How Forced Induction Works

Air. Fuel. Spark. Take one away from your car and you’re going nowhere fast. Increase one, air for example, and things get interesting. More air equals more power — the very principle behind forced induction. By compressing intake air prior to feeding it into the combustion chamber, forced induction squeezes more air in, along with a correspondingly greater amount of fuel. This results in bigger booms. Bigger booms equal faster, more powerful rotations at the crankshaft. In terms of horsepower and torque this is a good thing: forced induction engines will always up the ante over their naturally aspirated equivalents.

Two systems, supercharging and turbocharging, make this all happen. They differ chiefly in how they generate boost: where a turbocharger is spun by exhaust gases, a supercharger is powered by a pulley via the crankshaft. But before we examine their differences, discuss why they aren’t used everywhere and try to figure out if one is better than the other, let’s take a look at some boosting basics that apply to both systems.

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Boost Basics

Boost describes the amount of pressure that a forced induction system creates. A typical system found in that new whip you’ve been eyeballing typically creates around six to eight pounds per square inch (psi) of extra pressure. Compared to a naturally aspirated engine, that boosted beast sucks in almost 50% more air. For the reasons we mentioned above (namely, bigger booms) this enables smaller displacement engines to punch above their weight class, with 4-cylinder engines making horsepower usually reserved for 6 and 8 cylinder blocks, while hitting smashing EPA targets in the process. Sadly, there are some limitations.

The problem lies in the air-to-fuel ratio (AFR) that a motor can handle. An optimum AFR would provide just enough air to burn all of the available fuel. This completely efficient combustion is called a stoichiometric mixture, or stoich, and has an AFR of 13:1 — thirteen parts air to one part fuel. But stoichiometric mixtures burn extremely “hot” and can damage engine internals when the hammer’s down. Under high load, engines cycle at faster RPMs (revolutions per minute) and simply can’t dissipate the extra heat. This creates knock, or pre-detonation, which causes compression levels within the cylinder that are too high. To keep it together, engine management software causes engines to run on the rich side to keep cooler (cylinder) heads. In a forced induction engine similar solutions are applied to cool those bigger booms and soothe scorching cylinder walls. It’s also why your force-fed whip pines for premium at the pumps.

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Introduced in 1982 by the FIA, the Group B class of rally racing featured some of the most incredible vehicles and drivers to ever grace motorsports. During its short four-year run, Group B saw vehicle technology and horsepower levels grow exponentially. Boost levels were just about unlimited — and the winning cars’ horsepower numbers doubled in five years as a result.

The Lancia Delta S4 is prime example of Group B power excess. It was powered by a 1.8L 4-cylinder that used twin-charging to develop mad power. A supercharger served to aid acceleration in low RPM situations while a turbocharger would kick in, above its lag point, to keep the car pulling. The system could combine to produce 73.5 psi of boost and generate 1,000 horsepower. Reliability on the racetrack dictated that things be dialled back to a more manageable 500 hp (at a more reasonable 32psi), which translated to 0-60 times of approximately 2.0 seconds — on gravel.

Thankfully all hope is not lost: today, Volvo is starting to dabble with some Swedish twins of its own.

The Spin on Turbos

As an engine performs its normal cycle, exhaust gases exit through the exhaust manifold. In a turbocharged engine, these gases are re-routed to pass through and spin a turbine. The spinning of this turbine creates a vacuum, which sucks in and compresses air prior to forcing it into the intake manifold of the engine. As the engine spins faster, so too does the turbo, thereby forcing even more air into the motor. Unused air is released through a bypass.

Turbocharging has been around almost as long as the automobile. Though patented in 1905 by Swiss engineer Alfred Büchi, it took some time before an exhaust spun turbine found a home under the hood. The 1962 Oldsmobile Jetfire was the first to feature a force-fed engine, with the fabled Corvair Monza Spyder following suit shortly thereafter. Unfortunately, high compression and knock caused reliability issues and the turbo was torched after just one model year. BMW was the next manufacturer to experiment with exhaust driven induction almost a decade later with its iconic 2002 Turbo.

Turbo lag — the time it takes to spool up an exhaust-driven turbine — is the main difficulty associated with turbocharging. To hit optimal boost, the turbine must spin at or above a certain RPM. This only occurs when the right amount of exhaust gases pass through the system. During lag a car may feel almost anemic and then, all of sudden, full power kicks in. It almost feels like you’ve been rear-ended.

Modern turbos temper lag with technology — for example, twin-turbo set-ups in both sequential and parallel formats. In a parallel format, two smaller turbos on each exhaust bank spool quickly and combine to create the same amount of oomph of a larger, lag-prone, unit. Sequential systems use complicated plumbing to allocate one turbo to spin under all conditions, with the second unit bypassed. Then, at a predetermined RPM, the second unit spools to life to serve up the full Monty.

Variable Geometry Turbochargers (VGT) are highly effective at mitigating lag. VGTs act like a small turbo under low-load to deliver an efficient flow of power. As your right foot gets heavier, the turbine’s vanes shift position and the powerband increases in a linear fashion until you hit the redline, shift, and do it again. Because of their minimal lag and inherent low boost thresholds, VGTs are extremely efficient at doling out optimum boost throughout the rev range; this also makes them pricey and rare.

Supercharged, And at the Ready

A supercharger (also called a “blower”) is powered by a belt, similar to the accessory drive belt that turns your air conditioning unit. As the engine’s crankshaft spins, so does the supercharger. This spinning creates a vacuum that sucks in and compresses air which is then forced directly into the intake. The direct-drive relationship of the supercharger and engine create a very linear powerband. Superchargers’ direct-drive design means that boost is instantaneous, and by compressing and delivering a fixed volume of air in direct relation to engine RPMs, optimum boost is available at any and all throttle positions. That linear relation also means that the amount of boost created at 6,000 rpm is double what’s made at 3,000 rpm.

Mercedes-Benz was the first to feature a blown-motored (supercharged) car in 1921. Dubbed the Kompressor (a name still used today), it was the first road car to feature forced induction. Other manufacturers quickly followed suit, giving birth to an era of homologation specials, including the iconic Blower Bentley.

In general, superchargers suffer because they must “steal” power from the engine they’re strapped to in order to perform — sometimes requiring up to half of an engine’s naturally aspirated horsepower. The supercharger on top of a top fuel dragster needs the punch of a Bugatti Veyron’s full power just to start spinning. In all cases the power developed by the supercharger will create a net gain effect, but it also translates to efficiency concerns when used in normal vehicles.

Most manufacturers, even Mercedes’ AMG division, have started moving away from supercharging in favor of exhaust-driven experiments. A turbo’s reliance on kinetic energy that would otherwise be wasted through the tailpipe means there is no need to create additional EPA-robbing emissions to generate boost. Plus, with the various technological improvements made with turbochargers and other engine internals such as direct injection, manufacturers can achieve power gains without the sacrifices that traditionally sidelined forced induction. It was once said that in the search for horsepower, there’s no replacement for displacement. Fortunately for all us fossil-fuel-burning lead-foots, that is simply no longer the case.

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