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This, IMO, is an interesting article that I thought I'd share on Car Chat. It does a good job of explaining a rotary engine's characteristics and what eventually killed it off (Emissions).
Engineering Explained: Why The Rotary Engine Had To Die
Loads of power in a tiny, simple, lightweight package. There's a lot to love about the Wankel rotary engine, but not enough to keep it alive. Let's take a look at what went wrong
The 1964 NSU Spider was the first production car in the world to melt the rear tyres under the power of a Wankel rotary engine. The Wankel’s automotive debut was decades in the making, though its lifespan was relatively short ending with the 2011 Mazda RX-8. This leads us to several questions:
How does the rotary engine work?
What advantages does this engine have? (Why was it made?)
What disadvantages does the engine have? (Why did it die?)
1. How Does The Rotary Engine Work?
The process of the rotary engine is very similar to what occurs in a traditional piston cylinder engine. The difference is instead of pistons, there’s a triangular-shaped rotor, and instead of cylinders there’s a housing reminiscent of an oval.
Intake
As the rotor moves within the housing, a small pocket of air expands into a larger pocket, thus creating a vacuum. This vacuum is exposed to the intake ports, which air and fuel are then drawn from into the combustion chamber.
Compression
The rotor continues to rotate, compressing the air-fuel mixture against the flat side of the rotor housing. 1 MB Shout out to Ethan Smale for the epic GIF!
Power
Two spark plugs are used to ignite the air-fuel mixture, helping to speed up the combustion process and ensure the majority of the fuel burns, and this forces the rotor to continue to rotate.
Exhaust
Similar to the intake stroke, the rotor moves until exhaust ports are accessible, and the exhaust gases at a high pressure are then forced out as the rotor closes off the housing.
What’s important to realise is that unlike in a piston cylinder engine, within a single rotor housing all of these events are occurring nearly simultaneously. This means that while intake is occurring on one portion of the rotor, a power stroke is also occurring, leading to a very smooth power delivery and a large amount of power in a small package.
2. What advantages does the Wankel engine have?
Weight-to-Power
One of the biggest advantages of the rotary engine was its size. The 13B engine of the Mazda RX-7 took up about one cubic foot of volume, yet produced a significant amount of power for its small proportions.
Fewer Moving Parts
Often in engineering, the simplest solution tends to be one of the best solutions. The rotary engine drastically reduces the number of parts required for combustion to occur, with just three main components spinning in a two-rotor motor.
Smooth and High Revving
The rotary engine has no reciprocating mass, like valves or pistons in a traditional engine. This leads to an incredibly balanced engine with smooth power delivery, and the ability to rev high without concern of things like valve-float.
3. Why Did The Rotary Engine Die?
The 2011 Mazda RX-8 was the final production vehicle with a Wankel rotary, the 1.3-litre Renesis. Whether or not the RX-8 lived up to the rotary name, we all shed a tear at the loss of this innovative and unique approach to internal combustion. What dealt the final blow? The RX-8 was unable to meet Euro 5 emissions regulations, and thus it could no longer be sold in Europe after 2010. Though still legal in the states, sales had significantly dropped as the model had been around since 2004.
What disadvantages are there to the rotary design?
Just three major moving parts in a two-rotor Wankel engine
Low Thermal Efficiency
Due to the long and uniquely-shaped combustion chamber, thermal efficiency of the engine was relatively lower compared to piston-cylinder counterparts. This also often led to unburnt fuel leaving the exhaust (hence the tendency of rotary engines to
, which is obviously as awesome as it is inefficient).
Burn Baby Burn
By design, the rotary engine burns oil. There are oil squirters in the intake manifold, as well as injectors to spray oil directly into the combustion chamber. Not only does this mean the driver must regularly check oil levels to keep the rotor properly lubricated, but it also means more bad stuff comes out the tailpipe. And the environment hates bad stuff.
This hole in the housing is where oil is directly injected into during the intake "stroke" of the engine.
Rotor Sealing
Another issue which also can impact emissions: it’s challenging to seal the rotor when it’s surrounded by vastly different temperatures. Remember, intake and combustion are occurring simultaneously, but in very different locations in the housing. This means that the top of the housing is relatively cool, while the bottom of the housing is much hotter. From a sealing standpoint, this is problematic, as you’re trying to create a metal-to-metal seal with metals which are operating at significantly different temperatures. By using coolant jackets to help even out the heat load, this problem can be reduced, but never fully diminished.
Emissions
When you put it all together, emissions killed off the rotary. The combination of inefficient combustion, inherent oil burning, and a sealing challenge result in an engine that’s not competitive by today’s standards on emissions or fuel economy.
How does the RX-8 stack up against competitors?
The infamous apex seal from an RX-7 13B rotorIn my video describing the drawbacks of the RX-8, viewers rightfully pointed out that I compared model year 2015 vehicles to a 2011 model in terms of fuel economy, which was unfair on Mazda’s end. Let’s right this wrong, using the RX-8’s first model year.
CarEngine SizeWeightPowerMPG Combined Rating
2004 Mazda RX-8 1.3L Wankel 3053 lb (1385 kg) 197-238 HP (Auto/Man) 18 mpg (13L/100km) 2004 VW GTI 1.8L I4 2934 (1330 kg) 180 HP 24 mpg (9.8L/100km) 2004 Corvette 5.7L V8 3214 lb (1458 kg) 350 HP 20 mpg (11.8L/100km)As you can see above, the RX-8 doesn’t stack up favourably in terms of fuel economy. The Corvette, with a significantly larger engine, 47 per cent more power, and five per cent more weight still manages 11 per cent better fuel economy. It’s also worth mentioning that this was the first model year for the RX-8, while the Corvette and GTI engines had been around from previous years. Quite simply, there’s nothing good to say about the RX-8 in terms of fuel economy. Although the buyer may not necessarily view this as a negative point, without passing emissions there is no car to buy. Here’s to hoping Mazda’s beautiful RX-visionbecomes a reality!
Last edited by mmarshall; Apr 11, 2020 at 03:57 PM.
those warranty costs on the rx-8 were also no joke
Like it or not, Mazda pretty much had a monopoly on parts and service for the rotary engine, so, yes, parts for them often ran more than for equivalent piston engines.
Back in the early 1980's, I used to love rotaries, especially the first facelifted RX-7.
The 2nd facelift in 1984 used the 929/Luce/Cosmo's multi-spoke luxury alloy wheels - which I didn't think was as appropriate for a sports coupe.
However, these new alloys were 1" Plus at 14" and reducing the sidewall profile over the old 13" alloys.
Back around 2008, I test drove an used RX-8 with a manual gearbox for a friend, but the rotary seemed to lack bottom end punch, and was very loud and coarse - so I knew it was all over for rotaries.
Interestingly, the RX-8 had a superb handling/ride balance...
Last edited by peteharvey; Apr 13, 2020 at 02:51 PM.
Never really caught on as to what the big deal was about these. They lacked low-end power, were thirsty, and not super refined. For the time I followed it through articles and reviews (90's to the end), they were not gaining many accolades. Was there more to it than just rev-ability? And I'm not referring to the racing circuit, just civilian use/sales.
Never really caught on as to what the big deal was about these. They lacked low-end power, were thirsty, and not super refined. For the time I followed it through articles and reviews (90's to the end), they were not gaining many accolades. Was there more to it than just rev-ability? And I'm not referring to the racing circuit, just civilian use/sales.
they make very impressive power for their size, and the lack of reciprocating mass makes them very smooth
Never really caught on as to what the big deal was about these. They lacked low-end power, were thirsty, and not super refined. For the time I followed it through articles and reviews (90's to the end), they were not gaining many accolades. Was there more to it than just rev-ability? And I'm not referring to the racing circuit, just civilian use/sales.
Now you know why Mazda had a "monopoly" on rotary engines. No one else wanted to produce them.
Now you know why Mazda had a "monopoly" on rotary engines. No one else wanted to produce them.
Several other companies, including Chevy and Mercedes, did experimental and prototype rotary engines, but did not put them into production. NSU, a small German company from decades ago, was the only other company besides Mazda that actually put them into production.
Last edited by mmarshall; Apr 13, 2020 at 07:42 PM.
Why the Rotary Engine Died
Shout out to Ethan Smale for the epic GIF!
