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Understanding Superchargers & Turbochargers – The Inside Story

Understanding Turbochargers, Superchargers, & Naturally Aspirated power magic

Understanding Turbochargers, Superchargers, & Naturally Aspirated power magic

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The results of huge horsepower are awesome and entertaining, but we sense some confusion in the audience about how prodigious power is produced. Here’s a pop quiz: Compare and contrast turbo- and superchargers, then outline strategies to make a naturally aspirated engine’s output surpass forced-induction power. Would you like to read the Cliffs Notes before the test?

Turbochargers And Superchargers
Let’s start with turbos. A turbocharger, according to the Society of Automotive Engineers (SAE), is “a device for increasing the pressure and density of the fluid entering an internal-combustion engine using a compressor driven by a turbine that extracts energy from the exhaust gas.” Well, that certainly clears things right up!

If you thought “college frat” when you read “SAE,” try this liberal arts definition: Turbochargers (and superchargers) are air pumps. They increase a piston engine’s power by forcing more air (and, thus, oxygen) into its cylinders than can atmospheric pressure alone. More oxygen (and the associated effective increase in compression ratio) means more fuel can be burned, creating more power. It’s like the time you hooked up the vacuum-cleaner hose backward to blow air onto the charcoal grille to speed up the fire after you’d wasted so much time measuring the pressure and density of the carbonated malt beverage exiting the keg at the SAE frat house.

Manufacturer-installed super- or turbochargers can increase engine potency as much as 50 percent without either the weight penalty of shoehorning a bigger naturally aspirated powerplant into the same engine compartment, or, for example, the cost of developing a more complex overhead-cam powerplant.

Turbochargers employ otherwise wasted engine-exhaust gases to spin a fan inside a snaillike turbocharger housing. A shaft connects this fan, sometimes called the turbine wheel, to a compressor fan in the other, and separate, half of the housing. Turbos’ advantages over superchargers include lighter weight, smaller size (for easier underhood packaging), and better fuel mileage during easy driving. Turbos’ disadvantages include a delay of power delivery between when the driver stands on the gas and when increasing exhaust gas flow causes the turbo to spool up enough to pump a useful amount of air, a.k.a. “boost.”

To quell this “turbo lag” many manufacturers use two smaller turbos, which can be accelerated faster than a single large blower and together will pump enough air at high engine speeds to make big horsepower. The Toyota Supra Turbo’s “sequential” system employs a small turbo for quick response at low rpm and adds the output of a larger inline turbo for big power at higher revs. Another turbo disadvantage is greater sensitivity to the type of abuse you call normal driving, i.e. higher oil temperatures. (When was the last time you changed your car’s oil?)

In today’s automotive lingo, a supercharger is described as a mechanically driven-usually via a belt from the crankshaft-compressor. Most commonly, the compressor is composed of a pair of gearlike devices, turning in opposite directions, inside a housing that you never want to take apart for any reason-trust us. Turbos were known as turbosuperchargers when these exhaust-driven superchargers first showed up on military aircraft before World War II. (Remember the banjo-looking thingies just behind the P-38 Lightning’s propellers that, as an eight-year-old model builder, you thought were drum magazines for the machine guns? They were turbos!)

A leading advantage of supercharging is instant power availability at low engine speeds. Compared with turbos, superchargers’ disadvantages include higher weight and, since they’re pumping air even when you don’t need extra power, reduced fuel economy and increased engine wear. (An electronic clutch on the Mercedes-Benz SLK’s charger obviates this disadvantage.) Also, considerable power can be required just to drive a supercharger.

Although not currently employed as original equipment by automakers, centrifugal superchargers are offered by several aftermarket firms. While belt-driven like a Roots-type supercharger, these use a turbochargerlike vane-type impeller. Advantages include smaller packaging and, proponents say, a more efficient compressor that draws less horsepower and produces a cooler compressed charge.

The measure of a charger’s compressed air output can be confusing. Although most automakers talk in pounds-per-square-inch (psi) of air pressure, some European builders use the metric “bar,” while Indy car drivers talk of “inches of boost,” the latter shorthand for “inches of mercury manifold boost pressure.” When last year’s Indy cars were running 45.0 inches of boost (a mild level of pressure as compared with other racing turbos), that was equal to 1.52 bar or 22 psi. The Aston Martin DB7’s supercharger pumps 14 psi, just a hair shy of 1.00 bar, or in Indy-ese, 28.5 inches. The Lotus V-8’s turbo produces a modest 10 psi, equal to 0.70 bar or 20.4 inches.

The “intercoolers” found on many forced-induction cars are radiatorlike heat exchangers to remove some of the heat that compression puts in the intake air. Hot intake air isn’t conducive to maximum engine performance. Air-to-air intercoolers work just like conventional radiators, except that intake air, not engine coolant, flows inside the intercooler. Air-to-water intercoolers are surrounded by a jacket through which engine coolant is pumped.

Naturally Aspirated Engines
Naturally aspirated engines-those that rely on atmospheric pressure alone to fill the vacuum created by downward piston travel-need not fall behind their forced-induction relatives: Witness the 87.3-horsepower-per-liter output of the Ferrari 550 Maranello’s 5.5-liter V-12, which surpasses the 63.3 ponies per 1000 cubic centimeters produced by GM’s 3.8-liter supercharged Series II V-6.

Note the differences between the two engines: The forced-induction Series II is a low-tech, low-revving powerplant whose priorities include stout low-speed acceleration, good fuel economy, unquestionable reliability, and low production cost. The Ferrari is naturally aspirated Formula One technology come to the street, and its focus is neck-snapping acceleration and aircraftlike top speed, with cost virtually no object. The Series II’s single in-block camshaft opens but two valves for each cylinder through a tortuous path (including a 180-degree turn!) of pushrods and rocker arms. The Ferrari’s dual overhead camshafts-with two cylinder heads, that’s a total of four camshafts-directly open four valves for each of its 12 cylinders.

These differences, among others, mean the big Ferrari can rev safely to 7600 rpm, while the much smaller Series II redlines at but 6000 rpm.

Because a piston engine is essentially an air pump, creating more power is a fairly simple recipe: Make a bigger, faster, more efficient air pump. Without employing forced induction, one way to increase an engine’s ability to fill its cylinders with air and fuel is to increase the size of the valve openings. But, within the confined space of the cylinder bore, there’s a limit to what can be accomplished by increasing the diameter of a single intake and a single exhaust valve. Four smaller valves (two for intake and two for exhaust) per cylinder give more valve-opening area than two big valves, but require the more expensive, complex, and heavier overhead-cam arrangement. Five valves per cylinder, such as used in the Ferrari F50, flow even more air.

Another prescription for naturally aspirated power: More-aggressive camshaft profiles that open the valves higher and hold them open longer. This allows more good air in and more bad out. Computer-designed intake and exhaust passages not only flow more air, but also move it in ways that both pack more air and fuel into the cylinder and help draw burned gases out into the exhaust.

Increasing the compression ratio in turn increases the air-fuel temperature mixture before the spark plug fires. (A hot air-fuel mixture is unproductive on the way into the cylinder, but desirable-up to a point-at the end of the compression stroke.) The advantage: Minimal combustion energy is wasted preheating unburned gases, so most is focused on pushing the piston back down.

Computer-managed engines permit aggressive compression ratios and spark advance (firing the spark plug well before the piston reaches the top of its stroke) without danger of spontaneous combustion of the air-fuel mixture. Called preignition, or detonation, this spontaneous combustion can ruin the windshield wipers of a car following close behind when little droplets of molten aluminum from your pistons spray onto its windshield.

Also, when combined with these deep-breathing exercises, enhancing an engine’s ability to rev higher produces more power. Lighter, tougher, and more expensive moving components allow higher engine redlines and, assuming the torque curve doesn’t fall away, more horsepower.

Unfortunately, high-rpm power is difficult to exploit in everyday driving: With both engines turning at a leisurely 2000 rpm, that 477-horsepower Ferrari makes little more power than the 235-horse GM supercharged Series II.

Here’s a look inside a Garrett turbocharger, courtesy of Allied-Signal Turbocharger Systems. The main difference between a turbo- and a supercharger: The former is driven by exhaust gasses; the latter is driven mechanically by the crankshaft, usually via a belt.

The act of compressing air raises its temperature, limiting potential performance. Intercoolers, like the air-to-air units on this twin-turbocharged Mitsubishi 3000GT VR-4 engine, are actually radiators utilized to remove some of that heat. This 3.0-liter V-6 is a DOHC design with two camshafts mounted atop each of the cylinder heads.

A peek inside the Mercedes SLK’s Eaton M62 Roots-type supercharger reveals a pair of meshed gears that, working like a pump, force more air into the engine’s cylinders than can atmospheric pressure alone.

Four small valves can flow more air than two big valves. And as illustrated by this Ferrari F50 cylinder head, five valves-three intake and two exhaust- flow more than four.

GM’s Eaton-supercharged Series II OHV V-6 takes the reliable, cost-efficient, low-tech route to make gobs of low- and midrange torque. Note the conventional single in-block camshaft (arrow), which opens two valves per cylinder via pushrods and rocker arms.