Before we look at the Twin Turbo Zet electronic supercharger, let's spend a bit of time looking at the concept of supercharging.
An engine inhales air that - when mixed with petrol - is combusted inside the engine. The greater the mass of air that the engine can inhale, the higher the pressures that will result from this combustion. Higher pressures result in more force on the piston during the power stroke, giving more torque output at the crankshaft. And a higher torque at a given rpm equals more power.
So that explains the impact of having more combustion pressure during the power stroke - but what about the other three strokes? Prior to the power stroke is the compression stroke, and prior to that again, is the intake stroke. On the intake stroke, the piston descends, creating a lower pressure than atmospheric. The pressure of the air outside of the cylinder is higher than the pressure in the cylinder, so air flows into the cylinder. And as we've already seen, the greater the amount of air that flows in, the higher will be the combustion pressures a few strokes later.
The amount of air that flows into the cylinder, compared with the cylinder volume, is called the engine's breathing - or volumetric - efficiency. In a 3 litre six cylinder engine, each cylinder has a swept volume of 500cc. If the cylinder breathes in only 400cc on the intake stroke, the engine is said to have an 80 per cent volumetric efficiency (ie 400/500 = 0.8 or 80 per cent). Volumetric efficiency will depend on lots of factors (including how well the ports flow), but let's say that the VE of the example engine is in fact 80 per cent. If this 3 litre engine is revving at 6000 rpm full throttle, this means that it inhales 7200 litres of air per minute (remember, one intake stroke per two rpm), or 120 litres per second. To put it in different units, each minute this engine consumes 254 cubic feet of air. To put that into context, a little 60mm diameter PC cooling fan flows only about 18 cubic feet per minute. So, just to flow the amount of air that this naturally aspirated, 3 litre engine needs, you'd need an array of fourteen 60mm fans working flat-out. And that's without creating any boost at all....
Talking about boost, how can we improve an engine's VE? One way is to actively force the air into the engine, pushing it in with more than atmospheric pressure. If you shove in more air than the engine can consume, a boost pressure is developed. When the engine is being fed boosted air, VE can rise to 120 or 150 or even 200 per cent. And that spells good increases in power!
Superchargers are used to force-feed engines with air - to create boost. There are three main designs of superchargers, which can be divided into two categories - positive displacement and centrifugal. In a positive displacement design, every revolution of the blower pumps out a fixed volume of air. Centrifugal types have an airflow which rises as the square of their rotational speed - like turbos, they are more like fans than pumps.
One type of positive displacement blower is the screw supercharger, which uses two rotors turning at different speeds. Because of the relative movement of the rotors, the volume of air trapped between the rotors reduces along their length, compressing the air through the outlet. Mazda has used an IHI Lysholm screw-type positive displacement blower on the Miller Cycle Eunos 800M (New Car Test: Eunos 800 Miller Cycle). Mazda picked this design because the screw-type blower is very efficient - it takes relatively little power to drive it. Centrifugal blowers use compressor wheels spun quickly through the use of step-up gears which can be planetary or conventional in nature. Centrifugal compressors are also very efficient.
So, how much power does it take to drive an efficient supercharger like a screw type? The most efficient type of supercharger, flowing 265 cfm and developing a boost of 11.5 psi, takes 14.5kW to drive it. Figures aren't readily available for centrifugal blowers, but they'd be of a similar magnitude. So the best blower design (the same type that's used on the Mazda 800) takes about 14,500 watts to drive it on a modest-sized engine. This power is derived from the engine via a belt connecting the blower to the engine's crankshaft.
But let's say that instead of using a belt-drive from the engine, we power the supercharger by using a 12 volt electric motor powered from the car battery. For an electric motor power of 14,500 watts, we'd need a current flow of about 1000 amps (14,500 watts divided by 13.8 volts = 1050 amps). So, to supply the current to drive an electric supercharger having the same airflow output as the most energy-efficient type currently available, it would take 1000 amps. To generate this much electrical power would require at least 8 heavy-duty alternators bolted to the engine. Furthermore, to handle this current, the wires connecting the battery to the supercharger would have to be enormously thick - perhaps brass or copper bars 10mm square would be needed.
Centrifugal compressors need to flow a large amount of air to develop boost. Cast alloy blades with complex curved blade designs are used, with the compressor wheel mounted within a special compressor housing having an appropriate aerodynamic design. Any centrifugal supercharger (electric or mechanically-driven) needs this type of compressor wheel if it is to efficiently generate the required airflow.
The Twin Turbo Zet
When connected to power:
Reaction of all technical experts who examined the Twin Turbo Zet:
And, just for your interest, here's a pic of the Twin Turbo Zet together with a normal, 55mm electronics cooling fan.
For more information: www.turbozet.com