Frequently overlooked or regarded as primitive or
somehow unworthy, water injection – the flowing of very small drops of water
into the engine intake – has the potential to improve power, emissions and fuel
On engines with forced aspiration, water injection
allows more boost to be safely used. In naturally aspirated engines, the
ignition timing can be advanced. And in either type of engine, lower octane fuel
can be safely used at maximum power.
The primary advantages of water injection are:
That’s a pretty amazing list – some would say
literally incredible. So let’s take a look at each.
Water injection has the ability to suppress
detonation, allowing the use of higher cylinder pressures. And higher cylinder
pressures mean more power and improved engine efficiency.
So what’s detonation? Engine detonation occurs
when the air/fuel mix ignites within the combustion chamber in an uncontrolled
manner, instead of by the progressive action of a moving flame front. The terms
'ping' (a light, barely observable detonation) and 'pre-detonation' (detonation
caused by the ignition of the charge slightly before the full ignition of the
flame front by the spark plug) are also commonly used. 'Knock' is another
One definition of knock is "an undesirable mode of
combustion that originates spontaneously and sporadically in the engine,
producing sharp pressure pulses associated with a vibratory movement of the
charge and the characteristic sound from which the phenomenon derives its name".
If detonation is allowed to go on for more than
few seconds, the very sudden pressure changes within the cylinder can damage the
engine. In a worse case scenario, pistons, rings and even the head itself can
suffer major damage. Note also that the higher the specific power output of the
engine (ie hp per litre), the greater the likelihood of damage if detonation
occurs. In fact, in high boost turbo engines, detonation can destroy an engine
in a matter of seconds. Not a nice thought...
In everyday driving, detonation is most likely to
be heard when the driver is using a gear too high for the engine speed and load
conditions - like climbing a steep hill with the right foot flat to the floor,
while in third gear and travelling at 40 km/h. Depending on the engine,
detonation can sound like a 'ting, ting' noise, or even a little like coins
rattling in a coin tray. However, in some engines, when heard from the cabin the
audible note is much deeper.
In forced aspirated cars (turbo or supercharger),
or cars where the compression ratio has been substantially increased, detonation
can occur at high engine speed and high loads, making it very difficult for the
driver to hear it over the general noise level that's present at the time. It’s
high load detonation that is most dangerous – this is the form of detonation to
prevent at all costs.
The main causes of detonation are over-high
cylinder pressures caused by too high a boost and/or compression ratio, ignition
timing that is too advanced, intake air temp that is too high, or lean
Water injection works to stop detonation in four
Firstly, when the water is injected into the
intake system prior to the cylinder head, the small droplets absorb heat from
the intake air. Water has a very high specific heat rating (it can absorb lots
of energy while only slowly increasing in temperature) and so the intake air is
Thirdly, when the remaining water droplets and
water vapour reach the combustion chamber, steam is produced. This acts as an
anti-detonant by slowing combustion and reducing the peak cylinder pressures.
The area of water injection and intercooling can
be confusing. Here’s a brief rundown on some terms you should know:
Cools the Intake Air
As described above, water injection cools the
intake air. However, this characteristic is so important – especially on turbo
or supercharged engines – it’s worth covering in more detail. The reason it’s
important on boosted engines is that when a turbo or supercharger compresses
air, the air gets hot. Sometimes very hot...
This hot intake air creates two problems. Firstly,
as described above, it increases the likelihood of detonation.
Secondly, warm air has less density than cool air
- this means that it weighs less. It's important to know that it's the
mass of air breathed by the engine that determines power, not the volume.
So if the engine is being fed warm, high pressure air, the maximum power
possible is significantly lower than if it is inhaling cold, high pressure air.
So what affects how hot the air gets in a forced
The higher the boost pressure, the greater will be
the temperature increase. As a rule of thumb, if you are using a turbo boost
pressure level of more than about 0.5 Bar (~ 7 psi), intake air temps will be
much increased over ambient. A Roots supercharger boosting at over 7 psi or more
will greatly increase intake air temps (eg 100 degrees C above
The lower the efficiency of the compressor, the
higher the outlet air temp. So as indicated, less efficient superchargers (eg
older-type centrifugal compressors with straight blades and Roots blowers) will
cause higher intake air temps than screw-type designs. While a well-matched
turbo should be at peak efficiency most of the time, a heavily boosted factory
turbo will often be working at poor efficiency, giving increased charge-air
assessing intake air temps on a forced aspirated car, it’s always best to make
measurements. Measuring the actual intake air temp of your own forced induction
car under a variety of conditions will tell you more about what really happens
to intake air temps than any article like the one you’re now reading. Go to
LCD Temp Display!
for a cheap and effective LCD temp display that works very well at monitoring
intake air temp.
A common way of reducing intake air temps on a
forced aspirated car is to fit an intercooler, or if one is already present,
increase its size. But water injection can be also used to cool the intake
airflow – either instead of an intercooler or in conjunction with it. (See
breakout box below.) So is this cooling effect of water injection much the same
as intercooling? Is that why some people call water injection ‘chemical
For a couple of important reasons, water injection
doesn’t have the same effects as intercooling. In one respect it is inferior and
in the other, superior.
Firstly, the water that you’re adding is a new
substance takes up space in the intake system. Whether it’s in the form of
droplets, a fine mist, or water vapour, there is less room for oxygen. This
means that unlike intercooling, power doesn’t always increase with the lower
intake air temps. (However, the power output can then be increased by running
more boost or more ignition advance.)
Secondly, while both water injection or
intercooling will reduce intake air temps and so reduce the chance of
detonation, water injection is far better than intercooling at detonation
suppression. The effective increase in fuel octane rating with a good water
injection system is very high.
Reduces Emissions and Fuel Consumption
Water injection can dramatically reduce emissions
and has the potential to improve fuel economy. However, the results depend on
the actual strategy that is used.
In 1997 testing, Saab used water injection on a
2.3-litre Ecopower turbo engine to allow an air/fuel ratio of 14.7:1
(stoichiometric) to be maintained at full load. This approach was taken rather
than the more usual technique of enriching mixtures at high load.
This water-for-fuel replacement strategy dropped
HC emissions by 47 per cent, increased NOx by 142 per cent, and decreased CO by
92 percent. Further testing using twinned parallel cat converters brought the
NOx output down below that achieved when using fuel enrichment rather than
water. The Saab testing also indicated a stunning 25-30 per cent fuel saving at
full load! Water consumption varied from 0 litres/minute at about 5000 rpm to
0.5 litres/minute at 5500 rpm.
Typically, where the air/fuel ratio is not
leaned-out as dramatically as in the Saab case, it is the NOx emissions which
are reduced by the greatest amount. This is as a result of the lower combustion
temperatures. German testing carried out in 1971 on a 2.3-litre mechanically
injected six-cylinder engine showed that with water (injected with the fuel via
an emulsification agent) added at 30 per cent of the fuel volume, NOx emissions
could be reduced by 50 per cent.
injection is being widely used on diesel engines where it is very effective at
reducing particulates and NOx outputs.
is using high pressure direct water injection on its huge diesel engines used in
ships. This reduces NOx emissions by 50-60 per cent, important not only for the
environment but also for ship operating costs - some harbours are now setting
fees based on a vessel’s NOx emissions!
fuels containing a high percentage of emulsified water, or water dispersed in
tiny droplets, are also now being widely used in road vehicles.
And what about fuel consumption? If when water
injection is being used, the ignition timing is advanced and/or the air/fuel
ratio is leaned-out (both taking advantage of the greater resistance to
detonation), engine torque output can be improved. This can then result in
better fuel consumption.
Low Running Costs
A water injection system consumes (wait for it...)
That makes the on-going running costs of using a
water injection system very low indeed. A good water injection system will need
a very fine nozzle and so high quality
filtration of the water is needed. However, even if this filter is changed
annually, the weekly cost is still low – far lower than using an octane booster
or high octane fuel.
In times of tight emission controls, limited fuel
octane and rising petrol costs, water injection has major pluses. It is unique
in that it has the potential to both make a car greener and also allow more
power to be developed.
However, to be properly effective, water injection
needs to be a lot more than just a crude afterthought add-on. The system needs
to have engineering of at least the same quality as the main fuel system and to
be properly integrated into the engine management system.
Next week: how water injection has been used in
high power engines – from WRC rally cars to WWII fighter aircraft!
BOTH Intercooling and Water Injection?
turbo cars run intercooling from the factory. So is it possible to use water
injection in conjunction with an existing smaller intercooler to achieve a
better outcome? The short answer is: maybe.
boost levels increases the load on the intercooler in two ways – firstly, there
is more heat to get rid of, and secondly, the amount of intake air flowing
through the intercooler rises. By using water injection you can do something
about the heat load, but if the pressure drop through the intercooler is too
great (ie the intercooler causes too great a restriction to flow) then a new
intercooler will be needed. (The flow restriction of the intercooler is easily
worked out by measuring the peak boost level either side of the intercooler. The
difference shouldn’t be more than 2-3 psi.)
if the existing intercooler has sufficient flow for the new power level, there’s
nothing to stop you using water injection with it.