Supercharging with volumetric supercharger
Although seldom used in the automotive field compared to the turbocharger, it nevertheless has some advantages over the turbocharger. Let's see how it works and examine the differences between driving a turbocharged and supercharged car. If you have regularly followed our automotive blog, you will have noticed that we have already talked about turbocharging systems several times. We have discussed the different types of turbochargers (even the compound turbo system), the low-speed responsiveness of turbines, and their pressure control systems. We have also talked about multi-stage turbocharging systems, specific power records, and intercoolers while explaining the differences between air-to-air and air-to-water ones.
Now let's focus on supercharging with a positive displacement compressor. This is a solution that is not widely used on road cars and not used at all in heavy-duty trucks or in trucking in general. As a starting point, let's begin with the reason why positive displacement compressors are not used in heavy-duty trucks. The reason is related to the performance of this system.
How does a positive displacement compressor work?
There are several types, with the most common being the lobe compressor. This is a mechanical system in which two axes contain profiles that rotate together perfectly, like two gears but without teeth, which reduces noise and increases efficiency. The positive displacement compressor is so named because it compresses predefined volumes of air determined by its geometry and rotational speed.
The turbocharger uses a turbine moved by exhaust gases to make a centrifugal (not volumetric) type of compressor rotate very quickly. The first disadvantage of this (volumetric) compression system is that it uses mechanical force from the crankshaft. As you well know, the energy used to move the lobe rotors that act as a positive displacement compressor is not free and is subtracted from the overall power of the heat engine. A pulley attached to the crankshaft moves a (smaller) pulley attached to one of the compressor shafts (usually via a poly-V belt). This system, which actually subtracts power from the engine, is not optimal from an efficiency point of view. In fact, remember that the turbocharger works by recovering the energy of the high-temperature exhaust gases escaping from the manifolds. It does not subtract (nearly) any power from the engine, except for a small part due to the backpressure on the exhaust system, which is higher in the exhaust manifold and turbine tract than in a similar naturally aspirated engine
The positive displacement compressor does not impact the fluid dynamics of the engine exhaust system in any way. It subtracts some engine power in order to compress air in the cylinders and thus can inject more gasoline for more torque and maximum power. Like turbocharged engines, the positive displacement compressor is also combined with an intercooler system to lower the temperature of the air entering the cylinders. As with turbocharged engines, engines with a positive displacement compressor can have either an air-air heat exchanger (cooling of the intake air by air) or an air-to-water heat exchanger (cooling of the intake air by water). This system is not used on commercial vehicles, construction equipment, gensets, etc., because it is less efficient than what can be obtained with a similar turbocharged engine.
But what are the advantages of the positive displacement compressor?
The lobe or geared compressor has no response delay! The turbocharger has its own mechanical inertia, also at low rpm when the exhaust gases may be insufficient to drive high rotational speeds of the centrifugal compressor needed for generating boost. The positive displacement compressor is always engaging, and its supercharging is determined by its geometric conformation (the volume can handle the rotation of the lobes) and the gear ratio between the crankshaft and the compressor itself.
In order to act on the boost pressure of an engine equipped with a positive displacement compressor, the size of the pulleys connecting the crankshaft to the compressor shaft must be altered.
The greater the reduction in speed with respect to the heat engine, the lower the supercharger pressure will be, while the higher the rotational speed transmitted to the compressor, the higher the boost pressure will be. Engine tuning kits that adopt this supercharging system consist of an electronic modification (Eprom, remapping, additional chip tuner) and a pair of pulleys that vary the speed ratio between the engine and the compressor, the only means of increasing the air pressure sent to the cylinders!
Examples of cars using the positive displacement compressor.
The best-known ones are Mercedes, partly because of the Kompressor lettering on the back of cars that use this supercharging system. Jaguar uses a supercharger with 2 air-water intercoolers on some of its mighty V8s. Going further back in time, we can recall the VW Corrado G60 that used (at the turn of the 1990s) an 1800 gasoline engine with a supercharger and 160hp output. As mentioned, these engines immediately respond to accelerator pedal commands because they have no delay in generating boost pressure. A 2-liter engine with a supercharger can have the same power, torque, and response as a 3-liter naturally aspirated engine. The main difference lies in the type of sound the engine makes due to the presence of this accessory, which generates a kind of mechanical hiss, very different from the "whistle" of turbochargers.
If you liked this article, let your friends know about it too, and don't miss our other technical articles about other supercharger systems, how they work, and how they can be electronically tuned to increase performance!
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