Water makes you go further ?

[FullyFlapped]

A friend of mine, who is very into automotive engines was telling me yesterday that there used to be a contraption available for carburetted car engines which would squirt water into the petrol/air mix, the result being improved combustion and increased range due to a reduction in fuel usage. His proposition was that flying a carb'd plane in solid IMC should do something similar because of the increased water content in the air mass being taken in.

Is he correct ? And if so, is this viable in a GA engine ?

FF :ok:

[High Wing Drifter]

I thought water injection was related to wayback Forumlal 1 and had two benefits:

1) The legal grid weight of the car could be easily reduced during the race!!

2) The water helped keep insane head tempratures down.

I didn't think it had anything to do with just the fact that water makes things go better. Anyway, that seems counter intuitive to my limited knowledge.

Quick edit to say that my post is a not quite but almost: http://www.rallycars.com/Cars/WaterInjection.html

[The Nr Fairy]

And what would it do to the risk of stopping the engine due carb ice ?

[BroomstickPilot]

FullyFlapped,

During WWII some high performance aircraft had provision for water methanol injection into the petrol-air mixture, the (Fokke Wulf Ta152 comes to mind). The methanol was only included to lower the freezing temperature of the water.

It is definitely true that engines do run better when there is water vapour in the mixture: I'm sure you've noticed that your car runs better on a wet day. Why this should be, I'm not sure, (come on Gengis, you're certain to know).

Whether you would gain any discernible advantage flying in damp air, I rather doubt, and the increased risk of carb icing would, I suspect, make it unwise to do on purpose.

Broomstick.

[Andy_RR]

Water injection does several things:

- it cools the charge through the latent heat of evaporation, which increases the volumetric efficiency of the engine

- it cools the charge through the latent heat of evaporation, which increases the knock resistance

- it reduces the peak cylinder temperatures and pressures during combustion which also helps knock resistance.

On an aero engine which has fixed ignition advance, only point one is of relevance and even then not very.

A

[ShyTorque]

I agree with AndyRR.

The addition of water in small amounts helps prevent detonation, so that a higher compression ratio and more advanced ignition setting can be used. This can give the advantage of better power output and/or economy, although not necessarily in every engine. Some engines are prone to detonation, especially large, slow turning engines with large cylinder volumes.

In such engines, the ignition timing has to be set relatively retarded, which is bad for power and economy. If a way can be found to prevent detonation, more advantageous ignition settings can be used. It's here that water injection (or fuel with a higher knock resistance) may help.

The downside is that the amount used needs to be carefully controlled. Water vapour introduced into the inlet manifold ultimately displaces air, reducing the total mass flow of fuel that can be burned. If too much water is introduced, it will reduce the power output.

I've done quite a bit of research on this because I have a car engine that was built with a higher compression ratio than ideal, bearing in mind today's aggressive low-lead fuels and I was initially worried about detonation. In the event, the engine runs well without water injection. It is now possible to buy water injection kits, for turbo-charged road vehicles that run high boost. These are often designed to use use water/alcohol windscreen washer fluid.

See this web page, which explains this quite well.

http://en.wikipedia.org/wiki/Detonation_internal_combustion_engine

[White Bear]

Fully Flapped,
As has been mentioned this is relatively old technology, it has its drawbacks not least a weight penalty, and I’ve heard differing stories about internal rust problems:uhoh:

Anyway to learn more the NASA site below has some really interesting things to say not only about the original reasons for water injection (more power) but its research into water injection helping to lower exhaust emissions.
http://gltrs.grc.nasa.gov/reports/2004/CR-2004-212957.pdf
Regards,
W.B.

[FullyFlapped]

Thanks guys, some interesting comments - I'll pass them along !

FF :ok:

[bar shaker]

Water injection will offer very limited gains on a normally aspirated engine as the temp spread will be low and the water will be replacing oxygen by volume (the oxygen in the water is useless in combustion terms as it has already bonded with hydrogen).

In a turbo charged engine, the gains are measurable at high boost (~high inlet manifold temps) but adequate intercooling is more beneficial. Better still, a water spray onto an efficient intercooler will dramatically reduce intake temps.

The down sides with WI are: it introduces more mechanical parts, the need to map the WI into the engine's management system (so that its operation is matched to the engine performance) and the weight penalty of carrying the water.

[Piltdown Man]

However, I fear that your friend is incorrect. Moist air is less dense than dry air. Therefore, you actually get a performance reduction in moist. The perceived increase is probably due to his car being out in cold, rainy weather (the density increase due to the cold, not the rain).

[Sedbergh]

It should be remembered that the quality of the fuel available to the Luftwaffe in WWII for their piston engines was awful -

- hence the need for drastic anti-detonation measures like water injection

[White Bear]

Without going too deeply into this, I have always understood the main reason water injection worked is the heat required to turn water into super heated steam during combustion did 2 things:

1. It lowered the temperature of combustion thus lowering the flame front speed and therefore detonation, and for the same reason it reduced NOX emissions.

2. Superheated steam, takes up more room than the water that was used to create it, and therefore actually raised the compression ratio during combustion, allowing the engine to make more power, even though it has slightly less oxygen to burn.

It is because of these two effects engines run smoother and make more power when supplied with cool moist air.
Regards,
W.B.:D

[Andy_RR]

White Bear,
A phase change of water from liquid to gas during combustion would absorb energy (latent heat) so this energy is no longer available to do any mechanical work. Efficiency would go down.

This same energy going into a gas will increase the pressure a lot more, so higher cylinder pressures = more work/effort available.

The reason why water vapour decreases the temperature of combustion is because it is a triatomic molecule - it has more modes that the atoms can vibrate and absorb energy that doesn't translate into pressure than a diatomic molecule such as air (N2 and O2). This is expressed as 'gamma' or the ratio of the specific heats (c_P/c_V). If you take a look at the air-standard cycle formula for the otto cycle, you'll see that thermodynamic efficiency is a function of geometric compression ratio and gamma. As you decrease gamma, (more complex molecules) you decrease the efficiency, but in doing so, you decrease the peak cylinder pressure too.

The air-standard cycle assumes you are operating at the best igniton angle and doesn't allow for any knock limiting. So you might well say that how can you increase power with water injection if you are decreasing efficiency. Well, the answer is that if you are heavily knock limited, as tends to be in a supercharged engine, then any means of increasing the ignition advance will reap you more reward than the decrease in thermo efficiency that the lower gamma/ratio of specific heats will penalise you.

The corollary of this is that when you run lean, not only are you increasing efficiency by reducing pumping work, you are increasing the gamma by having residual O2 and N2 in the combustion products during the expansion stroke, increasing the overall value of gamma.

Apologies if my explanation is a bit too theoretical!

A

[bar shaker]

WB

If Andy was a bit heavy going, just go and read the Universal Gas Law to see why you wont have any steam in the compression or ignition process

:ok:

["aux vaches"]

Glider pilots have known for years that if you fill your wings full of water (in my case 200 litres) you go faster and further on a good day (strong, regular climbs) - the proposed hosepipe bans for 2006 are not looking good for the glider boys !!


"aux vaches"

[chevvron]

Another thing in common use by boy racers is Nitrous Oxide (NO2) injection giving a short term power increase at the expense of temporarily increased fuel consumption. Extra fuel has to be added as the NO2 adds to the amount of oxygen entering the combustion chamber, and thus extra burning medium is required to avoid leaning out.