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The Theory


The theory of supercharging at a basic level is to take air at atmospheric pressure, compress the charge and pump it into an engine in greater quantities than it would usually consume (adding appropriate quantity of fuel) thereby increasing power output. This is great but like most things in life “it’s not all gravy” there are consequences which must be understood. An engine responds to density, the greater the weight of intake charge the more output is achieved.

Heated Charge
Most enthusiasts would be aware that an engine breathing cooler air will produce more power - simply due to the greater density of the air. The same is true when supercharging. The trouble is, due to the physics of compressing a gas, all compressors will increase temperature. An easy illustration of heat being produced by compressing air is a bicycle pump inflating a tyre – the valve will soon get hot, yet let some air out - the air and valve now cools. So the aim should be to compress the air to the pressure required as efficiently as possible.

The degree of temperature rise depends on many factors, mainly :

1. compressor efficiency
2. level of boost
3. ambient temperature etc.

What must be considered is the total temperature effect in the combustion chamber. When heated intake is added to the heat created by compression within the chamber, the increased combustion chamber temperatures usually require higher-octane fuel, retarded ignition timing or excessively “rich” fuel to prevent combustion moving beyond the bounds of “controlled burning” into pre-ignition / detonation territory.

Higher octane fuel is more resistant to pre-ignition and/or detonation due to a slower burn time. The piston has more time to travel upwards before meeting the flame front. Thus reducing the time the piston is subjected to excessive pressure and subsequent overheating. Reducing the possibility of catastrophic failure.
Higher octane fuel is not necessarily easy to obtain any longer, but a good solution. See Ethyl Fuel additive

Trying to control abnormal combustion by retarding ignition timing or pouring in excessive fuel is not an efficient solution and generally leads to other problems.
Efficiency is sacrificed and potential power is lost.

1. Compressor Efficiency
Twin screw kompressors are generally known for their superior adiabatic efficiency. The efficiency of the supercharger (the adiabatic, or compressive efficiency) is a vital factor in determining performance of a supercharger system. This efficiency is based on the power consumed by the process, and discharge temperature of the air delivered to the engine. Adiabatic losses within a supercharger appear as power loss (due to drive effort required) and/or higher discharge air temperature (with a consequential decrease in charge density).

2. Level of boost
As boost levels increase, the temperature of the air in the intake manifold increases resulting in increased combustion chamber temperatures. Unless the level of boost is below about 6 psi. (0.4 bar) the delivered air will still be quite hot, so some form of charge-cooling (or inter-cooling) is desireable to bring the temperature down for best effect. The effect of passing fuel thru’ an efficient compressor can assist in cooling here.

3. Ambient temperature
Not much can be done about ambient temperature.

Air to Air Intercooling (Aftercooling)
The most popular method of cooling the intake charge is via an intercooler. Be it an air-to-air or water-to-air type. However, like most things in life, it’s not all roses, different types have advantages/disadvantages.

All current intercoolers suffer losses – some much more than others – Some older air-to-air types can lose around 4 psi of the available boost – not good if the set up only produces 6 or 7 psi. It is undersood even the major manufacturers allow for up to 2 psi “leakage”. Then there is the resistance to flow by the gas having to negotiate a rather torturous path thru’ the core etc. Try looking into your radiator neck to see a similar path to what the airflow is required to negotiate in a typical air-to-air intercooler.

Water to Air Intercooling (Aftercooling)
The extremely efficient laminar flow type (water/air) of intercooler does not suffer the same flow restriction and can produce a far cooler charge, but this system is also meant for fuel injected systems where only air is passed. Traditional intercoolers are best in ‘dry’ applications. That is, not passing fuel.
See Laminova intercooler

CHARGE COOLING (in ‘Wet’ systems)
Introducing a “wet” charge to the internal passage ways of an intercooler - encounters a large surface area where the tiny droplets tend to condense and ‘drop out’ of suspension, condense (or ‘puddle’) accumulating on the internal passageways forming large droplets which eventually dislodge and re-enter the airstream during manifold pressure change. Incidently, these large droplets do not absorb heat as well as a finely misted spray. Being heavier than air the large droplets are not carried in the airstream as well, with the resultant uncontrolled intake mixture.

This uncontrolled fluctuation in fuel/air mixture creates tuning problems and, in severe cases, can lead to the ‘big bang’ we all try to avoid. Bear in mind that in a severe backfire the intercooler will have a combustible mix in it.

Not good for performance or economy.
Or engine life for that matter.

An intercooling option for a ‘wet’ charge application is to introduce water* (or water*/methanol) to the mixture. *Demineralized Water preferable !!
An advantage of water injection over intercoolers - there is no loss of boost when fitted. In fact, some increase can be expected due to the increased density of a cooler charge.

The water injection (and water /methanol injection)concept is certainly not new, it has been around for a very long time and has been used on supercharged piston engine’d aircraft, rally and formula 1 cars etc.

Its main purpose being to suppress engine knock caused by uncontrolled and/or unintended combustion of the fuel mixture when charge and cylinder temperatures (or compression pressure) get too high for the fuel being used.

Water injection on it’s own does not produce masses of power but it allows tuning to take full advantage of the cooler, more dense intake air charge. Allowing higher boost without the need to reduce compression, running a ‘rich’ fuel mix or retarding ignition advance, all of which zap power potential!

Within certain limits, this actually can increase power, even in naturally aspirated applications.

Water exists mainly in a liquid state, its most stable inter-molecular structure. When heat is applied to it, its molecules begin to expand: a great deal of heat is absorbed during this process.

When the water changes from a liquid to a gaseous state, a large amount of heat energy is consumed in this process.
Taking this heat energy out of the induction charge has many benefits.

Water's specific heat capacity is 4.184 kJ/kg °C. This means that it takes 4.184 kJ of energy to raise one kg of water 1°C in temperature.

By comparison air ranges from 1.00 kJ/kg °C to 1.05 kJ/kg °C, in the temperature range applicable to engines. Water’s latent heat of evaporation is 2256kJ/kg, approximately six times more than petrol!

Therefore, if a little water is injected into the intake in the form of an ultra-fine mist, the latent heat of the water will cool the charge and increase volumetric efficiency.
There are definite limits, however, to the amount of water that can be injected. Too much will cause excessive cooling and misfiring.
With it’s inherent high specific- and latent- heat capacity, water is the perfect liquid for regulating excess heat in the induction / combustion process.

Potential power output is improved by:
An increase in charge density, potentially providing an additional 3 - 5 psi boost and,
Ignition timing can be optimized - no more retarded timing,
Fuel mixtures can be optimized - no more super “rich” mixtures,
Exhaust gas temperatures can be reduced,
Clean combustion chambers etc. by steam cleaning effect.

Water injection effectiveness is dependent upon the relative humidity in the atmosphere. Lower humidity will increase the effectiveness of water injection.

Concentrating only on factors relevant to Twin Screw Compressor installations, mainly draw thru’ carburettor type.
One of the advantages of passing fuel thru’ the supercharger is it increases the sealing efficiency by sealing the already tight clearances - rotor to rotor and rotor to body.
It also lubricates and lowers the discharge temperature at the source of compression - a small increase in boost at the same RPM may also be realized. Add to this the benefit of a cooler charge.

In fact, vaporization of the fuel/air mixture can drop inlet temperature by about 4.5°C (~40°F).

How does it work ?

How does Water (or Water/Methanol) assist charge cooling??
Water changes from a liquid state to a vapour state at its boiling point of 100ºC. Methanol changes from a liquid state to a vapour state at its boiling point of 65ºC, (i.e. as soon as it hits the compressed air mixture coming from the supercharger outlet). Which means that the water or methanol will try to keep the air / fuel mixture at a fixed temperature of 100ºC for the water phase change, and 65ºC for the methanol phase change, for a long time (until the entire fuel has changed state) while absorbing a very large amount of heat energy out of the compressed air.
Since the compressed air leaving the supercharger can be as high as 100ºC above ambient (so with ambient temperature of about 30ºC) inlet air temperature can easily be around 130ºC on some supercharger installations.
Once this 130ºC air meets the water and methanol mixture both the water and methanol will attempt to bring down the air / fuel mixture down to 100ºC (the boiling point of water) and if all the water has vaporized into steam, then further down to 65ºC (the boiling point of methanol).

Water or Water/Methanol mix??
Water alone will cool combustion temps well, methanol has the added benefit of increasing fuel octane.
For most uses distilled (or demineralized) water is adequate as pinging suppressant and is compulsory if clogged or chalked up nozzles are to be avoided. The very small jets (usually 0.3 - 0.6mm in size) with tiny water vanes within the jet will require stripping down and cleaning often if tap water is used. Imagine a clogged nozzle at the point of max. boost – just when the engine is wanting some cooling - not very comforting!!
Methanol is also used as an anti-freeze in water injection: 13% added will protect down to -7 ºC
24% added protects to -18ºC.
Methanol freezes at -96ºC, a 50% mixture will stop the mixture from freezing down to around -40ºC.

Water or Water/Methanol ratio and quantity??
A 50/50 water/methanol is usually suggested for charge air cooling, excellent detonation control, and controlling cylinder temperature.
Maybe a little extreme for road use as may be corrosive and is said to harm the oxide coating protecting aluminium.
Some suggest using less methanol such as 70/30 water/methanol to minimize the risk of it becoming flammable in use.

How much Water / Methanol?
The jetting is usually calculated to about 10-15% the total fuel flow of the system.
Sometimes as high as 25% or higher.

The main thing to watch at an extreme ratio is “saturation” which will also be affected by ambient relative humidity.

Anyway, we digress!! Again!!

So, a relatively simple method to determine an approx. fuel flow for road engines is to use the calculation generally adopted for EFI injector sizing.

Hp x 6 to give cc/min of fuel.
Therefore, an engine producing 150 hp @ 6000 rpm. would require a total fuel rate of 900 ml/min. or 0.9 litres/min.

So 10 to 15% of  900 ml/min = 90 to 135 ml/min injection nozzle rating.
These figures are a general “rule of thumb” theory and may benefit from some experimentation possibly with higher ratios.
Many factors must be considered in determining ideal water/meth flow.

Remembering, if 50% of the mix is methanol (a high octane fuel) the final fuel ratio will be richer by about 7% (or about 1 AFR) and re- tuning may be required to maintain optimum air/fuel ratio for optimum power.
Or the higher octane can be utilized by increasing boost.

The system should be configured so that water is only injected when there are high intake airflows.

Above all else, a water injection system must be 100% reliable.
The addition of water injection can permit the use leaner air/fuel mixture and more advanced ignition timing.
Not generally tolerated without water injection.

Consequences of a failed system, a blocked jet, or running out of water can have disastrous effect on an engine.

Due to petrol and water not mixing, the water injection requires a separate metering and injection system. There are various methods of injecting this substance into supercharged engines, obviously pressure must be employed.

The most popular methods are:

1. use manifold boost pressure to a pressurized tank, or

2. install a pump to supply the water (or water/methanol) at a reasonably high pressure.

All will show benefits in charge temperature reduction (hence greater charge density).

1. Pressurized tank system
Feeding boost pressure to a strong water tank generally dictates the nozzle must be placed on the atmospheric side of the supercharger.

2. Pressure Pump system
With the pump system, nozzle(s) can be as above or (depending on pump pressure) may be introduced after the supercharger (boosted side).
The latter method (after the supercharger) will cool the charge dramatically as this is where the highest temperature difference exists.

One drawback in both instances – the water is consumable – it must not run out if the installation is ‘running on the edge’.

A small price to pay for more power.

Water Injection Nozzles
The ideal nozzle produces a finely misted spray (or fog) rather than big droplets of water. 
Big droplets of fluid cannot absorb heat like billions of tiny droplets. 
A finely misted spray (or fog) is easily carried in the air stream and more evenly distributed.

Water Injection Nozzle placement
Where is the best place to install the injection nozzle?

There is no simple answer, no “one size fits all” or “x marks the spot”. hopefully the following will assist in making an educated decision.
Many inter-dependant factors should be considered when choosing the appropriate location and sizing of water (or water/methanol) injectors.
Among the the primary factors:

1. ‘Wet’ systems – pressurized tank feed

2. ‘Wet’ systems – pressure pump system

1. ‘Wet’ systems - Pressurized tank system Injection
Pressure relies on boost pressure applied to tank. Therefore pressure at the nozzle is limited to the manifold boost.
Depending on nozzle size and type, atomization of the mix may be compromised.

1a. Pre-Throttle butterfly
With this installation the nozzle(s) are usually fitted on the atmospheric side of the butterfly as the presence of vacuum on the engine side of the butterfly when idling, cruising etc. could result in siphoning of the mixture.
The distance to the intake port provides extra time for the fluid to vaporize and absorb heat.
By injecting a precise amount of water (or water/methanol) just before the carburettor improves supercharger efficiency, helping to cool and lubricate it.  Improved sealing efficiency and the higher density charge can increase boost at the same RPM.
Lower internal and discharge temperatures (particularly on high boost installations), significant cylinder cooling and detonation suppression can be achieved.
This installation should also include a method of controlling possible siphoning during instances where vehicle is in a position for the tank to be higher than the nozzle height.

2. ‘Wet’ systems - Pressure Pump system

2a. Pre-throttle butterfly
Same as above except pump pressure is usually well above boost pressure and a check valve (or solenoid) is recommended to prevent siphoning or “leakage”.

2b. Post -throttle butterfly
When fitting nozzle(s) after the butterfly, fitment of a check valve (or solenoid) is necessary to prevent siphoning during high vacuum (when idling, cruising etc.). Ideally this should be as close as practical to the nozzle (less than 24”) to limit the amount of fluid remaining in the hose.
Once again mixture distribution is not normally an issue as the water is introduced at a reasonable distance from nozzle to combustion chamber.

2c. Inlet Manifold
Fitting nozzle(s) in an inlet manifold or plenum raises concern about proper distribution of the water methanol injection spray. Intake manifold design and nozzle(s) placement must be evaluated to ensure proper distribution will be maintained.
It may be preferable to run multiple (smaller) nozzles (of similar output) in some instances.

The closer the injection to the inlet port, the less time it has to reduce air temps.