Occam's razor - water

Water injection has been looked at for decades. Back in the early 70's, when I was an undergrad at MIT, I had some friends who were in the mechanical engineering program. They said that an MIT lab had been studying water injection in internal combustion engines, and getting much better mileage, but the big problem was control - the optimum amount to inject changes dynamically with changing conditions, and it is very difficult to engineer.

 

Perhaps the effect is similar to using Seafoam or some other
engine-cleaner substance, wherein if you can get the engine to
stay running and suck it in, it changes combustion characteristics
enough to bust loose deposits. But products of this sort of
course say you have to repeat the treatment every 5000 miles
or whatever. Could an effective water-injection be done in the
driveway without having to install extra equipment, I wonder?
Stand there with someone helping mash the go-pedal while in Park
to bring the engine up to 2400 rpm and go at the throttle throat
with a plant mister?
.
Nobody addresses the potential issue of what happens to said
carbon once it's loosened up, i.e. does it then land in the
catalytic and start plugging *it* up?
.
_H*

 

Freed carbon will be burned in the converter. But a car in good repair burning E10 (as most of them do today) is unlikely to get carboned up.

 

Don't confuse water injection with electrolysis.

Water injection does have some positive effect by cooling intake gases as well as allowing higher compression ratios. Your engine has to be designed from the ground up to use it to get any real effect.

I've never seen any evidence to suggest that carbon deposits harm engine efficiency unless they are bad enough to cause pre-detonation. It's possible that an after-market water injection system would clean those out. But, the so would a can of fuel injector cleaner or an "engine shampoo".

As soon as you throw electrolysis into the mix you are at the mercy of thermodynamics.

 

Hi All,

Leaving the water injection in the intake manifold alone. There is a way to get allot better mileage out of a Prius with water injection. First you put on a turbo-generator. Which is a turbo part of a Turbo charger, hooked to a generator (brushless prefered). Then inject water into the exhaust gas flow. The secondary expansion caused by the water flashing to steam will greatly increase the energy in the gas flow by the turbo-generator.

 

I fail to see what water injection has to do with Occam's razor.
A friend on mine once used Occam's razor to solve a computer tracking problem.
The idea was that through two data points in a diagram you can always draw a straight line, and induce that all further observations will lie on that line. However, you could also draw an infinite variety of the most complicated curves passing through those same two points, and these curves would fit the empirical data just as well. Only Occam's razor would in this case guide you in choosing the "straight" (i.e. linear) relation as best candidate model. A similar reasoning can be made for n data points lying in any kind of distribution.

 

I'm sure this was a joke but:

Weight of components plus weight of water equals TANSTAAFL

 

Hi bbald...,

Not sure what TANSTAAFL stands for. But, no not a joke. In fact BMW and other manufacturers are investigating other secondary expansion steam engine methodologies to improve gas mileage.

 

TANSTAAFL = There Ain't No Such Thing As A Free Lunch.

If you can recover more energy than is lost by hauling around the components, it's all good.

I think the solid state heat to electricity converters recently announced are far more likely to yield useful returns.

 

Water injection is often used for one primary reason: Used appropriately (typically injected anywhere from 5-20% of the amount of fuel being injected), it significantly raises the octane rating of the air/fuel mixture so it is much more knock resistant. It does this by a combination of the water absorbing heat from the surroundings and also slightly slowing the burn rate.

What this means is that you can increase the compression ratio, run the engine at higher temperatures.

Really big gains in knock resistance can observed when using forced induction, such as turbo or super charging. The additional knock resistance will allow you to run higher boost pressures without detonation thus allowing you to extract more power from your engine. Typically these types of engines will dump extra fuel into the engine to try to keep things cool, and it works, but not as well as water and of course also wastes fuel.

 

Hi Again,

Well, think about it. A Prius engine is like 35 % efficient. Where is the other 65 percent going ? Mostly into to the exhaust gasses. Say you could recover half of that waste? That is almost like having two engines in the same car. So, it would improve fuel economy even if the recovery hardware and added structure weighed almost as much as the present engine.

 

I HAVE thought about it. Extensively. The question isn't whether there is waste heat, there certainly is. One question is whether you can recover enough of the heat to overcome the weight penalty of the components. Another question is, is there enough energy in the waste gas to use with current technology.

The first question is totally dependent on how you do the recovery. If you use something like steam, you must carry components to contain the steam which are heavy. You must carry the water to convert to steam, also heavy. And you must carry some kind of turbine/generator equipment, not as heavy as the first two but still. If the additional equipment weighs as much as the first engine and I was able to recover 50% of the waste heat, my net gain would be near zero. A Prius engine is 35% efficient in converting the energy in gasoline to usable mechanical torque. The energy contained in gasoline is MUCH more concentrated than that in the exhaust gas.

As to the second question, every time you convert from one form of energy to another you lose some energy to waste heat. That's a basic law of nature. Engineers refer to it as entropic loss. So avoiding unnecessary conversions is a must for efficiency. Direct conversion of heat into electricity, thermoelectric conversion, minimizes entropic loss.

Basically what I'm saying is that recovering energy from waste heat is a laudable goal. Using steam isn't the way to go. IMHO. YMMV. TANSTAAFL.

 

To recover energy from the exhaust you would need a exhaust turbine driving the flywheel because that energy in the exhaust pipe is mostly heat. Of course the radiator tends to dump a little heat too.

 

And you lose energy through frictional losses in drive train, radiant heat through the block, etcetera etcetera etcetera. So the energy content in the exhaust stream is much less than it might appear at first blush. Because typical ICE exhaust temperatures are near the typical inlet temperature of steam turbines, you would need near 100% efficient conversion of waste heat to steam temperature. And, we all know that isn't going to happen.

 

The Sterling Cycle engine shows some promise for extracting usable energy from the exhaust of internal combustion engines. As others have pointed out, the weight, volume, and complexity are negatives, but the Sterling Cycle is one of the least complex, and works well even with low heat deltas. Work is ongoing.

Some of the most efficient engines ever made were the old triple expansion steam engines used on ships. They recovered waste energy - twice over. The waste steam from the first cylinder was sent to another larger cylinder, where the steam was expanded again. The waste steam from this second cylinder went to a third, very large cylinder, where it underwent its final expansion. In theory you could use more than three cylinders, but the return from each diminishes. Triple expansion proved to be about the best they could do. With a ship, weight and volume are not a big issue, as compared to an automobile.

Tom

 

As much as I want to believe, it's not happening. I spent 20 years living with the thermodynamics of submarine engine rooms-- mostly theoretical & practical but occasionally applied directly to my face.

The Navy is always looking for "free" energy, and it's the most unprofitable of non-profit organizations. If any auto manufacturer (let alone Detroit) was able to make a penny by scavenging waste heat from a tailpipe then we'd have seen the equipment by now. (It's not locked up in the national archives with the 200-mpg carburetor and Indiana Jones' loot.) The fact that the tailpipe isn't glowing in the IR (and melting nearby body parts) indicates that all the usable waste heat is already being exploited by exhaust-gas recirc schemes. And if we did have a system to flash injected water to exhaust steam, it wouldn't produce enough kinetic energy to move the turbine against electromotive resistance. It certainly wouldn't generate enough energy to pay back the weight/equipment penalty.

BTW a Prius' "engineroom" is twice as thermodynamically efficient as the Navy's typical submarine nuclear reactor... 17% is considered pretty good by marine propulsion standards but 35% is just freakin' awesome.

 

Hi All,

Nords - we have seen it. BMW's prototype improved FE something around 20 %. And Gbee points out that it was conventional marine steam engine practice.

The BMW prototype had a condensor, the the water weight was fixed.

The Prius engine has a peak efficiency of 35 %, but at low power it drops down to 25 %. Other gasoline spark ignition automotive engines are down in the teens.

There is the vortex tube , which can increase the temperature of some of the gas, and cool the rest of the gas, but it requires much higher pressures than are available in an engine exhaust. If this could be done, thermo-photonic cells would be an option for energy recovery.

 

Okay, I'll accept that you've seen it.

Where is it? When was this done? Where are the engineering journal articles? Why hasn't it hit production?

 

Green Car Congress: BMW Developing Steam Assist Drive Based on Waste Heat Recovery
Combined cycle - Wikipedia, the free encyclopedia
Slashdot | Steam Hybrid Car from BMW
BMW unveils the turbosteamer concept

Why hasn't it hit production? It was being tested in 2005, so it's fairly new, and as posted above, the jury is still out as to whether the idea is practical. As you have pointed out, it gets harder and harder to extract additional efficiency. It's very likely that it's just not worth the expense, weight, and complexity to use this in a production automobile.

Tom