Rotary Titus

I was reading an article on the 8 saying the reason why it's so smooth is that with every turn of the driveshaft or something, it goes through 3 combustion cycles? So does that mean with every rpm, there's actually 3 X 1.3L (3.9L) worth of combustion going on? In a V6 engine like the G35, does every combustion (3.5L) turn the driveshaft or is it 2? Would this be why even though it's a small 1.3L engine, it eats gas much like a 4L engine? hmm.. just throwing out a though....

ByeByeSaturn

Sort of...
what's really happening is that a combustion event lasts through 90 degrees of the rotor's rotation/revolution. Since the eccentric shaft turns 3 times for every rotation/revolution of the rotor, each combustion event lasts for 270 degrees of the output shaft's rotation. In a piston engine, each combustion event lasts through 180 degrees of the crankshaft's rotation.

I had a part here about displacement vs. capacity, but I deleted it b/c I had the definitions wrong - see the link below to rotarygod's post.

A rotary is also smoother because there is no linear motion being translated into rotational motion, there is less friction, and there is no loss of energy like in a piston engine when a driveshaft has to push a piston back up for its compression stroke and ignition.

The worse gas mileage has to do with what used to be lower compression ratios (the Renesis is 10:1, so that's pretty much solved), unburnt gases leaving the exhuast port (that's partially changed with the Renesis as well), and the thermodynamic inefficiency of igniting an oblong volume.

sferrett

Look for rotarygod's post in this thread:
https://www.rx8club.com/showthread.p...threadid=15316

It's quite informative.

Rx8Past

I was reading an article in maxim about an organization called ELF that recently torched several Hummer Dealerships, and painted "I Love Pollution" on some of the cars costing several million in damages. You think if they found out how bad the mpg is on a small "so-called" 1.3L rotary engine, they might starting torching mazda dealerships? I hope not, and to prevent this from happening, maybe mazda should find a way to really fix the mpg issue.

ByeByeSaturn

OK, so I got it mostly right , except for the displacement/capacity definitions

Xyntax

Wow, if they start targeting individual cars and we're on their list, NRA will be my friend. It's all about headcount from there.

MP3Guy

I wonder what it is that makes people compare the RX-8's mileage with that of the Civic they traded in??? If you're getting 17 to 24 and you can do a sub 15 second 1/4 mile, there is nothing much to complain about. This whole mileage thing is getting overblown

Katchoo

ELF - Earth Liberation Front. An interesting group...they are the ones that torched that ski resort place a couple of years ago.

They also 'spike' trees in areas about to be harvested by the lumber companies...couple of people have been badly hurt by those actions.

Good ideas about saving the earth but bad methods of getting thier points across.

Len

Not to be confused with ELF the oil company, which is probably in favour of low fuel efficiency.

rjenk

May be overblown to you but to those experiencing the problem is it not. To be honest, I would love to have 17 and 24 instead of 14 and 19.

And just for the record, I am comparing my results to what the RX-8 is stated to get on average, not against my old car or any other.

MEGAREDS

As a non-physicist, all I really understand is that it's easier to keep somthing spinning than pumping up and down. Much easier... that basic truth seems at odds with our gas mileage. I suppose I must agree that its not reasonable to compare a 100 HP Civic to a 200 HP RX-8 (as I tend to do since my other car is a 100 HP Civic), but a large part of the coolness of the rotary engine is its potential... is there potential here to greatly improve the mileage or not? The car won the International Motor of the Year Award on a claim that it could achieve 30+ miles per gallon. What happened?

I've asked in several posts why the rotary isn't getting great mileage, and no one has really suggested a reason that makes any sense, other than the "thermodynamic inefficienty of igniting an oblong volume... ." Wow. With the weekend coming up, I plan on trying to re-read Rotarygod's post again...

rabinabo

It's pretty easy to understand the "inefficiency of igniting an oblong volume." The combustion of one "gas" atom sparks the combustion of the atoms around it, so combustions spreads radially in every direction. In a long chamber the flame would spread out as fast as in the sphere of the same diameter as the chamber (assuming the spark was in the center). Ok, maybe it is a little complicated

flatso

perhaps we get poorer gas mileage because of the larger friction area of the rotary. Do the rotors have a larger surface area then the pistons which it has to be overcome at each revolution?

ByeByeSaturn

Hmmm, maybe. I think it's more that the efficiency of an engine is determined by its ability to turn heat into work, or more specifically, to do work through the transfer of heat energy. This means transferring as much heat as possible into the motion of the rotor as it swings through a colder volume of air. An oblong volume has a lot of surface area to radiate the heat through, thereby losing efficiency. (maybe this is partly the reason why rotaries run pretty hot?)

ByeByeSaturn

Actually, I just read the whole thread linked above and the discussion between rotarygod and liveforphysics goes into more detail about this.

Just one correction to what rotarygod had to say - he said a hotter molecule is bigger. Not true. Temperature is simply a way of stating the root mean squared velocity of a group of particles in a volume. These velocities will follow a smooth distribution, which in normal, non quantum states (i.e. Bose-Einstein, etc.) will be a gaussian distribution. Higher temperatures mean a stretching of this distribution towards higher velocities, and therefore more faster moving particles which tend to want to occupy a larger volume relative to surrounding cooler air. Thus they are "hogging space" and reducing the amount of volume available for the cool dense gas/air mixture which provides the power upon ignition. The individual molecules are all the same size -these sizes are governned by electro-repulsive/attractive and quantum forces that are beyond the scope.

Vrimmick

yep byebyesaturn that's what I thought too

ProtoConVert

bump

just curious, but does electron density around a nucleus depend on either kinetic energy or temperature? if so, presumably some of them might get bigger, correct?

NickG

Fair point but putting this in context the RX-8's fuel economy is way, way worse than say a BMW E46 M3, which is considerably more powerful, faster and several hundred kg heavier. In real world conditions its economy is comparable with some 5 litre+ V8's we have here that weigh in excess of 1700kg (3750 lbs). It's clearly the car's one real achilles heal. Fortunately, however, there are so many other compensations for committed drivers....

Despite recent fuel price rises, Americans still enjoy amongst the cheapest fuel in the world. An Rx-8 in the US probably costs less to run than a 1300cc (reciprocating!) mini car in the UK and the average 2000cc tin box here in Australia....Enjoy it while you can!