Engines: Turbocharging and Supercharging

Since a disgruntled bunch of apes - commonly reffered to as homo sapiens - came down from the trees (or, indeed, since man was sculpted from clay) they have been striving for power in any form possible, not least of which is in the form of the engines they use in their beloved cars (curious creatures spawned from a scientific experiment involving the combination of large pieces of metal and round rubber things... oh, and dodgey CD players). But anyway, the point (yes, there actually is a point) is this: everyone from car manufacturers to your average petrol head wants a more powerful engine. The simple way to get a more powerful engine is - unsurprisingly - to buy a more powerful engine, however, this has long since been deemed both expensive and unnecessarily easy, so, instead, other means were thought up; and lo, the humble supercharger was born - soon to be followed by the mighty turbocharger. Now, you - the reader - may be thinking that these sound like something the power rangers might have, however, you don't need to dress up in a brightly coloured spandex suit to use one - they are, in fact, becoming increasingly more common in your normal everyday car as companies strive to increase performance!

Turbocharging

A turbocharger is a device fitted in various machines (namely cars) with either forced-induction or internal combustion engines. The turbocharges takes the exhaust gasses from the engine and passes them through a turbine that spins at roughly 100,000 rpm and is used to drive an air compressor. The compressor compresses (quelle surprise) air to a pressure greater than that of the atmosphere so a larger mass of air passes into the engine - allowing for more fuel to be injected - causing a greater force to be produced in the combustion stroke of the engine's cycle (assuming a 4-stroke engine), so more power is produced.

The critical value when it comes to combustion in engines is termed the stoichiometric ratio (the correct chemical balance between any two substances - in this case air and fuel). Using cars as an example, in petrol engines the accelerator pedal controls a throttle (rotating valve - when the pedal's fully down the valve is fully open) in the air intake pipe; this is combined with a petrol injection system that injects the appropriate amount of petrol to achieve the stoichiometric ratio as combustion will only take place at roughly this ratio. Hence, more air being allowed into the engine (at a higher pressure so that a greater mass of air can occupy the same volume as a lower mass at a lower pressure) means more petrol can be injected so a more powerful combustion takes place in the cylinders - increasing the power output. With diesel engines the air intake is constant (increased with the air pressure) while the accelerator pedal controls the diesel injection and is configured so that when fully depressed the necessary amount of diesel is injected to attain the stoichiometric ratio. (Diesel engines do not require a stoichiometric ratio to achieve combustion; however, maximum efficiency is achieved when it's attained).

Supercharging

The alternative to turbocharging an engine is to supercharge it - a much older method that's been used since the 1880's! (Turbochargers are far more hip and happening, having been first patented in the 1900s). A supercharger is a form air pump / compressor that’s mechanically driven by the engine (i.e. either directly or indirectly joined to the crank shaft) and performs the same function as the compressor does in a turbocharger.

Which is better?

Supercharging, despite increasing the performance at all engine speeds, runs the compressor directly off the engine and hence also decreases its brake thermal efficiency. Turbo charging, on the other hand, is used to boost the performance of the engine at higher speeds only (at lower speeds, the turbine doesn’t spin as fast so it has little effect on the power), and, because it uses exhaust gasses which would otherwise just be wasted, it has very little effect on the engine’s efficiency and produces more power than a supercharger when the engine’s running at the same speed. While turbochargers are considered more useful and are more commonly fitted as standard in cars, superchargers still do have their uses and are often found in dragstrip racers.

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dcasas 6 years ago

I also wrote a hub about turbos but not nearly as good as this one. Great job on explaining them and really giving a mental picture!

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