How Do Superchargers Work?
Power for the supercharger can be provided mechanically by means of a belt, gear, shaft, or chain connected to the engine's crankshaft.
Common usage restricts the term supercharger to mechanically driven units; when power is instead provided by a turbine powered by exhaust gas, a supercharger is known as a turbocharger or just a turbo - or in the past a turbosupercharger.
There are two main types of superchargers defined according to the method of gas transfer: positive displacement and dynamic compressors. Positive displacement blowers and compressors deliver an almost constant level of pressure increase at all engine speeds (RPM). Dynamic compressors do not deliver pressure at low speeds; above a threshold speed pressure increases exponentially.
Positive-displacement pumps deliver a nearly fixed volume of air per revolution at all speeds (minus leakage, which is almost constant at all speeds for a given pressure, thus its importance decreases at higher speeds).
Positive-displacement pumps are further divided into internal and external compression types.
Roots superchargers, including high helix roots superchargers, produce compression externally.
- External compression refers to pumps that transfer air at ambient pressure. If an engine equipped with a supercharger that compresses externally is running under boost conditions, the pressure inside the supercharger remains at ambient pressure; air is only pressurized downstream of the supercharger. Roots superchargers tend to be very mechanically efficient at moving air at low-pressure differentials, whereas at high-pressure ratios, internal compression superchargers tend to be more mechanically efficient.
All the other types have some degree of internal compression.
- Internal compression refers to the compression of air within the supercharger itself, which, already at or close to boost level, can be delivered smoothly to the engine with little or no backflow. Internal compression devices usually use a fixed internal compression ratio. When the boost pressure is equal to the compression pressure of the supercharger, the backflow is zero. If the boost pressure exceeds that compression pressure, backflow can still occur as in a roots blower. The internal compression ratio of this type of supercharger can be matched to the expected boost pressure in order to optimize mechanical efficiency.
Positive-displacement superchargers are usually rated by their capacity per revolution. In the case of the Roots blower, the GMC rating pattern is typical. The GMC types are rated according to how many two-stroke cylinders, and the size of those cylinders, it is designed to scavenge. GMC has made 2–71, 3–71, 4–71, and the famed 6–71 blowers. For example, a 6–71 blower is designed to scavenge six cylinders of 71 cubic inches (1,163 cc) each and would be used on a two-stroke diesel of 426 cubic inches (6,981 cc), which is designated a 6–71; the blower takes this same designation. However, because 6–71 is actually the engine's designation, the actual displacement is less than the simple multiplication would suggest. A 6–71 actually pumps 339 cubic inches (5,555 cc) per revolution (but as it spins faster than the engine, it can easily put out the same displacement as the engine per engine rev).
Aftermarket derivatives continue the trend with 8–71 to current 16–71 blowers used in different motorsports. From this, one can see that a 6–71 is roughly twice the size of a 3–71. GMC also made 53 cu in (869 cc) series in 2–, 3–, 4–, 6–, and 8–53 sizes, as well as a "V71" series for use on engines using a V configuration.
Superchargers are further defined according to their method of drive.
- Belt (V-belt, Synchronous belt, Flat belt)
- Direct drive
- Gear drive
- Chain drive
One disadvantage of supercharging is that compressing the air increases its temperature. When a supercharger is used on an internal combustion engine, the temperature of the fuel/air charge becomes a major limiting factor in engine performance. Extreme temperatures will cause detonation of the fuel-air mixture (spark ignition engines) and damage to the engine. In cars, this can cause a problem when it is a hot day outside, or when an excessive level of boost is reached.
It is possible to estimate the temperature rise across a supercharger by modeling it as an isentropic process.
- = ambient air temperature (absolute)
- = temperature after the compressor (absolute)
- = ambient atmospheric pressure (absolute)
- = pressure after the compressor (absolute)
- = Ratio of specific heat capacities = = 1.4 for air
- = Specific heat at constant pressure
- = Specific heat at constant volume
For example, if a supercharged engine is pushing 10 psi (0.69 bar) of boost at sea level (ambient pressure of 14.7 psi (1.01 bar), ambient temperature of 75 °F (24 °C)), the temperature of the air after the supercharger will be 160.5 °F (71.4 °C). This temperature is known as the compressor discharge temperature (CDT) and highlights why a method for cooling the air after the compressor is so important.
Note: in the example above, the ambient air pressure (1.01 bar) is added to the boost (0.69 bar) to get total pressure (1.70 bar), which is the value used for in the equation. The temperatures must be in absolute values, using the Kelvin scale, which begins at absolute zero (0 Kelvin) and where 0 °C is 273.15 K. A Kelvin unit is the same size as a Celsius degree (so 24 °C added to absolute zero is simply 273.15 K + 24 K).
So this means,
- = 1.70 bar (24.7 psi = [14.7 psi + 10 psi boost]; or 1.70 bar = [1.01 bar + 0.69 bar])
- = 1.01 bar
- = 297.15K (24 K + 273.15 K; use the Kelvin scale, where 0 °C equals 273.15 Kelvin)
- the exponent becomes 0.286 (or 1.4-1/[1.4]),
- = 344. 81 K, which is roughly 71.7 °C [344.81 K - 273.15 (since 273.15 K is 0 °C)]
Where 71.7 °C exceeds 160 °F.
While it is true that higher intake temperatures for internal combustion engines will ingest air of lower density, this only holds correct for static, unchanging air pressure. i.e. on a hot day, an engine will intake less oxygen per engine cycle than it would on a cold day. However, the heating of the air, while in the supercharger compressor, does not reduce the density of the air due to its rise in temperature. The rise in temperature is due to its rise in pressure. Energy is being added to the air and this is seen in both its energy, internal to the molecules (temperature) and of the air in static pressure, as well as the velocity of the gas.
Inter-cooling makes no change in the density of the air after it has been compressed. It is only removing the thermal energy of the air from the compression process. i.e. the inter-cooler only removes the energy put in by the compression process and does not alter the density of air, so that the air/fuel mixture is not so hot that it causes it to ignite before the spark ignites it, otherwise known as pre-ignition.
Two Stroke Engines
In two-stroke engines, scavenging is required to purge exhaust gasses, as well as charge the cylinders for the next power stroke. In small engines this requirement is commonly met by using the crankcase as a blower; the descending piston during the power stroke compresses air in the crankcase used to purge the cylinder. Scavenging blowing should not be confused with supercharging, as no charge compression takes place. As the volume change produced by the lower side of the piston is the same as the upper face, this is limited to scavenging and cannot provide any supercharging.
Larger engines usually use a separate blower for scavenging and it was for this type of operation that the Roots blower has been utilized. Historically, many designs of blower have been used, from separate pumping cylinders, 'top hat' pistons combining two pistons of different diameter the larger one being used for scavenging, various rotary blowers, and centrifugal turbo-compressors, including turbochargers. Turbocharging two-stroke engines is difficult, but not impossible, as a turbocharger does not provide any boost until it has had time to spin up to speed. Purely turbocharged two-stroke engines may thus have difficulty when starting, with poor combustion and dirty exhausts, possibly even four-stroking. Some two-stroke turbochargers, notably those used on Electro-Motive Diesel locomotive engines, are mechanically driven at lower engine speeds through an overrunning clutch to provide adequate scavenging air. As engine speed and exhaust gas volume increase, the turbocharger no longer is dependent on mechanical drive and the overrunning clutch disengages.
Simple two-stroke engines with ported inlet and exhaust cannot be supercharged since the inlet port always closes first. For this reason, two-stroke Diesel engines usually have mechanical exhaust valves with separate timing to allow supercharging. Regardless of this, two-stroke engines require scavenging at all engine speeds and so turbocharged two-stroke engines must still employ a blower, usually Roots type. This blower may be mechanically or electrically driven, in either case, the blower may be disengaged once the turbocharger starts to deliver air.
Supercharging Vs Turbocharging
Keeping the air that enters the engine cool is an important part of the design of both superchargers and turbochargers. Compressing air increases its temperature, so it is common to use a small radiator called an intercooler between the pump and the engine to reduce the temperature of the air.
There are three main categories of superchargers for automotive use:
- Centrifugal turbochargers – driven from exhaust gases.
- Centrifugal superchargers – driven directly by the engine via a belt-drive.
- Positive displacement pumps – such as the Roots, twin-screw (Lysholm), and TVS (Eaton) blowers.
Roots blowers tend to be only 40–50% efficient at high boost levels; by contrast centrifugal (dynamic) superchargers are 70–85% efficient at high boost. Lysholm-style blowers can be nearly as efficient as their centrifugal counterparts over a narrow range of load/speed/boost, for which the system must be specifically designed.
Mechanically driven superchargers may absorb as much as a third of the total crankshaft power of the engine and are less efficient than turbochargers. However, in applications for which engine response and power are more important than other considerations, such as top-fuel dragsters and vehicles used in tractor pulling competitions, mechanically driven superchargers are very common.
The thermal efficiency, or fraction of the fuel/air energy that is converted to output power, is less with a mechanically driven supercharger than with a turbocharger, because turbochargers use energy from the exhaust gas that would normally be wasted. For this reason, both economy and the power of a turbocharged engine are usually better than with superchargers.
Turbochargers suffer (to a greater or lesser extent) from so-called turbo-spool (turbo lag; more correctly, boost lag), in which initial acceleration from low RPM is limited by the lack of sufficient exhaust gas mass flow (pressure). Once engine RPM is sufficient to raise the turbine RPM into its designed operating range, there is a rapid increase in power, as higher turbo boost causes more exhaust gas production, which spins the turbo yet faster, leading to a belated "surge" of acceleration. This makes the maintenance of smoothly increasing RPM far harder with turbochargers than with engine-driven superchargers, which apply boost in direct proportion to the engine RPM. The main advantage of an engine with a mechanically driven supercharger is better throttle response, as well as the ability to reach full-boost pressure instantaneously. With the latest turbocharging technology and direct gasoline injection, throttle response on turbocharged cars is nearly as good as with mechanically powered superchargers, but the existing lag time is still considered a major drawback, especially considering that the vast majority of mechanically driven superchargers are now driven off clutched pulleys, much like an air compressor.
Turbocharging has been more popular than superchargers among auto manufacturers owing to better power and efficiency. For instance Mercedes-Benz and Mercedes-AMG previously had supercharged "Kompressor" offerings in the early 2000s such as the C230K, C32 AMG, and S55 AMG, but they have abandoned that technology in favor of turbocharged engines released around 2010 such as the C250 and S65 AMG biturbo. However, Audi did introduce its 3.0 TFSI supercharged V6 in 2009 for its A6, S4, and Q7, while Jaguar has its supercharged V8 engine available as a performance option in the XJ, XF, XKR, and F-Type, and, via joint ownership by Tata motors, in the Range Rover also.