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Diesel Wankels?

Diesel Wankels?

Diesel Wankels?

(OP)
With the advent of electronic controls on Diesel injectors, is anyone working on multiple injections into a Wankel type engine to "chase" the rotor around its path, coming closer to a constant pressure expansion cycle? Seems like a possibility is opening up here. The kludge way of doing it would be with multiple injection events from a single direct injector - the theoretically optimium might come from multiple injectors placed around the housing...

Related topic: Did Rolls Royce ever have success with their Diesel Wankels? It appears that the high surface to volume ratio of Wankel combustion chambers, plus the long, sickle shaped chamber would cause problems for complete compression ignition. Anyone have any comments on how Wankels do as Diesels?   

RE: Diesel Wankels?

I worked on a Wankel engined APU 15-ish years ago.  It had to be diesel because the vehicle it sitting in was diesel.

It needed spark assist and would only start with a copious squirt of "easy start" in the intake.

A horrible thing.

- Steve

RE: Diesel Wankels?

I've thought about this and actually worked out some math on it, but I have to warn you that without industry experience on this, everything I say should be taken with a pound or two of salt.

The huge advantage of a diesel Wankel would be the large area over which to spread multiple injectors for a fast, clean burn. (This is precisely the reason why the Wankel is so ill-suited for a single-injector or single-spark engine; the combustion chamber is flat, and the flame front has a pretty small surface area while the heat transfer boundary has a large surface area.)

I didn't even MENTION sealing...

One problem with sufficient compression on a diesel has a lot to do with rotor geometry. A high compression ratio results in spots of high wear and higher acceleration. Sealing would also be more difficult with the larger curvature necessary; the apex seal wouldn't just be sitting against a moving plane but would require flexure, rather like a man's razor.

RE: Diesel Wankels?

I did a quick calc on the geom a few years back, and if I remember correctly, I found that the highest compression ratio that wankel geom would allow was about 12:1.  That was with no pockets in the rotor.  Not ideal for a diesel.

 

RE: Diesel Wankels?

Diesels normally are used to increase fuel efficiency. A rotary has very large surface area to capacity, so it will always have lower thermal efficiency for a given output.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Diesel Wankels?

The application I was concerned with needed a high power density (power/engine mass) regardless of efficiency, using fuel from the donor vehicle's tank.  A bit of an odd situation really.  Almost (but not) military.

- Steve

RE: Diesel Wankels?

On the related topic...Rolls Royce discontinued research on their "cottage loaf" engine not long after it was announced.

No reason was given and the research efforts were then concentrated on conventional piston engined applications.

Pete.

RE: Diesel Wankels?

John Deere I believe studied a Wankel-style rotary, that was advertised as being capable of multiple fuels (from high-cut JP-4 down to low-grade heating oil).  As Sompting mentioned, it was interesting because of its predicted high power/unit mass, and a student chum was using its predicted performance curves in a design for a light plane as part of his senior project.  As far as I know, the engine was never produced, likely for reasons that Pat mentions.

RE: Diesel Wankels?

some of the problems that stopped the development of rotary engines in the past may now have been overcome (machining accuracy, better sealing materials with less wear). others however are still present: the high surface to volume ratio leads to problems with ignition/injection and to low thermal efficiency. the problems with ignition and injection may be solved with modern electronic systems, the low thermal efficiency problem can not be solved. geometrically the compression ratio is limited and therefore rolls royce tried a two stage design - that multiplied the sealing problems back then accordingly. also modern emission regulation poses new problems. all surviving wankel/rotary designs are used in applications where the drawbacks are not considered a problem that prevents commercial succes and therefore in some niche markets those designs live on - either because they are different and exclusive or in those instances where a high power to weight ratio is more important then longevity.

nevertheless, i still vividly remember a drive in a NSU RO80 car some 40 years ago - one of the most fascinating cars i ever had the pleasure to drive - and still quite modern, even compared to tadays standards.

RE: Diesel Wankels?

If I recall correctly there was some wording in the manufacturing / research agreement which meant engine makers were required to make their research information available to all other licence holders.

This may well have sent some makers to look for alternatives.

Pete.   

RE: Diesel Wankels?

I loved my friend's '84 RX-7 back in high school. They have a lot of potential as race engines (like in the Mazda 787B).

The two-stage engine gave me a few ideas....

RE: Diesel Wankels?

The Rolls Royce 2 stage diesel engine dealt with heat-issues. This is not surprising: The combustion stage is constantly exposed to high temperatures. The 'coldest' air it receives, is the adiabatically compressed hot air from the first stage (no intercooler). Keep also in mind the 'pressure ratio' of the compression/expansion stage is significantly higher than what you'd normally expect from a turbo.
http://www.der-wankelmotor.de/Motoren/Rolls-Royce/rolls-royce.html
A piston engine with a high compression ratio on the other hand does receive relatively cool air during every intake cycle.

If the 2 stage wankel engine was fitted with an intercooler and/or the combustion stage was 'miller-cycled', the heat-issues of this two stage concept could be overcome. However, this would not enable self ignition, but it would increase the overall efficiency of the wankel engine significantly (and reduce exhaust noise at the same time). Albeit with a reduction of the power to weight ratio.

RE: Diesel Wankels?

(OP)
Great responses, guys.

The volume/surface issue of Wankels always comes up, but there is the thought that at some sufficiently high rpm the residence time of the combustion gases would be short enough to get beyond this limitation...

The concept of 'chasing' the rotor around with multiple injectors & injections could attain a constant pressure combustion front - this would be pretty easy to do with modern controls & two or three direct injectors - no?

RE: Diesel Wankels?

you could certainly time the injection of fuel appropriately.  you might have trouble getting it lit.
 

RE: Diesel Wankels?

Speed does not change surface area. At twice the rpm, you only expose the charge to half the ime to lose heat, but you do it twice as often.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Diesel Wankels?

Pat, thanks for the insight.  I instinctively knew Flamefront's statement about residence time was a red herring, but up to now, given the limited space of my mental capacity and attention span, I could not put my finger on the flaw in his logic.

RE: Diesel Wankels?

"Speed does not change surface area. At twice the rpm, you only expose the charge to half the ime to lose heat, but you do it twice as often."

Is the inner wall temperature the same at 4000 rpm and 7000 rpm? If so, then you're losing less heat per cycle when you run it faster, which is more efficient overall.

RE: Diesel Wankels?

(OP)
The power per cycle is what is of concern, as <kevnindurette> correctly observes. The qdot/hp is the parameter of concern, not the total heat flux, in my opinion.

Re-stating the obvious: With less residence time for the combusting/combusted gases at higher rpm, more of the heat will be carried off in the exhaust constituents...for a given series of cycles, which means less heat is being lost through the "large surface area" (athough it is lost just the same). This leads me to believe that at some magical rpm, the difference between rotary and reciprocating engines converges...at least as far as the surface are argument goes.


I've read some engine designers poking holes in the "surface area" achilles heel issue on Wankels, but have not seen a complete argument yet, nor can I put one forth myself. Just a hunch....

RE: Diesel Wankels?

This argument cannot be fully resolved as at twice the rpm you have twice the power unless you throttle the engine and reduce the load, or you use half the capacity. There are just to many variables to get an all other factors being equal type situation.

Much larger surface area is not a good starting point for fuel efficiency, but high power density is a good thing for overall vehicle efficiency. To be objective rather than blindly partisan, we should consider all parameters and how they balance and how this balance changes everything else whan we change one parameter.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Diesel Wankels?

Wankel engines are relatively efficient at high loads and high rpms compared to piston engines. I remember reading somewhere ('can't find it anymore) that the Wankel engine 26B, which won the 24 hours of LeMans (1991) had a good fuel economy compared to other race cars.

The problem is: 95% of the time, people on public roads drive at low loads and/or low rpms.

But I think, if the plug-in hybrids actually become popular, there might be a benefit in using a Wankel engine, which uses less space, has little weight, doesn't vibrate and in series hybrid application a Wankel engine would obviously always be operated at high loads. It can also be provided with a turbocharger without having to worry about lag.

RE: Diesel Wankels?

So can you find any thermodynamic efficiency maps, or bsfc graphs to back that up? There are very good reasons why large Wankels won't be especially efficient and they apply in spades to small ones.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Diesel Wankels?

point 1:
Wankel engines are even less efficient at partial load than piston engines. The bad volume to surface ratio is even a bigger problem when these engines are operated at partial loads. More surface area per engine output.

point 2:
If a Wankel engine is supercharged: Less surface area per engine output.

point 3:
If an engine is used as back-up power in a plug in hybrid, the efficiency of the engine at partial load is irrelevant (no partial load operation) and its efficiency at high loads is of less importance, because the vehicle is mainly driven without IC engine - so the weight and volume of the engine has a higher importance regarding the actual vehicle efficiency.
But then again the costs of the IC engine are also a factor.



Literature:
BENSINGER, WD: Rotationskolbenmotoren. Springer: Berlin 1973

Bensinger was the head of the engine development at Mercedes Benz during their Wankel era.

RE: Diesel Wankels?

Btw, as opposed to a piston engine a Wankel engine can be perfectly balanced with one rotor only.
Having less rotors or less pistons also increases the volume to surface ratio (assuming the total displacement is kept constant).

(But I'm not claiming that this would really make a big difference.)

RE: Diesel Wankels?

How far along have ceramic coatings developed for internal combustion come along?

I can see an electronicly controlled ceramic diesel being interesting for some applications.




 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: Diesel Wankels?

As I understand it, at this stage, ceramic coatings are in wide use in very high performance engines.

They have to be applied as a very thin coating so as to avoid delamination. Being very thin limits their effectiveness as an insulator, but they still get small improvments which allows slightly higher cylinder pressures during the power stroke before piston durability is diminished and slightly reduce heat losses to the valve heads and combustion chamber.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Diesel Wankels?

Hrmm... I'd like to see a ceramic block Wankel.

RE: Diesel Wankels?

Actually, Mazda did make a ceramic rotary engine.
Well, sort of:
http://www.der-wankelmotor.de/Motoren/Motoren_Mazda/Mazda_26B/mazda_26b.html
The 26B race engine had ceramic coated titanium rotors and cermet coated side walls and peripheral surface. And the seals were made out of fiber reinforced silicon nitride. It also had 3 spark plugs per rotor, which also improved torque and fuel economy.

I guess the question is: Can anyone produce such an engine cheaply?
 

RE: Diesel Wankels?

Chase the rotor around?  Hmm  it may spin in a funny orbit instead of reciprocate but it still has to suck sqeeze bang and blow.  The surface to voloume is a drawback not overcome by its compactness of light weight as far as automotive is concerned.   

RE: Diesel Wankels?

Maybe the Wankel is not well suited for a production engine, but it still won the 24 hours of Le Mans against Porsche, Mercedes Benz and Jaguar with a relatively small budget.
Keep in mind: Having to run a valve train constantly at 10'000 rpms doesn't make a piston engine particularly efficient either. In addition: A small stroke/bore ratio worsens the surface to volume ratio of a fast spinning piston engine significantly (it's more a pancake shaped combustion chamber vs. a cylinder shaped combustion chamber).

RE: Diesel Wankels?

With one eye closed there is a lot you don't see.

Surface area is about thermal efficiency which has a big impact on fuel efficiency.

Valve train operation at high rpm is about mechanical efficiency.

High rpm is about power density. High power density is a big advantage of the rotary. Power density is more important to a race car than is fuel efficiency. I doubt a diesel Wankle has any future in auto racing unless a set of rules are written up to favour it.

 

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Diesel Wankels?

With one eye closed, there's a lot you don't see:

1. The Wankel engine I mentioned is a gasoline engine. I don't think Diesel Wankels have much potential in racing.

2. Engine fuel efficiency is always a combination of its thermal and its mechanical efficiency.

3. All fast spinning piston engines are short stroke engines and therefore drastically increase surface to volume ratio of the combustion chamber. A bad surface to volume ratio decreases thermal efficiency in a Wankel engine as well as in a piston engine.

4. Fuel economy of a race car is indeed important especially in a 24 hour race. Less weight and/or less time consuming fillings needed.

Of course, Wankel engines have no future in racing after the FIA prohibited its use after Mazda won LeMans 1991.

RE: Diesel Wankels?

Pat's statement: "unless a set of rules are written up to favour it" sums it up for me.

Unless the vehicles in a racing series have broadly similar engines, victory will always go to those whom the rules favour.

examples:

Ducati in WSB.
Diesels in Le Mans.
Turbos in F1
Two strokes in GP bike racing.
 

- Steve

RE: Diesel Wankels?

Very true and victory will especially never go to them whom the rules prohibit in the first place.

RE: Diesel Wankels?

Whatever.  

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Diesel Wankels?

Very nice discussion guys, I'm not sure how many of you are aware of the development discussed in this link

http://www.nextenergynews.com/news1/next-energy-news5.16.08b.html

I'm not very knowledgeable when it comes to the technical details since I'm not a mechanical engineer, and this being Moller I'm inclined to take it with a grain of salt, but it's still a very interesting concept

 

RE: Diesel Wankels?

I'm not sure I get the last bit, about "this being Moller..."  What's that mean?

RE: Diesel Wankels?

The Moller concept is probably the same as this Rolls Royce compound engine:
http://www.der-wankelmotor.de/Motoren/Rolls-Royce/rolls-royce.html
With the difference of using gasoline instead of Diesel as fuel.

This concept is more efficient but has the disadvantage of a lower power to weight ratio and probably costlier (as mention before).

RE: Diesel Wankels?

My comment regarding Moller is that they've been designing stuff for years and years and haven't managed to come up with anything marketable. I'm being sceptical and saying with that, that I'll believe their claims when they actually come out and run their engine in front of neutral 3rd party adjudicators which verify their claims.

I could go on and on about Moller, but this isn't the place to do it.

Don't get me wrong, I LOVE wankel engines, just like every other engineer out there, and I own an rx-8. This piece of news has been causing quite a lot of happiness in the rx-7 and rx-8 crowds who would like to see more cars made with rotary engines or at least much improved fuel economy in the next generation wankel which mazda is designing. They've already made some big steps forward in direct injection technology which has improved torque and fuel efficiency. The technology is now at the point where they could switch over to diesel... IF they could get the seals to take the compression.

RE: Diesel Wankels?

i have a question that might sound a little stupid.

Wankel engines fall under the Otto cycle correct? If thats the case, surface to volume ratio was mentioned a couple times and it affects thermal eff. apparently. I recall in Thermal Apps class that 1-(1/comp.Ratio^k-1)=EtaTh, so the higher the comp. ratio, the higher the eff. What is surface to volume ratio exactly, as it probably has something to do with the compression ratio?

 

RE: Diesel Wankels?

in case this needs to be cleared up from my last post:
k =(cP/cV)  assuming constant specific heats.
EtaTH = Thermal Efficiency

RE: Diesel Wankels?

surface/volume controls the amount of heat that goes into the head/block/piston rather than into the expansion work.  Those components have thermal limits and require cooling to stay inside the limits.  That cooling eventually goes out of the radiator and is lost.
 

- Steve

RE: Diesel Wankels?

There have been all kinds of odd Internal combusiton engine designs offered up through out the years. They are all pretty much failures as far as effciency and ease of manufacture. That is why the good old piston engine is still here.  

RE: Diesel Wankels?

anyone know the production or development status of the rand cam engine? that thing looked promising.

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