globi

I have a bunch of questions regarding the efficiency of the rotary engine and how it might possibly be improved. (I haven’t read the entire forum, so please be a little patient in case I miss something that has already been discussed before):

Anyway here are the questions/suggestions regarding efficiency improvement.

First of all: For different reasons (eg. moving and sickle shaped combustion chamber) the combustion of the air/fuel mixture is apparently not complete. So what if there was some sort of an post combustion directly after the engine outlet (eg. very short high-flow cat) and this extra pressure would drive a turbo in addition to the remaining exhaust pressure. Shouldn’t this increase the efficiency?

Secondly: We all know that if we can cool the air while it’s compressed we need less energy to compress that air (isothermal vs. adiabatic compression). (I think that’s why air compressors are sometimes serially connected to make them more efficient.) Anyway, so if the intake charge is partially compressed by the centrifugal compressor of the turbo and pushed through an intercooler not only should this reduce the ‘compression work’ (since less total heat is being generated) it would also allow to push more air into the combustion chamber without provoking detonation. Also this way the compression ratio could be reduced, the intake charge increased and at the same time the volume to surface ratio could probably be increased. This in turn should make the engine more efficient, shouldn’t it?

Thirdly: Mazda is apparently considering an electric turbo on a newer RX-8. This would mean that the turbo could basically be run like a mechanical compressor (generate pressure at zero rpm) the compression ratio could be further reduced and the volume to surface ratio probably further increased since the turbo could generate pressure at all rpm’s. In addition the electric motor could be used to brake the turbo and generate electricity, so the alternator might not be needed any more. Moreover the electric motor of the turbo could also make the waste gate obsolete und further increase the efficiency. (Heck, an electric compressor (turbo) could theoretically even start an engine).

And last but not least: Since an electric turbo could be run like a mechanical supercharger, the intake opening could be kept open for a longer period of time and therefore be run like a miller cycle engine (piston ‘works’ against compressor (turbo)) and probably further increase the efficiency.

Oh then there’s this question: Since the rotary engine is valve less shouldn’t it be less prone to detonation since there aren’t any hotspots (eg. exhaust valves). So how come a rotary engine is not being operated at a compression ratio of 12:1? This might partially answer the question but I’m not sure: Isn’t it the case that if one increases the compression ratio of a rotary engine the volume to surface ratio becomes worse since the recess in the piston, which probably improves that ratio somewhat, has to be reduced? (I remember reading a book where it claimed that rotary diesel engines can only be made in combination with a compressor since based on its design it cannot reach a compression ratio high enough.)

globi

Here's a link regarding the miller cycle engine (in case someone is not familiar with it): http://www.gizmohighway.com/autos/miller_engine.htm

I'd really be curious if the combination of post combustion, electric turbo, intercooler and miller cycle compression might be a feasible way to improve the efficiency of the rotary engine? (Well if not, the power to weight ratio could definitely be increased.)

rotarygod

Mazda has played with Miller cycle rotaries since the late 80's and we've never seen anything come out of them.

globi

More fuel efficient RX-8? (The same concept but gasoline instead of hydrogen).
http://just-auto.com/proActive/pdf/L...-issue%201.pdf

(Off topic: As long as hydrogen is produced with crude oil, there's no point in pursuing that technology. http://www.unh.edu/p2/biodiesel/arti..._hydrogen.html)

rotarygod

Hydrogen produced with crude oil? Hydrogen is 2 of the 3 atoms in a water molecule! A hydrogen burning vehicle's primary emissions would also be water. More specifically water vapor. You don't need to seperate these molecules with oil based technology. It can be done with electricity.

robertdot

I couldn't find any numbers comparing how much H it used vs how much gasoline it used or the tanks-per-mile or dollar-per-mile comparisons using gasoline vs H. I think it is really comparing apples to oranges, as H is a totally different type of fuel. But, if by fuel you mean gasoline, it is more fuel efficient because it doesn't need gasoline (just like an electric car is much more gasoline efficient than a Honda... because it uses zero gasoline to run).

But the Hydrogen powered RX-8 loses alot of power in Hydrogen mode. If Mazda wants to have a good hydrogen rotary vehicle, it needs to be redesigned from the ground up and probably need a designed-for-hydrogen-use rotary. The one in the Hydrogen powered '8 was meant to be a hybrid fuel system built from a engine that was designed to run on gasoline. So, it's been optimized for gasoline, and isn't going to work at the full potential it may be able to if the engine were designed for H and happened to also run with gasoline.

globi

With engine concept I meant the electric turbo and the electric motor that allows some regenerative braking. And any hydrogen driven engine can basically be run with gasoline. I didn't actually intend to compare hydrogen with gasoline directly regarding its fuel consumption. I just tried to make a point that this might be a way to make a rotary driven car that needs less gasoline. Which might have been Mazda's intention anyway (maybe they just added the hydrogen part for PR reasons only?).

Hydrogen production: Actually I'm not quite sure what fossil fuels they use to produce hydrogen, but they generally don't use electricity to produce hydrogen, since it's far too expensive. (Hydrogen plant: http://www.linde-process-engineering...0022/p0022.jsp)
So unless they don't come up with some plans how to generate these massive amounts of electricity needed to produce huge amounts of hydrogen, I don't see the point.

Yes hydrogen produces water only. A clean diesel or gasoline engine (with catalyzer, particle filter etc.) however produces mainly water and carbon dioxide. Carbon dioxide is essentially as clean as water as long as it has been extracted from plants. (Or in other words doesn't increase the amount of greenhouse gases.)

globi

Besides if it had to be a hydrogen driven car (no matter what), a fuel cell would probably be the preferable approach since it has a much higher efficiency.

robertdot

That has been my argument. I just don't see that combusting hydrogen is exactly embracing the technology that makes hydrogen worth using. It's an interesting concept by Mazda, but I think worthless if fuel cells can be made available to the mass market. Mazda's goal, too, was to offer an intermediary solution. It will burn gas, but you can fill it up with H (which I think it supposed to be cheaper "per-gallon", but, like I said, I couldn't find any comparisons of that nature to gasoline). If more people can fill up with H, more H refueling stations could be made while we slowly weed out the gasoline refueling stations. That is the beauty of the H RX-8, but, as a hybrid, I don't think the RX-8 should have been the starting point.

A hybrid gas-generator-electric / fuel cell hybrid would have made more sense, but then they couldn't say "look what else the rotary can do because it is so damn cool."

The problem is that you are just replacing fuel. It may need less gasoline, but it will burn a similar amount of any other fuel. Changing fuel types doesn't make it more efficient of an engine. In fact, since the power of the H RX-8 is so low, the efficiency seems to have dropped if it uses similar amounts of H as it does gasoline. Sure it's more efficient in terms of gasoline-fuel consumption, but not in terms of generic-fuel consumption.

globi

I agree and actually didn't say anything else (at least not intentionally). Again the whole hydrogen thing on a rotary is ridiculous. I'm mainly intrigued by the electric supercharger/turbo and the use of regenerative braking. I believe that this might be the concept for the first hybrid sports car (electric gasoline) or simply a more fuel-efficient sports car.
The extra batteries or capacitors might add some weight, but since they can be mounted anywhere, the cg can be lowered (and even the polar moment of inertia can be reduced, relative to the total mass of the car).

Actually the use of regenerative braking was prohibited in the Formula 1 umpteen years ago and they wouldn't have done this, if the race cars couldn't have benefited from it.

wakeech

the issue with using a rotary as a hydrogen fuelled engine is one which comes from the "impending shift" toward the fuel cell and electric motor bs (a hydrogen rotary would be better in just about every aspect), rather than as some kind of saviour fuel to make this car slightly more economical.

i don't see how a super/turbo charged engine is supposed to be more thermally efficient than than all motor engine, and a better balanced heaveir car is not better than a lesser balanced lighter car.
i'm curious about this "regenerative braking" thing, isn't that using magnets and dynamos on the wheels to generate electricity and slow the car down?? so i suppose it'd only be of use in a hybrid thinger. well, in reality it's just too cost prohibitive for the extremely slight amount of waste you reduce, which for me is the biggest reason why fuel cells and electric motors aren't the way of the 'future hydrogen economy'.

back on topic, right now the biggest thing with making the rotary more efficient, specifically this one, is getting the goddamned emissions stuff straight so that the motor can run its super lean duty cycles like it's suppost to and not burn the cat out (the motor in its current tune is not what was originally planned by Mazda).

robertdot

So, you are saying that a combustion engine that runs off hydrogen is better than a fuel cell powering electric motors?

I don't know about cost prohibitive. Toyota and Honda both use regenerative breaking in their hybrid gas/electric vehicles. From both sides, the cars cost ~20K.

If you mean fuel cells are too cost prohibitive to be worth using, give it time. When mass production starts, that will drop the cost for the consumer. When fuel cells have time to develop in cars, they will find what corners can be cut and prices will drop some more.

I agree.

History lesson for those who don't know: The problem was not burning up the cat, so they ran it richer. The cat should last 50K (if I remember correctly) miles as was suggested by the EPA when the '8 was in development. When the '8 came out required cat life was bumped to 100K by the EPA, so they dumped more fuel in the chamber to help cool the cat and increase the life. Theoretically, a good tune should increase your mileage and power.

globi

Again a hydrogen powered car (fuel cell or combustion engine) only makes sense if hydrogen has been produced with electricity. And producing hydrogen with electricity only makes sense if the electricity is produced with renewable energy sources (water, wind and/or solar power).
Imagine hydrogen would be produced with electricity that was produced in a cole power plant (35% efficiency of the cole plant times 80% efficiency of the electrolysis process times 25% efficiency of the car's engine gives you a single digit total efficiency.)

Have a look at this comparison:
http://www.unh.edu/p2/biodiesel/art...s_hydrogen.html)

Regenerative braking: The hybrid cars actually get a better mileage on city driving than on highway driving! That itself is a pretty convincing argument regarding the effectiveness of regenerative braking.
Also regenerative braking has nothing to do with what power plant the car runs on (diesel, gasoline, hydrogen, cole, propane, electricity, steam or pressurized air).

robertdot

Well, I think a hydrogen powered car makes perfect sense even if somewhere along the lines the power to get the hydrogen uses fossil fuels. Basically put, we are going to run out of fossil fuels in the next 50-70 years at our current rate and increase. Someone wants us to think that we should give up on cleaner fuels because they will still end up being powered by fossil fuels, which defeats the purpose ideologically. Well, it doesn't, depending on what your ideals are.

My ideals for H over gasoline concern emissions / pollution. If every car in America stopped shooting out toxins and started shooting out water vapor, pollution would drop substantially. Period.

The short sighted version of this, which "breaks the ideology", is that: "wow. If we started building H production plants, we would have to burn more fossil fuel that we already do to power them once they open!!!" The point of having H as a fuel source is NOT so that we have have two different fuel sources. It is so we can REPLACE the old fuel. So, if we started closing crude oil refineries and started opening H production plants, the amount of fossil burned will be the same over all. Even if we did it the short sighted way, the idea would be to eventually close most crude refineries.

If your ideals are to fully replace fossil fuels, then, yeah, your ideology is broken. If your ideals are to replace as much fossil fuel as possible, then, no, your ideology is fine.

But saying what does and doesn't make sense only matters depending on what sense you are trying to make. If the sense you are trying to make is 100% efficiency of fuels, then okay. But not even solar is 100% efficient. Solar panels are something like 15% (if I remember correctly). They WASTE alot of sunlight! So, again, it depends on what sense you are trying to make. Since solar requires only the things we have readily available with no work, you could say solar is 100% efficient, because it makes electricity out of "nothing".

AND the UNH biodesil site has an agenda, so I'd have to gather my own data before I jumped on that bandwagon. At any rate, I wonder what the emissions are on those and whether or not they were talking about combustion of H or fuel cells.

globi

This can be true for a gasoline or diesel engine also. You can make a gasoline engine or a diesel engine perfectly clean. Ideally both engines generate water and carbon dioxide. None of it is toxic. Otherwise we couldn't drink soda water which consists of water and carbon dioxide.

Of course, I was mentioning this regarding pollution free energy sources. After all that's the whole point of the hydrogen engine. Regarding solar panels: I'd wonder whether the earth has enough surface to produce enough electricity to fuel all the cars on the planet?
Anyway what I was trying to say: We were better off replacing fossil fuel with soy bean oil or algae oil (biodiesel) than with hydrogen. At least for now.
An ultra clean diesel engine (catalyzer and particle filter) does mainly generate water and carbon dioxide. Here's an example of a Diesel powered car with particle filter umweltbundesamt.de/uba-info-presse-e/presse-informationen-e/p3101e.htm. (Don't ask me why the US doesn't import it. Maybe because it's french?)

Here's a comparison biodiesel vs. hydrogen (I guess that link didn't work before): unh.edu/p2/biodiesel/article_biodiesel_vs_hydrogen.html

A few points regarding Biodiesel:
* Trucks could immediately switch to Biodiesel
* Reduced dependency on foreign oil
* Support of the US-farmers
* Cleaner (apparently Biodiesel burns cleaner than Diesel)
* Can be used on existing pump stations
* Lower flashpoint
* No net CO2 emission (greenhouse gas)
* Technology is already available
* and more

And actually you can run a Diesel engine on vegetable oil directly:
www.greasecar.com
Even if you don't believe the UNH data, this is not disputable. In fact the very first Diesel engine (over 100 years ago) ran on vegetable oil. And vegetable oil is easily obtainable and relatively inexpensive.

globi

Here's another Link with the hydrogen RX-8.
http://www.der-wankelmotor.de/wasserstoff.pdf

robertdot

I'm aware that H2O and CO2 aren't toxic.

It's the CO, VOCs, and NOx I'm worried about. That is why we have cats, to prevent this. But, even with CARB testing, there is a margin or allowable error in a cat.

Hydrogen vehicles (at least the fuel cell type) do not need a cat. Fuel cells NATURALLY produce water. There is no combustion, only the movement of H and the combination with O to make water. It doesn't get any cleaner than this. There is no margin for error as with a cat. It is just water, that's all. That's why I like fuel cells.

At any rate, I agree that adoption of H will be more costly than a move to biodesil. But, in the end, H is the most abundant element in the universe. We will eventually be able to conveniently convert water into fuel cells on a massive level. Even better would be if we could eventually build this into the car or have these type stations at our homes.

Biodesil will also be good for America's agricultural areas that are already being paid not to produce crops.

Anyway, there are good sides to both, but I think I like H fuel cells the most at the end of the day.

wakeech

yeah IC rules, it's less efficient than a fuel cell but the engines are smaller, lighter, cheaper to make, more serviceable, more powerful, and best of all noisey.

bgreene

Hydrogen may be the most abundant element in the universe, but that doesn't help us all that much here on the surface of the earth......

There are some serious logistical issues involved with replacing gasoline with H2 (not to mention some studies that H2 leaking from storage tanks could be as environmentaly harmful to the upper atmosphere as internal combustion emissions). Plus the fact that separating H2 from anything in large quantities (water, oil, methane may be a good candidate) will require major energy input, and that energy has to come from somewhere, under current conditions in the US that source is likely to be fossil fuel power plants, which means trading one emissions source for another.

Also, there are some big safety issues in hydrogen cars. You're pretty much stuck with a choice of whether you want an extremely high pressure tank (thousands of psi) which would turn into a cloud of shrapnel if it burts, or a tank of cryogenic fluid (liquid H2 is around -423F) riding around with you, and under your seat in a collision, plus the added bonus of huge tankers full of volatile cryo liquid cruising around as common as (or more common than) gasoline tankers, and large cryo-storage facilities interspersed throughout the suburbs and cities.

The other kicker is that virtually any type of fuel system will leak some H2 (the molecules are small engough to pass through solid aluminum at some thicknesses, which means effective gaskets get hard to design), meaning that unburned fuel "emissions" will likely be unavoidable.

We're probably headed to some sort of arrangement where many cars may be hydrogen powered, but that day is still somewhat far off. Building the cars is the easy part.

wakeech

some of those leaking predicaments are pretty interesting, but there are a bunch of different solid-state storage methods which eliminate many of the complexities of storing hydrogen on its own (like carbon microfibre or matrix whatever, metal hydrides, these neat things called 'power *****'). there are tons and tons of threads on hydrogen rotary stuff on this forum.

shawnio

The first diesel engine, I believe, was run on peanut oil.

Mercedez-benz had a hydrogen powered car that converted methane into hydrogen.

Hydrogen cars leak energy while they sit. There have been studies done as to the emissions produced during the life of the vehicle, production, and fuel source. The hybrid electric car was found to produce the least emissions over something like a 30 year period.

The biggest impedement to hydrogen use is that it's not priced competitively with gasoline right now. Are you prepared to pay more for a fuel that produces less power because it's theoretically better for the environment? Storing the hydrogen is a huge problem too. Which is why Mercedez-Benz decided on methane as the storage fuel and source of hydrogen. The reason they are getting excited about hydrogen fuel is that the price -is- coming down, and the price of fossil fuels is on the rise.

Of course, maybe something can be done about that ... http://www.changingworldtech.com/home.html
We'll have to see what becomes of this project.

Regenerative braking will likely never be used on a sports car. Talk to cortc about unsprung rotating weight and think about how heavy these motors/generators are. They are usually mounted on each wheel similar to rotors and calipers. It would be neat, if they could attach something elsewhere, maybe a driveshaft, or in the transmission. Perhaps something that worked like engine braking from the transmission, you lightly use the brakes while pushing in the clutch, a clutch to the generator is engaged and it slows down the wheels. Of course, this generator would charge a lightweight capacitor, which would discharge when you need to accelerate again.

This would add a bit of weight to the car.

Anyway this isn't going to make the engine more efficient.


as far as "since the rotary engine is valve less shouldn’t it be less prone to detonation since there aren’t any hotspots (eg. exhaust valves). So how come a rotary engine is not being operated at a compression ratio of 12:1?"
Rotarygod posted somewhere about someone not noticing any gains beyond 10:1

What I really wonder about a rotary engine ... is the path that the air takes while being pushed around by the rotor. Piston engines see an increase in efficiency by inducing turbulence inside the cylinders, one way to do this is to have uneven flow through the valves. Honda and Toyota engines do this by lifting one valve higher than the other. It's easy to see how air might spin in a cylinder. I have trouble imagining spin in a rotary engine. How would you want the air to spin anyway? What happens to the air with the current configuration at various RPMS?

Maybe someone knows.

Look to the way they increase efficiency in piston engines. Lightweight parts, check. Dynamic intake manifold, check. Variable "valve" timing, check ... opening and closing changes with rpm, although rotaries don't have valves. Throttleless intake, nope. Combustion swirl, dunno about this. Direct injection, nope.

So there you go ... 3 ideas

shawnio

"Also, there are some big safety issues in hydrogen cars. You're pretty much stuck with a choice of whether you want an extremely high pressure tank (thousands of psi) which would turn into a cloud of shrapnel if it burts, or a tank of cryogenic fluid (liquid H2 is around -423F) riding around with you, and under your seat in a collision, plus the added bonus of huge tankers full of volatile cryo liquid cruising around as common as (or more common than) gasoline tankers, and large cryo-storage facilities interspersed throughout the suburbs and cities."

You realize you're riding around in a car with an extremely volitile fuel right now? They've dropped those thousands of psi hydrogen tanks from something like 20 stories without damage. You can bet those tanks are expensive. If you think riding around with gasoline is so much safer than hydrogen, you're not entirely correct. Either way, measures have to be taken to prevent the fuel from hurting people. I remember a certain recall recently on Ford Crown Victorias, they found that if they were hit from behind at a certain speed, there was a chance of the gas tank exploding.

robertdot

Detonation can occur if you over compress gas. diesel fuel explodes when it is compressed. By design, this is how diesel engines work. Gasoline has preventative measures so that it can be compressed and ignited with the spark plugs.. The octane rating is your clue on how much the fuel can be compressed before it explodes. High-compression cars want high octane. If you increase the compression on a rotary without increasing the octane, the gas will eventually explode before the spark plugs ignite it.

ChrisW

Biofuels are essentially solar power - they convert the sun's energy into chemical rather than electrical energy, via photosynthesis. I think they are a lot less efficient than solar cells too. So I don't know what percentage of the world's transport you could power by this method.

ChrisW

Sorry, I've just read the link you posted:

"For an estimated $169 billion, enough algae farms could be built to completely replace petroleum transportation fuels with biodiesel"

If that's feasible then it certainly seems like the best solution to me - why invent all these new engine, fuel distribution and storage technologies for hydrogen if we don't have to?

Modern diesel engines are pretty clean now, and will get cleaner as the technology progresses. If the global warning predictions are true then even current toxic emissions from cars are an utterly trivial problem in comparison.