Diesel Wankels? 1

[Flamefront]

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?

 

[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[globi5]

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.

 

[globi5]

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.)

 

[kenre]

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.

http://www.retallickeng.com.au

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

 

[patprimmer]

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
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[kevindurette]

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

 

[globi5]

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?

 

[turbocohen]

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.

 

[globi5]

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).

 

[patprimmer]

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

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[globi5]

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.

 

[SomptingGuy]

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

 

[globi5]

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

 

[patprimmer]

Whatever.

Regards

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

 

[dalcazar]

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

 

[ivymike]

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

 

[globi5]

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).

 

[dalcazar]

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.

 

[Silverbullet86]

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?

 

[Silverbullet86]

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