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Background

There is a lot of opinion out there regarding the efficiency delta between wankel rotary engines and four stroke engines of various configurations (forced induction, etc).

This delta, whatever it is, seems to be a debate of how much. How much waste, how much efficiency, how much power.

What are the actual deficiencies of a rotary engine?

What is the delta of efficiency between a rotary engine and modern four-stroke engine?

What is the root cause regarding the configuration of a rotary engine that makes it so wasteful?

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NOTE: Wankel engines are still Otto cycle engines (re: intake, compression, ignition, exhaust), they just perform the strokes differently than a valve controlled reciprocating engine.

  • Lower Compression Ratio - It is known that a higher compression ratio provides more power without any other modification. This difference in compression ratio is said to be about 3% increase in output per 1 point of compression. This text book (page 14 of 21 [or page 274 of the text]) shows the compression ratio to be 6.98:1. That is typically giving up between 3-4 points of compression from what a stock piston engine can provide, with very little recourse to increase that number. (NOTE: While these figures show the lower compression ratio, Mazda has shown numbers to be as high as 10:1.)
  • Shape of Combustion Chamber - In the following image, you can see the basic Otto cycle in the Wankel engine. Due to the inherent shape, when the air/fuel is ignited (between #3 & #4 cycles), the flame front is chasing the leading edge of the rotor around the combustion chamber, but never completely catches up with it. This allows some of the air fuel mixture to escape out of the exhaust port as that port is uncovered. This is why you can sometimes see flames coming from the exhaust of a rotary engined vehicle.

    enter image description here

  • Rotor Sealing - On the rotor itself, there are three main seals which keeps everything in check: apex seals, face seals, & side seals. (Below is an image showing their locations.) Keeping all of the gasses in their respective locations is a chore for the seals and the seals will usually lose some of that integrity. One of the reasons for this is because the seals have to keep hot and cold unmixed at the same time.

    enter image description here

  • Engine Oil Injection - In order to help in keeping a good seal, engine oil is injected directly during the intake cycle and in the intake tract. This provides proper sealing and decreased wear, but introduces inefficiencies in the burn of the air/fuel mixture, as well as creating more pollutants in the exhaust.

  • Power Output - The below chart is a comparison between three different vehicles with their weight/power/displacement. Looking at the numbers, it becomes clearly evident comparing the vehicle weight, hp output, and mileage numbers, the rotary engine falls short. Just like the other engines, you can turbocharge the rotary engine (the MB GLA45 comes turbo'd from the factory). This will increase output, but will not increase efficiency proportionately due to how the exhaust flow occurs and not being able to control the intake/exhaust events like you can with piston engine. You will be blowing un-burned hydrocarbons out of the exhaust ports, which is inefficient. I'm not saying either of the piston engined vehicles don't blow hydrocarbons out the exhaust, I'm saying for the amount of air fuel you dump into the engine, the rotary engine dumps far more as a ratio than do the piston engines.
    Vehicle        Size    Power        Torque         Weight    City   HWY
    2011 RX-8      1.3L    232hp@8500   159lb-ft@5500  3064lbs   16mpg  22mgp   
    2015 GLA45     2.0L    355hp@6000   332lb-ft@2250  3457lbs   23mpg  29mpg
    2105 Stingray  6.2L    455hp@6000   460lb-ft@4600  3300lbs   17mpg  29mpg 
  • Regarding the 6.98:1 figure, this must be for an older design. The Wankel used by the RX-8 is quoted at 10:1 – Zaid Jan 18 '16 at 16:31
  • @Zaid - Show me the figures. I'm going off what I've found in print. – Pᴀᴜʟsᴛᴇʀ2 Jan 18 '16 at 16:35
  • Updated the comment with the link – Zaid Jan 18 '16 at 16:36
  • @Zaid - Let me update my answer ... – Pᴀᴜʟsᴛᴇʀ2 Jan 18 '16 at 16:43
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    @Anarach - It isn't the HP which is in question, it's the efficiency. Getting 232hp from the 1.3L and only getting 16/23 out of it, while you can own a Corvette with a 6.2L and get better mileage? There is an efficiency issue with the rotary engine, no way around it. – Pᴀᴜʟsᴛᴇʀ2 Jan 19 '16 at 14:30
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I couldnt help but notice,the much bigger rotary "piston" surface/per amount of fuel, was being somewhat overlooked in these other answers considerations. It might be a compensating factor in efficiency to some degree. Rotary engines are supposed to use hydrogen and probably hydrocarbon gases such as methane, more effectively than a piston configuration because the greater surface area can extract more motive energy from the fuel. This appears to have been a consideration for the Mazda RENISIS engines that use supplementary hydrogen,and Mazda claims that the Wankel rotary has better thermal characteristics than a piston engine allowing leaner combustion. http://www.mazda.com/en/innovation/technology/env/hre/ In theory,switching between exhaust valves spaced differently around the engines radius could alleviate some of the problems described because the exhaust part of the Otto cycle could be changed along with combustion timing.

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