I was wondering why do rotaries, which have small displacements, have relativley high fuel consumption. They are small light engines which produce 100hp (12a) which I would have thought wouldn't result in this. Can anyone tell me the reason for this?
Thanks

 

It's hard to get a complete fuel burn when the combustion chamber is moving past the plugs and is in the shape of a rectangle. That's also why rotaries (At least until the Renesis came along...) are horrific polluters...

 

i also think the fact that there is ports has something to do with it. i could be wrong tho

 

It's somewhat to do with unburned gasoline, rotaries are high in HCs. It's also due to the fact that a 12a, although rated at 1146cc, actually has three power cycles per engine rotation, boingers have a complete power cycle every two rotations. So there is a good deal more fuel running through a rotary per crank rotation than through a boinger.

 

The capacity of a rotary cannot be compared with a piston engine due to the way they operate. In some countries the capacity of rotaries is doubled for tax purposes.

The best comparison of fuel consumption is power output. On this basis the rotary engine performs quite well.

Pollution is a problem due to the shape of the effective combustion chamber, this is why the emmission controls are so complex so as to ensure it meets legal standards.

In reality the rotary engine badly tuned or worn has high fuel consumption and pollution compared with a piston engine primarily because of the design features of an apex seal versus a piston ring.

 

The shape and location of the combustion chamber are both to blame for the rotary's high unburned hydrocarbon and NOx production.

The general shape of the chamber is long and skinny with sharp outer corners, allowing for more cool zones, discouraging flame propigation compared to the relatively small, round combustion chamber of a cylinder engine. Since the rotary combustion chamber is also moving, the gasses are constantly introduced to new, cooler surfaces, further discouraging flame propigation.

NOx is formed in cracks and small spaces where the oxidation reaction does not occur as quickly and in the presence of less oxygen.

 

I think most of the wasted fuel is due to the seals, the large overlap, the moving chamber with large surface area to volume ratio, and at least in our cars, the carb. Stock these things did 24 mpg city if I remember correctly, which isn't too bad. Now add 120k mi, and cold air temps and suddenly your in the mid teens. So some still get good mileage, others suffer.

 

Overlap, I always forget about the overlap. Blowing fuel straight through the engine is never a good thing. I've never heard of what the scavaging ratio hovers around for a rotary.

 

Actually, from what I've read on my emission tests, NOx production on our engine is rather low. I know from looking at a good number of pollution vs lambda (A/F ratio) charts, and it stands to reason, that NOx is made when there is too much oxygen present (above stoic). Lots of heat combined with not enough fuel means the Oxygen is looking for a partner, and Nitrogen is abundant, so viola! NOx.

Other than that, I think your correct, it is a slower reaction.

 

You're right, I mis-spoke, NOx is formed in an excess of oxygen late in the combustion cycle.

I actually have a masters degree in this stuff. Unfortunately I don't get to use it much.

 

Purple82- excellent response. Can you answer another related question I asked on a different thread, why are the exhaust gases on a rotary relatively hot so causing rapid detoration of the system?

I spent 7 years at uni, but rarely use any knowledge taught as technology changes so quickly.

 

How have they fixed the fuel consumption with the renisis, is it just by reducing overlap or have they made other changes (it has a higher copression ratio doesn't it, does this have something to do with it as well)?

 

Paul-Three reasons that I can think of,
1) combustion gas is cooled by convection of the hot gas to the metal parts around it. This is effected by the amount of metal the gas is swept past and how torturous the path is, the more time the gas spends next to metal, the more heat it will transfer. The spent gasses in a piston engine certainly have a more complex path to get out the the cylinder than a rotary.

2) Power cycles, the rotary has six complete power cycles to the one of a boinger. There is simply more energy spent per amount of time in a rotary.

3) The whole reason for combustion is to expand gasses. The more a combusted mixture expands, the cooler it gets and the more energy from the combustion process goes into making the output shaft turn. The compression ratio of a rotary just isn't all that good. There is alot of enery left in the combusted gas that is wasted out the tailpipe that could have otherwise been used to push on the rotor.

Mazda worked on a Miller cycle rotary where some of the inlet charge allowed into the engine was released back into the intake port before combustion. In a Miller cycle, the charge let in is reduced to more efficiently use the energy of combustion to turn the output shaft. The exhaust gasses are much cooler in a Miller cycle engine.

browny-the renesis has the exhaust port on the side housing instead of in the rotor housing. This improves emmisions in two ways.

1) Imagine unburned fuel being pushed around the rotary engine by the tip seal. As soon as the fuel sees an opening in the rotor housing it will be dumped out, as it is in the exhaust stroke of any rotary up to now. If the opening is in the side housing, this fuel doesn't get the chance to leave and just goes around for another chance to be burned.

2) As you said, overlap. The side exhaust port eliminates overlap. This eliminates any fresh mixture exiting through the engine and it also greatly improves completion of the exhaust stroke.