- Used to stop detonation and allow more aggressive
ignition advance. - Used extensively in competition engines. - Requires careful mapping.
injection was first used on tractors and industrial engines in the 1920s to
avoid detonation. It received widespread use during WW2 in American war planes
and has been employed ever since for engines which need to operate at the peak
of power and efficiency. When the
speed at which the fuel burns, or the speed at which the cylinder pressure
rises, gets out of control the fuel mixture will explode uncontrollably causing
detonation. Mild detonation will destroy head gasgets and wild detonation will
quickly destroy pistons, conrods and cylinder walls, bearings, cranks and
everything else in the engine !!
order to extract max power and economy from an engine, we must find a balance as
close as possible to the point of detonation, without crossing the line. To eliminate
detonation we must limit combustion speed and pressure rise in as gradual a way
as possible so as to preserve max power and economy. One way to so this is to
improve the knock resistance of the fuel but this is rarely possible for road
use. This leaves
us with the option to either cut down on power and economy (no thanks) or to use some form of water injection.
injection isn’t about dumping water into the cylinders. If power is important
then water flow and atomisation must be carefully controlled to provide the
ideal quantity and delivery to stop detonation. Installing a
cheap and crude water injection system into a car is the same as choosing carbs
over fuel injection. You should never install any system based on substandard
parts like windscreen washer pumps and nozzles operated by some kind of pressure
switch. At best this kind of system will only prevent detonation.
promote more power or economy and will probably reduce power and contribute to
premature cylinder wall ware by contaminating the oil supply. An entry
level water injection system should be electronically controlled with a variable
pump controlled according to boost and rpm inputs as a minimum. The knock
control should be altered or removed to stop spark retard or boost limiting. A flow meter
for water and fuel is also helpful to maintain a ratio of 35% of water to fuel
flow (by weight). In volume terms this is 28%.
higher the intake temp and the lower the octane rating then the more water that
will be required to maintain spark advance (and
therefore power and economy).
On a normal
road engine running 95octane fuel you should see a 5% increase in power and a
15deg drop in intake temps. Obviously if you have much higher intake temps than
normal(eg.100deg) and worse fuel, you will experience even more gains by using
water injection. When we
inject water it gets converted to steam, which pulls heat out of the combustion
process, slowing the rate of pressure rise. These steam particles separate the
fuel and oxygen molecules which further slows combustion and pressure rise. As
this is happening, the piston is continuing to goto TDC, squeezing the gases
tighter but with less heat in the chamber, detonation is avoided. As the piston
descends a controlled burn occurs.
Water Injection Systems.
- Expensive and must be mapped carefully on the dyno.
Sophisticated water injection systems can be hard to justify because of the high
cost involved. But for max horsepower and max engine protection from detonation
then a top of the line water injection system is necessary. Such systems employ
water nozzles in each inlet port and a high pressure pump delivering water at
90psi to maximize atomisation. A fully programmable ECU must also be used to map
the system according to boost and rpm load, compressor discharge temp and engine
temp, turbo temp and overboost.
Location. - Equal water distribution and vaporisation is
important to equally distribute the water haze across the chamber. This is why
top end systems employ one nozzle per cylinder… but you can get close with one
or two nozzles. If we are using an intercooler the nozzles are best located in
the cooler outlet pipe. The turbulence here will help to disperse the water
uniformly in the charge air. This also leaves plenty of time for proper mixing
before the inlet. If the charge air is still hot after the intercooler it will
help in vaporising the water (which will draw heat out
ot the charge air early).
systems, nozzle position varies depending on the type or blower or turbo being
used. For centrifugal blowers and all turbos put the nozzle in the compressor
outlet. With other types of blowers its better to inject into the blower intake (if excessive amounts of water are used then the blower will
be damaged). However, the big gaps in roots blowers between the rotors and the
case is reduced when water injection is used so we have an increase in boost and
a drop in charge temperatures. Too much water will tighten the gap too much and
put excessive pressure on the rotors.
a lot in street-racing and sometimes in competition.. not good for the engine
advocate the use of a 50/50 water/alcohol mix. The alcohol adds a fuel value to
the mix and aids further in dispersal and atomisation. However, it leaves a
powdery residue on aluminum and causes corrosion of piston ring lands, the
rings themselves and the bore walls by washing away oil.
- Much better than alcohol, use between 30 and 50%
mix of methanol.
- Can be very poisonous, don’t allow skin exposure.
option is to use a 50/50 water/methanol mix. This has similar benefits as
alcohol without the downsides. Better results can sometimes be got by using a
70/30 mix. But methanol is very poisonous and can cause blindness and cancer and
eventually death if its not handled with respect. Never allow skin exposure,
never inhale methanol fumes etc…
- Too expensive and rare.
be used instead of water and has excellent resistance to detonation. But is mega