Why are Rotary's Fuel Hungry?
Full Member
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From: Newport News, VA
Aviator thanks for the explination. I knew about the heat disipation mod, but it just wasn't on my mind when I asked the question. Also the wider rotors and housings seems like a very good Idea, but didn't Mazda do this with the renisis motor? I believe the increased the width made the motor 100cc's larger than ours. I could be wrong. Maybe they need to make it a 1700cc or 1800cc motor instead to get us a better fule economy.
Rotary Freak
Joined: Mar 2003
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From: Calgary, Alberta, Canada
Originally Posted by BLK90TII
Aviator thanks for the explination. I knew about the heat disipation mod, but it just wasn't on my mind when I asked the question. Also the wider rotors and housings seems like a very good Idea, but didn't Mazda do this with the renisis motor? I believe the increased the width made the motor 100cc's larger than ours. I could be wrong. Maybe they need to make it a 1700cc or 1800cc motor instead to get us a better fule economy.
As for the benefit of wider rotors, fuel burn PER HP should be better--- at least in theory, since the displacement-to-combustion chamber surface area ratio would be reduced. But it seems unlikely that Mazda overlooked this option during research and development of the rotary. They obviously had their reasons for not choosing this route but the question remains, why?
Rotary Enthusiast
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From: Caldwell,ID
Originally Posted by Aviator 902S
If you compare the fuel consumption of a 1300 cc rotary to a 1300 cc boinger (used in cars of the same weight, drive line ratios and vintage) it would appear at first glance that the rotary's fuel economy was terrible--- only about 1/2 the distance covered for the same volume of fuel.
But what rednecks who use this comparison to justify their disdain for all things rotary don't seem to grasp is that displacement isn'tthe deciding factor here. HORSEPOWER is. The most accurate way to compare apples to apples here is to compare Specific Fuel Consumption (SFC). This refers to wieght of fuel burned per hp produced per hour at a given power setting, typically full power, but sometimes cruise power settings.
Typical boingers in the early '80s had an SFC of around 0.45 pounds of fuel consumed per hp per hour. The Mazda 12A of the same era had an SFC of around 0.5 lbs/ hp/ hr. Not as good as a boinger thanks to the elongated combustion chamber and heat issue, but not terribly worse either. In fact, the difference is neglegible. For example, the 1980 RX7 got about the same fuel economy as the 1980 Datsun (Nissan) 280ZX. True, the ZX produced a little more HP, but this was offset by its extra weight. The ZX was quicker off the line with more torque down low, but the 7 returned about the same zero-to-sixty times.
Modern boingers consume only about 0.43 lbs/ hp/ hr, while the Renesis 13B has an SFC of around 0.45 lbs/ hp/ hr, again, not as good as modern boingers, but the difference is neglegible. This means that the rotary may get 25 mpg on the highway but a comparable hp boinger may get 27. Big F&%king deal.
The problem is that the rotary's SFC at low rpms (city driving) is even more pronounced than it is at cruise compared to that of a boinger, ie: in city driving a comparable hp boinger might get 20 mpg while the rotary only gets 15.
Combined city/hwy driving produces a difference in fuel economy of only about 3 mpg, but ONLY if you compare apples to apples, ie: two engines of the same hp in cars of the same weight and drive line gearing.
I think I'll stick with my rotary.
But what rednecks who use this comparison to justify their disdain for all things rotary don't seem to grasp is that displacement isn'tthe deciding factor here. HORSEPOWER is. The most accurate way to compare apples to apples here is to compare Specific Fuel Consumption (SFC). This refers to wieght of fuel burned per hp produced per hour at a given power setting, typically full power, but sometimes cruise power settings.
Typical boingers in the early '80s had an SFC of around 0.45 pounds of fuel consumed per hp per hour. The Mazda 12A of the same era had an SFC of around 0.5 lbs/ hp/ hr. Not as good as a boinger thanks to the elongated combustion chamber and heat issue, but not terribly worse either. In fact, the difference is neglegible. For example, the 1980 RX7 got about the same fuel economy as the 1980 Datsun (Nissan) 280ZX. True, the ZX produced a little more HP, but this was offset by its extra weight. The ZX was quicker off the line with more torque down low, but the 7 returned about the same zero-to-sixty times.
Modern boingers consume only about 0.43 lbs/ hp/ hr, while the Renesis 13B has an SFC of around 0.45 lbs/ hp/ hr, again, not as good as modern boingers, but the difference is neglegible. This means that the rotary may get 25 mpg on the highway but a comparable hp boinger may get 27. Big F&%king deal.
The problem is that the rotary's SFC at low rpms (city driving) is even more pronounced than it is at cruise compared to that of a boinger, ie: in city driving a comparable hp boinger might get 20 mpg while the rotary only gets 15.
Combined city/hwy driving produces a difference in fuel economy of only about 3 mpg, but ONLY if you compare apples to apples, ie: two engines of the same hp in cars of the same weight and drive line gearing.
I think I'll stick with my rotary.
aren't the bsfc numbers rated at WOT?
that would create quite a bit of difference compared to a motor running at cruise
Rotary Freak
Joined: Mar 2003
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From: Calgary, Alberta, Canada
Originally Posted by rxspeed87
aren't the bsfc numbers rated at WOT?
that would create quite a bit of difference compared to a motor running at cruise
that would create quite a bit of difference compared to a motor running at cruise
But engines are most efficient at that rpm where the hp and torque curves cross when plotted on a chart. Above this point BSFC goes up, and rotaries burn slightly more fuel per hp in this range. Below this point on the graph (especially in stop-and-go traffic) BSFC goes down as well, but here is where rotaries don't do so well--- the BSFC compared to a piston engine is noticeably worse, on the order of 15 to 20% less efficient.
There may be other methods used that simply take an average over a variety of conditions and ranges, but I'm not familiar with them. It stands to reason that these methods would produce less favorable figures on both engine types, as well as an over all (slightly) larger difference in efficiency than other methods.
Rotary Enthusiast
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From: Caldwell,ID
Originally Posted by Aviator 902S
Different applications use different parameters. For example, in aircraft applications BSFC is often calculated at 75% power since this is typically the max cruise power setting. Others use the economy cruise setting of 65% power.
But engines are most efficient at that rpm where the hp and torque curves cross when plotted on a chart. Above this point BSFC goes up, and rotaries burn slightly more fuel per hp in this range. Below this point on the graph (especially in stop-and-go traffic) BSFC goes down as well, but here is where rotaries don't do so well--- the BSFC compared to a piston engine is noticeably worse, on the order of 15 to 20% less efficient.
There may be other methods used that simply take an average over a variety of conditions and ranges, but I'm not familiar with them. It stands to reason that these methods would produce less favorable figures on both engine types, as well as an over all (slightly) larger difference in efficiency than other methods.
But engines are most efficient at that rpm where the hp and torque curves cross when plotted on a chart. Above this point BSFC goes up, and rotaries burn slightly more fuel per hp in this range. Below this point on the graph (especially in stop-and-go traffic) BSFC goes down as well, but here is where rotaries don't do so well--- the BSFC compared to a piston engine is noticeably worse, on the order of 15 to 20% less efficient.
There may be other methods used that simply take an average over a variety of conditions and ranges, but I'm not familiar with them. It stands to reason that these methods would produce less favorable figures on both engine types, as well as an over all (slightly) larger difference in efficiency than other methods.
I'm confused on one point
you said that engines are most efficient at the rpm where torque and horsepower cross... that would be 5252rpms correct?
also though the amount of throttle used for bsfc can make a big difference as far as how reliable the number is
generally as the throttle is opened more you yield better bsfc numbers then at low throttle openings
that is another thign that confused me
you talked about gas mileage efficiency but yet also talk about 0-60 numbers I will be honest I don' see how the two mix together but then again I'm sick and not feeling all there so I donno
Rotary Freak
Joined: Mar 2003
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From: Calgary, Alberta, Canada
Originally Posted by rxspeed87
I'm confused on one point
you said that engines are most efficient at the rpm where torque and horsepower cross... that would be 5252rpms correct?
also though the amount of throttle used for bsfc can make a big difference as far as how reliable the number is
generally as the throttle is opened more you yield better bsfc numbers then at low throttle openings
that is another thign that confused me
you talked about gas mileage efficiency but yet also talk about 0-60 numbers I will be honest I don' see how the two mix together but then again I'm sick and not feeling all there so I donno
you said that engines are most efficient at the rpm where torque and horsepower cross... that would be 5252rpms correct?
also though the amount of throttle used for bsfc can make a big difference as far as how reliable the number is
generally as the throttle is opened more you yield better bsfc numbers then at low throttle openings
that is another thign that confused me
you talked about gas mileage efficiency but yet also talk about 0-60 numbers I will be honest I don' see how the two mix together but then again I'm sick and not feeling all there so I donno
This is the first I've heard about a wider throttle position improving BSFC. It doesn't seem to make sense that it would. (Can anybody verify this?)
As for the zero-to-sixty figures vs. fuel efficiency, you're right--- they don't mix at all. If you want one you give up the other. My point was that when people quote how fuel efficient their car is they always seem to quote it's best gas mileage figure--- and ignore the fact that this efficiency comes at a price, that price being that to get up to speed while returning anywhere near that mileage they need Nebraska. Even if a car gets 40mpg at constant highway speed it's not getting anywhere near that while under hard acceleration.
I guess the best way to describe it is to say that I'd rather have a car that gets only 25 mpg on the highway but will, on demand, hit 60 mph in under 9 seconds than have a car that gets 40mpg but takes 13 seconds to get up to speed when I'm coming down the on-ramp.
Rotary Enthusiast
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From: Caldwell,ID
5252 is always going to be where the horsepower and torque cross
no matter what the motor
horsepower is just more or less a math based thing and all motors are not at peak efficiency at 5252rpms.
as far as wot improving bsfc
first off your reducing pumping losses on the intake track since the motor doesn't have to fight as hard to pull the air in through the throttle body.
kinda like a dirty air filter reduces gas mileage becase it creates for more pumping losses.
also little more efficienct in how the cylinder(rotor whatever) is being fully filled or at least lcoser to fully filled then at partial throttle which again is goin to create a reducing in pumping loss. also comes dynamic compression ratio
ie you have a cylinder that displaces 100ci lets say with a 10:1 compression ratio
that means the compressed volume is 10ci
lets also say when cruising you are pulling in 20 ci of air
so our dynamic compression ratio is only 2:1
20ci/10ci
but now we open to WOT and lets say now th emotor is pulling in 80ci of air
our dynamic compression ratio now is 8:1 which would help increase pressure production as you would know creating more efficienct burn but also has further advantages a shelping with scavaging on the intake and exaust part
sorry if makes no sense I'm still not all here and at work so kinda slow still :-D
as far as performance vs gas mileage
some cars are able to pull that off though
take a look at your ls1 products that chevy was giving out
low 13's possible high 12 stock
yet still able to get 20mpg or so city and high 20's to low 30's for hwy
and with my rx7 once I was able to get 30mpg on a straight freeway cruise
filled up right before I left (at the freeway entrance) filled up again when I arrived (right after I got off the freeway) most the trip was done at around 80-85mph or so and that was with no tps either I know.
only happened once that I can recall but I know for sure it did happen
most the time I get around 22mpg avg between city and hwy. that is before my car starting being a bitch and not wanting to be nice to me
no matter what the motor
horsepower is just more or less a math based thing and all motors are not at peak efficiency at 5252rpms.
as far as wot improving bsfc
first off your reducing pumping losses on the intake track since the motor doesn't have to fight as hard to pull the air in through the throttle body.
kinda like a dirty air filter reduces gas mileage becase it creates for more pumping losses.
also little more efficienct in how the cylinder(rotor whatever) is being fully filled or at least lcoser to fully filled then at partial throttle which again is goin to create a reducing in pumping loss. also comes dynamic compression ratio
ie you have a cylinder that displaces 100ci lets say with a 10:1 compression ratio
that means the compressed volume is 10ci
lets also say when cruising you are pulling in 20 ci of air
so our dynamic compression ratio is only 2:1
20ci/10ci
but now we open to WOT and lets say now th emotor is pulling in 80ci of air
our dynamic compression ratio now is 8:1 which would help increase pressure production as you would know creating more efficienct burn but also has further advantages a shelping with scavaging on the intake and exaust part
sorry if makes no sense I'm still not all here and at work so kinda slow still :-D
as far as performance vs gas mileage
some cars are able to pull that off though
take a look at your ls1 products that chevy was giving out
low 13's possible high 12 stock
yet still able to get 20mpg or so city and high 20's to low 30's for hwy
and with my rx7 once I was able to get 30mpg on a straight freeway cruise
filled up right before I left (at the freeway entrance) filled up again when I arrived (right after I got off the freeway) most the trip was done at around 80-85mph or so and that was with no tps either I know.
only happened once that I can recall but I know for sure it did happen
most the time I get around 22mpg avg between city and hwy. that is before my car starting being a bitch and not wanting to be nice to me
Last edited by rxspeed87; Feb 3, 2005 at 11:14 PM.
84 & 85 GSL-SE
Joined: May 2004
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From: Atlanta, GA
Aviator 902S,
So what do the Power to Weight numbers look like between Rotory Engines and contemporary piston engines?
It would appear that the most salient engineering compromise is BSFC vs Power to Weight. The improved power to weight ratio provides engineering opportunities in the complete care design that are traded for fuel efficiency.
Decreased engine complexity is also a factor that is traded for fuel efficiency.
Mazada has seen these advantages and is exploiting them them to build cars with consistently outstanding balance and handling characteristics. But alas at a small cost in fuel.
When the RX-7 returns, I'm ready to pay the price!
Thanks for the outstanding discussion. I learned a lot.
So what do the Power to Weight numbers look like between Rotory Engines and contemporary piston engines?
It would appear that the most salient engineering compromise is BSFC vs Power to Weight. The improved power to weight ratio provides engineering opportunities in the complete care design that are traded for fuel efficiency.
Decreased engine complexity is also a factor that is traded for fuel efficiency.
Mazada has seen these advantages and is exploiting them them to build cars with consistently outstanding balance and handling characteristics. But alas at a small cost in fuel.
When the RX-7 returns, I'm ready to pay the price!
Thanks for the outstanding discussion. I learned a lot.
I swear in all that is holy that when I bought my car in Trussville AL and drove it back to Goldsboro NC I only used 90%-full tank (with a little to spare). -Cruised at 77-78 the whole way (2800rpms)
on mapquest it says -Total Est. Distance: 583.59 miles -
20 gallon tank right?
29 miles to the gallon
in my FD w/intake/downpipe/hollowed out cat/stock exhaust (how I bought it)
Something is weird about my car...
on mapquest it says -Total Est. Distance: 583.59 miles -
20 gallon tank right?
29 miles to the gallon
in my FD w/intake/downpipe/hollowed out cat/stock exhaust (how I bought it)
Something is weird about my car...
Rotary Freak
Joined: Mar 2003
Posts: 1,711
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From: Calgary, Alberta, Canada
Originally Posted by TEDDER1
I swear in all that is holy that when I bought my car in Trussville AL and drove it back to Goldsboro NC I only used 90%-full tank (with a little to spare). -Cruised at 77-78 the whole way (2800rpms)
on mapquest it says -Total Est. Distance: 583.59 miles -
20 gallon tank right?
29 miles to the gallon
in my FD w/intake/downpipe/hollowed out cat/stock exhaust (how I bought it)
Something is weird about my car...
on mapquest it says -Total Est. Distance: 583.59 miles -
20 gallon tank right?
29 miles to the gallon
in my FD w/intake/downpipe/hollowed out cat/stock exhaust (how I bought it)
Something is weird about my car...
Rotary Freak
Joined: Mar 2003
Posts: 1,711
Likes: 0
From: Calgary, Alberta, Canada
Originally Posted by fountaind
Aviator 902S,
So what do the Power to Weight numbers look like between Rotory Engines and contemporary piston engines?
It would appear that the most salient engineering compromise is BSFC vs Power to Weight. The improved power to weight ratio provides engineering opportunities in the complete care design that are traded for fuel efficiency.
Decreased engine complexity is also a factor that is traded for fuel efficiency.
Mazada has seen these advantages and is exploiting them them to build cars with consistently outstanding balance and handling characteristics. But alas at a small cost in fuel.
When the RX-7 returns, I'm ready to pay the price!
Thanks for the outstanding discussion. I learned a lot.
So what do the Power to Weight numbers look like between Rotory Engines and contemporary piston engines?
It would appear that the most salient engineering compromise is BSFC vs Power to Weight. The improved power to weight ratio provides engineering opportunities in the complete care design that are traded for fuel efficiency.
Decreased engine complexity is also a factor that is traded for fuel efficiency.
Mazada has seen these advantages and is exploiting them them to build cars with consistently outstanding balance and handling characteristics. But alas at a small cost in fuel.
When the RX-7 returns, I'm ready to pay the price!
Thanks for the outstanding discussion. I learned a lot.
Where the power-to-weight benefits of the rotary really become evident is in places where weight is the biggest enemy--- like in boats and aircraft. Quite often, a V8 is simply too heavy for the application no matter how powerful it is. But a rotary falls within weight limits (therefore requiring less power) and can be made to produce the necessary power quite easily.
Rotary Freak
Joined: Mar 2003
Posts: 1,711
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From: Calgary, Alberta, Canada
Originally Posted by robtackett
well I got 150 miles on 25$ worth of gas. I have to fill this thing up like ever 2 or 3 days. I guess with it being wet and cold dosn't help much.
what about the incoming air/fuel mix being diluted by old and dirty air/fuel being stuck in the combustion chamber at the trailing apex seal? Doesn't the exhaust port close, because it is positioned close to the center of the rotor, before all of the old air and fuel are expelled from the combustion chamber, and that makes it harder to burn the fresh air/fuel coming in? i remember reading something about this, but i'm not sure if it's valid or not...anyone wanna chime in?
the real port problem is exactly the opposite of that.... too much overlap. in other words, there is a time when the intake and exhaust ports are both open, so that some of the air/fuel just flows right on thru and out the exhaust. That is why the renisis gets better efficiency numbers, because they moved the exhaust ports to the side irons, so the overlap is gone.
Originally Posted by patman
the real port problem is exactly the opposite of that.... too much overlap. in other words, there is a time when the intake and exhaust ports are both open, so that some of the air/fuel just flows right on thru and out the exhaust. That is why the renisis gets better efficiency numbers, because they moved the exhaust ports to the side irons, so the overlap is gone.
Refined Valley Dude
Joined: Oct 2001
Posts: 2,283
Likes: 2
From: Kitchener, Ontario (Hamilton's armpit)
Originally Posted by NorCal90
So, this is a little off topic but still related, how would one get better milage? Would adding a S-AFC and a more powerful spark help that out? Will the S-AFC hurt emmisions? Personally, I'm getting aroung 16-17 in mixed driving which is so wonderful to me. My previous vehicle was a 90 Bronco that got maybe 10 mpg on the freeway and that's prolly wistful thinking.
Originally Posted by Valkyrie
S-AFC's only control the fuel flow when you're WOT...apparently.
So, no.
So, no.
Dunno how I missed this one.
Valkyrie - that is incorrect. If the S-AFC only controls fuel at WOT, then how in the world did mine manage to get programmed to lean out the mixture between 2500-3500rpm, and enrich it in the top end?
i am legendary
Joined: Sep 2003
Posts: 8,478
Likes: 1
From: Kirkland, WA
An S-AFC has two settings, low and high, that can be adjusted based on % of throttle. So of course you can adjust an S-AFC for when not in WOT, and just like Amur said it can be affected at any RPM you want!
Misinformation is bad
Misinformation is bad
ok, genius, correct that statement then. one of the things i really hate about this forum is the people who come on here and make smartass statements without backing them up with real information. are you so smart that we should just take your word as law, and not question it? i seriously doubt it.
pat
pat
Originally Posted by slpin
yeah.... LOTS of misinformation....
"piston engines are most efficient at 5252rpm"
Funny..... *sighs*
"piston engines are most efficient at 5252rpm"
Funny..... *sighs*
BSFC. Read about it. Has been desribed in this thread.