Aquamist

This article in the new contributions by experts in the industry feature is by Richard Lamb of Aquamist Aquamist provide water injection systems for a variety of WRC cars. Richard is recognised as being at the very forefront of the research and development of this innovative technology.

August 2005

Water Injection

This topic has often been mentioned in internal combustion engine publications and many SAE papers (Society of Automotive Engineering). I will do my best to explain what it is and how it can be implemented in automotive engines constructively. Please notify me of any mistakes or inaccuracies made so that I can correct them.

Why water?

Unlike any other liquid, water has the highest latent heat of vaporisation of all the known liquids that exist on this planet, naturally other than Mercury. Its strong inter-molecular bond is the main reason why it requires a large amount of heat energy to separate its molecules from each other. It requires 2240KJ of heat energy to fully vaporise one litre of liquid water into gas. Translating this into meaningful terms, a 3KW domestic electrical kettle will need 12 minutes or so to fully evaporate one litre of water, the equivalent energy to keep a 100W light bulb on for 6 hours.

A contardiction in terms?

You are now probably wondering why we are injecting water into any engine resulting only in taking heat energy (power) away since the sole purpose of a car engine is to convert air and fuel into heat energy to do the work of turning the wheels. Let us first look at the thermal dynamics of an engine – the energy produces is basically shared amongst three areas, pretty equally. One third is lost in the exhaust pipe; one third is transferred into the atmosphere via the normal cooling system. The remainder is for turning the wheels. As this document progresses, the reason for adding water injection will become more apparent.

Aiming for a highly efficient and powerful engine set up:

Having covered the engine basics, we can now examine how one can improve the efficiency of an engine by recovering some of the heat loss mentioned above. Engine efficiency can be greatly improved by increasing the compression ratio – the ratio that governs how much of the energy is channelled into useful power rather than just plain heat loss to the atmosphere. A normal spark ignition engine runs at an average compression ratio of between 9:1 to 10:1. The limitation here is the octane rating of fuel used, the higher the octane number the higher the compression ratio that can be used. In other words, an engine designed to run on 98-octane fuel will be more efficient and potentially produces more power than an engine designed to run on lower octane fuel. Octane rating is a measure of knock resistance – a higher knock resistance will allow an engine to run a higher compression ratio or more ignition advance hence producing more power.

How manufacturers contribute towards it:

Modern electronics has played a big part in allowing a normally aspirated engine to run a vast range of fuel grades by constantly adjusting the ignition timing until the knock limit is a degree or two ahead. Your engine is now getting the optimum timing ( by varying the effective compression ratio with the ignition trim) for a given grade of fuel. In order to stretch the efficiency of engine further with a given amount of air it can inhale, a turbocharger is added. The heated gas from the exhaust is harvested to power an air pump (compressor) to improve the volumetric efficiency of the engine. This arrangement will recover some of the wasted energy towards the wheels instead of being lost to the atmosphere.

Can we do more to shift more energy towards the wheels with a turbocharger?

Nowadays, modern electronics can further improve the power output of a Turbocharged engine by constantly modifying the boost pressure. Although a turbocharger can stretch the dynamic operation range of an ordinary engine it is still somewhat limited by the fuel grade used – over stepping this limit will result in the onset of detonation.

Stretching the limits on a Turbocharged engine–

The easiest way to extract more power output is by increasing the octane rating of the fuel (race fuel has that property). Any race fuel will allow you to run extra boost and extra timing thus further improving the efficiency of an engine. Unfortunately race fuel is not easily obtainable from normal commercial petrol stations. An intercooler will further improve the efficiency of a turbocharger by increasing the density of the compressed air from the turbocharger.

Extending the limit further.

So far we have outlined some existing methods to push the barrier further and most of the mentioned methods have already been implemented on some modern turbocharged engines. So how can the injection of water squeeze more efficiency out of a modern engine? The answer is - you can’t, unless you are prepared to increase the existing compression ratio further. In the case of a turbocharged engine, it means running a higher pressure-ratio. This is not new, car manufacturers are already using this technique such as timed over-boost during acceleration and boost tapering down over certain RPM etc. During these periods, the a/f mixture is enriched to assist in-cylinder cooling.

Power comes with a price.

With every modern factory turbo car produced nowadays, it is fuel-efficient until pressed hard. I have not seen any factory equipped turbo cars that doesn’t dump fuel under hard acceleration. It is not the preferred intention of the makers but they have little choice, since they have no other means to control the thermal loading within the combustion chamber. A bigger radiator or oil cooler is one answer however unfortunately the frontal area of the car doesn’t increase with the power output of the car.

How Water Injection can take you past the final barrier

We have finally arrived at the main topic – water injection. So far we are seeking a way to improve the efficiency of the engine and hence how much power can be squeezed out of an existing set up and of course without using extra fuel – we call it the final barrier. We will address this particular area of the turboed engine's dynamics and sum up how water injection can contribute in different areas and the reasoning behind the claim.

Water Injection will improve the turbo operating range:

A turbocharger has a designed operating range defined by a chart. Its flow characteristics are selected carefully to match the engines operating characteristics by car makers. Each operation range has an efficiency boundary denoted by percentage. Ideally, one should operate within the most efficient boundary area. When boost is increased, it tends to shift the operating range outside this area. Loosing about 5% efficiency each time it is shifted further away from the ideal boundary. It doesn’t mean that it stops working altogether but the loss in efficiency almost always translated into heat. The further away the shift, the hotter the charge the air gets. Almost all modern turbo cars use intercoolers to reduce those high charge air temperatures.

Introducing water injection will absorb any heat left over from the factory’s intercooler designed capacity when the turbocharger’s operating range is extended. High ambient and low vehicle speed will further tax the efficiency of the intercooler so having a water injection as a secondary cooling mechanism is very useful. Water does not have the problem absorbing heat as the intercooler in less-than-ideal conditions. Provided the water is well atomised and exhibits a great deal of surface area, it will grab heat very quickly. As the cooling effect is taking place, the air shrinks to accommodate more air coming out of the turbo or intercooler, resulting a extra throughput of charge air. The myth of water vapour displacing the air is sometimes over-stated – the volume of the water vapour occupied is minimal. It has to be noted that when the temperature in the inlet drops, the droplet size decreases and occupied less air space.

Water droplets working hard in the combustion chamber.

We have now arrived at the most important aspect of water injection in dealing with overall performance gain and fuel efficiency. Moistened air has the effect of reducing the temperatures of the surrounding engine components as it enters the combustion chamber allowing better volumetric efficiency of the induction stroke. Some cooling capacity is used during this cycle so the droplet size becomes smaller but only to the advantage of the next engine cycle. During the compression stroke, tiny water droplets are distributed amongst the charge air in the ratio of about 120:1 (based on water to fuel ratio of 10:1). This has the effect of regulating the flame speed thus promoting even flame propagation speed. The predicable burnt rate is essential for accurate ignition mapping to produce consistent power.

Any non-homogeneous a/f mixture exhibits abrupt frame-front temperature changes, this condition promotes the onset of detonation especially towards the end burnt period. Once detonation has started, it will tend to continue for the next few cycles even the subsequent condition is normal. The most common solution adopted by most engine management to avoid this from happening is by running an over-rich mixture. This has two effects – excess fuel cools the mixture and slows down the burnt rate. Slowing down the frame speed has the effect of shifting the peak cylinder pressure curve during the power stroke. When pressure is build up further away from the top dead centre (TDC) line it will minimise the onset of detonation but in expense of loosing some pressure that feeds the pistons to tuning the wheels. Rich mixture not only wastes fuel but it forms carbon monoxide molecules, a product that has only 30% energy release of the carbon dioxide, a fully oxidised carbon molecule. Every molecule of carbon monoxide carries an oxygen molecule out of your exhaust. Keep in mind that the engine only inhales about 20% of oxygen and should be treasured.

Now comes to the all-important reason why we are injecting water to rob heat energy - in order to push the power boundary further to obtain some meaningful figures, the boost pressure can be increased further (higher effective compression), aim towards MBT (maximum brake torque) timing and 12.5:1 a/f ratio. Since the use of race fuel is not on the menu. As we approach those tuning levels we will experience considerable increase in cylinder pressure and temperature, but at the same time more power. Since we cannot raise the melting point of the pistons, we must find a way to control and keep the engine components from overheating. Having six times the latent heat of fuel and huge expansion rate from liquid to gas (single molecule of water – not droplets) water is the ideal substance to inject as a coolant.Having absorbed and dealt with the destructive heat, the by-product (superheated steam at x1400 volume increase) becomes an active partner in adding force to the downward pistons. Water is converting the extra heat energy for wheel power. Without water, the excessive heat must be transferred to the cooling system and lost into the atmosphere. Since the quantity of water injected is relative small compared to injecting some six times the amount of fuel to arrive at the same cooling property. To all intent and purposes, we are not advising anyone to go this far but the possibility is there and achievable. In most cases, one can venture into the extreme gently. Since water in free, it cost nothing to continue your quest for a highly efficient engine that produce far more power than the fuel dumping method used by the makers.

One word of warning, a water injection with good failsafe capabilities must be your first priority when you are choosing one. So how much water should be injected to be effective? For those how want to extract the maximum power and not achieving maximum economy. To date, apart from the Audi FSI and some new Honda engines, it is considered an a/f ratio of 12:5:1 a/f ratio will yield maximum power and 14.5:1 for maximum economy. The exact a/f ratio vary from engine to engine, it has a great deal to do with how homogeneous is the mixture induced. We know that under WOT, full boost conditions most turbo car’s a/f ratio drops down to 10:1 to 10:5:1. 3% water to fuel will be have the same heat-absorption properly of the excess-fuel as running from 12.5:1 to 10.1:1 a/f ratio. We normally recommend 10% of water top fuel to make it extra safe.

Appendix.

The above document is a collection of information acquired over the last 12 year of my involvement of water injection with many from practical experiment. I believe the above description is accurate but I would like to hear from others that may have other experiences of running water injection. Please share with me with any new theory and findings.

Document on water injection by Richard Lamb 7th August 2005, free to be distributed in whole only with the correct reference to the author and source. www.aquamist.co.uk/phpBB2/index.php

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