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:
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 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:
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?
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.
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!