The maximum torque and power generated by an internal combustion engine depends, among other factors, on the amount of fuel that can be burned into the cylinders. The higher the amount of fuel burned, the higher the cylinder pressure, the higher the engine torque (power). The amount of fuel that can be burned inside the engine is limited by the amount of air (oxygen) available for combustion. This means that, even if we inject a high quantity of fuel into the engine, without the right amount of air (oxygen), the fuel will remain unburned and the engine will lack performance and have increased exhaust gas emissions.
In a naturally aspired internal combustion engine, also called atmospheric engine, the air is drawn into the cylinders by suction, when the piston moves towards bottom dead centre (BDC) and creates volume into the cylinders. In this case, the mass air flow depends on the throttling of the intake manifold, and the air pressure is always less than atmospheric pressure (1 bar/atm).
For a fixed engine size (displacement), by compressing the intake air to a higher density than the atmospheric air, before entering the cylinders, we’ll increase the torque (power) output of the engine. This is the main purpose of a supercharged engine. Therefore, a supercharged engine is a internal combustion engine which is using compressed air before cylinder intake, in order to increase the torque and power output.
By compressing the intake air, its density increases, which meas that, for the same volume, there are more oxygen molecules available for combustion. This means that more fuel can be injected and hence higher combustion pressure obtained, which translates into higher engine torque and power.
There are several methods of compressing the intake air of an engine:
- exhaust gas turbocharging
- mechanical supercharging
- pressure wave supercharging
Turbocharging is explained in detail in the following articles:
- How turbocharging works
- How turbochargers work
- Twin-scroll turbochargers
- Variable Geometry Turbocharger (VGT)
In mechanically driven superchargers, the compressor is driven directly by the engine. This means that the compressor is mechanically connected to the crankshaft, through a gear or belt, and takes power from the engine in order to compress the intake air.
Depending on the method of air compression, mechanically driven superchargers are split into two main categories:
- centrifugal superchargers
- positive-displacement superchargers
In this article we are going to focus on mechanically driven centrifugal superchargers.
The mechanical centrifugal supercharger uses the flow principle and the principle of momentum in order to compress the intake air. It basically consists of a compressor wheel (impeller) which is mounted on a shaft and driven by the engine through a gear set or a belt drive.The atmospheric air is drawn by the impeller (compressor) through the filter and compressed to a higher pressure. Before entering the engine (cylinders) the intake air is cooled, to further increase its density. Cool dense air means more fuel can be injected into the engine, which generates more torque (power) at the crankshaft. The boost pressure depends on the engine speed, the higher the speed of the impeller (engine), the higher the pressure of the compressed air.
Since the power required by the compressor to rotate is taken directly from the engine and not from the exhaust gases, as in the turbocharger case, the disadvantage is that the supercharger increases the parasitic loads on the engine. The advantage is that there is no temperature transfer from the exhaust gases to the compressed air, which translate into higher intake air density, compared to turbocharging.
The simplest drive gear for the centrifugal supercharger is using a belt, connected to the crankshaft through two pulleys. This simple and efficient method is however limiting the operation of the supercharger, since the output pressure is directly linked to the engine speed. There are also more advanced concepts, like Torotrak V-charge CVT driven supercharges, which are using full toroidal variators in order to control the speed of the impeller, independent of the engine speed. This method gives a better control of the boost pressure function of the engine operating point (speed and torque).
Another advantage of the supercharger, compared to turbochargers, is that the mass of intake air increases approximately in direct proportion to the engine speed, and therefore air requirement, of the engine. In the case of superchargers, since its range of operation is not limited by compressor surge (as in the turbocharger case), a much wider compressor operation area is possible. Also, having a direct mechanical connection to the engine, the response to sudden demand in intake air pressure is much faster.
- air filter
- centrifugal supercharger
- crankshaft pulley
- intake manifold
Since the compressor of the mechanical supercharger functions based on the flow principle, its overall efficiency is high, having also best ratio between dimension and volumetric flow, compared with other mechanically driven superchargers.
A mechanically driven supercharger can operate to speeds up to 100000 rpm or more. This means that, being directly driven by the engine, it needs quite significant gear ratios. First speed conversion is done at the pulleys (if driven by belt), with a speed ration of around 2:1. The second speed conversion is done internally in the compressor housing, through a fixed simple gear set or a planetary roller (gear) set. The internal gear ratio can go up to 15:1. This gives an overall speed conversion of around 30:1, which, for an engine speed of 2000 rpm, translates in an impeller speed of 60000 rpm.
The performance of the supercharger is measured function of its boost pressure. A pressure boost of 1.2 bar means that the supercharger has increased the intake air pressure with 1.2 above the atmospheric pressure (1 bar). This means that the absolute pressure in the intake air manifold, after compressor, is 2.2 bar. The boost pressure is not constant, it depends on the speed of the impeller, the faster the impeller rotates, the higher the boost pressure.
- compressor housing (volute)
- aeration oil pump
- transmission (simple fixed ratio gear)
The centrifugal supercharger contains an impeller which spins at high speed to draw air into a small compressor housing (volute). When air leaves the impeller, it is travelling at high speed while having a low pressure. This low-pressure, high-speed air is sent through a diffuser which converts the airflow so that it is high-pressure, and low-speed. The air is then fed into the engine, where the additional airflow (caused by increased pressure) gives the engine the ability to burn more fuel and have a higher level of combustion. This results in a faster, more responsive vehicle due to greater engine efficiency.
The impeller is the main rotating component of the supercharger and it’s the component which is creating air boost. The impeller pushes air into the blower and builds the pressure that translates directly into positive intake manifold pressure, also known as boost pressure. The impeller must be able to withstand high operating temperatures, and must be durable enough to continually perform at high engine speeds.
The snail-shaped design of the compressor housing is a trait unique to centrifugal superchargers. Technically known as a collector, the purpose of this compressor housing is to gather the airflow and deliver it to a downstream pipe. Although compressor housings can be made from a wide variety of metals or alloys, they are typically formed or cast from aluminum. Aluminum is typically used for supercharger housings/volutes due to the combination of strength, weight, and resistance to corrosion. After the housing is cast, it is then machined to match the impeller design. During assembly of the supercharger the housing is attached to the transmission with securing bolts or band clamps.
Located between the impeller and the volute is the diffuser. Downstream of the impeller in the flow path, it is the diffuser’s responsibility to convert the kinetic energy (high velocity) of the gas into pressure by gradually slowing (diffusing) the gas velocity.
Together with the step up ratio obtained through the (belt) drive system, a transmission step up is also required to obtain the impeller speeds necessary to create the desired boost. Additionally, the transmission contains bearings to support the shafts that are attached to the internal gears. Bearings are used throughout the system to help parts move smoothly and to reduce friction and wear. All centrifugal supercharger bearings must be able to withstand high-speed movement on a constant basis.
The simplest forms of supercharger transmissions are using a simple gear set or a planetary roller (gear) set. There are also more complicated designs, for example Torotrak V-charge CVT driven supercharges, which are using toroidal transmissions in order to modify the overall gear ratio of the supercharger function of the engine’s operating point (speed and torque).
- front housing
- input ring
- compressor housing
- compressor wheel
- rear housing
Torotrak’s V-charge is a mechanically driven centrifugal supercharger, with a variable ratio traction drive. The toroidal transmission, acting as a continuously variable transmission (CVT), is capable of changing the gear ratio, from minimum (0.28) to maximum (2.8) in less than 400 ms. It has a hydraulic low power actuation system, which is consuming less than 20 W when is changing ratio.
The V-charge supercharger is belt driven by the engine, with a pulley ratio of 2.5:1. Also, the output of the toroidal variator goes through a planetary roller set with a ratio of 12.5:1. The combination of the belt pulley, toroidal transmission and planetary roller set gives a minimum speed ratio 8.75:1 and a maximum speed ratio of 87.5:1.
Proper lubrication is essential for continued centrifugal supercharger performance. The high speed required for the supercharger to create boost demand adequate lubrication for all moving parts. There are several methods of lubrication used in centrifugal superchargers. Some designs utilise engine oil to provide lubrication for the supercharger. On enclosed (self-contained) systems, the lubrication is low weight synthetic oil which is specifically engineered for high-speed use. The lubricating oil is distributed throughout the transmission via an oil slinger/pump.
- oil canister
- oil filter
- centrifugal supercharger
- oil cooler
Centrifugal superchargers use a small portion the engine’s power to drive the movement of the supercharger’s internal components. Efficiency has both mechanical (power consumption) and thermal (heating of the compressed air) factors. A higher efficiency means the supercharger consumes less energy from the engine powering it, and produces less heat. There is substantial heat within an engine compartment, and some supercharger designs allow substantial heat transfer from the engine and other components to the supercharger. This in turn allows additional heat to be transferred to the air being compressed inside the supercharger, effectively decreasing efficiency.
In conclusion, let’s summarise the objectives of supercharging:
- for a given engine size (displacement), supercharging is increasing the torque and power output (e.g. a 2.0 litre naturally aspired (NA) engine has 110 kW peak power, while a 2.0 litre supercharged engine has 140 kW peak power)
- for a downsized engine (smaller displacement), with lower fuel consumption and exhaust gas emissions, supercharging is keeping the same level of power and torque output of the engine (e.g. a 1.2 liter supercharged engine has the same torque and power output as a 1.6 litre NA (atmospheric) engine)
Usually, superchargers are used in high performance engines. For fixed speed ratio superchargers, their overall speed ratio needs to properly match the compressor air flow map with the usable engine speed. This means that the belt pulley ratio and internal transmission ratio needs to be selected function of the engine speed and where the peak boost pressure is required.
- centrifugal supercharger
- rubber belt
- crankshaft pulley
Low speeds result in a lower air boost output, and the need for high speed to produce high levels of boost is one of the disadvantages of the centrifugal supercharger. Off idle acceleration produces low boost pressure, a result of low engine speed. To achieve the very high impeller speeds (above 40000 rpm) required for significant boost pressure, a combination of a smaller pulley on the supercharger belt drive, compared to the crankshaft, is needed.
Centrifugal superchargers have high flexibility in terms of mounting location, which makes them popular for aftermarket use. They can be placed before or after the throttle body. A discharge tube can be used to carry the compressed air to either the engine intake or an intercooler, rather than being attached directly to the intake manifold.
Centrifugal superchargers have minimal heat transfer, as a result of their low internal compression ratio. This high thermal efficiency results in an increased power gain, compared to positive-displacement superchargers.
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