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What’s the difference between turbo-charging and supercharging?
Let’s start with the similarities. They are forced induction systems used to increase the power of the engine. It is a fallacy that the engine sucks the air/fuel mixture into the cylinders. It is air pressure that forces the mixture in and this renders the engine a sort of piston pump, one of the least efficient types.
Therefore, by simply increasing the air pressure more mixture can be forced in and power increased. It is not a directly linear benefit so you will not double the power by doubling the pressure but the increase is substantial. All well set-up forced induction devices will increase volumetric efficiency and thereby decrease fuel consumption for a given rate of acceleration and speed. This will, in turn lower emissions.
The difference between supercharging and turbo-charging is that the former is normally driven in the same way as an alternator, by a belt from an engine driven pulley,
or less frequently by gears, whilst turbo-chargers are driven by the energy of the exhaust gasses. Turbo-charging is often said to be ‘free’ in that the power in the exhaust, generally given as 30-40% of that produced by the engine, would otherwise be wasted but this is not strictly true. A certain amount of back pressure must be generated by the turbo installation and this does ‘cost’. However, the power loss is much less than a supercharger. Figures of 40% have been quoted for some supercharger installations but this is a bit fanciful and 10-15% is about right for a road car with up to 5% for turbo-chargers. The conclusion to this is that turbo-charging is more efficient and therefore, all things being equal, fuel consumption should be lower.
The major advantage of a supercharger over a turbocharger is that the latter suffers from turbo-lag. This is because there has to be energy in the exhaust, i.e. the engine has to be working against a load, before it can make a significant addition to the power. Merely pushing the revs up against the limiter without a load does not generate much compression as the engine is not ‘working’.
There are ways of reducing the lag but no way of actually eliminating it. For instance, as the turbo rotates, there is a certain delay whilst it is accelerated up to speed. This is why exotic materials are used for the bearings and ultra lightweight ceramic material is used for the vanes. Further, the turbo is kept as small as possible to make the rotating parts lighter so in order to do the same amount of work as a supercharger it has to run at very high speeds which means a shorter life, or even more exotic materials, or very careful machining, or all three. Whichever way you go, it is expensive.
There is a slight lag in some supercharger installations that have intercoolers as the run is longer. However, this is largely ignored as it is much less than that of a turbo-charger.
A supercharger has an additional advantage, in that it is much easier controlled than a turbo, since there is no turbo lag. Therefore, boost, and thus emissions can be controlled with greater precision.
The advantage of superchargers at lower engine revs is well known. Once a turbo-charger is under pressure the two systems compare very well, with a certain advantage going the way of the turbo-charger given that it is driven from the exhaust. Once up to maximum revs, the supercharger comes into its own again when the turbine in a turbocharger becomes a restriction and thus effectively uses petrol/gasoline to build up excessive heat. Because superchargers are on the ‘cold’ side, it doesn’t suffer from the turbine restriction, and thus none of the heat-sink issues especially prominent for petrol/gasoline applications of turbo-chargers.
A CAD drawing of a Rotrex supercharger, C30 range. The pulley is the circular object to the right with ROTREX on it. The unit has an up and down to it, the flat bottom needing to be within 15 degrees of horizontal. The compressed air comes out of the vent at the top of the picture. See below for one fitted to a car.
Our thanks to Rotrex for permission to use the CAD image.
Image copyright Rotrex.
This photograph is doctored to make the installation of the supercharger clear. This is an early installation by SC Power. The pulley is arrowed. As can be seen it is on the same plane as the other belt-driven items. The compressed air goes over the engine and into the standard plenum.
Other items of the installation have been washed out but will be explained in a later edition of Resource.
Our thanks to SC Power for kind permission to use the image.
Image copyright SC Power.
Supercharging/turbocharging
A supercharger is a much simpler and cheaper system to ‘bolt-on’. However, its downside is that if you use the extra power you use a considerable amount of extra fuel but that goes regardless of which system you go for.
With ever increasing limitations on emissions, the supercharger, and to a lesser extent the turbo-charger, come into their own. Greater efficiency means less emissions and the easiest way of going about this is by supercharging.
There are various ‘types’ of supercharger and turbo-charger but in essence they all do the same thing: compress the air/fuel mixture.