6 Stroke technology

The capacity for original thought is a mark of genius. In the past century, it has delivered us countless advances. Often, they have been achieved against the odds, in the face of reactions ranging from disinterest to scorn: almost every engineer has to deal with dismissive responses to new ideas, usually on the basis that something so different obviously can’t be good. On that basis, we should all be riding single-cylinder side-valve motorcycles. Daring to be different is obviously essential. Malcolm Beare, a 53-year-old Australian wheat farmer is indeed an original thinker. Designing a better motorcycle engine has been his mission since 1981, when he built his first prototype. Beare has created an innovative hybrid design of the internal combustion engine, combining a two-stroke top with a four-stroke middle section of sorts. An exciting V-twin Ducati-based BEARS/Sound of Thunder racer incorporating Malcolm’s self-built frame, fitted with a 90deg V-twin version of the Beare six-stroke engine was the first multi-cylinder application of six-stroke technology. Beare began working 23 years ago on a stratified charge opposed-piston two-stroke, he recalls, “but after building a couple of prototypes I realized flow control was a problem, with too many ports going everywhere.” He thus simplified it into a six-stroke, with the objective of improved efficiency by overcoming the drawbacks of poppet valves, with rotary valve application (commonly used in two-stroke engines). This overcomes the problems of poppet valve inertia that inhibits flow, especially the exhaust valve hotspot within the combustion chamber. While various rotary valve designs made in the past were advantageous because they didn’t restrict flow or absorb as much energy as poppet valves, they had significant overheating problems, with associated drawbacks in sealing, lubrication and oil consumption. So Beare decided to produce the best of both worlds, taking the basic components of a rotary disc-induction two-stroke engine, and mating it with a four-stroke motor. “After a great deal of work and experimentation since building my first six-stroke prototype in 1990, I truly believe we’ve refined the concept, proved its reliability and established its worth, so now we have it fully patented — even in the USA — and ready for commercial use in the variety of applications it obviously lends itself to —including motorcycles, stationary engines and propeller aircraft engines. Below the cylinder head, everything is conventional. So one advantage is that the Beare concept can be transplanted on existing engines without redesigning. But above there, he’s thrown away the cylinder head, complete with poppet valves. To replace the camshaft and associated valves, Beare has retained the cam drivebelt and fitted an ultra short-stroke upper crankshaft complete with its inverted piston, the belt driven at exactly half engine speed. This piston slides up and down in a sleeve; past inlet and exhaust port cavities set into the cylinder wall, very much like on a two-stroke. Continuing the two-stroke analogy, the Ducati-based V-twin is fed fuel-air mixture via a reed valve, thus preventing exhaust gas exit through the inlet port. Beare technology recently has been paired with a new form of fuel injection known as the Pulse Fuel Injection system, which has been tested by the US Naval Air Warfare Center, among other international authorities. Unlike conventional fuel injectors, compressed air is used to atomize liquid fuel and pre-mix it with air, increasing efficiency and reducing emissions. Interestingly, this nozzle technology is being developed for emission control systems in diesel exhausts for Ford Motor Company.At the other end of the upper crankshaft is a two-stroke type rotary disc valve that regulates exhaust timing, cutting off the exhaust flow at the appropriate time to stop gas returning into the cylinder, and thereby creating the inlet cycle suction. This being its only function, the valve bears a light load, reducing lubrication and sealing problems. The Ducati I rode uses hardened, anodized aluminum rotary disc valves, which work well. Then, during the compression and expansion strokes, the upper piston seals both ports, leaving pressure between the two pistons. The lower piston is a conventional flattop three-ring design, while the conical upper one (so shaped to aid gas flow during both inlet and exhaust cycles, by guiding it towards the ports) has two rings — one compression, one oil. In the combustion phase, twin sparkplugs provide ignition. Not only does the engine run on pure petrol (no need to add oil, because it is positively lubricated, in spite of the application of two-stroke technology), it’s very happy on low octane unleaded fuel — a boon for the Indian market. OK, so the Beare six-stroke has impeccable credentials but that just gives a hint at the commercial and technical advantages Malcolm seems justified in claiming. This new technology has some strong backing — Sir Jack Brabham, the renowned three-time World F1 Champion, keenly supports the Beare ideology and his eponymous firm has gone to the world market along with CEO Alan Casey. The obvious benefit in eliminating poppet valves on a four-stroke is that it removes any chance of valve float, which in turn implies a far higher safe rev limit for the six-stroke — an eye-popping 28,000rpm in theory, given the half engine-speed operation of the upper crankshaft, and the fact that GP reed-valve two-strokes peak at 14,000rpm. But Malcolm Beare says the rev limit, primarily depends on what the crankshaft can bear and so he’s arbitrarily limited the Ducati-based 6S-V2 to 9000rpm, at which point he says 86bhp is delivered at the rear wheel. Instead of absorbing about 10 percent of engine power in driving the camshafts, the cam belts now deliver about nine percent net power back to the main crankshaft, via the upper, conical porting piston. If you factor in a 1997 model Ducati 900SS delivered 73bhp at the rear wheel in stock form, that’s quite an impressive claimed power increase. But there are other, much more significant apparent spin-off benefits from the Beare design. It is these that make it seem so very viable for a demanding Indian market, the most important being the ubiquitous fuel economy. Beare claims his engine is 35 percent more economical at low revs/throttle openings than the equivalent conventional engine, and 13 percent less thirsty at high rpm/full throttle. Therefore, there are reduced hydrocarbon emissions, because you use less fuel to achieve the same performance. Next, there’s improved torque at lower revs proven on all prototypes, which will surely help in the stop-and-go nature of congested Indian traffic. Beare discovered the six-stroke version produced the same torque as the four-stroke conventional motor 1000rpm lower down the rev scale, as well as making exponentially more torque as revs rose. But in a commercial application, perhaps the most attractive benefit is the significantly reduced number of moving parts, compared to a four-stroke — not as few as a two-stroke, but what you appear to get is improved performance and torque, coupled with four-stroke inherent advantages. Fewer moving parts also mean reduced manufacturing costs. My riding impression of the 6S-V2 revealed that while the Beare V-twin may appear a little rough at the edges, it’s obviously built to a budget as well as acting as a rolling test bed for continuous modification. So let’s ignore the looks and worry about the sizzle. For starters, it has an absolutely unique engine sound, like nothing else I’ve ever heard in my life. You clearly hear the ring-ding two-stroke signature above the trademark four-stroke V-twin boom from the twin megaphone exhausts. An uncanny two engines in one. Malcolm claims there’s much reduced mechanical noise from the engine, because of fewer moving parts. Without any tappet noise, it’s remarkably silent — and that’s in spite of being air-cooled. Heat dissipation is not a problem, so while it’d be easy to water-cool the engine, there’s honestly no need for the extra complication on grounds of noise or thermal efficiency. Riding the bike for a dozen laps showed up three significant assets for the six-stroke engine and one negative. The question mark hung over the power curve: while there’s no doubt it revs reasonably high by four-stroke standards, there seemed to be no direct benefit in doing so — there was no more power up high at, say, 8000rpm than there seemed to be 2000 revs lower. One thing Beare certainly doesn’t need to worry about is torque, a vital ingredient for the Indian rider, which together with the crisp, responsive throttle pickup and the reduced vibration are the three strong points of the six-stroke motor. The Beare engine is unbelievably muscular in terms of torque, and importantly from very low revs. Ironically, you appreciate this because those of a tube-bender do not match Malcolm’s evident talents as a bush engineer: the square-tube chassis was pretty awful in terms of handling. But the fact that the bike jumps around over bumps and doesn’t steer very well actually helps you appreciate the engine’s responsiveness, because you need to back off the throttle in, say, the middle of a chicane to allow the chassis to recover. But when you get on the gas again — get ready for a surprise. Not only is there instant response when you twist the wrist, there’s tractor-pulling torque at seemingly whatever revs the engine’s turning over at — even as low as what felt like 2000-3000rpm or so. There’s also no transmission snatch at very low revs, indicating a responsive, torquey motor, while the manner in which it pulls hard from way down low bears all the hallmarks of a long-stroke engine design, whereas the opposite is in fact the case: the lower ‘four-stroke’ crankshaft assembly has 86 x 57mm dimensions, while the upper ‘two-stroke’ measures 60 x 25mm. Yet the inherent nature of the Beare motor is one of a torquey slugger, belying these measurements. And allied with the so-responsive pickup and a wide spread of usable power, makes this bike ridiculously easy to ride. You hardly need to use the gearbox, just park it in top gear and ride. Even backing off the throttle in the middle of a turn doesn’t require hooking down a gear — just crack it open when you’re ready and feel the front wheel start to aviate on you. My hands-on assessment of the Beare six-stroke leads to some inescapable conclusions. What we have here is a hybrid engine design with the advantages already recounted above, allied with good squish, the ability to run radical bore/stroke ratios, quiet mechanical operation, and no exotic materials such as ceramic coatings or costly alloys required, nor complicated machining operations.As such, the Beare design surely seems worthy of serious consideration for powering low-cost transportation in developing countries like India, where expense of manufacture, fuel economy and torque (delivering carrying capacity for people and/or goods) are all paramount factors, yet where emissions matter too. The industry trend away from cheaper two-stroke power in favor of costlier but cleaner four-stroke engines in both Europe, Japan and South East Asia makes a concept like the Beare six-stroke, which offers the best of both worlds, project a strong case towards volume manufacture. At the time of going to print, Autocar India had learnt of a six-stroke small capacity single-cylinder prototype being in the pipeline for production by a leading motorcycle manufacturer in China, but few details are available. What is known is that a 150cc six-stroke Taiwanese single-cylinder bike from Yingang is ready to roll. The questions of how the technology will work in actual practice, whether it does make it into production reality at all in India, and how reliable it proves under harsh and demanding use remain to be answered. Watch this space.