Thursday, 28 June 2012

Mazda Builds Its Last Rotary Engine... For Now?

Mazda rotary engine cutaway
While 99% of cars in recent years (leccy cars aside) have used reciprocating piston engines of some sort, the efficient-yet-inefficient rotary engine, devised in the 1920s by Felix Wankel, has powered NSUs, motorbikes, and, all the way up to yesterday, Mazda sports cars. The idea of converting pressure into rotating motion rather than reciprocating motion (i.e. pistons going up and down in a cylinder) creates a significantly higher power output per litre of displacement, so while a 1300cc piston engine in your little hatchback might make around 70bhp, the 1300cc "13B RENESIS" rotary engine in the RX-8 makes 230bhp. Because there's no huge force bouncing up and down repeatedly, it's also a much smoother engine than a piston unit, one that can rev higher and weighs a lot less, owing to its diminutive size. So what are the drawbacks? They are threefold. Not only do they not produce very much torque, but they suffer from two inherent flaws that the modern motoring world can no longer tolerate: poor fuel economy and emissions. As such, the RX-8 was killed off the the UK when the new Euro V emissions rules came in revently (the same rules that axed the 5-cylinder Ford Focuses and the FN2 Honda Civic Type-R), and has now reached its end globally due to a lack of sales. Thus, it seems somewhat doubtful that we will see them again as regular engines in regular cars. After 45 years of proudly standing by their clever and unusual engines, the Mazda rotary is dead...... right?

I could go on and give you a short history lesson on how Felix Wankel started developing the engine with NSU in the 1950s, and how when they built a complete engine without him knowing he said to them (in German) "You have turned my race horse into a plough mare." I could then go on to say that as NSU came and went, giving us the advanced-but-chronically-unreliable Ro80 in the process, motorbike companies like Norton and Suzuki used the light, smooth-running rotary engine, and that when they were all done with it, only Mazda was left, proudly championing their RX models and putting a three-rotor unit in the Eunos Cosmo (think Lexus SC or Merc E-Class Coupé), eventually bringing the Wankel engine its crowning achievement in 1991 with the tremendously loud and fast 787B, winner of the 24 Hours of Le Mans that year, before finding favour with tuners in the turbocharged RX-7 and ending up in the odd-but-cool-but-slightly-underpowered RX-8, its final resting place, but... oh, I think I just did. Well now you know. At any rate, the benefits of this engine couldn't be ignored over the years, but then by the end, neither could the disadvantages of fuel economy and emissions. It has also become to Mazda what 2-strokes and V4s were to SAAB - their way of being proudly different - except that Mazda's engine was actually successful for quite a long time.

Smint, Spirograph, spinning. Simple.
But how does it work? Well, it follows the same basic principles of a four-stroke piston engine, but does it differently, and using much fewer parts. As the gif on the left shows, fuel goes in at the top, gets compressed by the big rotor that looks like a Smint, is then ignited by the spark plugs and the resulting explosion forces itself out of the exhaust exit, propelling the rotor as it does so. The constant spinning of the rotor as opposed to the jerky up-down reciprocating of pistons means that rotary engines are inherently smoother and can rev to 8500-9000rpm with relative ease, which is good, as they have to be revved high to get to peak power. Compared to direct-injection turbo piston engines, they feel pretty gutless if you don't, as there's little in the way of torque. Of course, turbocharging a Wankel goes some way to sorting this out. Either way, you'd think the compactness and lightness would be good in an equally compact and light MX-5, but Mazda has never made such a car before. Previously their excuse would probably have been that it would encroach on the RX-7/8, but that car is now dead, so now there's no excuse! Oh wait, yeah, emissions...

So it won't be going in an MX-5 then (not officially, anyway, but it's a somewhat common engine swap among MX-5 tuners), and the RX-line has officially reached its end this week. So is that just it? Well not entirely, as there are other applications for Wankel engines, like in snowmobiles, where they appreciate the fact that a failing rotary engine is much less sudden than a failing piston engine, or in aeroplanes. A more unusual use for it is as a seatbelt pre-tensioner in VWs and Mercedes-Benzes, as this patent shows. Rather than running the whole time, it's driven by and explosive charge when the seat belts need pre-tensioning (i.e. in a crash).

But surely Mazda aren't giving up on their USP? In 2007, the strange-looking Taiki Concept featured a ~1600cc engine dubbed "16X", a proposed replacement to the long-serving 13B that featured a lengthened stroke and reduced-width rotor housing - making it taller and thinner - along with direct injection and aluminium side housings instead of cast iron ones. The results of these Various Things being added were a supposed jump in fuel economy (by 20%) and more torque across the rev range. What's more, the one in the Taiki Concept is said to be capable of up to 350bhp. Crikey. Unfortunately, budgetary constraints have meant that not long afterwards, 16X was put on the back-burn, and since then Mazda has become preoccupied with its new Skyactiv technology, which in a nutshell is all about efficiency through lightweight engineering and engine witchcraft. They have said that the money they gain from this technology - which has already been rolled into the surprisingly not-lame CX-5 crossover and will henceforth appear across the range - will be partially rolled into 16X along with, supposedly, a smaller rotary engine for use as a range-extender. For an explanation of an Extended-Range Electric Vehicle (E-REV), click here and scroll about halfway into the fourth paragraph, but the point of using a rotary engine in such a vehicle is that the aforementioned benefits of simplicity, compactness and lightness are teamed with the fact that not only is the lack of torque negated by the engine never powering the wheels, but Wankels are very efficient when running at a constant rpm, which a range extender does (if you've driven a GM Volt/Ampera or a Fisker Karma, that's what that monotonous humming is), so in many ways it's the ideal power plant for such a vehicle. Assuming E-REVs catch on then, this would be an ideal avenue for Mazda to pursue, as they're the rotary experts nowadays and they could benefit hugely from a rotary E-REV. Of course, it's all if's and when's at the moment, though.

Another avenue that Mazda's already looked at is the idea of a Hydrogen Rotary Engine (HRE), first seen a year before 16X. When you think "hydrogen car", you might think Fuel Cell car, like the million-dollar Honda FCX Clarity, or if you're somewhat ignorant, you'll thing "Hindenburg/H-bomb on wheels", which is just silly. Would Honda put an H-bomb on the road? No. It's perfectly safe. Anyway, the fact is that hydrogen-burning engines have been tried and tested before, because of course hydrogen can be stored as a liquid just like petrol and diesel can, and combusts just as easily, in fact far more easily. The key difference of course is that burning hydrogen with air only produces water vapour. Emissions problem solved, instantly. Mazda actually leased out a small number of RX-8 H-REs in February 2006, two years before the Honda FCX Clarity came about. But what if you weren't near a hydrogen filling station? It's perfectly likely. Well, never fear, because the H-RE was actually a "duel fuel" engine, which could run on good old fashioned dead dinosaurs as well. In fact in many ways, the hydrogenability (which is now a word) of the engine was just sort of plonked on top of the normal RX-8 powertrain. A second fuel tank for H2 was added in the boot, and hydrogen was directly injected into the top of the engine using a system later adapted for 16X, meaning you could switch between regular RX-8 and hydro-powered RX-8 as and when you needed. Useful.

Yet again, this is an application where the Wankel's many advantages come into play. You see, the ease with which hydrogen combusts means that less energy is needed to burn it, so in a piston engine, where fuel is injected through the top of the cylinder right near the spark plug, the Hydrogen being injected could combust on contact with the spark plug even when it's not sparking, due to the plug's general temperature. However, in a rotary engine, the fuel is injected in the top and then combusted further round the cycle, as you can see to your left, so there's no risk of "abnormal combustion". Running a juicier fuel than petrol also means more power, as the E85-powered Koenigsegg CCX-R and Agera R prove (on that note, the mentalist Furai Concept ran an E85-powered Wankel and sounded like the bees of hell had been kicked in the groin). So if Mazda make plenty of money and a hydrogen infrastructure appears - a key drawback with H2 cars at the moment - they could end up with a hydrogen-rotary-E-REV in their range! That would be cool. What would they call it, though? RX-H-RE-E-REV? That would be silly. How about the RotER-H* (pronounced "roter eych")? Something like that.

*I know it's spelt Rotor, but the ER is short for Extended Range.

But of course, this is all potential. If potential things always happened, Toyota would've won Le Mans, SAAB would be alive and well and making brilliant innovative cars, I would've won the Euro Millions all for myself one week and, well, so many other wonderful things would've happened. For now, we can only hope that the weirdly wonderful Wankel rotary engine will one day return, revving away with pride under the bonnet of an RX-9 of some sort. Oh, and the RotER, of course. I think I might have to copyright that name so they have to buy it off me for three million pounds (or yen, whichever is more valuable by then...). I refuse to believe that after 45 years of thick and thin, Mazda will just give up on what is in many ways its crowning achievement. Cheer on the Skyactiv diesel LMP2 car at Le Mans next year, and maybe even test drive a new Mazda road car and see if that lightweight engineering and "Zoom-Zoom" spirit touches you enough to grab your heart and wallet in equal measure... for that shall revive the rotary!


  1. Perhaps a different form of revival for the rotary engine?

    Though my heart will always be with the Wankel!

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