DamonB

The very first response is correct. As usual someone takes the time to explain exactly why and people stick their head back into the sand and come up with excuses.

The rotary has poorer combustion characteristics compared to a piston in a cylinder. End of story. Period. Finito.

Not only does the rotary have a horrible combustion chamber shape but an enormous amount of material exposed to the flame pulls heat out of the burn. This makes the burn less efficient (that's where all the heat in your coolant comes from and we all know rotaries run hot. That's why).

No matter what you argue about displacement, horsepower, emissions etc you are always left with the same fundamental problem!

rs 1101 and turbojeff already explained why. Anyone who disagrees with them doesn't know what they are talking about.

Valkyrie

Would it be more efficient if the metal between the coolant ducts and the area of the rotor housing where the power stroke takes place was thicker?

That way I assume it would remove less heat from the burn... although the removed cooling efficiency might just cause even more problems.

rs_1101

i actually pull down 25mpg on the freeway, but it drops to about 15 when i floor it.

Aviator 902S

First off, there is no argument that rotaries burn more fuel than boingers of the same output, and for all of the reasons mentioned by rs1101 and turbojeff: elongated combustion chamber, large surface area per volume, and intake/ exhaust overlap. (although the Renesis engine has no overlap. Yet it still has only slightly better SFC figures than previous 13B engines).

But again, the question is the DEGREE of fuel efficiency difference between the two engine types. On a per hp basis (the basis that counts most), Specific Fuel Consumption is not that far off, at least not enough to be a major factor in driving costs. The .45 lbs/ hp/ hr (13B) vs. .43 lbs/ hp/ hr (boinger) aren't figures that I pulled out of my ***--- they were arrived at by no less authority than the likes of NASA and SAE (Society of Automotive Engineers)--- people with far more knowledge on this subject than you, or even most of us on this forum for that matter.

Jeff's minivan vs. RX8 example further illustrates this point. 240hp (minivan) vs. 238hp (RX8). 4300 lbs and large frontal area (Equivalent Flat Plate Area in aeronautical engineering terms) for the van vs. 2900 lbs and small EFPA for the RX8. 22- 23mpg hwy/ 18mpg city (minivan) vs. 25 mpg hwy (rs1101) / 17mpg for "spirited driving" in the RX8.

Those are pretty damning figures for a 2900lb car against a 4300lb van. Except for one factor not mentioned: Acceleration performance. Actually, Jeff did give the figures for the minivan-- 0 - 60 in 7.7 seconds, 1/4-mile in 15.9 seconds. Pretty damned impressive, especially for a 4300 lb van with the aerodynamics of a bank vault. But the RX8 does 0 -60 in a shade over 6 seconds, with a 1/4-mile figure around 15 seconds.

These numbers are considerably better than those of the minivan, but they'd damn well better be, considering that the RX8 weighs 1300 lbs less and is far slipperier than the van. So these factors largely cancel each other out, with the van putting up efficiency figures that are slightly better than those of the rotary-powered RX8.

This means that the rotary's fuel consumption is a little worse than that of a boinger on a level playing field comparing apples to apples. A LITTLE worse, not a LOT worse. Which has been my point all along.

Besides, the rotary's biggest drawback isn't higher SFC. It's weaker torque at low rpm vs. that of boingers. This can only be overcome by boost (shortens engine life, just like it does in boingers) or by adding a 3rd rotor.

Oh yeah, Jeff: about the guy claiming 28- 31 mpg hwy in his 160hp 2nd gen: Those figures of his are definitely optimistic--- unless he's talking IMPERIAL gallons (Canada and Europe, larger than U.S. gallons), and a constant cruise speed of about 60 mph. Even then, 28mpg would be about the best we could expect. If he's talking U.S. gallons, he's doing something that borders on PFM (Pure ******* Magic). Or PBS...

hornbm

You know this is what I dont get, I get the same gas milage no matter how hard I drive my car. As a matter a fact my best tank was 17MPG doing over 100MPH for 50 miles and shooting fireballs at this second gen rx7 the whole way!

Aviator 902S

Them's good figures for 100 mph (around 5000 rpm). My question is, do you ever drive a steady 60 to 70 mph on the highway for any length of time? If you do, and the trip lasts long enough to burn up a tank of gas, you should be getting somewhere on the order of 25 mpg. Unless of course you've done some serious mods.

In 1994 I made a Vancouver, BC to Woodstock, NB (cross-Canada) trip at an average speed of over 60 mph. But this took into account slowing down for towns and cities as well as the odd 90- 100mph sprint on desolate long stretches in the middle of nowhere. When I tallied up the hours, miles covered and fuel burned my final mpg worked out to 25.2 miles per IMPERIAL gallon. This was with a stock 1982 12A.

DamonB

I stand by my all previous statements.

You're already confused yourself:

and then you jump to:
You can't have it both ways. How much fuel you burn vs how much power you make IS fuel efficiency and you already said there is no question that rotaries burn more fuel than piston motors of the same output. That means the rotary is less fuel efficient!!

Too much smoke and mirrors. I've come to expect no less.

DamonB

Your comparison has severe flaws. Namely that fuel mileage figures do not come from vehicles cutting the fastest 0-60 times they can but yet you use them in a comparison anyway. Your example means nothing and is completely bogus and misleading without the actual fuel efficiency numbers from each vehicle while completing the 0-60 run.

I can guarantee you that at maximum power (such as your 0-60 data) the rotary is guzzling far greater amounts of fuel than the van. The rotary has less fuel efficiency than a boinger just tooling around town and when you decide to also compare their consumption at high power outputs you'll find the rotary drops behind even further.

Your 0-60 comparison neither supports nor condemns your fuel efficiency argument; it just flat out doesn't apply without the fuel consumption levels during the 0-60 run.

Aviator 902S

HOOO boy, one of us desperately needs to get laid in the worst way, and it ain't me. (You should see my drop-dead gorgeous oriental wife. The sex btw, is frequent and great! )

You really need to lighten up before you give yourself a hernia, hemroids and a heart attack.

But back to the subject at hand:

Please re-read my posts so that you understand the point I'm trying to get across and you'll then understand that there are no contradictions in my points, and that I am in no way confused. I don't know how much clearer I can make it, but just for you, I'll try:

First, I've said it before and I'll say it again: Rotaries ARE NOT as fuel efficient as boingers. Period. Happy now are we? And yes, under hard acceleration the SFC goes straight into the crapper--- Just like it does when you punch a minivan up to 60mph in 7.7 seconds. That vehicle won't return 22 to 23 mpg (or even 18 mpg for that matter) under those conditions either. Simple, isn't it?

But again, the rotary engine at a given hp setting burns not a hell of a lot more fuel than boingers do. However (as I've stated in earlier posts) their SFC during low rpms that fluctuate in city driving, especially when shifting at upwards of 6000 rpm, does go up by a greater margin than boingers in the same scenario.

Unless you drag race everywhere you go the difference in fuel costs is minor. I've owned four of these cars over the last 15+ years, am involved in aircraft maintenance and aviation use of the Mazda rotary, and have worked on these cars since before you entered junior high. But you don't have to take my word for any of this if you don't want to. Just take your argument to NASA. I'm sure they won't lie to you.

This is not to say that I think you're an idiot. In fact I think you know more about this subject than some people on this forum. But your comments are not only immature and venemous, they also are unprovoked. At least mine and others are retaliatory.

But I respect your right to your opinions, and while I find them biased and hostile, I've come to expect no less from you. Ciao...

DamonB

Are you by chance basing much of your specific fuel consumption info on aviation applications where the engine runs at constant power levels (and given the fast that hardly any auto people know what SFC is)? Because then of course you understand that running any engine at a constant level of power returns much better fuel efficiency than constantly changing the power setting of the engine in asking it to accelerate the vehicle and thus comparison of any two engines at constant power vs load settings would make the differences always appear smaller.

I'm glad you enjoy screwing your wife repeatedly. I'd hate to think there are married men out there who don't enjoy it.

turbojeff

iTrader: (30)

Brake Specific Fuel Consumption or BSFC is what we should be all talking about.

Test the Honda 3.5 V6 w/240hp or the S2000's 240hp 2.0 4cyl against the the RX8's 238hp Reneisis on a engine dyno and you'll see that the RX8 motor just can't win the BSFC under any condition of throttle, 10%-100%.

I used the Honda motor as an example/comparison since the HP rating was very similar to the RX8 motor. I used my van because I see the mileage as equivalent or very similar to the RX8 but my van is carrying around an extra 1400lbs and the frontal area is much larger, both hurt the mileage of the van. If I could chop the top down about a foot and take out the 1400lbs of the van I'm pretty sure that it would both beat the RX8 to 60 mph while returning better mileage. This is where the torque curve comes into play....

live2drive15

iTrader: (4)

[QUOTE=turbojeff]Brake Specific Fuel Consumption or BSFC is what we should be all talking about.QUOTE]

What we should be doing is having fun with our rotaries and stop arguing about why they don't get good gas mileage cuz it's not going to change now or probly ever. So get out and drive and for god's sake have some fun.

Aviator 902S

If I wasn't any good at it my wife would see to it that those encounters were few and far between. Thank you for burying the hatchet, and thanks for not burying it where I part my hair.

But to answer your question, yes, I realize SFC is not a commonly-known term in automotive circles. But once explained, it's not hard to grasp and it is an effective way of communicating the info. I'm also aware that constant-rpm settings used in aircraft while in cruise do not compare to the up-and-down rpm ranges experienced in cars 90% of the time, ie: while travelling 15 minutes to and from work (very poor fuel economy here, since for half that time the engine is cold and getting only half the distance per gallon as when warm), stop-and-go city driving, etc.

The only scenario where auto engines come close to the SFC figures experienced in aircraft use are at constant power settings on the highway, and only as long as said highway is relatively flat like in Nebraska, Kansas, etc. Even then, typical power settings on the hwy are only at about 35% for piston engines and (due to lower final drive ratios used by Mazda to offset the rotary's lack of low-end torque) 40+% for rotaries.

In aircraft applications these same engines would be cruising at 65 to 75% power. SFC here is very close for both engines, with the piston engine still slightly more efficient than the rotary. In fact, there is no power setting or condition where the rotary returns better (or even as good) SFC figures as the boinger. It simply can't. And the further away from the constant high power setting scenario we get, the wider the efficiency gap becomes.

This is what I meant several posts ago when I stated that in city driving an equivalent hp piston engine could return upwards of 18mpg while the rotary could only return 15. On a percentage basis, this represents a larger efficiency gap than comparing hwy figures of say, 27mpg vs. 25mpg.

So yes, the piston engine is more fuel efficient. But my idea of a gross difference in fuel efficiency seems to differ from yours and others. If the rotary could only return 10 or 12 mpg in town vs. 18 to 20 in town for an equivalent-power piston engine, that would be what I consider a terrible lack of efficiency. As it stands though, a 5% to 20% less fuel efficient (than boingers) rotary is a fact of life that I and others find very easy to live with.

But there are still those who don't know what you and I know about these things, and they continue to compare the fuel economy of their friend's 1300cc shitbox (which gets 40mpg) to the 1300cc Renesis' measily 17 to 25 mpg--- with no regard for hp produced. This perpetuates the myth that rotaries are gas-guzzling monsters that should be banned.

7Langit

The new Renises rotary gets better mileage and lower emissions because engineers & metallurgists solved the side seal & carbon buildup problems. Now the engine doesn't put out so much unburned fuel because the SIDE intake ported inlet porting is maximized for efficiency & emissions in a way the the PERIPHERAL intake ported 13b's and its predecessors could not do.

We just have to live with an inefficient cumbustor but its small size and light weight give it chassis placement advantages no piston engine can match.
And its power per ENGINE weight is still better than the "boingers". It's just a "power dense" little unit with not anywhere near the years or hours of engineering development the boingers have enjoyed. Thank Toyo Kogyo (Mazda) for all the development they did to get it where it is after the big car makers gave up on it in the 60's.

Aviator 902S

Not sure about the side seal/ carbon buildup problem (first I've heard of it being a major issue, let alone solved in the Renesis and somehow improving fuel economy). The buildup in the side seals on prior rotaries didn't occur until many miles accumulated on these engines (unless the owner never changed the oil but rather merely added it when necessary), and those running pre-mix had little if any of this buildup.

What Mazda did do to this engine that helped SFC (as well as emissions) was to go with side exhaust ports, eliminating overlap between intake and exhaust. This not only got rid of the problem of raw unburned fuel exiting the exhaust port but also sent some of it back around for another trip through the combustion chamber.

And yes, the power to weight ratio of the rotary may be better than that of boingers, and this is a real benefit in aircraft or racing boat use. But the overall difference in the average car can easily be offset by cutting weight in other areas of the overall vehicle design.

dragula53

not to rain on the parade...

But there is not much difference in weight between a rotary and a piston engine, either. The 1.8 liter toyota engine used in the lotus elise actually weighs less than the renesis.. by about 22 pounds.

It is not remarkably more fuel efficient, considering the power and weight of the entire car though. 22-27 miles per gallon, from 190 horsepower and 1970 lbs of car. An extra 40some horsepower and over 1,000 more lbs..

Rotaries are not fuel efficient, but they aren't as terrible as some make them out to be. My 227 horsepower wrx gets 20-22 miles per gallon, (26 on highway/long trips).

In the grand scheme of things, that extra 4 miles per gallon is not going to make a big difference in total cost of ownership. When you are paying $400+ a month in car payment, $150+ in insurance, an extra $30 a month in gas is insignificant.

I may have rambled off on a tangent somewhere, but I get tired of hearing how light rotaries are, and complaints about their fuel efficiency.

Thermodynamics say that a rotary can not be as efficient as a piston engine.

AREITU

You guys are flaming like honda forums.

Aviator 902S

Nothing to really disagree with here, you've pretty much nailed it. And there are exceptions to the weight rule. For example, formula 1 engines are low-displacement boingers that put out huge amounts of power.

But I think the point being made was that typical normally-aspirated auto engines producing the kind of hp figures that the renesis does weigh a bit more, since most of those are V6 and V8 engines displacing upward of 2.6 liters.

Amur_

How about an 87 NA?

https://www.rx7club.com/showthread.php?t=212848


Consistent 31 mpg right here.

Aviator 902S

Still sounds pretty optimistic, but then again imperial gallons that you buy in Ontario are about 15 to 20% larger by volume than U.S. gallons.

Amur_

Imperial gallons? We're on the metric system (just like you - we're both in Canada, yes?)


9L/100km, give or take a few tenths. The more I leave the cruise control alone, the further she goes.

Cool_ManX

14 mpg in the city stock engine ... see my signature

Aviator 902S

Yeah, I know. But when converting to mpg, the gallons in question on our side of the border are imperial rather than U.S. gallons. 31 miles per imperial gallon = about 25 miles per U.S. gallon--- a figure that I've only been able to attain if driving a constant 100km/hr over flat terrain, which is almost never.

Most of us still measure our fuel economy in mpg rather than liters consumed per 100km. BTW, an easier method to use in this country would have been km per liter. But remember, the feds had a say in the matter so this is what they stuck us with...

Aviator 902S

That sounds about right for a T-II. Especially during rush hour.

Amur_

So what do you get when you convert 9L/100km plug into US gallons? I quickkly plugged it in here to get the 31:

http://www.hicloneqld.com/data.htm



And my numbers come from the same trip, over and over, across the Niagara Escarpment. A little up and a little down, over and over again.