water injection = Higher Efficiency?

[jtbo]

Good for me, I can adjust ignition with all my cars, one I could not, but I put programmable fuel injection that solved the issue, two are also diesels which work bit differently.

[serialk11r]

Er t vago, the exhaust stream is at relatively low pressure (maybe like 3 bar if NA) but high temperature, so you need a heat transfer of some sort to be able to pick up the majority of the power. A turbo creates a greater pressure drop using the piston's power.

A good turbo system has less backpressure than boost pressure, but there's always a bit of excess pressure left in the cylinder if the expansion and compression strokes are equal so ideally we'd have no backpressure at all.

[t vago]

3 bar? Seems awfully high to me for NA. I'd submit that if you have a pressure reading of 3 bar in your exhaust piping, and you're running NA, you have something seriously wrong with your exhaust piping.

A turbo causes relatively low velocity high pressure gas to become a high velocity low pressure gas. Since the pressure drops, its temperature drops, too, as all that thermal energy gets converted into kinetic energy. The slight drop in efficiency of the engine is offset by the amount of thermal recovered by the turbine. Isn't that the entire idea? Making more thermal energy available to work? Geez, this sounds too much like making the perfect the enemy of the good.

With that turbosteamie thingy, the exhaust would have to slow down anyway in order to sufficiently heat soak the heat exchanger. That also means higher pressure than for a purely NA application.

[serialk11r]

Yea if you look at charts that graph in cylinder pressure as a function of crank angle you'll see that when the exhaust valve opens an NA engine is at about 3 bar. It's not that much pressure if you think about it, adiabatically expanding that only gets you a pretty small amount of energy.

Turbochargers all increase backpressure. The problem is that the high pressure exhaust gas blows out of the cylinder in a short wave rather than continuously, so when it hits the turbine housing it gets reflected back and stuff. First of all this limits how much energy can be extracted, which is why turbocharger housings add restriction to increase the shaft power available for the compressor to use.

The other issue is that a turbine is not actually converting heat into work, it is only scraping up leftover uncollected work, if you get what I mean. Expansion in a turbine is adiabatic, the amount of heat energy ideally stays the same while the net "work" done on the gas goes down (pressure drop). In real life some of the work turns into heat because of friction (blanket term for all of the losses).

If we run some calculations or look at F1 turbocompounding (which nets 7% efficiency gain), we see that at full load the engine is spitting in the neighborhood of 10% of its output into the exhaust pipe in the form of useful kinetic energy (pressure). Since the gas engine (non-Atkinson cycle) is running at ~35% efficiency optimistically, the pressure leaving the exhaust only accounts for 3% of total fuel energy. Using the 30% of fuel energy going out the exhaust figure, we see that the vast majority of the energy is leaving in the form of pure heat.

Something like the turbosteamer will cause a little bit of restriction because it cools the exhaust and perhaps reduces the exhaust gas velocity a bit. However the exhaust valve opening before the bottom of the stroke means that most of the exhaust leaves before the piston even hit the bottom, of its own accord. Scavenging does not deliver very much energy back to the piston since the wave coming back is weaker and needs to go through the exhaust valve anyways. I believe the gains in a custom exhaust system are from 1. removing excessive muffler restriction (sometimes the cats :O) and 2. tuning for pulse interference and scavenging with the headers, but the stuff that happens before the collector that merges the pipes together is what's most important since that's where the pulse effects are strongest.

[tjts1]

All the chicken or the egg arguments in the world are not going to resolve this. It would be relatively easy for me to test this idea with megasquirt.

Set the engine to a 2000 RPM fast idle in closed loop at 14.7 afr, engine hot.
Start recording a datalog
Let it idle unmolested for 30 sec
Spray water mist into the intake for 30 sec

Megalogviewer will record exact RPM, AFR (xx.xx), injector pulse width (x.xxx milliseconds) and wide band sensor correction (x.x% correction). If the water has any positive or negative effect, it will show as a change in RPM, pulse width or ego correction or most likely all 3.

[t vago]

Originally Posted by serialk11r

The other issue is that a turbine is not actually converting heat into work, it is only scraping up leftover uncollected work, if you get what I mean. Expansion in a turbine is adiabatic, the amount of heat energy ideally stays the same while the net "work" done on the gas goes down (pressure drop). In real life some of the work turns into heat because of friction (blanket term for all of the losses).

Leftover... uncollected... work.

You really have no idea how a turbine works, do you? Tell me what happens to the pressure of a gas, when it forced through a nozzle. Then, tell me what happens to that gas's temperature.

And do please try to explain all those percentages you keep throwing around. 10% = 3%? 35% efficiency out of an internal combustion engine? And why do you mention full load? Everyone knows that most internal combustion engines do not run at full load all the time. In fact, that's why they're dinged with such low efficiency.

[t vago]

Originally Posted by tjts1

All the chicken or the egg arguments in the world are not going to resolve this. It would be relatively easy for me to test this idea with megasquirt.

Set the engine to a 2000 RPM fast idle in closed loop at 14.7 afr, engine hot.
Start recording a datalog
Let it idle unmolested for 30 sec
Spray water mist into the intake for 30 sec

Megalogviewer will record exact RPM, AFR (xx.xx), injector pulse width (x.xxx milliseconds) and wide band sensor correction (x.x% correction). If the water has any positive or negative effect, it will show as a change in RPM, pulse width or ego correction or most likely all 3.

I trust you're going to use pure water for this, and not a 50% mix of ethanol and water?

[tjts1]

Why mix it with anything?

I have a pressure vessel somewhere in the garage, fill it up with some distilled water, pressurize to 43psi with my air compressor, hook up a spare EFI injector at the other end and aim it into the open throttle body. The car has six 22# injectors so adding another one running the exact same PW would give a 6:1 fuel to water ratio.
easy

[t vago]

Originally Posted by tjts1

Why mix it with anything?

Typical water injection systems use a 1/1 mix of water and ethanol. The ethanol provides freeze protection, and also serves as a fuel source. It's the fuel source part that I was concerned with, as it would obviously skew injector pulse width measurements.

Originally Posted by tjts1

I have a pressure vessel somewhere in the garage, fill it up with some distilled water, pressurize to 43psi with my air compressor, hook up a spare EFI injector at the other end and aim it into the open throttle body. The car has six 22# injectors so adding another one running the exact same PW would give a 6:1 fuel to water ratio.
easy

Excellent. You have provided a way to accurately measure the amount of water being sprayed into the intake manifold.

[IamIan]

Originally Posted by t vago

Using your example of 1034 kPa and 700 K (or 427 C, if you will), and assuming that the liquid water was near 100 C, we find that:

The mass of your mystery gas in that closed volume is unknown. However, for a simplified exhaust gas containing 80% diatomic nitrogen, 11 percent carbon dioxide, and 9 percent steam, it's 0.2 grams.

It holds about 150 J of thermal energy. Of that, 91 J is available that can be used to vaporize water.

That gram of water needs 2260 J to completely turn into steam.

Roughly 4% of that gram flashed into steam, bringing the temperature of your closed system to that of the water, i.e. 100 C.

The pressure dropped, too. Ooooh, lookie - 101 kPa.

And you still have 0.96 grams of liquid water to deal with.

So, now you have 39 ccs of mystery gas, 1 cc of water, and it's all at 100 C and 101 kPa. Oh, excuse me - 212 F and 14.7 psia.

Not about the point you were making itself ... just wondering what I am missing about this result you work out here ... please let me know what I'm missing or what error I've made in the bellow thoughts.

#1> You can't inject the liquid water into that system without the application of the liquid water being at a higher pressure than the system was before you added the water... unless that was ignored for simplicity?

#2> Even before the temperature change takes effect ... displacing the volume of the added liquid water itself in a fixed volume will increase the pressure of the system ... plus the additional pressure ( energy ) added to the system that was needed to inject the water in the first place... unless that was ignored for simplicity?

#3> The end system result you gave looks like it is just the effect of the initial gas in the fixed volume ... which contracts as it cools from the transfer of energy to the liquid water ... where did you account for the expansion effect of the % of liquid water that is converted to steam in the system ... that liquid to steam expansion counters the initial gas contraction you showed ... even just 0.04 grams of liquid water to steam is a significant increase in pressure for a fixed volume ... the if not contained in the fixed volume the initial 1cc of liquid water would be 0.96cc of liquid water + about ~64cc of steam.. confined by the 40cc fixed volume and the other contents in that volume ... I don't see how you got the 0.04 grams of steam being that low of a final system pressure.

So I was guessing the first two were skipped for simplicity ... but I don't see what I am missing for the 3rd?

Please clarify... thanks