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?

 

[SomptingGuy]

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

 

[kevindurette]

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.

 

[ivymike]

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.

 

[patprimmer]

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

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

 

[SomptingGuy]

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

 

[Peter7307]

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.

 

[btrueblood]

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.

 

[romke]

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.

 

[Peter7307]

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.

 

[kevindurette]

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....

 

[globi5]

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.

 

[Flamefront]

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?

 

[ivymike]

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

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[Lou Scannon]

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.

 

[kevindurette]

"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.

 

[Flamefront]

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....

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[globi5]

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.