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A guide to rotary engines

Joining the rotary club Published: 20th Jun 2011 - 0 Comments - Be the first, contribute now!

A guide to rotary engines Felix Wankel was the man who made the engine work but it got a poor reputation
A guide to rotary engines Exposed cut-away shows the sophistication of a rotary yet they’re also fairly simple. DIY is possible, too
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A guide to rotary engines Mazda RX-7 of the late ‘70s and ‘80s proved the unit could be fast and reliable, albeit hardly frugal
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A guide to rotary engines
A guide to rotary engines
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A guide to rotary engines
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Super smooth but so suspect – that’s still the view of the rotary engine! it’s time for the truth…

Everyone knows that Felix Wankel invented the rotary engine – except he didn’t. He was born in 1902, yet by 1910 there were over 2000 patents already filed for various rotary engine designs. But none of them were workable; what Mr Wankel did was take the idea and make it work more reliable than ever before – even though his initial ideas bankrupted his employer, NSU! Wankel’s rotary engine was incredibly smooth, but it was also very thirsty and fiendishly unreliable, thanks to rotor tip seals that wore rapidly, leading to a loss of compression. The NSU Ro80 was given first place in the 1967 Car of the Year award, but with engines needing to be replaced at around the same time as a piston engine is just getting run in, it was inevitable that NSU would go belly up. VW bought NSU, but despite the fate of the latter, as the 1960s became the ‘70s, car makers went rotary crazy, licensing the technology in case the piston engine should be consigned to history. Mercedes- Benz, GM, Ford and even Rolls-Royce all bought licences, but aside from Mazda, it was Citroen which went the furthest, putting its Birotor into production. Based on the GS, just 874 were built before Citroen offered to buy them back, to reduce future liabilities. Only one car manufacturer stuck to its guns and continued to develop the Wankel engine, and that’s Mazda. More of this company’s first-generation RX-7s were built than MGBs, so the technology can work. But before the RX-7 there were plenty of other rotary Mazdas, including a whole series of RX models, plus the delightful Cosmo 110. Mazda even built the world’s only rotary-engined bus between 1974 and 1976, while the company has enjoyed a fair degree of success in motorsport over the years. The company took first place in the Singapore Grand Prix 40 years ago with a Familia Rotary Coupe and throughout the Seventies, Eighties and Nineties Mazda achieved a raft of circuit racing first places. However, in motorsport the rotary engine is probably best known for an outright win at Le Mans in 1991, when the 787B came across the line first. Which just goes to show that the rotary engine can be made to work!

How it works

The rotary engine works in a way that’s fundamentally differentfrom conventional internal combustion engines, even though it does the same job ofburning petrol to produce power. Instead of a piston there’s a rotor which spins in the centre of a housing, which is shaped like an oval, pinched slightly in the middle. The rotor has a set of internal gear teeth cut into the centre of one side. These teeth mate with a gear which is fixed to the housing. This gear mating determines the path and direction the rotortakes through the housing. An eccentric shaft is turned by the rotors in a way similar to a handle turning a winch. With every 360-degree turn of the rotor, an output shaft turns three times. The rotor itself is triangular, and its three points are in constant contact with the housing wall through an apex seal – the new term for what used to be called the rotor tip seal. The shape of the housing ensures that the rotor’s centre point forms a closed circle with every complete turn. The three flanks of the rotor combined with the inner surface of the housing form three working chambers, whose volume constantly changes during a single turn of the rotor. This architecture makes a normal crankshaft and valves unnecessary. The only moving parts are the rotor and the eccentric shaft. These characteristics mean that a rotary engine is lighter and more compact than a traditional reciprocating engine. And smoother. While a normal four-cycle piston engine needs four cycles to facilitate two turns of the crankshaft, rotary engines achieve all four cycles with only one turn of the rotor. The rotor itself produces the power of the rotary engine and applies it to the eccentric shaft, which fulfils a function comparable to the crankshaft of a traditional piston engine. It’s because of the small number of moving parts that rotary engines with more than a single rotor are incredibly smooth. With a twin-rotor engine the lobes of the eccentric shaft are placed 180 degrees in relation to each other, which ensures an almost perfect balance of mass. A twin-rotor engine operates more smoothly than a sixcylinder piston engine with hardly any vibrations at all. The rotary engine is also characterised by uncomplicated architecture, with a minimum of components. The extremely compact dimensions of a rotary engine allows designers to place the engine further back, lowering the bonnet line for better aerodynamics and weight distribution.

Felix Wankel

Major development of the rotary engine began with the German inventor Felix Wankel. A man without an engineering degree, or even a driving licence, Wankel wrote automobile history and has taken his place alongside other automotive engineers such as Nicolaus Otto, Karl Benz, Gottlieb Daimler, Wilhelm Maybach and Rudolf Diesel. Wankel was fascinated his entire life with machines, even though he did not possess a technical education. He was never an abstract thinker, but an inventor who had a different view of, and kept a certain distance to, mathematics. No one formulated Wankel’s importance to the automotive industry better than Kenichi Yamamoto, Director of Research and Development, and later Chairman of the Mazda Motor Corporation. “The automotive world lost one of its greatest thinkers,“ Yamamoto said upon hearing of the death of Felix Wankel on October 9, 1988. On that day, Mazda Motor Corporation announced that it would continue to develop engines without valves and connecting rods according to the Wankel principle. Mazda kept its word and, since first starting to work on Wankel’s engine in 1961, has built around two million million rotary engines, most of them for its RX-7. Today Mazda is the only major carmaker to continue the legacy of Felix Wankel’s rotary engine concept with the latest Renesis unit, under the bonnet of its Mazda RX-8.

Buying a classic

Apart from Mazda, nobody has offered a rotary-engined car since the early 1970s, which rather limits your choices somewhat. The three key factors to bear in mind if you fancy a Wankel is that these cars are generally very rare, most of them have dissolved by now, and early engines proved rather fragile. When it comes to tracking down a suitable car, you’ll need to cast your net far and wide, which means looking in Europe as well as Japan. Talking to the guys at Rotechniks might be of some help of course, but there are only so many cars about – and frankly there just aren’t enough to go round, which could force the prices up. The next issue – of corroded bodywork – is the same as with any other classic. That means you’ll have to be handy with a MIG and as none of the older rotaries are supported in terms of panel availability, your fabrication skills will have to be good. The final consideration relates to that rotary powerplant, which needs to be mollycoddled to some degree, to get the best out of it. The two main issues with the Wankel is a loss of compression, caused by the rotor tip seals wearing, plus overheating. Early rotaries are notoriously weak, but Mazda made this type of powerplant reliable. With proper servicing, Mazda’s rotaries will sail past 100,000 miles; it has been known for 400,000 miles to be racked up. However, to get that sort of mileage out of it, the engine’s oil would have been renewed every 3000 miles. Most of these cars are now used very rarely, so they’re more likely to have an annual oil swap. Owners tend to be fastidious with them, as RX-7s that are used every day are few and far between. There are various signs that a rotary powerplant will soon need some TLC, the two key ones being burning oil and a loss of compression. However, as well as having an appetite for petrol, the rotary also uses a fair bit of oil, even when it’s healthy. This is because oil is injected into the combustion chamber to lubricate the rotor tip seals, to stop them from wearing prematurely. The rate at which oil is consumed will vary depending on how the car is driven; if thrashed it can use up to a pint every 1500- 2000 miles. However, conservative driving should see proportionally less oil use. While the rule of thumb was not to use fully synthetic when these cars were new, the technology has moved on enormously and modern fully synthetic lubricants can be fine – but be very careful. One of the few that’s definitely okay is Royal Purple, which is produced in the United States. Some rotary specialists sell it, but it’s not that easy to track down in the UK. While using Royal Purple does help the engine to burn more cleanly, increase its lifespan and reduce emissions, using a decent quality mineral oil such as GTX is recommended, as long as it’s changed frequently. Once an engine is ready for a rebuild, it may start to foul its plugs – although this can happen just by starting the car from cold. The powerplant refusing to start when hot is more indicative of a loss of compression, and it’s a sign of big bills looming. That’s why it’s important to get the engine up to temperature then switch off, before trying to restart it ten minutes later. Don’t fret dearly about smoke from the exhaust while the engine is cold. Because oil has to be injected into the combustion chambers to prevent rotor tip, side andcorner seal wear, it’s perfectly normal to have a smokey exhaust for up to half a minute after starting. What you do need to worry about is a powerplant that overheats, because this is the quickest way of killing a rotary engine. A rotary unit consists of a series of metal castings all bolted together, some of which are alloy and some of which are cast iron. These expand and contract at different rates, potentially leading to warping and even contact between the rotor seals and the housings. Once this happens, your best bet is to find a decent used engine. That’s why it’s essential you ensure that the thermostatic fan cuts in and also that the engine fires up and runs happily. The key thing is that the rotary engine is far simpler than a piston engine, so DIY maintenance is easy – as are rebuilds. Things are made even easier by the acquisition of one of Rotechniks’ DVDs, which explains fully how to overhaul a Wankel powerplant – you can buy one for £54 and it’s money well spent. You don’t need special tools to rebuild a Wankel engine, but if you don’t have the time or inclination, Rotechniks will do the work for you. Get your own unit fettled and you could pay as little as £1000, but if you opt for an exchange unit off the shelf, it’ll be closer to £2500, although these units do come with a two-year 24,000-mile warranty. Remember also that many older cars like R080s used alien engines such as Ford V4s! Thankfully now Mazda units are used. So why not join the Rotary Club?

How best to join the rotary club

Mazda RX-7

It’s the biggest-selling rotary car ever, offered across three generations between 1978 and 2002. With more than 800,000 RX-7 built, in theory there are lots about – but sadly that isn’t the case thanks to rampant rot wiping out most of the first editions. While those first cars were merely sports cars, by the time the third take on the RX-7 formula arrived in 1992, the model had become a genuine supercar, with its twin-turbo rotary under the bonnet. Now the first edition of the RX-7 has moved into classic territory, there are a few things to watch out for besides the generic info already given: The biggest problem the Mk1 RX-7 has to contend with is corrosion, so check all the metalwork very closely, especially the rear wheelarches, checking for some crafty filler work. Also check the sunroof surround, footwells, front inner wings and bulkhead, along with the front suspension turrets and metal around the windscreen wiper spindles. Ask to see the service history; the engine should have had fresh oil every 3000 miles or annually. Expect any rotary to use oil, but clouds of blue smoke from the exhaust suggest all is not well. If an RX-7’s powerplant has had it, you could slot in an RX-8 engine, as they’re essentially the same unit. UK cars were never officially available with leather trim. Velour was the order of the day, and after at least 20 years there’s a good chance it’ll be showing its age by now.

Mazda RX-8

Newer than most cars in these pages, the RX-8 is most definitely a modern classic, thanks to its fabulous looks, brilliant chassis and consequent great dynamics. Even better, unlike most other rotaries, you don’t have to worry about finding a good car because over 25,000 have been sold in the UK so far. However, despite the RX-8 being so modern, there are still a few weak spots towatch out for:


  • On early 2003 cars, the lower suspension arm ball joint can fail, so make sure the recall has been actioned.

  • Idling for too long can lead to the exhaust ports melting, allowing molten metal to wreck the catalytic converter.

  • Make sure there’s a heat shield between the exhaust pipe and plastic fuel tank; it sometimesgets left off after a routine service.

  • Make sure the clutch isn’t slipping, as premature failures aren’t unknown and a new clutch is very costly at around £1000.

  • Check that the pipe from the oil cooler is intact, as it corrodes, leading to the engine losing its oil – then a seizure.

  • Ensure the xenon headlights are working properly, as water gets in then wrecks everything. Replacements are very expensive.

  • The anti-roll bar links are prone to failure; this isn’t dangerous but it will leadto the car leaning in corners more heavily.

  • It might be worth having an emissions test done before purchase, as the catalytic converters are prone to failure; a new one is over £1000.
  • Others

    The best-known of the Sixties rotaries must surely be the NSU Ro80, a car which bankrupted its maker even though it was voted Car of the Year in 1967. With its windcheating design, clutchless manual gearbox and brilliantly spacious cabin, this was meant to represent the future of motoring. So advanced was the car that in fact there’s still a lot of NSU DNA in current Audis! It’s a shame the Ro80 went pear-shaped instead, but despite NSU’s woes, Citroen wasn’t deterred from offering its own rotaryengined car, the Birotor (using Ro80 running gear). Based on the GS but using entirely different exterior panels, nearly 900 examples were made beforeCitroen realised the car would prove to be a liability. It offered to buy back the cars and crush them, which is why few have survived. There are a few about though – but you’ll probably have to go to Europe to find one as it’s reckoned just three are in the UK and only one of these is running. Other rotary oddities include the NSU Spider (the first rotary-engined car ever to be offered for sale), along with the Mazda 110S Cosmo and various other Mazdas such as the RX-2, RX-3 and RX-4 plus a raft of saloons. You’ll be doing well to find a decent (or indeed a tatty) example of any of these though – especially as some that survived have been converted to piston power!

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