Tech Information > Water/Methanol Injection
For and against water injection
NathanStewart:
--- Quote from: Milwaukee Midget on June 06, 2010, 04:58:15 PM ---I've asked in another thread if water injection is helpful for a normally aspirated engine, but got no response. Anybody care to chime in, and if so, what would be a target ratio of air/fuel : water?
--- End quote ---
Definitely helpful if you're trying to run a high(er) compression motor on a fuel with a lower anti-knock index (pump gas for instance). Typical water:fuel ratio for n/a applications is 10-15%.
--- Quote ---I've read where water injection can cost power because the cylinder volume has less air/fuel mixture.
--- End quote ---
If this were really true I probably wouldn't have a job. :-D
jl222:
--- Quote from: Milwaukee Midget on June 06, 2010, 04:58:15 PM ---I've asked in another thread if water injection is helpful for a normally aspirated engine, but got no response. Anybody care to chime in, and if so, what would be a target ratio of air/fuel : water?
I've read where water injection can cost power because the cylinder volume has less air/fuel mixture.
--- End quote ---
I've got to learn how to take fotos and post, my chemistry book [ typical effect of temperature on gas volume at 1 atmosphere] two of the 5 examples
300 deg kelvin =-----570 ml volume
V2=VI X T2 DIVIDED T1
610 deg kelvin ==== 1180 ml volume
This shows that as air is heated [supercharged in our case] the volume increases but its just expanded air with no extra oxygen molecules and when it is cooled it shrinks, again no extra molecules but it is denser. the shrunk air from cooling [water injection--intercooler--alcohol] allows more air and molecules to be packed into the same space.
This is why roots style blowers [they heat air more than turbos or centrifugals] are so much better when ran on alcohol than gas.
JL222
hotrod:
This is a subject that has many twists and turns.
The usual rule of thumb for NA engines is to start at water injection rates of about 10%-20% of fuel flow. As mentioned above, ADI used in WWII aircraft like the Pratt and Whitney R2800 engine used about 50% of fuel flow, with a mixture of 50% methanol and 50% water, and in extreme cold conditions they also had a mixture of ethanol, methanol and water, if I recall correctly it was about 60% alcohol by volume.
http://www.enginehistory.org/Frank%20WalkerWeb1.pdf
Water vapor actually assists in the combustion process, as it facilitates conversion of carbon monoxide to CO2. In lab tests you can take a mixture of dry carbon monoxide gas and oxygen and it is nearly impossible to ignite, but add just a bit of water vapor and it ignites easily. This is one of the pathways that water injection uses to help make more power.
It cools the fuel air mixture by evaporation and due to the high specific heat of water (how much energy it takes to raise a water droplet one degree compared to a similar weight of gasoline). It also takes considerably more energy to evaporate the water droplet, to gas (steam) than it does to evaporate the gasoline droplet.
This means that during the compression stroke the fuel air mixture does not heat up as much due to compression, so the work that used to be spent fighting rising pressure due to heat in the cylinder is spent evaporating the water alcohol mist. This gains engine power by reducing compression losses.
The water does slightly retard the effective ignition due to its cooling effect but in many engines that moves the time of peak cylinder pressure slightly later in the combustion stroke closer to its ideal 12-14 degrees after top dead center for maximum power recovery. Also during the burn phase the water gives back the energy that was invested to evaporate it. It lowers peak cylinder pressure (cooling effect during compression and higher heat capacity), but the steam generated during combustion stretches out the pressure peak so although the peak cylinder pressure is slightly lower the total effective pressure goes up because the pressure does not fall as fast as the piston descends down the cylinder due to waters high specific heat (it has to expand more to cool off the same amount as a normal fuel air charge).
The slightly lower peak cylinder temperature, lowers heat stress, and heat losses to the cooling system, and helps keep valves alive at power levels that would melt them without the water injection.
The evaporative cooling as the fuel air water mist enters the cylinder also increase volumetric efficiency of the engine as the fuel air charge heats up less as it passes the hot head of the intake valve, and cools off hot spots like spark plug electrodes and any other possible sources of pre-ignition.
All these effects allow you to produce more useful work out of the same or even more fuel than you could burn before without killing the engine due to high heat loads.
Larry
jl222:
Hotrod...that reference article about Frank Walker and his work on water injection is one of the best I've ever read.
It's a must read for anyone interested in water injection and clears up alot of myths.
150 in [60 lbs boost] and 3800 hp wow!! Also testing for 100 hrs at full military power :-o
Hey guys if somebody says water injection doesn't make any power, just say how much you wana bet :-D
JL222
P.S. Wish I knew all about the hydraulic drive for the superchargers on that E model.
hotrod:
Yes and that is on an engine that started out rated at 2000 hp, which gives good field validation to test results by NACA that water injection was capable of increasing MEP and power up to 160-180% of gasoline only power on supercharged aircraft engines of the day.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930093187_1993093187.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091074_1993091074.pdf
The primary limit to engine power for most internal combustion spark ignition engines is knock limits imposed by fuel octane and heat limits posed by combustion temperatures. If you don't melt it you break it due to detonation. Water injection attacks both of those problems at the same time, by internal cooling and raising the effective fuel octane very substantially. That is where most of the power gain is. By increasing the effective fuel octane you can stuff more fuel and air into the engine than is physically possible on gasoline only.
Larry
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