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The H2O Way Part 1

Often spoken about but seldom done - we look in detail at water injection

by Julian Edgar

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At a glance...

  • Water injection suppresses detonation
  • Water injection cools intake air
  • With other changes, water injection improves emissions and fuel economy
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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 economy.

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:

  • Suppression of detonation

  • Cooling of intake air

  • Improved fuel consumption

  • Improved emissions

  • Very low running costs

That’s a pretty amazing list – some would say literally incredible. So let’s take a look at each.

Suppressing Detonation

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 synonym.

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 mixtures.

Water injection works to stop detonation in four ways:

  • 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 initially cooled.

  • Next, the small drops of water start to evaporate. Water has a very high latent heat of evaporation (its change of state absorbs a lot of heat) and so the intake air charge is cooled still further.

  • 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.

  • Finally, the steam keeps the combustion chamber very clean, so preventing the build-up of carbon “hot spots”.


The area of water injection and intercooling can be confusing. Here’s a brief rundown on some terms you should know:

  • Water Injection – small droplets of water added to the intake air that passes into the engine

  • Intercooler – a heat exchanger positioned between the turbo (or blower) and the engine intake, designed to cool down the intake air

  • Air/Air Intercooler – an intercooler that exchanges the heat of the intake air with the atmosphere (ie outside air)

  • Water/Air Intercooler – an intercooler that exchanges the heat of the intake air with water that is then pumped through its own special radiator to cool it

  • Intercooler Water Spray – water sprayed onto the outside of an air/air intercooler to help cool it

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 induction engine?

  • 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 ambient).

  • 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 temps.

  • Thirdly, the actual temp of the air will depend largely on how the car is driven. A 10-second squirt of full boost will not give anywhere near the intake air temps that are reached if climbing a long open-road hill on only half boost.

Measure It!

When 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 intercooling’?

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.


Water injection is being widely used on diesel engines where it is very effective at reducing particulates and NOx outputs.

Wartsila 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!

Diesel 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...) water!

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!

Using BOTH Intercooling and Water Injection?

Most 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.

Lifting 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.)

So if the existing intercooler has sufficient flow for the new power level, there’s nothing to stop you using water injection with it.

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