Thorlifter
Captain
How does water injection provide more horse power? Is water actually being injected into the cylinders because to me that would hinder the combustion process.
Explain water injection
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parsifal
Colonel
my two bobs worth.....
In a piston engine, the initial injection of water cools the fuel-air mixture significantly, which increases its density and hence the amount of mixture that enters the cylinder. The water (if in small liquid droplets) may absorb heat (and lower the pressure) as the charge is compressed, thus reducing compression work.
An additional effect comes later during combustion when the water absorbs large amounts of heat as it vaporizes, reducing peak temperature and resultant NOx formation, making use of the heat byproduct for the production of HP, and reducing the amount of heat energy absorbed into the cylinder walls. I believe this slows the burn rate for the fuel and a slower burn rate will deliver more power compared to a more spontaneous combustion, more akin to an explosion, which is what you tend to get when the fuel is hot and tending to vaporise. Cooling the combustion chamber also converts part of combustion energy from the form of heat 9basically a useless by-product of the internal combustion process) to the form of latent 9and useable) potential energy in the form of heightened pressure.. As the water droplets vaporize by absorbing heat, they turn to high pressure steam, which is much more efficient in building pressure than the standard fuel/air mix. The alcohol in the mixture burns, but is also much more resistant to pre-detonation than other fuels,. The net result is a higher octane charge that supports very high compression ratios or significant forced induction pressures before onset of detonation.Higher octane ratings also actually slow down the detonation speed, assisting in using the full stroke of the piston during its power stroke.
In a piston engine, the initial injection of water cools the fuel-air mixture significantly, which increases its density and hence the amount of mixture that enters the cylinder. The water (if in small liquid droplets) may absorb heat (and lower the pressure) as the charge is compressed, thus reducing compression work.
An additional effect comes later during combustion when the water absorbs large amounts of heat as it vaporizes, reducing peak temperature and resultant NOx formation, making use of the heat byproduct for the production of HP, and reducing the amount of heat energy absorbed into the cylinder walls. I believe this slows the burn rate for the fuel and a slower burn rate will deliver more power compared to a more spontaneous combustion, more akin to an explosion, which is what you tend to get when the fuel is hot and tending to vaporise. Cooling the combustion chamber also converts part of combustion energy from the form of heat 9basically a useless by-product of the internal combustion process) to the form of latent 9and useable) potential energy in the form of heightened pressure.. As the water droplets vaporize by absorbing heat, they turn to high pressure steam, which is much more efficient in building pressure than the standard fuel/air mix. The alcohol in the mixture burns, but is also much more resistant to pre-detonation than other fuels,. The net result is a higher octane charge that supports very high compression ratios or significant forced induction pressures before onset of detonation.Higher octane ratings also actually slow down the detonation speed, assisting in using the full stroke of the piston during its power stroke.
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Thorlifter
Captain
So basically it works like Nitrous in a race car, correct? Lower the temperature, increase density, increase fuel in the combustion chamber.
parsifal
Colonel
I think so
Nitrous also adds oxygen to the charge if I'm not mistaken; more oxygen more fuel is burned, more power. Hence why Nitrous was used at high alts once your above the FTH of the engine.
Using water/MW50 injection also involves a de-enrichment valve that when water is used the fuel/air ratio is reduced (don't need the over rich fuel ratio to cool the radial engine). Atleast on the P&W 2800 and I'm pretty sure once the Germans figured out on the BMW 801 to inject the MW50 into the eye of the supercharger. The first attempt at water they injected it into the cylinder as they did fuel; which had no cooling effect to the charge and no real advantage.
More than likely the Japanese also had water injection systems set up the same way.
If I remember correctly from reading.
Using water/MW50 injection also involves a de-enrichment valve that when water is used the fuel/air ratio is reduced (don't need the over rich fuel ratio to cool the radial engine). Atleast on the P&W 2800 and I'm pretty sure once the Germans figured out on the BMW 801 to inject the MW50 into the eye of the supercharger. The first attempt at water they injected it into the cylinder as they did fuel; which had no cooling effect to the charge and no real advantage.
More than likely the Japanese also had water injection systems set up the same way.
If I remember correctly from reading.
GregP
Major
You also have to keep the mixture at the correct ratio, which is around 13 : 1 air - fuel. If you get much leaner, or richer, it gets bad. Since NO2 has more Oxygen than air, I'd guess they had to make the water injection also run a richer air-fuel mixture automatically to keep the air-fuel mixture correct.
I can tell you from personal experience that if you do NOT add the extra fuel for the NO2, you burn pistons and valves almost immediately. My guess is the aircraft engine would do the same thing, probably slightly less rapidly since the piston have thicker crowns and the engine are turning slower than an LS-1. It might take a minute or so but, if it is wrong, it will grenade quickly at full rattle.
I can tell you from personal experience that if you do NOT add the extra fuel for the NO2, you burn pistons and valves almost immediately. My guess is the aircraft engine would do the same thing, probably slightly less rapidly since the piston have thicker crowns and the engine are turning slower than an LS-1. It might take a minute or so but, if it is wrong, it will grenade quickly at full rattle.
parsifal
Colonel
absolutely. nitrous is exciting stuff, but it can significantly reduce your engine lif if it is used incorrectly
I'm really wanting to hear from the true airborn petrol heads in this place to see how much, if any difference there is to the ground based information...I would expect the theory to be the same, but will see......
I'm really wanting to hear from the true airborn petrol heads in this place to see how much, if any difference there is to the ground based information...I would expect the theory to be the same, but will see......
Water injection and nitrous oxide work in two quite different manners: the former (like initially explained by Parsifal) happens before blower to cool air during its compression, the latter adds chemically oxygen (which requires additional fuel to be burnt with, like pointed out by GregP).
More in detail, principle of water injection is to move what naturally tends to be an adiabatic compression (virtually with no heat exchange with exterior, so keeping all warm generated by compression itself, giving hotter and less dense air as a consequence) to an isothermal-like compression, having a more dense air as output. Denser air means more oxygen to burn more fuel, therefore more power (in the end power is always a matter of how much fuel is burnt in the unit of time).
Nitrous oxyde way to work is even simpler: N20 is an unstable compound that tends to separate in ordinary molecular nitrogen and free oxygen (O), which is more reactive than common molecular oxygen (O2). It soon reacts with extra fuel, again to give more power.
More in detail, principle of water injection is to move what naturally tends to be an adiabatic compression (virtually with no heat exchange with exterior, so keeping all warm generated by compression itself, giving hotter and less dense air as a consequence) to an isothermal-like compression, having a more dense air as output. Denser air means more oxygen to burn more fuel, therefore more power (in the end power is always a matter of how much fuel is burnt in the unit of time).
Nitrous oxyde way to work is even simpler: N20 is an unstable compound that tends to separate in ordinary molecular nitrogen and free oxygen (O), which is more reactive than common molecular oxygen (O2). It soon reacts with extra fuel, again to give more power.
Water injection is also used on the Hawker Harrier
The maximum take-off thrust available from the Pegasus engine is limited, particularly at the higher ambient temperatures, by the turbine blade temperature. As this temperature cannot reliably be measured, the operating limits are determined by jet pipe temperature. To enable the engine speed and hence thrust to be increased for take-off, water is sprayed into the combustion chamber and turbine to keep the blade temperature down to an acceptable level.
Water for the injection system is contained in a tank located between the bifurcated section of the rear (hot) exhaust duct. The tank contains up to 500 lb (227 kg, 50 imperial gallons) of distilled water. Water flow rate for the required turbine temperature reduction is approximately 35gpm (imperial gallons per minute) for a maximum duration of approximately 90 seconds. The quantity of water carried is sufficient for and appropriate to the particular operational role of the aircraft.
Selection of water injection engine ratings (Lift Wet/Short Lift Wet) results in an increase in the engine speed and jet pipe temperature limits beyond the respective dry (non-injected) ratings (Lift Dry/Short Lift Dry). Upon exhausting the available water supply in the tank, the limits are reset to the 'dry' levels. A warning light in the cockpit provides advance warning of water depletion to the pilot.
The maximum take-off thrust available from the Pegasus engine is limited, particularly at the higher ambient temperatures, by the turbine blade temperature. As this temperature cannot reliably be measured, the operating limits are determined by jet pipe temperature. To enable the engine speed and hence thrust to be increased for take-off, water is sprayed into the combustion chamber and turbine to keep the blade temperature down to an acceptable level.
Water for the injection system is contained in a tank located between the bifurcated section of the rear (hot) exhaust duct. The tank contains up to 500 lb (227 kg, 50 imperial gallons) of distilled water. Water flow rate for the required turbine temperature reduction is approximately 35gpm (imperial gallons per minute) for a maximum duration of approximately 90 seconds. The quantity of water carried is sufficient for and appropriate to the particular operational role of the aircraft.
Selection of water injection engine ratings (Lift Wet/Short Lift Wet) results in an increase in the engine speed and jet pipe temperature limits beyond the respective dry (non-injected) ratings (Lift Dry/Short Lift Dry). Upon exhausting the available water supply in the tank, the limits are reset to the 'dry' levels. A warning light in the cockpit provides advance warning of water depletion to the pilot.
swampyankee
Chief Master Sergeant
- 3,954
- Jun 25, 2013
Not quite; nitrous oxides is reactive and an oxidizer; water isn't. N2O breaks down during the combustion process to release oxygen
Shortround6
Major General
I think all of you have pretty well explained it.
I am somewhat doubtful about the effect of the water turning to steam in the cylinders however.
The temperature in the supercharger and intake duct/s can be well above the boiling point/vaporazation of water.
RR was figuring the temperature rise in the supercharger of a Merlin XX in high gear as 148 degrees C. That is the rise in temperature over the temperature of the ambient air.
Granted the Merlin XX was never fitted with water injection, I just used it because I have the figures for the temperature rise, but some (most?) of the water was turning to steam before it ever got to the cylinder.
It is the latent heat of vaporization that causes the cooling of the intake charge. It takes a bit over 4 joules of energy to raise 1 gram of water 1 degree C in temperature until you get to the vaporization point. You need 2230 joules of energy to turn 1 gram of water (at 99.99 degrees C) to steam or vapor.
You also have about 1/16th of second for this to happen, the time it takes for the air to go from the carburetor/injector at the eye of the supercharger to the spark plugs in the cylinders.
Water will keep absorbing heat even after it reaches the vaporization stage but the big change is right at vaporization.
RR figured in the Merlin XX that the evaporation of the fuel was good for lowering the intake charge temperature by 25 degrees C.
I would note that some engines were using excess fuel as a coolant at high power.
However water has a heat of vaporization 5-7 times higher than most of the compounds used in gasoline.
I am somewhat doubtful about the effect of the water turning to steam in the cylinders however.
The temperature in the supercharger and intake duct/s can be well above the boiling point/vaporazation of water.
RR was figuring the temperature rise in the supercharger of a Merlin XX in high gear as 148 degrees C. That is the rise in temperature over the temperature of the ambient air.
Granted the Merlin XX was never fitted with water injection, I just used it because I have the figures for the temperature rise, but some (most?) of the water was turning to steam before it ever got to the cylinder.
It is the latent heat of vaporization that causes the cooling of the intake charge. It takes a bit over 4 joules of energy to raise 1 gram of water 1 degree C in temperature until you get to the vaporization point. You need 2230 joules of energy to turn 1 gram of water (at 99.99 degrees C) to steam or vapor.
You also have about 1/16th of second for this to happen, the time it takes for the air to go from the carburetor/injector at the eye of the supercharger to the spark plugs in the cylinders.
Water will keep absorbing heat even after it reaches the vaporization stage but the big change is right at vaporization.
RR figured in the Merlin XX that the evaporation of the fuel was good for lowering the intake charge temperature by 25 degrees C.
I would note that some engines were using excess fuel as a coolant at high power.
However water has a heat of vaporization 5-7 times higher than most of the compounds used in gasoline.
Peter Gunn
Master Sergeant
Nothing to add really except I remember the best explanation on water injection I got was from my freshman high school drafting teacher Mr. Delevanno who had been a Thunderbolt crew chief in the South Pacific during WWII. Excellent teacher, spent much time in his office talking aviation.
This thread for some reason stirred some great memories of a wonderful man, thanks.
This thread for some reason stirred some great memories of a wonderful man, thanks.
One of the great things about the forum is it introduces subjects I never thought about. I always thought that water turned to steam and that was that. It does turn to steam at 100C but that is wet steam. With water, it is a vapour up to its critical point of 374C (647K) after which it behaves like a gas or a supercritical fluid depending on the pressure. My conclusion is that steam will still absorb heat or provide cooling at any temperature up to 374C.
Critical point (thermodynamics) - Wikipedia
It brought back a vague memory of my father discussing steam engines, he told me steam was drawn from the dome at the top "because it has the driest steam" I now have an idea what he was talking about.
Erich
the old Sage
was told by Horst Petzschler while in 2./JG 3 flying G-6/AS 109's when using MW 50, they had about 10 minutes of spped before the pistons would blow. same from 2 vets in 10 (N)./JG 300 flying 109G-6 and G-14/AS at night hunting Mossies.
MIflyer
1st Lieutenant
Note that in the F-82 they had water injection. That appears to have been a way to compensate for what I consider to be a bad design mistake - the lack of an intercooler/aftercooler on a two stage supercharged engine. So the water injection cooled the fuel/air charge instead. A liquid cooled intercooler between the two supercharger stages would have been far better.
The KC-135A, for example, used water injection on its J-57 engines. And I have heard it said many times in the USAF that the KC-135A using water injection on takeoff was the loudest airplane around, even louder than a B-52 using water injection on eight J-57 engines. I don't know why; maybe the B-52 was climbing faster so it was higher when it passed you. Water injection is out of style in the airplane world. Some years ago when a B-52A carrying a Pegasus space booster from the West coast to the East coast stopped at Carswell AFB to refuel, Orbital Sciences Corp had to haul out many bottles of commercially procured demineralized water to service the airplane.
The KC-135A, for example, used water injection on its J-57 engines. And I have heard it said many times in the USAF that the KC-135A using water injection on takeoff was the loudest airplane around, even louder than a B-52 using water injection on eight J-57 engines. I don't know why; maybe the B-52 was climbing faster so it was higher when it passed you. Water injection is out of style in the airplane world. Some years ago when a B-52A carrying a Pegasus space booster from the West coast to the East coast stopped at Carswell AFB to refuel, Orbital Sciences Corp had to haul out many bottles of commercially procured demineralized water to service the airplane.
fastmongrel
1st Sergeant
Yes and no some Steam engines (Locomotives) didnt have domes but had a steam collector pipe usually but not always above the firebox. On a 19th Century Loco the dome was very high to collect the steam high enough to prevent water being picked up and carried over into the valves and cylinders. On a 20th C Superheated Loco a high dome was not as necessary as after the steam was collected from the Dome/Pipe it went into Superheater tubes which were small bore tubes fitted inside the Fire tubes which heated the steam till it was a true gas. There was a lot less chance of water drops making it through the superheater tubes and into the valves and cylinders. Early superheaters were called steam driers till engineers realised they also increased power and fuel economy.
The Packard Merlin on early P-82s (and on P-51Hs) was also with provision for water/alc injection. Along with lower CR of the Merlin, it helped a lot when people were pushing the boost to 90 in Hg.
Intercooler works well on any altitude, and also increases power in lower regimes. .
fastmongrel
1st Sergeant
In that case they would most likely not have been superheated and your dad was right.
My favourite Steam Dome a South Eastern & Chatham Railway D Class 4-4-0 built in 1901 when young boys with lots of Brass polish and rags were cheap and easy to employ
