Why do we need Water Injection? isn't intercooling enough?

There's no doubt that turbocharging without intercooling is totally substandard. You increase boost and instead of extra power you only get extra heat.
Raising the boost on a stock turbocharged car only really brings results for a handful of psi - after that intake temps get so high that the ECU has to pull back the ignition. So power-wise we're back where we started, and as a bonus we've eaten up the manufacturer's safety margins.

 

But is water supposed to be inside the engine? Won't it rust everything, is it not an abomination?

Natural thing to ask. In fact large quantities of water are produced during combustion anyway. Even the air the engine breathes contains more water than people think.

Here's an interesting fact: on a rainy day, 1Kg of air at 25C contains about 33ml of water. So a typical 2 litre turbo engine consumes over 200cc of water per minute at 6000rpm on a rainy day at 1 bar boost. That's not far off the quantities water injection normally operates at.

Below is a high-output engine using water injection instead of intercooler. Bold move, but it goes to show that the concept works!

Big block TransAm: had a massive intercooler and pipes, trying to cope with the hefty output from the two turbos. It was making well over 1000bhp (claimed 1300)

Here the intercooler has been dumped in the search for better throttle response! It has been replaced by multi-nozzle water injection. Not only were the resulting temps lower, it could run 2 extra degrees advance, and got rid of the messy thick boost pipes too. Interesting...

 

Here is a nice twin-nozzle installation on a 300zx (local copy here)

My first experience with a primitive Water Injection setup

I had turbocharged a 1100 bike without intercooler - space was far too tight for any size of intercooler I could find at the breakers at the time. As a result, at high boost the intake manifold would get very hot, so the fuel tank (right above it) would absorb so much heat that I could feel it through my belly. I had nightmares that the whole thing would explode in a ball of fire. The only viable way to cool that air was Water Injection. It was primitive (compared to the systems available today) and the water tank was tiny (bit over a litre big, concealed as a licence-plate bracket. But the difference it made to the running of the bike was amazing. At high boost it would manage to run smooth, going through the revs without gaps, clicks or pops.

Very nice for short bursts (no more, because running out of water was always a threat). And thirsty it was. The more water I would add under boost, the better it felt. It wouldn't saturate! I doubt that any of the water ever made it to the chambers, because the whole intake was so hot under boost that every droplet would evaporate long before it reached the intake valves. The system was crude, the pressures involved low - but it was intercooling that WORKED!

 

Yeah, right, but that was a non-intercooled engine. I've got a mother of a Cooler up front, and my charge temps are very close to ambient. Would Water injection still have such a dramatic effect?

No, not all the time anyway. It depends on your particular setup.

You say that your charge temps are within 10C of ambient. You know that because you've installed a charge-temp-gauge and it very rarely goes up by more than 10C from ambient. Or so you think. During full boost runs, (when it really matters) especially when the throttle is sustained wide open, the charge temps will go well above that. You're not likely to notice it on the road, because you'll be risking an accident by going that fast and looking at the boost gauge and the temp gauge instead of the road ahead. It's best done on a RR in a controlled environment.

Then you'll see a temperature spike that could be 30-40C above what you think is the norm. Sustained full boost will lead to the intercooler core suffering temporary heatsoak, so it's efficiency drops sharply. This will lead to the cyl head suffering from temporary heatsoak, and suddenly you'll find one or more of the following

1. the ECU pulling back ignition retard in a major way, desperately trying to avoid detonation (this will further aggravate EGTs though!)

2. the ECU pulling back boost

3. detonation has damaged the engine anyway

4. the head has warped, or a couple of exh valves have been burnt

5. the car is very sluggish afterwards, because the ECU has shifted to a lower-octane map and intends to stay there for a while, just to be on the safe side. The Motronic for example takes a minimum of 9 minutes of continuous operation to switch back to the higher-octane map.

The whole saga would have been avoided perhaps if the tank was full of fuel with a MON rating of 10 extra points, but for a road car that is not an option.

Water/methanol injection, triggered at the right moment, could have an equivalent effect.

 

How will my mixture be affected by Water Injection?

Will it become leaner, richer or stay completely unaffected? Water doesn't burn after all, does it?

Good question! Although water steam itself won't burn, it will alter the conditions under which fuel will burn. Also steam coming out of the exhaust is perfectly stoichiometric, so the oxygen sensor will give a reading that will err towards stoich. In real life water is only injected under high boost, where the mixture is meant to be quite rich. Therefore the water injection will make the Oxygen sensor to read leaner than it really is, in other words not as rich.

This is important for the avid tuner, as they may be tempted to dial in more fuel, fearing that the engine isn't rich enough under boost. Beware - only check the fuelling with water injection switched off (same as NOS). Oxygen sensors, even Wide Band ones will be fooled by the new levels of excess oxygen produced, and they will also show leaner.

In any case you do not need to be running pig-rich when W.I. is operational. For maximum power you don't need to go richer than 13:1. I've got maximum power at 14:1 under full boost with W.I., whereas otherwise it would be 12.5:1.
In other words, a well setup W.I. system will save you fuel, allowing to run almost stoichiometric at full boost.

Water doesn't burn, but neither does excess fuel (AFRs lower than stoich indicate just that: unburnt fuel) The excess fuel is used mainly as a cooling agent --- a task water can perform MUCH better (having twice the specific heat and six times the latent heat!).
Here is a similar take on the issue, you CAN be too rich (local copy here)

Here is a video of a similar system being used on a 2Lt Cosworth engine running 2 Bar boost. The custom made intake manifold has a “sight glass” on the end so the water can be viewed during dyno runs. (local copy here)

 

Water Injection installation on the C20LET:

Water Injection installation on the 2JZGTE:

 

Injecting before the compressor

There are cases when we want water injected in more than one locations. We can use a 'splitter' (shown at the middle of this pic) for this purpose. The Aquamist pump can handle a couple of the smaller nozzles - although total flow will be capped at the pump's total flow capacity (around 330cc/min)

Here is a nozzle fitted on the air pipe before the turbo. This is meant to help increase the efficiency of the compressor. The chargecooling function is of secondary importance here, although it will be massive as the charge air temp at that point will always be over 100C (before the intercooler and at 1bar or over) Note the extra metal clip that has been inserted around the pipe and drilled, so that the nozzle will stay in place. Just sticking it on the silicon pipe is not good enough, the water pulses will work it loose.

This is a 'universal' hose clip set that can be used to make a custom clip at the diameter we want (larger than most commercial sizes)

Note that injecting before the compressor actually decreases the efficiency of the intercooler, because that depends on the temperature difference between the incoming air and ambient air.

I've found it to be a decent compromise though, as the compressor now behaves as a slightly larger one. Technically speaking it operates now a bit more in isothermic fashion, compared to the adiabatic normal operation. (More on this advanced subject later).

Also note that the 'main' nozzle on the inlet manifold will now flow a bit less too, which should not be an issue, as the total amount of water injected into the airstream has increased by almost 50%

This chart shows the datalogged temps from a fellow enthusiast's runs. He used a variety of nozzles, all before the compressor.

Dark blue are the pre-turbo temps, hovering just above ambient (15C).
Magenta are the temps right after the turbo. They are nowhere near as high as we'd expect them because of the cooling effects of the water spray. However this compromises the efficiency of the intercooler, and this is quite clear in the Yellow line (post-intercooler). The larger the nozzle, the smaller the effect of the intercooler.
Light blue are the throttle plate temps. It's interesting how these are below ambient, especially with the larger nozzles.

The effects of in-cylinder cooling, which are the most important, cannot be displayed here of course.

The time lapse between runs is long enough for most effects of heatsoak to be irrelevant. But it clearly illustrates the point that small nozzles can work better: post-intercooler temps are lowest with the smallest nozzle. In fact, the largest jet had a cooling effect as no WI at all!

So when should the Water Injection be triggered then?

There are no hard-and-fast rules, it all depends on the specific engine setup.

You want it to trigger as late as possible, but soon enough so that excessive in-cylinder temps are avoided. Below are some factors that would tend to push the 'trigger' point upwards:

1. improved intercooling. Lower ambient temperatures, too.

2. lowered compression ratio (either static or dynamic)

3. higher octane fuel

4. lowered max boost

5. increased ambient humidity

6. running rich under boost ( 12:1 or richer)

In a 'safe' Water Injection setup, the trigger point is 3-4 psi below the level where the ECU would start to take precautionary measures. For OEM turbocharged cars running high boost, that would typically be the max boost level of the stock engine. For example, if a stock car runs 11psi overboost and now it's configured to run 20psi without decompressing the engine, it's wise to have the water injection activated around 11psi. In the summer maybe that trigger level could go down by a few psi, to compensate for the higher charge temps.

According to Richard Lamb (Mr Aquamist) we could use this rule of thumb for the onset of water injection for normal unmodified factory car: just below the factory boost pressure.

Furthermore, 10% of water will normally allow a compression ratio increase of one full point (say 8:1 up to 9:1)or around 3.7psi extra boost pressure. Other factors such as ambient temperatures and fuel grade would (of course) affect these figures.

So how do we know when we've reached our 10% water/fuel goal?
Easy: as (another) rule of thumb goes, we require approximately 500-600cc/min of fuel per 100BHP. So a 300bhp (target) engine flows around 1.5 ~ 1.8litres of fuel per minute. The water flow rates for each nozzle are given from the manufacturer, the ones below are for the basic system 1s:

Caution: The above map provided by Aquamist is incorrect!

I've always had the feeling that something's wrong with the Aquamist deliver, but couldn't put my finger on it. The theory just didn't tally 100% with my experience - the engine bogged down more than it should (or when it shouldn't). The answer is that the flow-rate graph provided by Aquamist is wrong. It underestimates the volume of water delivered.

For example I've measured the 0.5mm nozzle and it flows about 200cc/min on 12V and 240cc/min at 14V (where it will operate in real life). So for a 2lt turbo engine pushing out 280bhp, you probably want to go for the 0.4mm nozzle, and nothing bigger.

On a double-nozzle application (yes, the pump can do this) the delivery is capped by the pump's max flow capacity. I measured a 0.5mm and a 0.6mm twin setup to flow 320cc/min, which is as far as the pump can go. (with a priming pump it can deliver more, but that's not the issue here).

Once we've got the water flow sorted, it's easy to size up the nozzles. For a badly-intercooled engine the 10% goal can easily be upped to 15%, reaching up to 20% for non-intercooled engines.
Please note that these are not the Ten Commandments, just the basis for experimentation.

Another practical example:
A 300bhp engine will flow fuel at a rate of 1500-1800cc/min. If it's 4-cylinder then that's around 400cc/cylinder.
So the water-injector add-on we would like to be about 150cc/min (one-tenth of total fuel, may just a bit less if our intercooling is good).
A 0.4mm nozzle would do then (looking at the above graph).
But if the injection is activated at 10psi, then we get this sort of water while the engine is only making (say) 220bhp - far from 300. So water/fuel ratio at that point is over 15%, and it will reach our target of 10% only at full boost.
It gets more interesting: most people use the 0.5 or the 0.6 nozzle for this sort of horsepower, and that's over 230cc/min when at that sort of boost we'd really want 100cc/min (200bhp is 2000mm/min worth of fuel).

If the water injection is not boost-sensitive (more boost -> more water) then we get either too much water at an early stage, or not enough water at full boost.

The bottom line here is: be careful, it's easy to swamp the engine with water and lose power, if your nozzle is too big and the trigger point is too low. For a 300bhp engine we may want the smallest nozzle (0.4mm) and a trigger pressure of not much below 1 bar.

Below is my rough guide to setting up the boost trigger for the Aquamist pump. It is a small, handy spreadsheet to help you calculate more accurately the boost trigger point, depending on your engine's state of tune. Just click on the Water Injection tab.

 

Water to Fuel ratio

We've seen that excess fuel is traditionally used as in-cylinder coolant. We've also seen here that water is a much better alternative for this purpose, since its latent heat is 6 -7 times higher than fuel (energy absorbed as the tiny droplets evaporate)

Latent heat of fuel = 350KJ/kg
Latent heat of Methanol = 1109 Kg/kJ
Latent heat of water = 2256KJ/kg

Below are charts compiled by Richard Lamb. It appears that all you need is 3% water/fuel to replace two points of AFR (10.5-12.5). There is no performance gains in going richer than 12.5:1, and a lot less mass of water is needed to cool things down.

On high output engines more than one nozzles may be needed for proper atomisation and adequate water flow. This is a '1000bhp' Supra setup with port water injection. Each of the six nozzles is 0.4mm, driven by two pumps (one for every 3 nozzles) Note the water surge accumulator, looking like a dump valve next to the pumps.

Above is a NACA graph enhanced with fancy colours to make it more presentable. The cyl head temps were measured at various locations, and they all followed the same pattern, with two notable exceptions:

1. EGTs, which were largely unaffected

2. Exhaust valve guides, where temps went UP. This has to be taken into context of the higher horsepower that was also produced at the higher water/fuel situations, and they were still lower than those achieved at those power levels with only fuel used as a coolant.

 

Wet compression paper

Swirl Flash paper

 

 

Graphs above posted on the Water Injection forum. They are based on the following assumptions:

10Kg of air, Gasoline's latent heat capacityof 350KJ/Kg
Water's latent heat capacity of 2256KJ/Kg
Methanol's latent heat capacity of 1109KJ/Kg

Note how steeply heat absorbtion drops as water is substituted by methanol. In real life methanol actually helps water evaporate better by reducing the surface tension of the mix, but the ultimate heat absorption capability during the compression stage doesn't change much.

 

A few more points:

Ideally we want the water injection pressure to be compensated for the extra boost pressure of the engine - same as with fuelling.
We would also want the water injection to be rpm-sensitive. 1 bar at 3Krpm would need less water compared to 1 bar at 6Krpm.
Finally we'd like the water injection to trigger only when the charge temperature is over 50C - it would be counterproductive otherwise. This is the idea behind raising the boost trigger point in the winter.

In an 'optimistic' water injection setup, the ignition timing would be advanced a few extra degrees for the duration of the water injection. This will result to power gains, sometimes considerable ones. The downside is that the life of the whole engine now depends on the water injection working properly. If one considers the number of things that could go wrong, this would only really be sensible on a competition setup.

Every turbocharged car has a pattern of the engine temp going upwards after a few full-boost runs. Good FMICs reduce this a bit, but it's still there. With water injection it doesn't happen though. Despite repeated full throttle abuse, it's hard to make the engine temp rise above a 'cool' average. This must be very refreshing for the combustion chambers and the exhaust side in general. EGTs should also drop considerably, to the tune of 40-50C. If they do not, then the system has not been setup properly.

Below is the result of using a 1:1 water/fuel ratio in a cylinder that is overstressed (too much boost/too high compression ratio)

Red line is without WI, blue with.
We can see max cylinder pressures drop from 57bar down to 37. Water drastically reduced peak pressure (which is what breaks things), but maintained total power by having higher pressure through much of the power stroke.
Although exaggerated a bit, this is the effect we are after.

 

I'm running pig-rich. Will adding Water Injection make more power?

Nope.

Maximum power is achieved at around 12.5:1. If you're running 10:1 or richer, then a lot of your fuel doesn't get burnt at all, and the speed of combustion is slower than optimal anyway. If you add water into this situation, then you're making matters even worse. Water is counterproductive when you're running too lean or too rich, see here for more details (Here is a local link)

Beware, some people have reported increased EGTs while the water injection is operating. This is usually because their trigger level is set too low, so at low revs/low boost they're injecting too much water in relation to the fuel used at that moment. The excess water slows down the flamefront, effectively retarding the combustion. This raises EGTs and lowers horsepower as well. Using smaller jets and/or raising the activation threshold addresses this problem.

 

I'm running fine as it is. Will I make more power with Water Injection?

Nope, you'll probably lose power if everything else is left as it is. If the engine is operating well within it's operating parameters (i.e. not retarding like mad, or not operating in the middle of the Sahara under 70C heat) then increased humidity will sap power. Water injection tends to go further than merely increasing humidity, so power loss can be substantial.

Below is an extract from the excellent book Internal Combustion Engine in Theory and Practice, Vol 1, Second Edition Revised

(I have combined and simplified examples 12-3 and 12-4)

Effect of Humidity:

Suppose we have an 8cyl engine producing 300bhp at 4500rpm, 60F and dry air.

Power at humidity 80% and temp 120F (very hot and humid): 230bhp (dry air: 268bhp)

Power at humidity 50% and temp 20F (very cold and humid): 318bhp (dry air: 324bhp)

Note how power is always down when humidity is present.

Another effect of humidity, example 12-5:

Engine producing 300hp, at 100F, humidity zero.

Same engine only produces 243hp, still at 100F but in saturated air.

Here just the effect of reduced air density lost us 57hp, yikes.

All the above are for n/a engines, but it doesn't matter: a well-running turbocharged engine will also experience a similar power loss. The only way to turn this into a gain is when the air is saturated (and beyond!) while the operating parameters are WAY OFF the ideal -- this means charge temps way too high, detonation conditions forcing the ECU to be pulling out timing, too much boost creating abnormally high cylinder pressures.

This translates into the Water Injection only being active when the engine is operating BEYOND the normal parameters. Over and above the manufacturer's maximum boost, most likely, while taking out the extra fuel used for detonation prevention. This is a brave move for the non-believers, running AFRs of 13-12.5:1 while under full boost (much higher than stock boost actually) is not for the faint of heart. Increasing ignition advance at the same time looks suicidal to the 'normal' school of tuning, so very few people will actually do it.

...hence the typically poor power results from the use of water injection.

To recoup: the extra power will come from running MORE BOOST, MORE ADVANCE and LESS FUEL. If you don't do that, you'll get less power, and in that case you may not bother with water injection at all.

 

Very nice. What mixture should I inject though? Surely not tap water?

Tap water might be a bad idea if you leave in an hard water area. The water should be soft and well filtered (via a home-purifier perhaps). 50% methanol mix is being touted as the best compromise of power/pump reliability/heat containment.

However, in most cases I don't see this happening. Methanol is a nasty chemical and you don't want to touch it or smell it. But if you use the windscreen washer bottle, you ARE going to inhale it, every time you wash the windscreen. It's very toxic. It's also very flammable, and it boils well below 100C. Therefore I wouldn't want more than 10% of the thing in my windscreen tank, and that's only because some form of antifreeze is needed in the winter.

Commercial windscreen fluids rarely contain methanol nowadays, for the same health reasons. They contain other kinds of alcohol, that will swell your pump's seals and kill it. So they, too, are out of the question.

Ah, but I'll use a separate water tank I hear you say. I've got a cool aluminium 2lt tank, so I'll use that for the 50/50 methanol/water injection. Listen carefully then: methanol attacks aluminium and the swarf will end up clogging your filter and the pump will then run dry. Nasty stuff this methanol, it also attacks some forms of rubber and plastic. You've been warned.

Large Water Injection tank in the boot: That will last for a while...
An even more interesting Water Injection boot tank: a couple of thermos bottles. They keep liquids hot, but cold too! Imagine storing a 50/50 methanol mix at -50C. All the charge cooling of nitrous oxide without the increase in cylinder pressures. Cheap DIY refills, too. How cool is that?

 

When it comes to the antifreeze properties of these mixtures, it's nice to know what these concentrations can do. Below are two tables, courtesy of AaronC on the (now defunct) TurobICE forum.

Methanol \ Water Mixtures

Methanol Conc. Vol %	Freezing point (C)	Flash point (C)
0	0	no flash
13	-7	54
24	-18	43
35	-26	35
46	-40	29
56	-54	24
66	-71	21
75	< -73	16
83	< -73	13
92	< -73	13
100	< -73	13

 

Isopropanol \ Water Mixtures

IPA Conc. Vol %	Freezing point (C)	Flash point (C)
0	0	no flash
10	-4	41
20	-7	29
30	-15	24
40	-18	21
50	-21	18
60	-23	18
70	-29	18
80	-37	18
90	-57	18
100	< -73	12

 

Below is an indication of the ability to absorb heat in the chamber. Note that water is more than 4 times better than petrol, and twice as good compared to methanol or ethanol

Water -- Oxygen content 0% (ignoring dissolved oxygen) Latent heat of Vaporization 2259 KJ/kg . K = 539.6 BTU/lb . F

Methanol -- Oxygen content 49.9% by wt, latent heat of vaporization 1099 KJ/Kg . K = 262.6 BTU/lb . F

Ethanol ----- Oxygen content 34.7% by wt, latent heat of vaporization 854 KJ/kg . K = 204.1 BTU/lb . F

Isopropyl --C3H8O--- Oxygen content 26.66% by wt, latent heat of vaporization 666 KJ/kg . K = 159.24 BTU/lb . F

Petrol -- About 586-628 KJ/kg . K ---- 140 -150 BTU/lb . F depending on fuel blend

Nitrous Oxide -- 376 kJ/kg. As far as in-cylinder cooling goes, it's worse even than petrol. Of course this is not the main purpose it's used, see here for more.

 

Surface tension of water/methanol mixtures

Interesting document on surface tension (thanks to a fellow enthusiast Abner from the Water Injection forum)

 

Here is a NACA paper on the knock-suppression properties of various injection mixtures (local copy here)

Local copy of the well-known water injection paper that used to be on the TurboICE forum

Copy of the classic SAE paper 1999-01-0568 Water injection effects in a single-cylinder CFR engine

Frank Walker using water injection in RAF planes in the war, running 3 times the normal boost pressures.

Magnetic and Electric effects on water:Weird. (local copy here)

Another water injection model is here along with some experimental data (local links)

In the age of the internet, there are forums and newsgroups catering for every conceivable interest.

Water Injection has it's own forum:

It's not terribly busy, but it's there!

 

Here is another commercial Water Injection site. Make of it what you will.

 

Back to Intercooling

 

 

 

 

 

Water -- Oxygen content 0% (ignoring dissolved oxygen) Latent heat of Vaporization 2259.197 KJ/kg . K = 539.6 BTU/lb . F

Methanol -- Oxygen content 49.9% by wt, latent heat of vaporization 1099.45 KJ/Kg . K = 262.6 BTU/lb . F

Ethanol ----- Oxygen content 34.7% by wt, latent heat of vaporization 854.62 KJ/kg . K = 204.1 BTU/lb . F

Isopropyl --C3H8O--- Oxygen content 26.66% by wt, latent heat of vaporization 666.7 KJ/kg . K = 159.24 BTU/lb . F

Gasoline -- About 586-628 KJ/kg . K ---- 140 -150 BTU/lb . F depending on fuel blend

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pre-compressor WI

Compressor tips from a VF24 on a scoobie.
Upon closeup the erosion from precompressor injection is clear

 

 

datalogging temps using various jets

Normal compression is adiabatic, which to a first order means that as the gas is compressed, it gets hot. Now this heat is one of the biggest problems with forced induction, for 2 reasons. Firstly you have to get rid of the heat, and secondly you need to take power out the turbine shaft to perform the heating.( heat is work and work is heat).

With the right level of water injection, the heat is removed before it builds up, pushing the compression closer to isothermal (not all the way, but closer). In round terms this is about 30% more efficient (less exhaust gas required for the same boost, or more boost at the same exhaust flow).

Problem 1 is that water on its own, whilst having a very high latent heat of vapourisation, doesn't have that good a saturation partial pressure. You can only put so much in before the air is at 100%RH. Above this the water will not cool the air until you compress it in the engine. However, if you add a second fluid, say methanol, Dalton proved with his law of partial pressures that the methanol doesn't know that the air is saturated with water vapour and goes on to vapourise as well. Add a 3rd, such as acetone (all available chemicals) and you get a 3rd tranch of cooling.

The net result is that you may be able to cool the air to slightly below ambient with the right mix. So your compressor becomes more efficient, you can throw away the intercooler, increasing your flow, and in some cases significantly improve the flow from turbo to inlet.

If you leave the intercooler in, you are generally warming the air back up again, so you have to take the plunge and remove it to gain the best benefit.

 

------------------------

You are working with a complex system of mechanical devices that interact with each other in many ways. Even though on first blush injecting infront of the compressor or between the compressor and the intercooler might appear to be less effecient you need to account for ALL the interactions. In many cases we simply don't have enough information to predict the results so frequently experimentation will give you better data in a matter of minutes, than all the incomplete computer simulations you can afford.

Injection in front of the compressor accomplishes several things. A turbocharger is a constant pressure variable volume "DYNAMIC" compressor.

A turbocharger only knows 2 important properties of the gas it is compressing. The density of the gas at the compressor inlet and the pressure ratio it is operating at, which is determined by the rotor rpm and the gas density. If you increase the pressure or reduce the temperature at the inlet you will modify both of those parameters. In both cases (increased inlet pressure, or lower inlet temperature) you increase the apparent density of the gas passing through the compressor. At a given rotor rpm with a given gas density you will flow a very specific volume of gas and it will be compressed to a specific pressure ratio on exit. That is what the compressor map is based on. If you change the inlet conditions (gas density) you in effect slide the compressor map left and right. This is the "corrected flow" of the turbocharger.

By injecting water/alcohol ahead of the compressor two things happen. You cool the inlet air substantially, this in effect moves your true operating point to the left on the compressor map. (in most cases for max performance this is a good thing, although on some turbocharger conditions it can cause compressor surge.)

You also change the pressure temperature profile inside the compressor wheel itself. You probably actually change the shape of the compressor map. As the gas moves outward and is compressed, heat that would have gone into heat and increased pressure is absorbed by the WI mist and so the compressor has less work to do since it is no longer fighting this temperature driven pressure increase, it can achieve more mass flow at that pressure ratio. The cooling should also modify the speed of sound in the gas and the mach number of the compressor blade tips should also change. This should change the choke flow characteristics of the compressor but I don't have the information to comment in detail on that.

Net result is, you increase the mass flow through the compressor --- in effect you make it act like it is bigger than under normal conditions.

During WWII this was the way ADI (anti detonation injection --- the common term in the aircraft world for WI ) was set up on military aircraft in most cases. The water and the fuel was injected into the eye of the centrifugal supercharger.

Errosion of the compressor blades is not a problem if steps are taken to ensure the mist is very fine when it arrives at the compressor inlet so that it follows the airflow and does not imping on the blades with a high differential speed.

The ideal is to get drop size down as close to 10 microns as possible but due to the brief periods of use and intermittent nature of most WI systems, in reality you can live with larger drop sizes in real world systems.

If you have ever ridden a bicycle or motor cycle in a rain storm you know how sharp the impact of a large water dropplet can be, but a fog or drizzle will not cause the same painful experience because the droplets are small enough they are strongly influenced by the direction of flow of the air stream around you and impact with much less velocity and obviously also have lower momentum.

For people in hot dry climates that lose a lot of turbocharger performance in hot weather, pre-turbo injection should be looked at.

In regard to the effects on the compressor mass flow, maximum results appear to occur with mist flow of about 3% - 10% of the air mass, so you will likely need to inject additional WI near the throttle body to reach maximum detonation suppression and best power.

--------

If you mean that the ambient temp air flow is affected, I wouldn't expect that to be much. Say ambient is 20C, how much lower can it get by injecting mist of water that's 30C+ (the bottle is in the engine bay!). I suspect that after the pump the water is even warmer.

Actually it can be very substantial. The cooling is by evaporation, and typically will exceed 10 - 20 deg C . The initial temp of the water is of little consequence. The cooling due to evaporation (latent heat of evaporation) is very much larger than the latent heat of the liquid water.
As you can see below the evaporation of a gram of water will absorb 542 x the heat energy required to lower that same quantity of liquid water one degree in temperature. That means that even if the water were nearly at boiling temps when injected, evaporative cooling would reduce the air flow temp below ambient temperatures long before all the water changed to vapor.

Specific heat capacity of liquid water - 4.187 kJ/kgK
Latent heat of evaporation - 2,270 kJ/kg

Since the evaporation of the water and the alcohol will essentially stop when the air becomes saturated (ie. 100% humidity) there is a practical limit to the cooling or around 20-30 deg C for alcohol water mixes, and in real systems you seldom get much more than about 15-20 deg C.

For every 5 deg C you drop the inlet air temp you will increase mass flow by about 1% due to density increases.

I would have thought that this 'efficiency gain' is the effect of water droplets inside the compressor blades, as they try to squeeze the air.
Am I right?

Only in a very crude sense. Your thinking in a mechanical piston pushes on air sort of way, but what happens inside the compressor impeller occurs at a molecular level.

Think of it this way If you could freeze frame time, and stop what was happening inside the impeller while its spinning at 120,000 rpm. Each impeller passage between adjacent pairs of compressor blades contains a wedge shaped parcel of air. When spinning at 120,000 rpm the air is subject to huge centrifugal forces as it moves away from the hub of the impeller and toward the rim of the compressor. The trapped air would like very much to be slung out of the impeller but like a crowd at a stadium after a match it simply cannot all get out as fast as it would like. As a result it stacks up (compresses) as it gets near the exit. In this process a lot of internal friction occurs. The air near the tips of the compressor might be moveing near the speed of sound at maximum flow, this heating makes the air try to expand. This increases the pressure which fights the outward movement of the air. Eventually a balance is achieved between the centrifugal forces trying to throw the air out of the impeller and the pressure build up due to the compression and the pressure build up due to the heating. The addition of the water mist removes a very large fraction of the pressure gain due to heating. As a result more air can exit the impeller over a given period of time, and more of the pressure gain is real compression rather than waste heat. The net result is a more isothermic compression which is always more effecient than an adiabatic compression.

 

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Injecting methanol really complicates the tuning process. Complete combustion (stoich) for gas is about 14.7 A/F. For methanol it is about 6.3. Methanol has less fuel energy than gas, but adds considerable quantities of oxygen. Adding, 10% methanol, to gas creates a new unknown stoich value somewhere in between, maybe closer to about 13.8...who knows? The oxygen sensor is primarily measuring the very, very small amount of remaining oxygen from the combustion process. A sensor calibrated for gas will read leaner once significant alcohol is injected (more oxygen is available). The issue is calibration. "How do you know exactly what the A/F accuracy is with this new fuel mix?" Unfortunately, you don't.

It gets even more complex when using a rising rate of alcohol injection. My SMC hardware starts at about 6 psi boost with 70 psi injection line pressure, and ramps up to 100 psi by 16 psi boost. So, now you have not only a new fuel mix, it is also changing. Trying to tune using conventional wisdom with target A/F values is a bucket of worms that may have no practical solution. It's even tougher, when there is no access to an AWD dyno.

 

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A compressor moves a certain VOLUME of air. Even though most compressor maps list airflow by mass (lbs/min), it's actually a matter of VOLUME (CFM), that has been translated to mass by assuming a certain temperature and pressure (~25C and 1 atmosphere).

So, what this means is that a compressor that maxes out at say 55lb/min airflow on it's compressor map, should be able to flow about 10% more (60 lb/min) if the inlet temperature was 0C (0C air is about 10% denser than 25C air).

I imagine another big part of the limitation of the compressor is the amount of force of stacking up the air and compressing it. Although the ENGINE benefits from a intercooler, the compressor really doesn't care. It's still working hard, and making hot temps. These hot temperatures which result naturally from compression, work to expand the air, while the compressor works to contract it.

So, what if you could "intercool" the compressor itself? Add a mist of methanol/water to the inlet, which would vaporize and cool the air as it passes through the compressor? I'm still unsure on this, but wouldn't that mean a even bigger jump in the efficiency and upper limit of a given compressor, seeing as how it has to work against the air far less?

My guess here is that a compressor works on relative volume in vs out. When you see a compressor map, and one section says say 75% efficient at 60lb/min and a 3 pressure ratio. What REALLY matters, is how much volume is going in, vs going out. You get a 3x reduction in volume through compression, and gain 30% volume through heat expansion (25C in, 150C out). That's a mass boost of 2.1:1.

Now, IF the out temp of the compression process was reduced to say 40C through methanol vaporization, look what happens:

Pressure ratio of 3:1 remains
Temperature gain now only expands the air by 5%.

3*.95=2.85

So, if I'm not missing anything, the same compressor, operating at the same 3:1 pressure ratio, will be able to flow 2.85/2.1=35% more air if the air only heats up to 40C during the process instead of 150C.

Combine the gains of pre-compressor cooling to freezing temps, and the integrated cooling (actually heat gain reduction) during compression, and a compressor rated to only flow 55lb/min could flow: 55*1.1*1.35=81.675lb/min.