss, pressurizing the intake charge to something close to sea level pressure. (Superchargers are popular for high-altitude aircraft piston engines for precisely that reason.)
Q: Does adding a supercharger or turbocharger burn a lot of fuel?
A: All else being equal, yes. Engines burn air and fuel at an ideal (stoichiometric) ratio of about 14.7:1, which means that if you burn more air, you must also burn more fuel. Even when the supercharger isn’t producing much — or any — boost, a supercharged engine is somewhat less fuel efficient than a non-turbocharged (normally aspirated) engine of the same displacement and configuration. On the other hand, a supercharged engine tends to consume less fuel in day-to-day driving than a larger displacement, normally aspirated engine of similar power. For example, a 2.0 L turbocharged four-cylinder engine with 240 hp (179 kW) will generally be somewhat less thirsty than a 3.5 L normally aspirated V6 engine of the same output, at least in relaxed normal driving. As any owner of a Subaru WRX will tell you, though, if you use the boost a lot, you’ll pay the price at the pump.
Q: Why use a supercharger instead of just using a bigger engine? Wouldn’t that be easier?
A: To some extent, it would be. A supercharger significantly increases an engine’s specific output — the amount of power generated per unit of engine displacement (usually quoted in terms of horsepower per cubic inch or horsepower per liter), but, as the old adage says, there’s no substitute for cubic inches (or, more pithily, no replacement for displacement). Increased displacement provides more power without the added cost, complexity, and sometimes nonlinear behavior of superchargers. Still, a large-displacement engine usually ends up being bigger and heavier than one of small displacement, which makes it harder to fit under the hood, and does unfavorable things to weight distribution. A smaller, supercharged engine can provide similar power with less bulk and somewhat lower average fuel consumption and carbon dioxide emissions. In addition, some countries, like France and Italy, levy prohibitive tax and licensing surcharges on cars with large-displacement engines, making a smaller displacement, supercharged engine much cheaper to buy and operate.
Furthermore, any existing engine design can only have its displacement increased so much without a major redesign, so the addition of a supercharger or turbocharger can be a useful way to pep up an existing engine that has reached the limits of its growth potential. It’s a relatively easy “bolt-on” power increase that doesn’t require a vast amount of engineering work.
Q: Why don’t superchargers produce maximum boost all the time? What’s “turbo lag?”
A: The amount of boost any supercharger produces is dependent on the size and rotational speed of its impeller(s), as well as the type of compressor it uses. (For example, centrifugal superchargers, whose output increases proportionally to the square of the rotation speed, are most efficient at high speeds, while Roots superchargers are most efficient at lower speeds.) The peak operating speed of a typical automotive supercharger is more than 30,000 rpm, more than 100,000 rpm for some turbochargers. The compressor does not produce its full boost until the impeller has reached that speed.
Let’s look at a specific real-world example: the belt-driven Paxton Model SN supercharger used on Studebaker’s R2 engine and offered as an option on some GT-350 Mustangs. Described in detail in the July 1966 issue of Car Life, the Model SN was a centrifugal supercharger with an impeller 5.78 inches (147 mm) in diameter, geared to rotate at just under six times the speed of the engine crankshaft. Maximum boost was 5.0 psi (0.35 bar) at an impeller speed of just under 30,000 rpm, equivalent to an engine speed of 5,000 rpm. As we mentioned above, in a centrifugal supercharger, the boost is proportional to the square of the rotation speed. At an engine speed of 2,500 rpm, therefore, boost drops by a factor of four, to about 1.25 psi (0.09 bar). Cut it in half again, to 1,250 rpm, and the boost is only about 0.31 psi (0.02 bar).
What did that mean in practical terms? When the supercharger was making full boost, it provided a significant amount of extra power, on the order of 20-30%. At very low speeds, however, boost was negligible, probably not enough to make up for the power used to run the compressor. This was borne out by contemporary road tests of the supercharged GT-350, which found the supercharged car no faster than its normally aspirated counterpart under about 40 mph (65 km/h), but noticeably quicker to 60 mph (97 km/h), with significantly higher trap speeds in the standing quarter mile (0-402 meters).
Turbochargers, which usually use centrifugal compressors, have the same limitations as other types of supercharger, further complicated both by the turbo’s higher peak speeds and the fact that the speed of the impeller is dependent on the speed of the exhaust stream, not engine speed. Unlike an engine-driven supercharger, the turbine speed isn’t fixed; it varies with throttle position (among other things). At steady cruising speeds, the turbine is often spinning well below its boost threshold, that is, turning too slowly to provide any boost. When you press the gas pedal the speed of the exhaust gases increases and the turbine begins to accelerate, but there’s a delay while the turbine overcomes its own inertia and accelerates (spools up) to peak speed. Since that peak speed is usually quite high, this produces a brief but annoying delay, known as turbo lag or boost lag, where not only does the turbocharger not produce any extra power, it actually reduces output slightly because of the increased back pressure the turbine creates in the exhaust stream.
The severity of turbo lag often depends on how much boost the turbocharger produces. More boost requires either a bigger compressor — which has more inertia — or a higher operating speed, either of which take longer to spool up. Engineers have developed various tricks to reduce turbo lag, including reducing the mass of the turbine blades, changing their shape to improve their acceleration, and even adding movable nozzles that change the direction at which the exhaust stream hits the turbine blades, depending on their speed. (Porsche recently introduced a “variable geometry” turbo system, claiming it to be a first, but Chrysler had something very similar on its short-lived VNT turbo engine back around 1990.) Some sports cars have also used two or more sequential turbochargers of different sizes, a smaller turbo offering good low-speed response and a bigger one that takes over to provide maximum boost at higher speeds. The limited-production Porsche 959 used sequential twin turbos, as did the third-generation Mazda RX-7 and fourth-generation Toyota Supra Turbo.
Reducing turbo lag is easier with turbochargers whose maximum boost is relatively low; the “light-pressure turbochargers” used by some Saab and Volvo engines, for instance, don’t produce a great deal of boost, but they have little lag and a fairly linear power curve.
A more complex alternative is a two-stage turbo-supercharger, which uses both an engine-driven supercharger and a turbocharger in sequence. The supercharger is designed to produce its maximum boost at low speeds; at higher speeds, a clutch disengages the supercharger and the turbocharger provides the boost. This was not uncommon on aircraft engines in the 1940s, and Volkswagen recently reintroduced the concept with its “twincharger” engines, now used on some European Polo and Golf models.
Q: Are there other disadvantages of superchargers and turbochargers?
A: Definitely. The most obvious are cost and complexity. Aside from adding a bunch of extra parts to the engine (which means more to break), the moving parts have to be precisely machined and quite strong. Turbochargers require fairly exotic materials to withstand both the high temperatures of the exhaust system and their very high operating speeds. Forced-induction engines need to be well lubricated, as well, and they tend to put a big strain on the engine’s oil system, requiring good-quality oil and frequent oil changes to avoid a build-up of sludge.
Another potential problem is detonation. If you increase the pressure of the intake air, you also increase its temperature. When the mixture enters the cylinders it is compressed before it burns, raising its temperature even further. If the mixture is too hot, it may be prematurely ignited by hot spots within the combustion chamber (this is called detonation, preignition, or knock). Detonation can cause severe internal engine damage. To reduce the risk of detonation, forced-induction engines often have their compression ratios reduced, so the pistons do not compress the mixture as much prior to burning. This avoids detonation, but it means that the engine’s power output is reduced, particularly when the supercharger is not producing much boost. Many forced-induction gasoline engines require higher octane fuel, which is less susceptible to knock, but costs significantly more.
As we’ve mentioned above, superchargers consume a certain amount of engine power even when they aren’t producing useful boost, and turbochargers increase back pressure in the exhaust, which also costs power. The compressor can also create a certain amount of internal drag at low speeds. At maximum boost, the increased power provided by the compressor far outweighs these parasitic losses, but they make the engine less efficient off-boost.
Superchargers and turbochargers also take up a little more space in the engine compartment and add a certain amount of weight. These penalties are modest compared to the benefits (bolt-on superchargers typically weigh less than 50 pounds and fit fairly easily under the hood), but they’re not negligible.
Moral of the story: there is no free lunch.
Q: What’s an intercooler?
A: As we said, increasing the pressure of the intake air raises its temperature, which is really not desirable. Not only does it increase the risk of detonation, it lowers the density of the intake charge, which starts to defeat the whole purpose of supercharging. There are several tricks that can reduce that temperature. One is to inject a little bit of liquid (such as water and alcohol) into the intake manifold; some of the thermal energy of the compressed air goes to vaporizing the liquid, reducing the air’s temperature. Another method is to add an intercooler, which is a heat exchanger — basically, a small radiator — that removes some of the heat from the pressurized air before it enters the engine’s intake manifold. A properly designed intercooler dramatically reduces the intake air temperature, avoiding detonation and, as a bonus, increasing the density of the intake cahrge. Intercoolers aren’t really necessary for low levels of boost (less than about 5-6 psi/0.3-0.4 bar), but they’re virtually mandatory at very high boost pressures.
Of course, adding an intercooler further increases cost, complexity, mass, and bulk. The heat the intercooler extracts also needs somewhere to go. If it’s an air-to-air intercooler, it needs a good flow of cooling air; if it’s an air-to-water intercooler, the engine needs a bigger radiator to cope with the extra heat added to the engine’s cooling system.
Q: How long have superchargers and turbochargers been around? Did Saab invent the turbocharger?
A: The Roots-type compressor has been around a lot longer than the automobile — that type of blower was patented back in the 1860s for mine shaft ventilation, and the first patent for an automotive version was filed by Gottlieb Daimler (of Daimler-Benz fame) in 1900. Rudolf Diesel patented the supercharged diesel engine in 1896, while the centrifugal supercharger was patented by Louis Renault in 1902. The turbocharger, meanwhile, was invented by Alfred Buchi and patented in 1905. Turbochargers were used on diesel engines starting in the 1920s, but the difficulty of manufacturing turbines able to reliably endure the higher exhaust temperatures of gasoline engines kept turbochargers from wide use on petrol engines until the mid-1930s. Turbochargers became common on aircraft engines shortly before and during World War Two.
Every so often you’ll find someone on the web claiming that Saab offered the first production turbocharged gasoline engine, which is simply not true. Turbocharged race cars began to appear in the early 1950s, but the world’s first production car with a turbocharged engine was the 1962 Oldsmobile Jetfire, followed a few weeks later by the Chevrolet Corvair Monza Spyder. The Olds turbo lasted only two years, the Corvair turbo for four, but both beat Saab by more than a decade.
Turbochargers are nearly universal on modern diesel engines, but forced induction for petrol engines has fallen in and out of favor over the years. Turbochargers became popular in the late 1970s, then nearly faded away in the early 1990s in favor of bigger, normally aspirated engines. With increased political pressure for better fuel economy and reduced carbon dioxide emissions, they’re again becoming common — some engineers think that light-pressure turbochargers will eventually become standard, just like fuel injection. The motivation, as before, is to try to extract big-engine power from smaller, less thirsty engines.