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?
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?
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?
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?
RE: Diesel Wankels?
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RE: Diesel Wankels?
- Steve
RE: Diesel Wankels?
No reason was given and the research efforts were then concentrated on conventional piston engined applications.
Pete.
RE: Diesel Wankels?
RE: Diesel Wankels?
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?
This may well have sent some makers to look for alternatives.
Pete.
RE: Diesel Wankels?
The two-stage engine gave me a few ideas....
RE: Diesel Wankels?
http:/
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?
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?
RE: Diesel Wankels?
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RE: Diesel Wankels?
RE: Diesel Wankels?
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?
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?
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.
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RE: Diesel Wankels?
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?
Cheers
Greg Locock
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RE: Diesel Wankels?
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?
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?
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?
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.
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RE: Diesel Wankels?
RE: Diesel Wankels?
Well, sort of:
http:/
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?
RE: Diesel Wankels?
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?
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.
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RE: Diesel Wankels?
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?
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?
RE: Diesel Wankels?
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RE: Diesel Wankels?
http:
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?
RE: Diesel Wankels?
http:/
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?
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?
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?
k =(cP/cV) assuming constant specific heats.
EtaTH = Thermal Efficiency
RE: Diesel Wankels?
- Steve
RE: Diesel Wankels?
RE: Diesel Wankels?