The Fundamentals of Superchargers and Turbos

The basics on how engines get force fed air and power gets boosted

centrifugal-supercharger-Kawasaki-cutaway

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Kawasaki H2R centrifugal supercharger cutawayCourtesy of Kawasaki

Unsupercharged, or “atmospheric” engines are those whose intakes are open to the atmosphere. Because atmospheric pressure is only about 14.7-psi, this puts an upper limit on how much air an unsupercharged engine cylinder can take in. If we want more power, we must burn more fuel/air mixture per second. We have two obvious choices in how to do this; to increase engine displacement (bigger or more cylinders) or to increase engine rpm (filling cylinders more frequently).

A less obvious choice is to force air into engine cylinders to charge them above atmospheric pressure by using a pump of some kind. This is called supercharging because it is cylinder charging at a pressure above (super means above) atmospheric pressure. There are many ways to do this, and here are some of them:

  1. Roots blower — This is the familiar type of rotary blower seen atop classic AA Fuel dragster engines. A pair of rotating meshing lobes sweep air through a vaguely figure-8-shaped casing. This is called a "positive displacement" blower because each rotation delivers a geometrically fixed volume of air or mixture. Roots blowers are relatively simple but are inefficient because they do not compress air – they deliver it at atmospheric pressure.
Roots blower illustration

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A labelled cross-section of a Roots blower with 2-lobed rotors. The rotor form is cycloidal, and in this drawing is drawn accurately. In fact, it is part epicycloidal and part hypocycloidal, and in both cases the smaller generating circle is 1/4 the diameter of the circle it rolls around. Roots blowers find uses in superchargers and gas pumps which need a high volume at a low differential pressure.By Inductiveload (Own work Based on File:Rotary_piston_pump.svg) [CC BY-SA 3.0], via Wikimedia Commons
  1. Vane blower — This too is a positive displacement machine, but in it, a radially slotted cylindrical rotor, offset inside a cylindrical housing, carries radial vanes in its slots. As the offset rotor turns, the vanes move in and out in their slots to keep their tips in contact with the inside of the housing. The volume between vanes varies as the rotor turns. By placing an intake where volume is increasing, and an outlet where volume is at a minimum, air or mixture can be drawn into the machine, compressed, and delivered to the engine.
Powerplus vane type supercharger diagram

Powerplus-vane-type-supercharger-diagram

Cross-section diagram of a Powerplus vane-type superchargerBy Oscar W. Schey & Herman H. Ellerbrock, jr. [Public domain], via Wikimedia Commons
  1. Centrifugal blower — A fast-spinning disc carrying radial vanes on one or both of its faces is contained in a close-fitting housing. Air enters on the central axis of the disc, is accelerated and flung outward by its vanes, acquiring the velocity of the tips of the radial vanes. Surrounding the housing is a scroll-shaped diffuser, in which the fast-moving air (tip speeds of 1100 ft./sec. are typical) is decelerated to convert its kinetic energy into pressure. Centrifugal blowers can be highly efficient but their output rises steeply with impeller rpm. This is called a dynamic machine because its output pressure depends not on geometry but on velocity. Kawasaki's recent H2-R employs a gear-driven centrifugal supercharger.

  2. Piston compressor — Once common but little-used today for supercharging is the piston compressor. The German DKW firm used piston compressors in some of its successful prewar racing two-strokes. Two-stroke engines of the 1975-2001 Grand Prix era used the undersides of their power pistons as scavenge pumps, delivering mixture compressed in their crankcases. Basically, picture most any air compressor you'd buy to drive your nail gun or impact wrench.

  3. Axial flow compressor – air gains energy by passing through 7 – 12 alternating rows or stages of rotating and non-rotating circular arrays of vanes. Although commonly employed as the compressors on jet engines, axial flow machines have occasionally been used as superchargers.

Compr Assiale diagram

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Low pressure axial compressor scheme of the Olympus BOl.1 turbojetBy F l a n k e r (made by uploader) [Public domain], via Wikimedia Commons

Most of these blower types are mechanically driven by belts or gears, but when exhaust energy is recovered by passing it through a turbine, and the turbine shaft carries a centrifugal blower, the result is called a turbo-supercharger, or simply a turbocharger. Engines powering thousands of US WW II combat aircraft such as the P-47, P-38, B-17 and B-29 were tubosupercharged to boost their performance at high altitudes, but since the 1960s it has also been common to turbocharge auto and bike engines to increase their power for sport and racing use.

MAN Diesel & Turbo turbocharger

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Turbocharger for two-stage turbochargingCourtesy of MAN Diesel & Turbo

Turbochargers are highly efficient and work very well at constant speed, but when they must serve engines that operate at varying speed, problems arise. A turbocharger of a size for a motorcycle engine must spin as high as 150,000-250,000-rpm to deliver high output pressure (called “boost”), and because it takes time for the exhaust turbine to drive turbo shaft speed that high, the problem of “turbo lag” appears. The vehicle operator opens the throttle, but because it takes time to spin up the turbine/blower assembly, engine torque may take one or two seconds to appear. Various tricks are used to shorten turbo lag, such as variable exhaust nozzle area (like putting your thumb over the end of the garden hose to get a faster-moving stream of water), lighter-weight turbine wheels made of ceramic, or use of two smaller turbines in place of a single larger one.

Because the compression of air raises its temperature, any form of supercharging or turbocharging can make engine knock or detonation more likely. To reduce this tendency, the air or mixture output of a supercharger may be passed through a charge air cooler (common name is “intercooler”) to reduce its temperature. This is easy to provide on a car, but not so easy to find room for on a bike.

2007 Ecotec Turbo 2.0L I-4 (LNF) Turbocharger

2007 Ecotec Turbo 2.0L I-4 (LNF) Turbocharger

Turbocharger for 2007 Ecotec Turbo 2.0L I-4 (LNF), David Kimble Illustration.Photo from Sloan Automotive Laboratory, MIT

The term “boost” refers to the pressure above atmospheric delivered by the supercharger. It may be given in pounds per square inch (psi) or in atmospheres (one atmosphere is 14.7 psi). Low boost in the range of 3-5 psi can usually be handled by a modern engine with little or no modification, but higher boost makes it necessary to reduce the engine’s compression ratio to prevent engine knock or detonation. Typically horsepower rises directly with boost — a one atmosphere boost roughly doubles horsepower.

Automakers are using turbochargers to allow smaller, more fuel-efficient engines to perform as well as larger unsupercharged engines, but it is far from clear that this technique will in future be applied to motorcycle engines.

Kawasaki Ninja H2R engine cutaway

Kawasaki-Ninja-H2R-engine-cutaway

Kawasaki Ninja H2R engine cutawayCourtesy of Kawasaki