Ivegonemad recently brought up the topic of a diesel rotary, something that I had been wanting to try myself for a long time.

Before I get out the flame suit, here are a few of my thoughts:

The combustion chamber would have to be filled almost completely with material, leaving only a small dish down the center of the rotor. This would allow a high enough compression ratio to compression-ignite the diesel fuel. (Balancing would obviously be necessary, both E-shaft and rotors.)

The fuel injector could easily be threaded into the leading spark plug hole, giving a good view of the dish in the rotor for atomization.

Now for starting... This is a problem, as we all know the rotary does not produce full compression at cranking speeds. Nor does it increase the air temperature as much during the compression cycle as a piston engine (moving past a metal surface that cools it, rather than being compressed in one spot.) This results in insufficient activation energy for compression-ignition (AKA: It won't start.) Modern Diesel engines have two means to work around cold starting problems, grid heaters and glow plugs. A grid heater is easily installed anywhere in the intake manifold, Glow plugs must be near the actual point of combustion.

The awesome thing about a rotary is that it has two spark plug holes. The trailing plug hole could house a glow plug that allowed the engine to start.

I don't know how fast the engine could efficiently turn on diesel fuel (this depends on injector on-time, fuel rail pressure, injection timing, boost, etc.)

Speaking of boost, Since a turbo rotary will take 15 psi or so without much work on the internals, I don't see why the additional compression required for the diesel engine would damage the bearings or apex seals.

Ok, the flaming can now begin <puts on flame suit> I really do want to hear your thoughts on this, positive or negative. It is much better the hear about problems in the ideation phase than learn about them in the testing phase.

 

i was thinking that if compresion can be raised it wouldn't need to be at high boost..i maybe wrong??

as far as where the injectors can be placed, it would be great to place it on the leading plug hole, but wouldn't it create a problem with the A/F ratio?? since the port is on the oppisite side of the spark plugs,it would be efficient to leave the injectors as they are but placing them a little lower to the intake port to fill the whole bowl of the rotor??

im glad im not the only one thinking about this...

 

There have been diesel rotaries. Rolls Royce made a 2 stage diesel rotary. It was basically two engines. One engine as a pump to compress the air and the other for further compression and combustion.

I don't think you can get that high of a compression ration out of a regular wankel because of the geometry. Maybe if it were supercharged... but it would be dependent on the supercharger if you got it to work.

 

Hmm, before I thought about why I havent seen one, and answered my own question, cant have high enough compression in one to ignite the diesel fuel.

BUT, from what you just said, this now seems like a feasible idea... maybe using a electric assist turbo to generate high boost at cranking speeds to help ignite it as well?

But then, I dont know **** about engines... realy, as much as I want to think I do, I realy don't. =[

 

i read somwhere on this forum that someone was able to weld enough material into the dish of a rotor to get slightly above an 11:1 CR while still leaving a sufficent pocket from his research to allow flame propagation to occure somewhat efficiently, i belive he/her/they started with a 9.7:1 rotor

without major changes to the engine's geometry (diff shaped rotor coupled with a shorter stroke) a very high compression ratio is from what i have read impossible to attaine, there just isnt enough movement

 

even though a supercharger will possibly work, it will only be atleast a low boost which i dont think it will be able to compress the fuel substantially enough to atomize or ignite the fuel..

 

Current day diesel's atomize there fuel by injecting it at a very high pressure (over 150,000psi) though a specialized injector

 

Common rail diesel fuel pressures, while very high, are nowhere near 150,000 psi. Currently you'll see 24,000 psi with race engines hitting near 30,000.

 

I have always wanted to do this. I would be willing to toss up on of the motors lying around if someone can figure out how handle the computing needs for the motor. I have a turbo from a 7.0 liter marine diesel sitting in the shed just waiting for me to do something with.
I have a couple of kegs that need to be torn down. If one of you guys is confident enough to fill it in with metal. Ill have it balanced. I think some 3mm apex seals would be in order. Probably want to have the thing doweled. That ain't cheap. I don't think it would be necessary to have the electric compressor.
I know that some of the smaller diesels for generators have as little as 9:1. Granted that is a single piston, 305cc, operating at one speed.

Here it is guys. I'll do it if you can figure out the computer.

 

alexdimen, you are the closest to bing on track. The 2 stage rotary, one as a compressor dumping into the second as a combustion stage is a solution. A diesel needs between 15 to 20 to 1 compression ratio to function. Therefore a turbo will not work well with an 8 x 1 rotor motor, it doesn't develop a high enough compression. A supercharger or compressor that develops between 30- 45 # boost is need for efficient functioning. The other ideas for injectors and glow plugs are good.

Worked on the development of a totally new diesel engine design in the mid 80's, it was called a turgin. A 3 stage engine. Stage 1 compressor (supercharger), stage 2 combustion, stage 3 expansion, similar in concept to a turbo jet engine, but a positive displacement design. It was like a rotor motor with 3 rotors and the fuel/air/exhaust flow was parallel to the eccentric. Ports between the stages passed the gases from one stage to the next. It received 13 patents, 7 were basic patents, the most for any one device in 50 years. Projected output was for an engine about 2 foot cubed could produce about 2,000 hp. Design was so radical, even tough we had a semi-functional model the major auto makers couldn't imagine how it could work. It would have taken between 5 & 10 million dollars to build the working model.

I got on the development team because of 9 years with GM research and building hi-per rotor motors starting in 73 and understood the eccentric principal and sealing used in the rotor motors.

Doc-Watt Just another outdated old fart.

 

so a turbo wont work at all..then how does a supercharger help to build compression??

was this at the time that GM was working on rotaries??

 

You have 2 types of compression ratio. The one everyone thinks about is static compression ratio. This may be 9:1, 9.4:1, etc. It is based on what rotors you are using. The other compression ratio and the only one that matters is the dynamic compression ratio. This is what actually happens inside the engine. Think about it this way. Our engines are almost never at 100% volumetric efficiency. If they were then static and dynamic compression would be the same. Let's say we are cruising and our engine is at part throttle and only at about 50% volumetric efficiency. The actual amount of compression inside the engine is far less than what the static is. If we had a 10:1 static compression ratio but our engine was only running at a load where it was 50% volumetrically efficient, it's dynamic compression ratio would also only be 50% of what the static is. It would be the equivalent to a 5:1 compression ratio. Although the chamber physically changes in size the same way, we aren't filling the chamber completely before we compress the air. It may only be half full. Port timing, load, and rpm all play a big role in this.

A supercharger or turbochargers goal is to increase the dynamic compression ratio. This is how you make more power. The nice thing about a diesel is that they don't use throttlebodies. This means they are always at a higher load level and therefore running at a higher volumetric efficiency level. If you keep a throttlebody, you will also keep a lack of efficiency and therefore lower dynamic compression ratio at all but full throttle.

If you want a place to start when it comes to making a rotary work as a diesel, think simple. Go back in time 30 years. You need to start with something that works before you make something that is modern. Baby steps. Get ahold of an old mechanical diesel fuel pump. Not the modern kind but rather those that are responsible for metering the fuel. Older diesel injection pumps work very much the same way as a distributer does on ignition. It times the fuel mechanically to inject fuel at a certain time. There will be no need for crazy computer controls this way. Once you can make this work decently, then you can worry about taking a modern approach to it.

 

ok now i get...but even diesels do use turbochargers, why would a supercharger work best for this application for a rotary??

 

I think it would probably be a supercharger/ turbo combo. after the supercharger gets things going, the turbo take over.

 

hmm..so a supercharger can do the start up and once the exhaust gases get to thier full expansion a turbo can kick in??

 

Actually guys, I know some guys that have access to common rail components and ECU, etc. So the old stuff is not necessary, we can have an electronic common rail diesel rotary if we can overcome the big question: Can the compression ratio exceed the 16:1 that is required for reliable diesel starting.

Theoretically, a diesel engine will compression-ignite at 13:1 dynamic compression ratio (at normal operating temperature, not cold.) When starting, the air does not heat-soak from the engine, so at least 16:1 is necessary to start at about 70 F. To start on a cold day (say 20 F) 24:1 or so is better. (This is due to the semi-isentropic compression of the air increasing the temperature to auto-ignition temperature of atomized diesel fuel.)

Ok, that is about all I know about diesel compression ratio, which I learned from my friends.

Supercharging - Alexdimen is a genius.

The reason that a turbocharger will not work to start the engine is simple: Being a cetrifugal pump, it does not function at low speeds and since it is exhaust driven, the exhaust is cold and there is very little flow at starting speeds, it is not spinning fast.

A supercharger (at least a roots-type supercharger) is positive displacement and engine driven. This means that it puts through a specific quantity of air per engine revolution (very much like an rotary or piston engine.) If the supercharger puts through 2.6 L/E-shaft revolution (this quantity is dependent on belt drive ratio) the engine will be running at 14.7 psi boost (assuming 100% Volumetric Efficiency.)

To address the difference between effective (dynamic) and mechanical compression ratio:

Diesel engines do not have a throttle plate. They regulate the fueling to increase or decrease power output and displace the same air per stroke all the time. Because of that, the effective compression ratio is pretty close to the mechanical compression ratio times the volumetric efficiency.

Turbocharged diesel engines displace more air after the turbo gets spooled up, but do not require a blow off valve to let the boost down with a release of the throttle, the engine just puts very little fuel into a lot of air.

The lack of throttle plate is the reason that a diesel engine is so much more fuel efficient than a gas engine. (Which is why I want a diesel rotary.)

Without the throttle plate, compression ratio is always high (efficiency goes up with higher compression ratio.) The diesel also runs very high A/F ratios (like 60:1) when under light throttle, allowing low fuel consumption.

I apologize if this is long and confusing, I am pretty excited about this being a reality.

As far as parts go I have several old rotaries lyinng around that I don't mind welding on, I also have some old turbos and superchargers removed from diesel engines.

Spark Notes:

Diesel parts can be found.
Supercharger should be effective means of increasing compression ratio.
Want diesel rotary for fuel economy.

 

That's why I want one.
I am also working on a plug-in diesel hybrid motorcycle. I don't mean to hijack a thread, But check this out.
a 6.6 HP water cooled diesel turning a 24v generator. going to a battery pack that's connected to a 24v electric motor. that is connected to a constant variable snowmobile transmission, which then goes to the rear sprocket. Based on the current efficiency of one of these motors I'm looking at. 200MPG+. If I take out all the electric stuff I'm looking at 150+ MPG.
Back to the 7...
Is the wall of the rotor housing strong enough for 16:1 compression? Will it just pop like an old condom?
Will it being a rotary diesel limit it's RPM? Every diesel I know of has very low red line. If we get a working DR, will we have a higher than normal (for a diesel) red line? Or will we be limited from injector speed or other fuel system limit?
Sorry I'm jumping ahead in the game.
Anyone confident in their welding ability to try this on my rotors?

 

The problem with really high compression ratios on rotaries is that fact that we not only compress the air, we are also flowing air around the engine. That may sound weird so bear with me. On a piston engine, we bring in air and compress it in one spot. For the most part it goes nowhere. In a rotary as we all know, air is moving from one side of the engine, around to the other side and back again, compressing air as it goes. This may not sound like a big deal as compression still takes place. The problem comes from the fact that as we are actually compressing the air, it flows through the little rotor dishes in the face of each rotor and actually passes from the top to the bottom of the motor through this opening. We need air to flow through this little pocket.

When you make this recess smaller as in raising the compression ratio, you make it harder for air to flow through this and this problem is independent of compression itself. It is another matter altogether that just happens to occur during compression. There is a Mazda development chart that shows various tests on engines with different compression ratios. I believe it is somewhere in the book by Kenichi Yamamoto that is floating around online somewhere. In the graph it shows that power doesn't appreciably change between 9.0:1 and 11.0:1 compression ratios. Above 11.0:1 and below 9.0:1 power rapidly falls off. On the low end it falls off due to an obvious lack of compression but above 11.0:1 it falls off due to losses from trying to flow too much air through too small of an opening, namely the rotor face dishes.

This may sound like it can't happen this way and it would seem that this phenomenon is disproven by the fact that turbocharged engines cram way more air into the engine and can still make more power. There are 2 things about that to remember though. One is that we've already compressed the air to get more in a smaller space which means the turbo does much of the added compression work done to the incoming air so the engine doesn't need to. This is why 1 bar of boost isn't equivalent to doubling the stress on an engine. The stress increase is far less than that since the engine itself isn't being forced to do all of the extra compression. The second and most important thing that explains why boosted (even highly) rotary engines don't seem to suffer from issues with airflow throgh the rotor dishes is that boosted air also has more oxygen in it. More oxygen gives off a bigger bang. If we were to just increase compression without boost, we wouldn't have any more oxygen to offset the extra effort necessary to move the air through such a small space. Compression in one location is one thing but compression by forcing air through a smaller and smaller orifice is another.

This is a huge reason why we have never seen a compression ignited diesel rotary that has had any degree of success. We've seen low compression diesel rotaries that have used superchargers and spark plugs but the efficiency understandably wasn't really much of an advantage. The only diesel rotary out there that has come anywhere near optimal is the apu from PATS. It is a small 1 rotor engine based on an aircooled Rotapower unit. Instead of a spark plug, the have machined a prechamber along with a glowplug. Fuel pressure is lower at an older 200 or so psi. This type of system is referred to as IDI or indirect diesel injection. Fuel is injected into a pilot chamber where it ignites and is then sent out into the main combustion chamber. Diesels were done this way for years. The advantage of this to a rotary is that much of the initial ignition shockwave is dissipated in the prechamber which makes life easier on the apex seals. One thing about the engine though is that although it uses a glow plug and diesel fuel, due to the lower than optimal compression ratio, the glow plug is on full time. Without it there isn't enough heat to spontaneously light off the mixture.

With the aid of forced induction weither through a turbo or a supercharger, you should only need a glow plug at the lower rpms. Once you get some boost you should be able to get ignition temperatures. Finding a way to shut off the glow plug above a certain rpm or boost level might be a novel idea.

Where you inject the fuel is going to be huge. You don't just inject it anywhere. Diesel fuel is rated in Cetane so we don't normally assiciate it with octane level or resistance to detonation. Many people wrongfully assume that a diesel runs on detonation. They do not. Detonation is just as bad on a diesel as it is a gasoline engine and it is detonation or the control of it that has led to the huge leaps in diesel technology in recent years. In the 70's it was easy to find diesel engines that had low compression ratios. Caterpillar had engines with 8.0:1 compression ratios but positive displacement roots blowers to make up for the lack of compression. Exactly like what is being discussed here. The reason for the low compression ratio was for the lack of fuel control. Those engines had carburetors which injected the diesel fuel in the intake manifold as opposed to directly or even indirectly in a prechamber. This is a problem and another phenomenon whih many people can't seem to grasp. Diesel fuel has an octane rating of somewhere between 20 and 40. That's low. Decades ago when gasoline was of lower grade and quality with lower octane ratings, engines also had lower compression ratios. How many hot rods and muscle cars do you see with low compression ratios? Lots! As the years have gone on and fuel quality improved, compression ratios have gone up. In recent years as better fuel and ignition control have advanced, soo too has compression ratio also been creeping up.

Back to the Cat engine and the concept many have a hard time with. Compressing fuel and air HEATS it up! Heat is bad. Getting the charge too hot leads to spontaneous combustion which can either be preignition or detonation depending on where in the ignition cycle it occurs. This is bad. If you compress the air and then inject the fuel into the compressed air, it doesn't get as hot. This is how modern direct injection and common rail diesel engines have progressed so far. Since you have less time to inject fuel than just dumping down an intake runner, you need more precise control. The shorter amount of time to get it injected means you also need higher fuel pressures. Through higher fuel pressures you need to also find a way to control the spray pattern better. Etc, etc... You get the idea and can now see the progression of modern engines. Higher fuel pressures, better fuel control, higher compression ratios.

I've had people argue that fuel in the air cools it down which is why people run extra rich on turbocharged cars. There comes a point where going overly rich or overly lean will actually result in cooler temperatures. If we just compress air with no fuel, it doesn't get as hot as compressing air with a stoichiometric amount of fuel. If we try to compress fuel only (lets not get into the logistics associated with this) it will not heat up as much as air does or fuel and air at the perfect mixtures. Of course we can't compress liquids but we can increase their pressure.

It will take more than just pure guess work to find the perfect fuel injection point. The rotary makes this very hard in regards to direct injection (whether it be gasoline or diesel) due to the fact that we don't compress in one location but rather have a moving compression region. At one point in the compression stroke air actually stops and moves backwards for a moment! You'd really have to see a rotor in a housing up close to grasp this but it does happen. On modern diesel engines, they can control the amount of "knock" just by altering the fuel injection event in milliseconds. It's actually quite complicated and much harder than just simply getting ahold of common rail equipment and slapping it on. The results will be practically guaranteed to not be very good. The best thing to do is to start with older technology to overcome it's many limitations before you try to tackle more advanced tech. Sometimes it's faster and easier to go backwards first.

Just some stuff to think about.

 

wow that was long, but informative...lol...i think i might have to read that a couple times more to get it all through.....

so in theory, the design used by mazda for the rotary will change, with it bieng a diesel??

 

http://www.regtech.com/

this company has a running rotary diesel.although still a prototype right now.check the video.doesn't look like what you might be expecting but same principle.



that black fb in the video is the same guy being interviewed who's heading up the r/d for the project and was the first person to get the prototype running.

that video was posted on bcrotary.

 

That engine is the randcam. While it is rotary in nature, aside from that it has nothing in common with the wankel engine. Check out the "liquid piston" rotary engine. It's pretty neat too.

 

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The Gm ? first. No it was after both GM and Curtis Wright gave up on building rotaries, neither were willing to hold or build to the tolerances necessary to make a rotary work. Even the Germans were unable to make a reliable Wankel, though they produced one in a production car in the 70's. My time with GM was before the turgin design stint.

a. With a turbocharger, it is more difficult to develop enough boost to raise the dynamic compression high enough for efficient operation. High boost with turbo requires high rpm.

b. The dynamic operating range of a diesel is under 2500 rpm and that is solely determined by the flame propagation rate within the combustion chamber, which is in turn determined by the type of fuel.

c. Gasoline has a much faster flame propagation rate than diesel. Therefore it can run at a much higher rpm. With a diesel it takes longer for the fuel in the chamber to be completely consumed, consequently a slower operating range.

d. Most superchargers are positive displacement devices that can give higher pressures at lower rpm, and that can be controlled by pulley size and ratios. Making the supercharger more practical for raising a low dynamic compression to a higher compression ratio, necessary for diesel operation.

 

[QUOTE=Doc-Watt;7310671]----------
My time with GM was before the turgin design stint.
QUOTE]

what design was this?? but i can understand now...

 

I can't remember the name of the book I read this from (from the 70s) but Felix Wankel came of with over a dozen types of rotary designs. Some could make compression ratios of over 100:1

He chose the triangle rotor design I believe because it provided the best compromise of average seal life and could be run on contemporary compression ratios

 

^really?? i would love to check this book out....