Forced Induction Series - Part 1: The Supercharger
You’ve probably heard the old axiom “there is no replacement for displacement”. The thinking behind it is that the only real way to effectively see a significant boost in horsepower is to swap in a larger engine. Back in the 60s through the early 70s, this was mostly true, which ultimately lead the big three to roll out flagship muscle cars featuring big block behemoth V8 engines.
That rule was eventually displaced by a technology nearly as old as the internal combustion engine itself called forced induction. The earliest version of forced induction was called supercharging. Simply explained, supercharging means turning two screw-shaped rotors against each other, to compress air and hence increase the density of air going into an engine. With a higher air density, an engine can increase its effective compression ratio and produce more horsepower and torque during the power cycle. To power a supercharger, a pulley is attached to the supercharger’s compressor and it is hooked to the engine via a drive belt, gears or chain. This helps deliver power over the entire engine rpm range including in the crucial lower rpm range that is needed for both fast acceleration and better fuel economy.
Early versions of the supercharger were all very similar, they contained two screws that would turn against each other and compress the air at a constant volume no matter the engine RPM. This type of supercharger is called a positive displacement supercharger, and it is normally bolted right over the intake side of the engine. Many V6 and V8 engines have this type of supercharger sitting inside the V of the engine. There are varying types of positive displacement superchargers, the most common are roots type and twin screw. Dynamic compressors work similar to turbochargers; they accelerate air into the engine where the high velocity is exchanged for density as the air's path narrows. This allows for different pressures over the RPM spectrum as speed of the compressor can be varied through internal gearing between it and the pulley.
Although superchargers allow for a net gain in power, supercharging comes with some negative effects. First of which is that any air that is compressed will be heated, the hotter air can cause fuel to pre ignite or not fully burn. This issue can be alleviated by having a radiator that will exchange the heat to either outside air or coolant, but going with the countermeasure is harder to implement with positive displacement superchargers. Another downside is that superchargers add parasitic mechanical loss to an engine. You are using engine crank power to turn the supercharger thus taking away power that can otherwise be delivered to the wheel. Although there is still a net benefit, it is not as great as with a turbocharger that uses the power of exhaust gas that would otherwise be wasted. In recent years there has been a shift away from supercharging in favor of turbocharging as the technology for turbos has become cheaper and more reliable.
Due to weight and size factors, not many cars used superchargers before the 1970’s, although companies started building kits to add superchargers to the popular muscle cars of the era. It would not be until the 1980’s that materials and manufacturing technology caught up to allow more road cars to come with superchargers from the factory. The real heyday of factory-equipped forced induction, i.e. superchargers and turbos began in the mid 80’s. With vehicles like the Toyota MR2 and Ford Thunderbird starting an era of small displacement engines making V8 power with the help of a supercharger.
Many performance variants of vehicles would come with superchargers bolted onto upgraded engines, these models would produce the same power as engines with more cylinders and displacement. Nearly two decades ago, Mercedes Benz began on a path that would make the company the world leader in supercharged cars as it spread its Kompressor line of engines to almost every vehicle in its lineup. Many top of the line AMG vehicles including the Mercedes Benz SLR McLaren would use a supercharged V8 engine.
Even now, as turbocharged engines take over to become the standard for fuel efficiency and big-power, there is still a place in the market for the supercharger. Volvo and Volkswagen are both producing twin-charged engines that have a supercharger for low rpm power and turbochargers for higher rpm power on smaller displacement engines. This unique setup allows for each forced induction system to provide a benefit while offsetting the negative impact of the other.
Come back next week as we delve into the magical world of the turbocharger.