labrat

In considering the questions raised in the following, I would like it to be understood that I truly and deeply love my car without reservation (genuflects before the car, and makes the Sign of the Rotary by touching the tips of index fingers and thumbs of opposing hands).

Why is the RX-8 so fuel inefficient? To put it in the words of the great Prof. Sumner-Miller, "why is it so?". Can some one with some knowledge of automotive thermodynamics explain it to me? We have an engine where the primary motive force is rotary, rather than up and down pistons requiring a crank shaft to translate the mechanical energy. Should be more efficient, right? Forget about all the performance car hoo ha for now, let's look at the case of just pootering around town doing the day to day speed limit stuff. My previous ride was a boring as bat **** old Celica, 110kW dripping wet, weighing about the same as the RX-8. I always recorded under 8L/100km around town. On the same route to and from work, I'm lucky to get say 11.8L/100km, and by the comments from others I do really well. There's no spirited driving, no jerk and go, just mild anticipatory driving with lots of gear work and little braking.

Petrol is just liquid chemical energy. We burn it, and we get heat. Some of this heat is converted into mechanical energy. Why is this conversion efficiency so low in the case of a rotary motor?

Revolver

I'm no scientific genius labrat but could it have something to do with the fact that even in normal driving the renesis is usually revving higher than most piston engines?

As for me, I know why I go through fuel - I rarely have the tacho below 4000-4500rpm unless I know there's no chance of any fun. :D

Tim Tam

I was thinking the same thing yesterday (as my fuel light came on again). It's only 1.3litres, even if you rev the ring off it it shouldn't be using that much.

bellyboyau

I'm pretty sure there are some (long!) posts by RotaryGod in the main forum that explain the working of the motor in great detail, including the whole fuel consumption thing. Explains the 1.3L concept very well. I'm thinking you'll find some good answers there..

Cheers :D

timbo

Oh Labrat!! I was hoping you'd explain!

Actually, is the Renesis less 'efficient'? Look at the numbers: 1.3L; 177Kw; combined cycle fuel economy 12.1L/100k (which my records confirm).

Alfa 147 GTA 3.2L v6 -- 180Kw -- 12.1L/100k (Wheels data)
Audi A3 3.2 v6 -- 184Kw -- 10L/100k (") :o
BMW 330 L6 -- 170Kw -- 10.5L/100k (") :p
Holden Monaro -- 5.7L v8 -- 160Kw -- 12+?
Mitsubishi Verada 3.5L v6 -- 155Kw -- 12.1L/100k (")
Nissan 350z 3.5L v6 -- 206Kw -- 11.1L (")
Subaru Liberty 3.0L f6 -- 180Kw -- 12.4L/100k

OK, above cars are sort of a list of vehicles in the same segment/power range/considered by some buyers (ok, just kidding with the Verada). The 8 is pretty comparable, IMHO.

Note, I have excluded FI motors, such as the Subarus, EVO 8, Audis, VWs, Saabs and Volvos. Why? Well, FI clearly improve the thermodynamic efficiency of any engine, and FI on the rotary would do likewise, so a further, different range of comparisons would be necessary.

Sorry, ducked the issue by redefining its context

My 1.3L's worth

BVD

Let's try and clear up a few misconceptions in the above posts:

Firstly:

You're kidding Labrat! You can't just forget about the performance aspects of an engine just by driving slowly. Would you expect a race tuned Lotus to do the same mpg as a 4cyl shopping car if you drove them both at the same speed for the same distance? No. Doesn't work for a rotary either. The engine has been designed and optimised to GO - not to be a fuel miser.

Secondly:

No.

There is a 1 to 3 ratio between the rotor and the output shaft on our engine. The shaft might be revving at 9,000rpm, but the rotors are actually "only" doing 3,000rpm. The "high revving" thing is a bit of a furphy.


Calling it a 1.3 litre is misleading. It's comparing apples with oranges. It's usually referred to as being the equivalent of a 2.6 litre. But 3.9 is also correct! :D Each rotor chamber has the equivalent of 650cc (and there are 2 chambers, so 650 x 2 = 1.3 litres) but each chamber has 3 rotor faces in it - so you could say 3.9 litres. Comparing rotaries to piston engines is like comparing turbine jet engines to rocket motors, or whatever. i.e. you need different ways to rate them.

See RotaryGod's explanation here, or search here for when we discussed it before.

https://www.rx8club.com/showpost.php...24&postcount=7


Apparently the rotary is somewhat less thermally efficient due to the size of the areas involved where useful heat is lost. But the main reason that our car isn't especially fuel efficient is still because it's designed to go quickly, not to squeeze the most out of a litre of fuel. As Timbo points out, buy any fast car and expect to buy more fuel...

auzoom

Now let me state that its been a while since I thought about this but!!!

I thought 1.3L was one combustion cycle, not 1 engine revolution. For each Engine Revolution there are 2 combustion cycles. This is the reason why the 13B or 1.3L engine is compared with a 2.6L Piston engine.

Andrew

timbo

Good on you, BVD -- I knew someone had written an extremely long post on this...forgot it was rotarygod

Revolver

Thanks for the clarification BVD!

BVD

Hope it helped.

For some reason we tend to get hung up on "cubic capacity" as a useful figure, but it has limited value when comparing different types of engine.

All it measures is the "swept volume" of an engine. In other words the volume covered by the pistons as they travel up and down the bore, (in a piston engine).

It doesn't take into account the size of the actual combustion chamber, the compression ratio, how efficient the engine is at filling the chambers with fuel mixture, or any number of other important factors.

Importantly, it doesn't even say what the pistons are doing at the time. It just says "swept volume of one cylinder x number of cylinders = cubic capacity".

Consider a 4 stroke engine. As the piston comes down the cylinder it can be either on a power stroke or an intake stroke. It needs two revs to complete a full series of cycles for all cylinders. With a two stroke engine however you get the full "suck, squeeze, bang, blow" on EVERY revolution.

So cubic capacity is of limited value even comparing two strokes to four strokes. They use different principles, they use that area in different ways, and they are not equally efficient at filling and/or exhausting the combustion area. It's even more confusing if you try and use "cubic capacity" as a meaningful way of comparing piston engines to rotaries.

auzoom

Did I miss something? I just read that whole thread an nowhere does it state if there are 2 or 3 combustion cycles (4 or 6 faces) per engine revolution (eccentric shaft revolution). 2 means 2.6L 3 means 3.9L.

Andrew

labrat

OK, one thing I did glean out of the foregoing posts is that the rotary is not as thermally efficient as a "normal" motor, and that goes to the heart of fuel economy. The conversion of chemical energy>heat energy>mechanical energy is pretty miserable anyway in an internal combustion motor, so reducing thermal efficiency will further impact adversely on potential fuel economy. Are fuel economy and performance totally mutually exclusive? Is the trend towards common rail diesel engines an attempt to address this?

Another question: would it be possible to have dual-range fuel maps, one for say suburban driving, and another for highway/fun/track?

I guess at the back of all my musings is the feeling that manufacturers will be doing their utmost to close the gap between fuel economy and performance, and the rotary might not be a suitable candidate for future development as it might be fundamentally incapable of competing in this particular race. I hope I'm wrong, but I think that the rotary could be seen in the future as an interesting evolutionary branch of automotive technology which became extinct due to being unable to compete in a changing environment.

auzoom

I lied https://www.rx8club.com/showpost.php...7&postcount=81 so my earlier post was incorrect. I give up :-(

As for efficiency and future development, Mazda have been researching the Rotary Engine with Hydrogen fuel for a number of years now http://autoweb.drive.com.au/cms/news...doc=maz0310282

Andrew

labrat

Concerning hydrogen vehicles, during a recent trip to Switzerland, I had the opportunity to catch up with an old friend, a scientist who has just retired from the ETH in Zurich ("Einstein's University"). He has worked most of his career on catalysts associated with hydrogen use in vehicles, and he's pretty much up on the latest technology. He was telling me that the Germans are now seriously suggesting that to get over the low fuel density of hydrogen, you put an 800 bar hydrogen tank in your car! I think sitting on a just sub-critical mass of plutonium could be safer!

Forget about Mazda and a hydrogen 8. It's a publicity furphy to show that their heart's in the right place. According to Yoda, happen it will not.

Hymee

iTrader: (1)

I you look at how much air (or displacement) is drawn in to the engine per revolution, compared to a 4-stroke piston engine, then a 1.3L Rotary engine is equivalent to a 2.6.

Surely the amount of air being drawn in, and the amount of horsepower produced are the main "characteristics" to gauge this thing by?

Also, I have it on good authority that the rotary engine produces 10 to 14% less power for the amount of air it draws in. (Or that might be it uses 10-14% more air for the power it produces). Hence, this might help explain the so-called inneficiency.

But you gotta weight that up in the overall context of thing. It is a very small motor, and that provides a whole bunch of advantages. We just gotta except the associated disadvantage.

Cheers,
Hymee.

BTW - My wife has a 4L Prado. All the VVTI and DOHC in the word, all the O2 "efficiencies" touted by marketing, at the end of the day it gets about 1100km out of 160 litres of fuel. It is after all, a 4.0L motor dragging around 2.5 tonnes of vehicle.

Hymee

iTrader: (1)

The car essentially already does that. While you are "cruising" along, the PCM is in closed loop mode, doing it's very best to maintain the ideal fuelling to provide the optimum combustion. I guess the only way to cut consumption further when in cruise would be to lose some of the displacement. "Displacement-On-Demand" is a big thing with the larger capacity engine makers now.

I guess unless big Bernie makes a rule change to F1 stating that reciprocating engines are illegal, and rotary engines are OK, then we will never get there. The piston engine has the benefits of how many years of research and development gleaned from F1? Unless the rotary engine gets the same amount of development thrown at it, who knows if will ever be able to be compared in the same light.

Cheers,
Hymee.

BVD

Fuel economy and performance are different goals that require different design solutions and not just different fuel maps (although those are of course important).

Consider a man who wins the 100 metres gold medal. Everything about him is designed for speed. Over 100 metres his "energy economy" is of minor importance. yet put his "perfect" body in a marathon and not only won't he win but he probably won't be able to even complete the course. Wrong design for the task.

So our motors have been primarily been designed to go fast and accelerate well. This involves all manner of design decisons about the shapes of ports, pipes, chambers, etc. as well as many other mechanical and thermal issues.

They could be redesigned for greater economy if that was seen as a viable market.

As Hymee points out, if you gave the rotary the same amount of research and development as the piston engine has had over the past century you could make it faster, more economical, or more efficient in whatever way you aimed at.

I imagine that both rotaries and piston engines are probably headed for the design scrapheap - but unfortunately my crystal ball isn't telling what will replace them. :D

Revolver

This has been very interesting. Thanks guys for sharing your knowledge.

The comparison stats posted by Timbo were revealing. Although I get where labrat is coming from on a theoretical level, the stats indicate that while the 8 is at the thirsty end of the scale it is by no means off the charts. As a practical matter, when choosing which sports car you want to buy (where economy is rarely a prime motivator) the 8 is by no means uncompetitive in its price bracket.

I also found it strange how few of you have mentioned things like kerb weight, aerodynamic drag and tyre choice - all of which have large impacts on fuel efficiency in real world driving. Or are you just looking at the engine alone in this discussion?

labrat

I think Revolver has a point in mentioning the value for money. When you think about the cost of "performance" diesels around of European manufacture, the price difference is such that over a reasonable lifetime of the car, you'll never make up the savings that the RX-8 has to offer.

I thought Hymee's observation about relative air usage was instructive. That would explain some of the thermal inefficiency. Shame that all that heat lost is not recoverable in a practical sense.

Still, it would be nice if you could have perforance and economy...

timbo

Well, I could be persuaded to look more closely at one of the new generation diesels for a future car. The BMW 330dci coupe has a reputation, as do the Golf GTdi's. Torque is a beautiful thing (ducks for cover)

Gomez

Now I haven't seen Star Wars since 1977, but I have absorbed enough of it through osmosis over the years to appreciate this quote as being somewhat humorous . Now throw out a smilie occasionally, will ya labrat?? :D :D

Love,
Gomez.

TeamRX8

iTrader: (25)

My Z06 Vette got 30 mpg @ 80mph,

I'm lucky to get 19 mpg @ 75 mph in the RX-8, I get 16 mpg towing it so it doesn't make sense to drive it when you consider the cost of wear and tear ...

zoom44

at 75 mph on a long cruise where you use the whole tank and you have had the cruise control on the major majority o fthe tiem you should get 23.5 mpg at least and if its a fairly flat or down hill stretch 24.5 or better mpg. if you cant get over 19 mpg on an entierly highway driven 6th gear cruise for a whole tank then you should turn your car in and pay them to do an mpg test on your car. you could have one of several issues that can cause improper fueling. such as a bad tstat which keeps the car in open loop all the time a bad o2 sensor or a bad/dirty mafs etc etc

on the displacement on demand thing mentioned by hymee. we had a thread going about it soem time ago where several of us worked out how it could be done well on the renesis. the question was then put to the president of Mazda by Rotarynews.com and he stated it would never happen with the Renesis and that they were looking in other directions

Nubo

One of the original theoretical "strengths" of the rotary engine was the elimination of "all those moving parts". The idea was that simplicity would breed efficiency and durability. But, in the real world, improvements in manufacturing, materials and design have largely negated this supposed advantage. Modern piston engines whirl along just fine with their hundreds of moving parts.

As far as efficiency, the moving combustion mass and odd-shaped volume mean it will probably never be as fuel efficient as comparable piston engine. Now, during the Renesis development cycle the engineers were predicting mileage somewhere in the low 30's, IIRC. Not too shabby. So, assuming they weren't just blatantly lying, the engine probably has that potential, but other things holding it back whether to meet EPA requirements, longevity, driveability, etc. And definitely true that R&D for rotaries is a drop in the ocean compared to the worldwide efforts in developing piston engines.

That doesn't leave the rotary without a strong suit, however. Power per cubic inch and smoothness of power delivery remain strengths. And the RX-8 was built to take advantage of those.

If, somehow, the engineers could actually realize those initial mileage predictions this car and rotaries in general would really open some eyes.

guy321

Yoda wasn't in the original Star wars. His first appearance was in 198o in the Empire Strikes Back.