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fuel efficiency problems

 
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rpm5252



Joined: 11 Jul 2005
Posts: 2

PostPosted: Mon Jul 11, 2005 9:47 pm    Post subject: fuel efficiency problems Reply with quote

no ones probably out there to answer this yet but ill try anyway. I'm wondering what the deal is with engineers not being able improve the design of rotaries to burn all of the fuel. If it's the low compression ratio then wouldn't turbo chargers help that? If it's the long combustion chamber then wouldn't maybe 4 spark plugs per rotor help burn all the fuel? Or am I just way off?
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Jingyee
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Joined: 11 Jul 2005
Posts: 85
Location: Portland, OR

PostPosted: Mon Jul 11, 2005 9:51 pm    Post subject: Reply with quote

Hi there, today is monday 9:51pm, I will be in the rotary shop tommorow.

I'll see what information I can get for you tommorow.

-wesley
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Blake
Been there, done that


Joined: 12 Jul 2005
Posts: 135
Location: Portland, OR

PostPosted: Tue Jul 12, 2005 10:27 am    Post subject: Re: fuel efficiency problems Reply with quote

rpm5252 wrote:
no ones probably out there to answer this yet but ill try anyway. I'm wondering what the deal is with engineers not being able improve the design of rotaries to burn all of the fuel. If it's the low compression ratio then wouldn't turbo chargers help that? If it's the long combustion chamber then wouldn't maybe 4 spark plugs per rotor help burn all the fuel? Or am I just way off?


The ideal combustion chamber is a sphere. That's just a fact of life. Piston engines come closer to that ideal that rotary engines, which have long, skinny rectangular cumbustion areas. This means piston engines are potentially more thermodynamically efficient and, thus, potentially more fuel efficient. Rotary lovers should not dispair too much, however. We tend to be more volumetrically efficient, due to the lack of valves and tight turns in the intake tract. Ports just flow better and have more cross sectional area, thus the chambers can fill easier, realizing more of their volumetric potential. The chambers also expand and contract more gradually, owing to the 3:1 gear reduction of the rotor and stationary gears, thus giving that intake mixture of air and fuel more time to fill. Finally, don't forget that a 1.3 liter rotary engine is litterally the volumetric equivalent of a piston engine rated at 2.6 liters. This is simply due to the particular way that rotary displacement is measured; displacement equals "capacity" (the rating of an air pump's ability to move air...all engines are simply a particular type of air pump!), whereas piston engines are rated at double their capacity due to requiring two revolutions per cycle (as opposed to our one). In other words, rotary engines "move" twice as much of their displacement per revolution as a piston engine and, thus, are equivalent in volumetric terms as a piston engine of double the rated displacement. Anyway, you should not compare the fuel efficiency of a 1.3 liter rotary engine to that of a 1.3 liter piston engine; it is really equivalent to a 2.6 liter piston engine.

Overall, while rotary engines are not the most fuel efficient engines in the world, they still hold their own in that regard, and more than make up for it in performance. And don't forget that rotary engines are used almost exclusively in sports cars, so fuel efficiency is secondary and few of the cars are driven conservatively.

I hope that answers your question.
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pkghost



Joined: 11 Jul 2005
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PostPosted: Tue Jul 12, 2005 10:51 pm    Post subject: Reply with quote

go blake! *cheers*
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rpm5252



Joined: 11 Jul 2005
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PostPosted: Wed Jul 13, 2005 8:05 pm    Post subject: Reply with quote

I'll make my question more specific. I'm wondering why simply having more spark plugs wouldn't help burn all the fuel.

I mean, it's the fuel toward the outer edges of the combustion area that the explosion doesn't have time to reach right? Add a couple more spark plugs and you'll have 4 places for the combustion to start shortening the furthest distance from a spark plug thus shortening the time it takes for the explosion to spread everywhere.
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Blake
Been there, done that


Joined: 12 Jul 2005
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Location: Portland, OR

PostPosted: Thu Jul 14, 2005 11:04 am    Post subject: Reply with quote

rpm5252 wrote:
I'll make my question more specific. I'm wondering why simply having more spark plugs wouldn't help burn all the fuel.

I mean, it's the fuel toward the outer edges of the combustion area that the explosion doesn't have time to reach right? Add a couple more spark plugs and you'll have 4 places for the combustion to start shortening the furthest distance from a spark plug thus shortening the time it takes for the explosion to spread everywhere.


Your question presumes that there is (1) unburned fuel, (2) oxygen, and (3) that combustion will happen at a time that is useful. All of those assumptions are tenuous. Indeed, there are times the engine can run rich, but the nature of being rich is the lack of oxygen to permit combustion. As far as a proper air/fuel mixture, say at the trailing edge of the combustion chamber, not burning...you are really talking about burning it at a time that is useful; it does, eventually, burn late in the cycle. The problem here is that the gas flow inside the chamber exceeds the speed of the flame-front, so that the flame front created by the Leading (lower) plug cannot burn past the Minor axis of the engine. That's why they adopted a Trailing (top) plug. But the Trailing ignition adds very little power; some, but not much. Mazda also adopted the practice of firing the Leading plug a second time, called Wasted spark (as it's accomplished by firing Leading plugs of both rotors together, thus "wasting" one), but that is purely to deal with those few nasty molecules of fuel litterally butted against the trailing apex seal, which are detrimental to emissions. Mazda did adopt a "late trailing ignition" on their 4-rotor R26B in the Le Mans winning 787B race car, but at best the system added fractional gains and required quite a bit of engineering to adopt. Apparently, they decided it was not worth using in any sort of street engine, probably due to cost, complexity and impracticality. And, don't forget that in the last 30 years, a bunch of independant rotary engine builders, like Rob Golden, have been trying to exploit any possible path to more power. Ignition was not overlooked. Even if Mazda found gains from a third or even fourth plug but couldn't justify the cost for production, these guys would have spared no expense to find any gain whatsoever. Guess what, it was basically a bust. Been there, done that.

Here is an old picture of one of Rob Golden's experimental engines, if you don't believe me.


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JoshuaS



Joined: 21 Jul 2005
Posts: 1

PostPosted: Thu Jul 21, 2005 1:38 am    Post subject: Reply with quote

rpm5252 wrote:
I'll make my question more specific. I'm wondering why simply having more spark plugs wouldn't help burn all the fuel.

I mean, it's the fuel toward the outer edges of the combustion area that the explosion doesn't have time to reach right? Add a couple more spark plugs and you'll have 4 places for the combustion to start shortening the furthest distance from a spark plug thus shortening the time it takes for the explosion to spread everywhere.


The flame does get to the edges of the combustion chamber. Where the flame doesn't get is the space around the side of the rotor before the side seal. That area has so much surface area for that little volume the relatively cold housing/irons and rotor quenching the flame so the fuel in that small space doesn't burn. In piston engines they call that crevice volume. To reduce crevice volume and unburned hydrocarbon in a piston engine they just move the top ring upward. The side seal isn't so easy to move and because the combustion chamber is so long the crevice volume is greater than on a round piston.

Getting back to fuel efficiency; it's the early opening of the exhaust port that is largely responsible for the lower fuel economy. The combustion gasses don't get to expand and give up their heat energy before the exhaust port opens. This is why the exhaust gas temperature of a rotary is much higher than a piston engine and all that wasted energy is why they are so damned loud. Crevice volume is more of a concern for emissions than economy. The Renesis engine with side port exhaust delays the exhaust port opening so the power stroke lasts longer and the heat from the fuel does more work before it goes out the exhaust.
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Blake
Been there, done that


Joined: 12 Jul 2005
Posts: 135
Location: Portland, OR

PostPosted: Thu Jul 21, 2005 2:24 am    Post subject: Reply with quote

Josh,

Nice of you to join us. I can't say that I agree with what you stated about the exhaust port timing, nor that I have ever read anything supporting that idea, but I'm just happy anyone is here and joining in the conversation!

Again, just let me reitterate that, besides the non-ideal combustion area shape, the gas flow in the chamber is well documented (since the publication of "Rotary Engine" by the Mazda engineer Kenichi Yamamoto in 1969) to flow faster than the flamefront can travel, so that "upstream" A/F mixture burn does not contribute to cylinder pressure at the ideal time (when the rotor has the greatest leverage over the crank). In other words, it burns too late in the cycle to do much but make heat. Now, if the exhaust timing were retarded, all you accomplish is putting that heat into the internals, coolant and oil, rather than expel it out the exhaust. The Renesis may retard the exhaust timing, but it's doing it for the sake of emissions; not power. And, just think of it logically: when the exhaust port is opening, where is the crank journal in relation to the rotor? Almost 180 degrees away! If there is any "push" left, it's not going to do anything constructive.
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Blake Qualley
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DMRH



Joined: 28 Oct 2005
Posts: 5
Location: Sydney, Australia

PostPosted: Fri Oct 28, 2005 4:30 pm    Post subject: Re: fuel efficiency problems Reply with quote

rpm5252 wrote:
no ones probably out there to answer this yet but ill try anyway. I'm wondering what the deal is with engineers not being able improve the design of rotaries to burn all of the fuel. If it's the low compression ratio then wouldn't turbo chargers help that? If it's the long combustion chamber then wouldn't maybe 4 spark plugs per rotor help burn all the fuel? Or am I just way off?


Mazda have experimented with multi plug 2-rotor engines. This is in an effort to ignite the complete mix properly due to reasons well stated above.The 13B-MSP engine is a classic example of this & pic's can be found in the Jack Yamaguchi "Mazda RX-8" book.

The far-trailing plug is a good design for completing the burn & reducing the HC content of the expelled charge. Mazda engineers noted the far-trailing plug contributed almost nothing to power (for logical reasons) yet its ability to burn that stubborn mix of fuel sitting on the trailing edge of the compressing chamber was well documented.

Although unpractical on the street the engineering involved is well worth it in racing circles as a more complete burn allows a more precise tune of the fuel/ignition map & less reliance on closed loop operation were a 1% or 2% alteration during running can become detrimental.

REgards
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