Water injection and alcohol fuel 14

[winfieldblue]

140Airpower, I would have thought that a cylinder filled with a larger mass of air would have more expansion given the same amount of fuel/btu's. The compression ratio would be higher with cooler air.....and that would explain why my idi diesel engine suffers a large power loss when breathing pre-heated intake air!

 

[Lou Scannon]

BTW, the earliest turbo auto engines were the Oldsmobile Jetfire (not buick) and Corvair Monza Spyder of 1962, 12 years before Porsche.

Yes, those were the two I had in mind, I mistakenly wrote Buick since the Olds 215 was simply a derivative of the Buick 215. Thanks for pointing that out.

I guess our common ground is that the early (2nd gen, 70's) auto turbo applications I was alluding to (turbo Trans Am, Buick Regal, Ford 2.3 e.g.) were attempts at economical performance, not economy cars per se, which is all I was trying to say.

 

[Lou Scannon]

140Airpower, I would have thought that a cylinder filled with a larger mass of air would have more expansion given the same amount of fuel/btu's. The compression ratio would be higher with cooler air.....and that would explain why my idi diesel engine suffers a large power loss when breathing pre-heated intake air!

winfieldblue, can you give us the P-V and T-S diagrams for that?

 

[140Airpower]

1gibson, water, H2O, is a lighter molecule than either nitrogen, N2, or oxygen, O2. To the extent that water vapor replaces air, the charge becomes lighter by mass. Furthermore, the replacement of an amount of air by water vapor lowers power by that exact amount.

The mechanisms by which water helps have been measured, calculated, surmised and speculated on by many authors.
From what I can tell....
The benefit of water as a VAPOR is that by replacing some air it lowers combustion pressure from charge dilution. At high enough temperatures its specific heat increases above that of the burned and unburned parts of the charge, thus reducing temperature and pressure and it also begins to dissociate, absorbing even more heat. The presence of water vapor then tends to slow the burn and reduce the peak temperatures and pressures. It is a good anti-knock agent with any and all fuels in an SI engine. It has somewhat different beneficial effects in diesel engines and jet engines.

Water introduced as a LIQUID, sprayed into the manifold or direct injected, will absorb a LOT of heat from its very high heat of vaporization. In the manifold, it will cool the charge air to the extent that it evaporates (charge air has to be hot) and will thereby increase the charge density. What water that does not evaporate in the manifold or if it is direct injected it will retard the temperature and pressure rise on the compression stroke from the heat it absorbs while evaporating and then reduce the maximum temperature and pressure during combustion from the mechanisms described above.
Water is usually measured as a percent of fuel by weight. Popular water injection systems can be set to inject water up to some low value like 25% of fuel. However, I have not seen anything that says there is a limit to how much water can be injected under high boost up to 360%, 3.6 x the amount of fuel by weight direct injected. In WWII 60% of fuel was common and 150% of fuel was documented. The combined anti-knock effects run anywhere from 1.5 to 3 octane numbers, ON, per 10%. One test gave 20 ON for 60%.
Later in the power stroke water returns the energy it has absorbed. In a turbo setup, I believe the return of energy should be greater compared to an open exhaust setup. However, water lowers the thermal efficiency of an engine overall (in terms of BSFC) even while increasing the maximum boost allowable and power output (the figures I've seen on efficiency do not account for the decrease in the required cooling power).
Also, there is a great benefit in mixing 50% Methanol with the water in terms of much better evaporation, cooling and increased charge density in the manifold and therefore more power. The merits of Methanol were discussed before.

 

[140Airpower]

winfielblue, I think you are correct.

 

[140Airpower]

1gibson, I think that thermal efficiency is a tricky thing to measure accurately. BSFC is a standard yardstick since Ricardo's early days, but measurements of BSFC on a dyno can be skewed by such things as coolant temps and whether the engine runs its own water pump and provides its own power for the radiator fan. Heat rejection is a factor and ignition or injection timing, mixture, CR, back pressure, etc come into play. The use of an intercooler is part of the cooling budget. I am well aware of the undeniable advantages, but I wonder about the thermal accounting.

 

[winfieldblue]

Hemi, I have no diagrams for that, I'm trying to figure out why my N.A. idi diesel makes more power with a cooler intake charge while using the same fuel quantity (full throttle). This engine has no computer,so intake air temp doesn't change injection timing or the amount of fuel injected. Where does the extra power come from? 3 possible explanations spring to mind, one being a higher compression ratio from more air density, two being more heat transfer into the cooler combustion air rather than to the cylinder/head/piston,and three I'm wondering if the expansion rate of air is linear in regard to temperature. Any other offerings would be appreciated!

 

[azmios]

Slim,

I think the SAE paper 2009-01-2808 and 2009-32-0047 will shed some light on why water injection makes more power. There are also many gas turbine that use water injection to make more power, lower fuel consumption and reduces NOx.

 

[Slim3]

Thanks azmios, I will try to find those SAE papers and read them. I am about to finish up my direct injected compressed air starting system on my show car and plan to enter it in the Oct 13th British car show in Nashville.

 

[azmios]

Slim, if you go over the paper, water has a high specific heat capacity and latent heat of evaporation. This simply mean that it will absorb a lot of combustion heat thus minimizing the combustion heat from being wasted. Furthermore, the gas constant R for steam is much higher than nitrogen or CO2, at high temperature, the pressure increase is also much bigger than the CO2 or N2 gas expansion.

 

[Compositepro]

Er... the gas constant is just that, constant. You are probably referring to the molecular weight effect where one gram of water vapor has more volume than one gram of CO2. However, the effect of temperature and pressure change is the same on both gases.

 

[azmios]

Compositepro, Not really, you first have to read the SAE paper. The gas constant R is independent of the molecular weight. The constant R is relevant according to the P=mRT/V. With higher specific heat capacity of water, you will get higher T. Once you plug in that T, the higher R will give you higher P too.

 

[Lou Scannon]

Azmios, water injection, along with other substances and methods of internal cooling, does not inherently increase power. In fact, with no other changes, it does the opposite. The benefit of internal cooling is that it permits more power to be made, due to its various direct effects on physical limitations on maximum power, which are application dependent. The most obvious effect is it reduces local peak temperatures in the combustion chamber and exhaust system. Taking advantage of this effect, the performance engineer can increase the maximum charge flow, gaining net power while not exceeding material temperature limits.
A secondary effect of the cooling is that combustion rates are reduced, which tends to reduce peak pressure. Taking advantage of this, the performance engineer can take steps to move the combustion phasing closer to TDC, or increase the charge flow, gaining net power while not exceeding power cylinder and cranktrain stress limits. In Otto-cycle applications, maximum power is typically knock-limited. Internal cooling typically has a very strong effect on suppressing knock. Taking advantage of this, the performance engineer can re-optimize compression ratio, charge flow, air/fuel ratio, and combustion phasing for substantial gains in net power.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

Hemi.

An excellent explanation.

Regards
Pat
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[Lou Scannon]

Thanks Pat. Every once in a while, a blind squirrel finds a nut.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

Even if only by bumping into it and hitting his head hard. ;-)

Regards
Pat
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[azmios]

hemi, i think you should rear paper below

"High output supercharging without intercooling: theory and results" by J W G Turner and R J Pearson.

There is also an explanation that Slim is looking for on why the mixture of methanol and water returns better power in Charles Taylor's engine text book.

As for your explanation about water in the combustion chamber not producing any power. I beg to differ because of the paper below: -
1) Lestz, S.J., Melton, R.B. Jr., Rambie, E.J.,“Feasibility of Cooling Diesel Engines by Introducing Water Into the Combustion Chamber,” SAE paper no. 750129, 1975

The paper above is done by the US Army researchers decades ago,the paper will tell you why you're wrong. The power went up a lot when water is present in the combustion chamber.

In case you are not aware, Nasa also published several papers on the use of water injection to achieve extreme thrust in the jet engine. These papers will also tell you that you are really wrong.

I wish that I can tell you a whole lot of other wonderful things that can come with water in combustion chamber of piston and turbine engines, unfortunately for me most of them are classified. There are so many things that people outside of the organizations that do skunkwork under the government funded research projects will never understand. Even if others fail to understand, we dont simply insult them, instead we simply say "ignore him he's not one of us, dont bother to explain".

Anyhow, I bet you wont bother to read all these papers right that I quoted right, so I wont bother to explain any further.

 

[Slim3]

Thanks again azmios, for the clear explanation. I didn't know why it worked, I just know for sure that it does work and works well from my experiments and practical application. Those in doubt need to actually try it.

Slim3

 

[azmios]

Slim3, here is another proof to support your claim

http://www.lotuscars.com/gb/engineering/lotus-exige-270e-tri-fuel

 

[140Airpower]

The effects of water introduced in piston engines has been studied since the very early days of the 20th century and comprehensive, complete and definitive answers are not all in hand. Studies are being done to this day. However, of everything I have found, nobody says that water releases energy and produces power and in fact water CANNOT produce power in these types of engines. That is not to say that the ENGINE cannot produce more power with the introduction of water for several reasons. The most obvious reasons have already been mentioned by other posters.
From what I've read, a normally aspirated engine that is performing correctly will suffer a loss of power if water is injected into the manifold. For a supercharged engine there are a variety of results that you can get depending on circumstances and especially if the engine is designed for water injection. Gas turbines are different.