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Electric Supercharges
1. Engines burn fuel and use the energy of that combustion to do work. The more fuel that is combusted in any given time then the more energy is available to carry out the engines task. Fuel requires air (or the oxygen contained within air) to burn so if there isn't enough air mixed with the fuel it will not burn. This also means that the amount of air entering an engine determines how much fuel can be burnt and consequently how much power an engine can produce. Superchargers are essentially an air pump designed to cram extra air into an engine allowing it to combust more fuel than would otherwise be possible.
Mercedes pioneered automotive superchargers on their race cars during the 1920's. These were simple reciprocating compressors attached to the engine by an electrically operated clutch. A switch activated by the accelerator pedal turned the pump on when extra power (full throttle) was required. A flurry of engineering endeavor ensued in order to reign in Mercedes advantage on the racetrack. Within a few short years most of the basic designs for modern superchargers had appeared.
During the 1930's superchargers were largely the preserve of marine engines, aircraft and race vehicles but gradually found their way into commercial diesel engines by the 1950's. It has been common for truck engines to be supercharged for decades but car engines have proven to be more difficult to effectively employ this technology.
2. Superchargers mostly fall into one of two categories, mechanically driven superchargers and turbo superchargers driven by exhaust gasses. A third category is starting to make an appearance and that is electrically powered superchargers.
2a. Turbo superchargers (a.k.a. turbochargers or turbo's) are relatively compact, lightweight and efficient but suffer from turbo lag and heat stress. By turbo lag we mean the amount of time it takes for the turbo's rotor to speed up to full efficiency. Some of the earliest turbo charged vehicles took so long for the turbo to produce a usable amount of boost that they were all but useless. Modern turbo chargers are much better in this regard but turbo lag is still a problem. Heat is another bane of turbo chargers. Exhaust gasses are extremely hot and can cause so much heat to build up in the turbo that oil will burn and congeal within its galleries leading to a bearing failure. This is why many turbo chargers have a turbo timer. The timer will cause an engine to continue idling for a few minutes after it is switched off allowing excess heat to be dissipated.
2b. Mechanically driven superchargers usually don't suffer from turbo lag and can often produce more boost than an exhaust driven charger (turbo). On the negative side they are generally bulky, heavy, and have a cumbersome drive mechanism (usually belt drive). Furthermore most chargers of this type have to supply air at all engine speeds and loads making them difficult to match various engine conditions precisely.
3. Heat exchangers (intercoolers) are frequently used in conjunction with superchargers. Compressing air increases its temperature thus making it less dense. By re-cooling the compressed volume of air before it enters, density is increased allowing even more air to be forced into the engine. Intercoolers are more important for turbo superchargers as there are two heating sources present, the act of compression and heat from exhaust gasses both increase air temperature.
4. Two words immediately come to mind when discussing the difficulties of supercharging.
1. Pressure
2. Volume
Superchargers squeeze (pressurise) air into the engine so that additional fuel can be combusted creating extra power - see previous article for a more complete explanation. People sometimes ask why can't you just put a big fan in the engines inlet? Fans are good at accelerating a mass of air but in general they cannot produce substantial amounts of pressure. Without pressure the accelerated blast of air will come to a grinding halt at the first sign of resistance. Sometimes people get confused between high flow rate (i.e. the blast of air from a large fan) and pressure. Imagine if you will filling an otherwise empty container with air (we will ignore for the moment issues with vacuum and pressure differential). Using a large fan would fill the container more quickly but once full the flow of air would stop. As fans generally struggle to produce pressure no more air can be pushed into our vessel. Pressure is required to force extra air in which is exactly what superchargers do to effectively overfill an engines cylinders. It is this extra air that is required to facilitate more combustion and hence more power.
Not only do we need to pressurise air for efficient supercharging we have to pressurise lots of it, huge amounts in fact. Even without supercharging an engine draws vast quantities of air in through its inlet so to make an increase in the inducted amount of air an enormously efficient pump is required. These pumps (superchargers) require a considerable amount of power to drive them typically 10 to 15 kW (13 ½ to 20 HP) for mechanically driven devices. Turbo superchargers (a.k.a. turbochargers or turbo's) overcome this problem to a small extent by using the engines out rushing exhaust gasses to drive them. There is a common misconception regarding turbo chargers whereby it is believed that they impart no load on the host engine, the boost is free so to speak. This of course is not true as the chargers turbine impedes exhaust gas flow which in turn requires the engine to do work to overcome. Overall turbochargers are more efficient than mechanically driven superchargers. There are shortfalls however, turbochargers are limited in the maximum amount of boost achieved by the total amount of drive force obtainable from exhaust gas flow. Heat is another problem to be dealt with as exhaust gasses are extremely hot when they exit the engine. Turbochargers are fan like in their design and have to spin at incredible speeds to be efficient. Achieving these high rotational speeds takes time delaying engine response (often called turbo lag).
The pump (supercharger) also has to be variable in its delivery. That is to say the engine only demands full boost when maximum power is required. Conversely a supercharger must not restrict the flow of air when not required to produce boost. This is especially difficult in the case of mechanically driven superchargers as their rotational speed is fixed to engine speed not to the engines load. For example, when driving down a steep incline the engine may well be spinning quickly (high revs) but is not required to produce large amounts of power. Mechanical superchargers cannot be simply disengaged at idle or light load as air will not pass through most designs when the rotors are not spinning thus choking the engine into a stall.
Superchargers are not just a challenge to design, building them usually requires absolute precision machining and only the finest quality materials will suffice. Considerable modifications to the engine are usually required when fitting a supercharger. Hence superchargers are costly to build, install and maintain ($10,000 is a common purchase price).
5. A relatively unexplored area of supercharging is the use of electric motors to drive a charger.
Theoretically speaking electrically powered superchargers should have enormous advantages over conventional types. They can deliver boost at any engine speed or load the designer wants, can be fitted wherever there is space, do not suffer from excessive heat build up and are mechanically efficient (low rotational mass and efficient motive power). Or at least that is how the theory goes, in practice electric supercharger designers have some enormous obstacles to overcome.
Mechanically driven superchargers typically require 10 to 15 kW (13 to 20 HP) to drive them. That would require an electric motor 10 times more powerful than a starter motor. Can you imagine the wiring and charging circuit required to service a motor that big? It has been done with the Thomas Knight system pictured left. Although an impressive engineering feat (using three electric motors) and almost certainly the most powerful electric supercharger available it is much more costly than others in this category and is technically challenging to install.
There are a number of alternatives many of which are based around ducted fans as used in model aircraft and bilge pumps. Although these chargers cannot generate much in the way of boost they do provide some performance increase for a tiny fraction of the cost of a conventional supercharger. Ducted fans are very good at accelerating a large volume of air but struggle to generate any amount of pressure.
Technically speaking superchargers using ducted fans and their like are axial compressors, they drive air along the line of their axis to generate flow momentum. Another type of compressor is the radial fan type. Turbocharger impellors (fans) are radial compressors, they draw air in along their axis then blast that air out radially (sideways) to achieve compression. Radial compressor characteristics are almost the opposite of axial compressors; they achieve pressure relatively easily but have a low volumetric efficiency. Turbo chargers have to rotate at fantastic speeds to be functional (typically 200,000 RPM or faster).
Electric superchargers have the advantage of a very lightweight (usually plastic) impellor unlike the massive compression elements of a mechanical compressor or the metal rotor of a turbocharger. Comparatively little energy is required to power an electric supercharger for any given amount of boost making them very efficient.
Electrically powered superchargers are unlikely to achieve the massive amounts of boost supplied by conventional types but have a viable application as a low cost intermediate performance enhancement. Their real advantage is that boost can be supplied at low engine speeds increasing power and torque in this range thus making the engine more flexible and responsive. A proportional lack of output is something of an advantage in that large scale (i.e. expensive) modifications are not required to realize the benefits of electric superchargers.
6. There are a number of methods used to indicate a superchargers output. Some will provide more flattering figures than others while a few are positively misleading. Naturally marketers are not prone to letting facts get in the way of good hype so we will try and make clear the situation before describing our supercharger.
Let's begin by defining boost. A superchargers job is to increase the amount of air being inducted into an engine and does so by boosting its pressure. Boost is a measure of that increase in pressure. Although engineers normally measure gas pressure in kPa (Kilo Pascals) we have used the more universally understood psi (pounds per square inch). There are two basic means of measuring boost, relative and absolute.
Absolute boost is the measure of pressure above or below mean (normal) atmospheric pressure whereas relative boost denotes the difference in pressure caused by supercharging. Note that this definition refers to absolute boost not absolute pressure. When a normally aspirated engine is running vast amounts of air is being drawn in. As the throttle assembly forms a barrier to this airflow (even when wide open) there is always a low pressure region (partial vacuum) down stream from the throttle body. Lets say that this part of the inlet manifold has a pressure of 1.5 psi below atmospheric at full throttle. If a supercharger is fitted and supplies 4 psi of boost then its relative boost is 4 psi whereas its absolute boost is 2.5 psi above atmospheric pressure (4 psi minus 1.5 psi lost in the throttle body). Real world fluid dynamics are a lot more complex than this but the model is a good approximation of what actually happens. Conventionally engineers only measure absolute boost whereas product marketers prefer the more flattering relative boost. It is relative boost however that relates to real performance gains most closely particularly when dealing with small capacity superchargers.
7. Gas flow is probably the most misunderstood and abused measurement of all. Describing gas flow would take another web page just to skim the subjects' surface. The problem being, that superchargers operate in a dynamic environment where by it is extremely difficult to separate flow rate attributed to the engine from that provided by the supercharger. Even simply quantifying the flow rate difference between a supercharged and naturally aspirated engine tells surprisingly little about the superchargers overall performance. Measurements taken, when the supercharger vents into an open space are meaningless, a large fan would outperform a supercharger in this test yet fans are for the most part unable to generate significant pressure. All in all we think it is best to leave these figures out completely rather than step into a quagmire of potential controversy for the sake of very little in the way of worthwhile information. Boost pressures along with power and torque gains are the benchmarks that best describe a superchargers performance.
8. An initial consideration is under bonnet space (under hood space in the US) as a lot of modern vehicles have very little room to fit additional objects in the engine bay. Our superchargers vary in size but on average would occupy a cavity 300 mm x 300 mm x 200 mm (12" x 12" x 8") on top of which must be added room for an air filter and ducting. In most cases we removed the original air filter box to make space.
In general carbureted engines responded well to our superchargers with a slight exception of some Mitsubishi carby's that tended to run quite lean under boost.
9. Fuel injected engines calibrated by air flow meters, hot wires, Karmen Vortex chambers etc. behaved in a very similar manner to carbureted engines and performed well with our charger. Karmen Vortex type air flow meters are often fitted inside the air filter box, and need to remain there thus severely limiting available under bonnet (hood) space. Many more recent vehicles use MAF (Mass Air Flow) sensors (e.g. some Fords and late model Toyotas). These sensors must remain in their original housing and upstream from the supercharger to work properly. This again uses up valuable under bonnet (hood) space.
MAP (manifold absolute pressure) sensed fuel injection systems behave slightly differently to the above. MAP sensors can only detect pressure differences below atmospheric pressure so absolute boost can confuse them. In most cases the engines ran sufficiently rich at full throttle to accommodate the superchargers but some ran a bit lean under boost.
Two stroke engines are very difficult to supercharge. Gas flow through a two stroke engine is critical to its operation and any changes to that gas flow require substantial alterations to the engine.
Diesel fuel systems need to be recalibrated if a forced induction system is fitted. This is a technically difficult and expensive operation that is hard to justify for electric superchargers.
SU and Stromberg CD type carburetors need to be mounted upstream from any form of supercharging. This means there would be a very large volume of combustible fuel/air mixture in the manifold, ducting and supercharger itself. Should this mixture be ignited a substantial explosion would occur. Our supercharger is electrically driven and could ignite the mixture (unlikely to happen but still distinctly possible).
LPG (liquid petroleum gas) and CNG (compressed natural gas) fuelled engines have much the same problem as SU and CD carbureted engines. Those that introduce fuel to the inlet manifold via a diffuser or spud port require superchargers to be mounted downstream from the fuel source. Exceptions to this are LPG and CNG engines that use a multi point fuel injection system. These engines are suitable for supercharging.
Many electric superchargers (including our own) are not designed for long periods of continuous engagement. People who live in mountainous regions will often consider a supercharger to offset the effects of altitude or to provide more power when climbing long hills. In most cases this will over tax the current source and lead to premature failure of the electric motor.
10. There are numerous different types and makes of electric superchargers available on the Internet. Many do exactly what they claim to do while some are nothing short of useless. There are a few however that could be considered positively fraudulent. For example we've seen a couple of computer power supply fans mounted in a plastic housing masquerading as a supercharger. These fans couldn't pressurize a matchbox let alone an engine yet the seller was asking $US300 per unit. All genuine supercharger vendors are keen to see these fraudsters exposed and make the Internet a more trustworthy business medium. The Internet is awash with legitimate traders and rouges all rubbing shoulders and all very difficult to tell apart. While we do not intend commenting on any of them directly there are ways to tell the good from the bad.
Some supercharger vendors will claim dramatic improvements in fuel consumption while simultaneously achieving large increases in engine power output. This is almost a contradiction of terms when large gains are said to be realised as a massive advance in engine technology would need to occur to make this possible. We contacted one vendor who claimed that a static piece of metal resembling a wind chime could instantly yield an extra 35 Hp and at the same time improve fuel consumption by 20 mpg. The seller was asked why none of the major manufacturers used this device to which they answered "the manufacturers don't know about it yet". A further question of 'how is it that we know about the device but they don't?' was never answered. We for our part find claims like this difficult to believe and would doubt the veracity of anything said by the claimant.
Most electric supercharger vendors will publicize various figures and statistics relating to their products performance. These are of course difficult to verify and may or may not accurately reflect the superchargers worth. An approximation of an individual electric superchargers real performance can be obtained from details of their current and voltage requirements. A supercharger is essentially a pump and has to do work to pressurize an induction manifold. They have to be a powerful pump as a huge volume of air needs to be pressurized to keep up with an engines demand. Even the most efficient electric supercharger designs need a substantial electric motor to drive them and such a motor requires an equally substantial amount of electricity to power it. Simply multiplying voltage (Volts) and current (Amps) together will tell you how much electric power (Watts) the supercharger uses. Our experiences showed that it is difficult to achieve worthwhile results with anything less than about 560W but gains down to 480W are at least conceivable. If for example an advertised electric supercharger runs at 12 volts and its power supply wires are obviously very thin (therefore can only conduct very small currents) the said supercharger cannot deliver much of any thing. Our electric supercharger has its operating voltage boosted from the host vehicles 12 volt system to 24 or 36 volts (depending on which version is used) yielding power ratings of 720W to 1800W.
An individual superchargers ability to generate boost reduces as engine size increases. It has to do more work just to keep up with a large engines inherent capacity to displace air. Furthermore a larger cubic volume requires more energy to pressurize hence a correspondingly larger supercharger is needed just to maintain the same mean gas pressure. Electric superchargers generally have a lower volumetric efficiency than conventional chargers and are more sensitive to engine capacity. An electric supercharger that can blow the socks off a 900cc minnow may only be able to tickle a 5000cc beast (depending on its total volumetric efficiency). This is an important aspect to keep in mind when evaluating published performance figures and it is worth querying the test conditions and engine used.
Unfortunately gas flow figures (CFM or L/m) are often misleading. Here is an excerpt from our performance page that details our thoughts on this -
11. Gas flow is probably the most misunderstood and abused measurement of all. Describing gas flow would take another web page just to skim the subjects' surface. The problem being, that superchargers operate in a dynamic environment where by it is extremely difficult to separate flow rate attributed to the engine from that provided by the supercharger. Even simply quantifying the flow rate difference between a supercharged and naturally aspirated engine tells surprisingly little about the superchargers overall performance. Measurements taken, when the supercharger vents into an open space are meaningless, a large fan would outperform a supercharger in this test yet fans are for the most part unable to generate significant pressure. All in all we think it is best to leave these figures out completely rather than step into a quagmire of potential controversy for the sake of very little in the way of worthwhile information. Boost pressures along with power and torque gains are the benchmarks that best describe a superchargers performance.
Whilst electric superchargers are a very cost effective and efficient performance enhancement they do have limitations. It is unreasonable to expect that a low cost electric supercharger will produce the same performance gains as a hugely expensive conventional supercharger. If they could match conventional chargers yet still only cost a tiny fraction of the price then electric superchargers would put conventional chargers out of business. Vendors who claim performance parity at vastly reduced cost are probably being less than truthful.
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