Compound expansion plus water injection to increase fuel efficiency of a Diesel cycle engine? 4

[TSLexi]

I was recently reading the research of Prof Dr-Ing Gerhard Schmitz into compound expansion ICEs, and Mr Bruce Crower into water injection. So I'd like your advice as to whether this is feasible, as I am merely a community college student planning on entering automotive or mechanical engineering.

Most ICEs waste a whole bunch of power by having the exhaust expand into the outside world. And they require massive radiators to keep the parts from getting heat-damaged. And they also require camshafts to open and close the valves, which adds a whole bunch of weight. And Otto cycle engines require spark plugs and have to use lower compression ratios to prevent detonation, which adds weight as well and decreases power.

We can solve this by starting with a basic four stroke Diesel cycle engine. We replace the camshaft with electronically-controlled solenoids to actuate the valves. Then we can dispense with the timing belt as well.

We then add a low pressure cylinder that the exhaust is directed to. This cylinder will be at TDC when the exhaust valves on the combustion cylinder are open, and when it's at BDC, it's contents will be directed into the air. Steam engine designers figured out the concept of compound expansion centuries ago, why has it taken ICE engineers this long?

Finally, the next time the combustion cylinder is at TDC, instead of injecting diesel, we inject distilled water. This will a) provide an extra power stroke, as water expands 1600x when it turns into steam, and b) cool the engine. We could also increase power-to-weight ratio, depending on the weight of water needed to replace the weight of the radiator. Once again, steam engines don't need radiators, as they produce power, they also dissipate heat. You'd just have to ensure ) that the water is very pure, and b) use steam engine lubricant.

And to ensure the engine will always be operating at peak efficiency, we couple it to a continuously-variable transmission.

So we have the benefits of a) the very efficient Diesel cycle which doesn't require spark plugs and produces more torque, b) use electronically-controlled solenoids to actuate the valves, which saves the weight of the camshaft, c) compound expansion to extract extra power from the exhaust, d) steam power, which keeps the engine cool and adds extra power, and e) a CVT, which eliminates the inefficiency involved in gear-shifting.

If we build the engine out of carbon fiber, this engine will be very light and strong, and able to survive the high temperatures involved. And a lighter engine means it can rev higher and have larger cylinders. It's better to produce torque at a higher rpm, because then you can take full advantage of the CVT's gearing to produce optimum power, and there's no replacement for displacement.

So thanks for advising me!

Lexi

 

[TSLexi]

Well, my idea of having a steam stroke is to decrease the amount of diesel needed. Twincharging can be utilized to decrease the amount of diesel needed, as higher oxygen density means more complete combustion, which means more work can be extracted from a given amount of diesel, and less emissions of UHCs. And to reduce power losses from the chargers, I think we can use carbon fiber, as it's lighter and very heat resistant, which means it will take less work from the engine to spin it fast enough. GE uses it as fan blades in their GEnx family of turbofans, and it's used on F1 brake pads and clutches.

Having the compression ratio able to differ from the expansion ratio also allows more work to be extracted from a given amount of diesel (although forced induction is required to keep power density the same).

The cooling effect from the steam strokes will decrease cylinder temperatures, so less NOx will form, and the engine can rev higher, since it runs cooler, although we do have to work out the effects of thermal shock and fatigue on the parts from the constant heating and cooling.

And since we're using diesel to provide the main power stroke, and steam to provide an additional power stroke, 33% of the six strokes are devoted to power, making it more efficient then a normal four stroke IC, where only 25% of the strokes generate power. And since we use steam to provide one of the power strokes, we can use less diesel, so the exhaust would be mainly steam, which will decrease the total percentage of exhaust that is PM, UHC, and NOx. The solution to pollution is dilution.

I think this could be workable, but that's just me. I wish I had a friend with a CAE package to virtually prototype this.

I guess with engines their are four things, power, fuel efficiency, weight, and cost, and you can only have three.

 

[TSLexi]

I guess I could get some money together to build a small W4 motorcycle engine from this concept.

 

[TSLexi]

I think carbon fiber-reinforced silicon carbide would be a great material to build this engine, as it has a high melting point, high thermal shock resistance, and a high strength/weight ratio. I mean, if it can be used in high-performance brake pads, why can't you use it for the engine's moving parts?

 

[berkshire]

There was an SAE paper on this subject.
B.E.

You are judged not by what you know, but by what you can do.

 

[TSLexi]

So, from reading that, water injection increases power while decreasing BFSC, and water contamination of the oil can be avoided by aiming the water at the piston head rather than at the bore walls. And it decreases NOx. And I believe the problem of SOx and UHC and CO emissions can be solved by using ULSD, and using forced induction to increase the amount of oxygen present to fully-combust the diesel.And having a higher dynamic CR will increase the oxygen density and temperature, leading to more complete combustion.

And that paper was written in 1975, and fuel and engine technology has drastically increased since then. Does that paper indicate the effect on BMEP?

And, no matter what, I still stand by that using a CVT will make any engine more efficient, as the engine can always operate at peak efficiency.

 

[MikeHalloran]

Having the engine go from idle to screaming WOT in a quiet residential parking lot is not necessarily a desirable feature.

How do you sell the idea of adding water at every fuel stop, when people are already getting annoyed about adding DEF?
How do you keep the water from freezing? How easily do you repair your system after it does?



Mike Halloran
Pembroke Pines, FL, USA

 

[TSLexi]

Keep it from freezing by adding methanol to it. And the sound...that's a feature. some people like loud engines. You could dampen it with active noise cancellation technology as an option.

 

[TSLexi]

I think you could sell the idea of having to add water by the savings on fuel.

 

[TSLexi]

I also wonder if placing a squirrel cage generator between the turbines of the chargers, will help recover some of the energy lost, as we could use it to charge the car battery, and when the engine's exhaust speed is too low to drive the turbine fast enough to provide the desired boost, we could use the battery to run the generator in reverse to spin the blades fast enough to provide boost.

Then we solve the whole problem of not having boost during low rpms, but we also drain the battery a bit when we're at low rpms.

 

[waross]

Back in the '40s, water injecting carburetor were available for gasoline engines. I haven't seen a water injecting carburetor since I was a little boy and the one I saw was in a box, not on a vehicle.
NOX generation is related to the dynamic effects of the high temperature in the center of the burning mixture in the cylinder. The farther from the cylinder walls, the higher the temperature of the burning gasses may be with more NOX produced. Water injected during a previous or subsequent cycle will have little effect. Water may be more effective if it is injected with the diesel fuel.
Many statements here may demonstrate a lack of understanding of the basic laws of the conservation of energy.
More study of basic principles may yield negative answers to many of the questions here.
Over-spinning the turbo at low speeds may have merit. Faster forward, not in reverse. Google "super charger".
In practice the actual savings rather than the percentage savings may be so little as to be almost pointless.
Taking energy from the turbo will, in many cases, reduce the efficiency of the turbo.
There may appear to be a saving at WOT if energy extraction is used to reduce dumping of excess volume of charge air, but only if the extracted energy may be used efficiently. Batteries on a running engine are usually back up to full charge shortly after the engine is started. The excess energy extracted from a turbo may be used to supply electrical loads such as daytime running lights and other loads, but a charged battery is not an effective place to store recaptured energy.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TSLexi]

Yeah, perhaps trying to use the turbo as an alternator would be wasteful. Although using an ultracapacitor may have merit. A variable-geometry turbo would be better.

So in order to reduce NOx emissions, we need to reduce hot spots in the cylinder, by making sure the air and fuel are thoroughly mixed. I was reading Somender Singh's research about roughening the combustion chamber to ensure maximum turbulence of the air-fuel mixture, and thus maximum mixing.

And the way to minimize energy loss is to make sure all reciprocating and rotating parts are as light as possible. Carbon fiber-reinforced silicon carbide is extremely light, extremely strong, and has high thermal shock resistance. If the parts are lighter, more torque is transmitted to the crankshaft, and if the stroke is also shortened, they can rev higher.

Also, my main reason to have two extra strokes for steam power was to cool the engine in a way that also increase BMEP, and potentially decrease BSFC wrt diesel. That's it. Water is already used to cool engines, why not have it do some work as well. I mean, all you have to do is, after the first power stroke, keep the exhaust valve closed, inject water when the piston is at TDC, and then open it when the piston is at BDC.

I wonder why so many people don't like CVTs? They're more efficient, and you don't have to waste time shifting.

 

[tbuelna]

Please explain how injecting water into the cylinder long after combustion has taken place will reduce peak combustion temps and reduce NOx formation in a diesel engine.

The reduced exhaust gas temps will also complicate the performance of the various exhaust emission devices used on modern automotive diesel engines.

 

[waross]

It may be difficult to get the piston back o top dead center with the exhaust valves closed. Something will probably break.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SomptingGuy]

As this is a blue-sky discussion, I don't think it makes sense to assume that contemporary exhaust emissions technology has a place. Where we stand now is a result of incremental reaction to legislative changes. Evolution does not always provide the best solution. The gasoline people saddled with the three-way cat and all the compromises that brings will attest to that.

This statement: "If the parts are lighter, more torque is transmitted to the crankshaft" is flat-out wrong though.

Regarding CVTs. Not everyone hates them. It's mostly those that have driven them. Human brains find the disconnect between the senses disconcerting. Personally I find even a typical torque converter auto fitted to a small engine to be impossible to live with.

- Steve

 

[TSLexi]

tbuelna, my reasoning is that the exhaust gases from combustion are quite hot (EGTs are routinely 1200+ degC), and by having the piston recompress those gases, and then injecting water, the water will be vaporized, which will provide force to push the piston back down, and also reduce EGT. High EGT is a sign the engine is not as efficient, as energy that could be used to perform work is instead being rejected as exhaust heat.

This means that both BSFC wrt diesel can be increased, as you can use less diesel to provide a given amount of power, and emissions are decreased because of the same.

And SomptingGuy, my mistake. Lighter parts will allow the engine to rev higher, as less mass needs to be accelerated. So we get more power. If we have enough power at enough speed, gearing can solve the issue of torque. And waross, I believe the flywheel will conserve enough angular momentum to return the piston to TDC with the exhaust valve closed. Also, lighter engines are good in general, as they, all other things being equal, decrease weight, increasing power to weight ratio.

Bruce Crower of Crower Cams has actually built an engine on this design, with great results, and unfortunately he has fallen ill, so someone needs to continue development for him.

http://www.popsci.com/scitech/article/2007-05/invention-awards-six-strokes-genius

High BMEP and low BSFC is what all engine designs strive for.

 

[BrianPetersen]

Recompressing the gases takes work - the work that you just extracted by expanding them! Error - does not compute!

I appreciate the enthusiasm shown here but I sense a lack of understanding and appreciation of why some things are the way they are and why they have become that way over the last century or so of development, and a certain lack of understanding of thermodynamics and heat transfer.

Ceramics have been tried in engines. One thing is for sure; they are expensive. Even if you can get certain parts of the engine to work with ceramic, it is unlikely that you can make the WHOLE engine out of ceramic. Piston rings may be a challenge, for example. Valves would be another. If there is a ceramic material out there that is not brittle, I have not seen it. Just because a given material has the temperature resistance doesn't mean it will play nice with everything else in the engine. Run things at very high temperature, and lubrication becomes a challenge.

And then ... Running things at high temperature might not necessarily raise the efficiency of the engine, anyhow. Just because you are not taking heat out of the engine doesn't mean that heat transfer is not occurring. It just might be transferring heat to the intake charge (bad) rather than to the cooling system.

Diesel engines nowadays have been trying to RAISE the exhaust temperature in order to make particulate filters work, and a diesel engine that conforms to today's emission standards without a particulate filter has not been developed. I'm not saying it's impossible, but it's something that thousands of engineers have been working on for the last decade or two, and they have not found a way yet. If you need that particulate filter then you need to have the exhaust temperature to regenerate it, at least under SOME engine operating conditions.

I would encourage the original poster to look up HCCI. While the truest form of HCCI has practical implementation problems, close relatives of it - call it "almost-HCCI" - are where internal combustion engines are headed. Those have at least the potential for getting away from the diesel engine DPF and NOx aftertreatment systems, although a 3-way catalyst is likely still needed (and needs a certain amount of heat in order to operate ...)

The HCCI and similar combustion systems that I am aware of, rely on re-using a portion of the previous cycle's exhaust gas to raise the temperature of the intake charge of the next cycle, as a means of controlling the self-ignition timing in the next cycle.

 

[TSLexi]

OK, but my theory based around recovering some energy from that hot exhaust. As long as the EGT is 500-800 degF, the catalytic converter will operate. Any temperature above that is wasted energy.

Also, perhaps a way to extract more work would be to have opposing pistons linked together by their heads (i.e. the piston heads are facing on another) with a lightweight yet strong, heat resistant rod, and the strokes timed so that whenever one cylinder is being forced down in it's cylinder, it pulls the other one up in it's the cylinder. Not like the Stelzer cycle though, that's asthmatic.

So the diesel power stroke of one is compression stroke of the other, and the steam power stroke of one is the exhaust stroke of the other. The size of the engine will have to increase a bit to make up for the lost combustion surface due to the rod taking up some space in the combustion chambers, but if we can find a material with a high enough tensile strength to specific volume, this can be minimized.

You know, if you can use less diesel to get the same power, emissions decrease, all other things being equal, because it's happening in lower quantities.

 

[SomptingGuy]

@OP: If you like opposed-piston diesel engines, take a look at what those crazy people at Achates Power are doing. In common with all the start-ups, their

http://WWW site

is packed with fun and games. Educational too. They might even be hiring.


- Steve

 

[TSLexi]

Wow, Achates Power and me have the same idea kinda (theirs seems more thought out though)! I mean, about the cylinder design.

 

[waross]

The turbo charger goes a long way to recover some of the otherwise waste heat of the exhaust gases. It also cools the exhaust gases and improves the efficiency of the engine. Water cooling of the exhaust gases will interfere with the operation of the turbo and lose some of the efficiencies that the turbo delivers.
You are not the first one to look at the hot exhaust gas as a possible source of energy. The turbo is one solution.
Another solution is to use the hot exhaust to boil water in a steam generator. The steam is then used to drive a steam turbine and generator. This is often used with gas turbine generators. The principle is valid for diesel sets also, but diesel installations are seldom large enough to make exhaust gas steam generation economically feasible.

Bill
--------------------
"Why not the best?"
Jimmy Carter