ALT-1 Engine Configurations on Motorcycles
Motorcycles vary in use and performance, and due to the power-to-weight ratios involved, research and development has often far outstripped the technology used in cars and other vehicles. The motorcycle engine is small and powerful and readily accepts development. In the pursuit of power and performance, many configurations of engine have been developed, and the more popular and reliable designs are still with us and still being improved today.
Single cylinder: Chronologically speaking, the first engines were single-cylinder and restricted in power output and reliability by capacity. This means that if you go too far in capacity, the vibrations and balance of the engine will work against the design, causing problems. Most singles are no bigger than around 650cc. Early singles were mounted either upright , perched on top of the crank-case, or horizontally forward.
Upright engines lacked clearance between the top of the frame and the cylinder head, making bigger engines tall and top-heavy, and were further developed to be slightly sloping to increase installation versatility and reduce the center of gravity. Horizontally forward cylinders were similarly disadvantaged by capacity restrictions, the front-wheel clearance being of concern in handling and high-speed stability situations.
The single offers reliability, fewer moving parts than a multi-cylinder engine, and ease of maintenance and repair. The single is suitable for either two or four-stroke design and available for dirt or road application. Cooling is good, as there is only one piston to consider, fuel economy is excellent and the single-cylinder bike is narrow and light for twisty roads and sporty performance. Good examples are the Yamaha XT and TT range, the Suzuki DR series and most of the early British bikes.
Twin Cylinder: When the capacity demands of the motorcyclist went beyond the reliable range of the single, the twin cylinder appeared. This designation is still produced today by many manufacturers, like Harley Davidson, Ducati, Moto Guzzi and the Big Four Japanese brands.
There are four basic designs for twins - Parallel Twin, Vee Twin, L-Twin and Horizontally Opposed twin. The parallel twin was developed by makers such as Honda, Norton, Triumph and most other manufacturers as a quick-fix to capacity woes suffered by singles. . Basically, one just welded two engines together and widened the crankcase to accept the two cylinders. The parallel twin may fire both cylinders at once, fire them in 90 degree opposition, or may have a well-developed crank-timing designation to take advantage of engineering foibles too detailed to go into here. A mid-eighties Kawasaki two stroke ,the KR250, trialed an in-line water-cooled twin, one cylinder mounted behind and above the forward cylinder, but this was fraught with problems
Advantages of the Parallel twin are mainly due to increased capacity over singles, but still retain most of the benefits of a single-cylinder engine. This was one of the earliest developments in pursuit of extra power, and the capacity increase was tempered by balancing difficulties, doubling up of parts (and therefore maintenance), cooling concerns and, once again, capacity restrictions. Good examples of a parallel twin are made by the Big Four, early CD and CB Hondas being some excellent bikes.
The Vee-twin was the next incarnation, installed North-south in the frame. This design was also due to the need for more CC's. If you take a Harley motor and imagine the height of the frame should the cylinders be mounted parallel or straight up, you can imagine the handling difficulties involved. Other advantages include room to mount intake and exhaust manifolds, valve and cylinder head development, and balancing of the engine under load. Capacities went from sub 700 to over 1600cc over the history of Vee twin evolution. Most Vee twins are four strokes.
Moto-Guzzi is one of the few manufacturers (Honda made an attempt in the seventies and early eighties) to mount the Vee twin east-west across the frame. This design has the same benefits and drawbacks of the north-south Vee, but is less restricted by frame or fuel-tank clearance. Earlier designs suffered from the one-sided torque reaction from the crank rotating in one direction - bikes leaned to one side in high-revolution situations, but this has been ironed-out by balancers and other methods over time.
Problems with carburettors firing either one or both cylinders, vibration and power-to-the wheel were still being encountered due to the fact that there were still only two cylinders, and the triple was next to emerge.
The L-Twin is mostly associated with Ducati motorcycles, as their first twin was, in simple terms, a single with another cylinder welded to the front of the crankcase, pointing forward. Various degrees of opposition were tried, and nearly all of the Big Four have produced a Japanese L-twin. The torque and power outputs of an L-twin are renowned in engineering folklore, as is the distinctive sound and narrow, almost single-cylinder nature of the engine. As with the Vee twin, the L-twin was designed for high capacity bikes, and is usually a four-stroke.
The drawbacks are similar in nature to the Vee twin, but the balance issues are not so apparent. The forward cylinder is restricted by the front wheel as mentioned earlier, and carburettor balancing is sometimes tricky. Frames must be purpose-built for the L-twin as intake and exhaust manifolds must have clearance. Rugged power output requires a strong, balanced frame to accept the bigger "thumps". Damage to the front cylinder and exhaust-pipe is a concern on rougher roads.
BMW have built their motorcycling empire with the enduring flat-twin design, developed during the forties. The flat twin has two cylinders emerging east-west from a central gearbox. This is a nicely balanced design, offering capacities up to around 1000cc. Many imitators in Germany, China and the Soviet union still produce forties and fifties designs copied from bikes left behind or captured during World War 2, such is the enduring reliability and robust engineering inherent in the flat-twin.
Problems include ground clearance, leg clearance due to carburettors and cylinder placement, and vulnerability in drop or slide crash scenarios. Similar torque reaction to the Moto-Guzzi have been nearly eliminated by BMW over the years. The flat twin does keep one's feet warm in winter.
Triples: The three-cylinder configuration was the next logical step after the twin reached it's performance limitations. The triple eliminated most of the balance issues suffered by the twins due to developments in crank-timing, and the early two-strokes gave the rear wheel three kicks per rotation! Various lay-outs including north-south (currently employed by Triumph for the Rocket 3) east-west (Kawasaki, Suzuki, Yamaha) and two forward, one back (and vice versa). Companies like Laverda jumped on the Triple design and produced smooth, fast, reliable bikes and the triple is still used today.
Most of the advantages of the triple are related to solving the problems of the parallel or Vee twin, being balance, capacity, and improvements in smoothing out useful power delivery. Triples can be utilized in two or four-stroke designs. Three-cylinder bikes can take advantage of a power-stroke followed by a balancing stroke, or just have a nice three in a row combustion cycle. Most triples are designed for the higher end of the road bike market, as they are too big and powerful for dirt-bikes and have often found favor in the designing engineer's workshop due to their advantages without the need for a forth cylinder.
Any design of motorcycle engine incorporating more than two cylinders is usually very reliable due to the reduction of combustion shock effects to the frame and its lazier operating capacity. Problems are few, but beware of hastily built mid-eighties four-stroke Japanese machines which suffered due to limited R&D when the engineering advances of the triple became better known. The early Kawasaki two-stroke triples were incredibly powerful, but were under-braked and handled poorly, gaining a reputation as suicide machines.
Four and More: Very early bikes sometimes used four cylinders, but with limited success due to upright cylinder placement, intake and exhaust problems, cooling failure in north-south mounted engines and difficulties in tuning multiple banks of carburettors. With the release of the Honda 750 four, the world was shown a glimpse of the Superbike, problems were solved and capacities were to reach well over those available with the twins or triples. Kawasaki's Z900 is renowned as a ground-breaking machine, using an east-west four-stroke air-cooled design. Power outputs and reliability were only restricted by model and manufacturer, and the in-line four is now one of the most popular configurations in motorcycle manufacturing.Most four-cylinder bikes are four-stroke, though a few radical and prototype developments in two-stroke technology (TZ Yamahas for instance) were built prior to the environmental panic which surrounded two-stroke emissions. The options available to the four-cylinder owner include carburetion, exhaust management, crankshaft timing, and a range of capacities from 250 to 1600cc to get just the right power and torque requirements for the task at hand.
The drawbacks of a four-cylinder engine are related to multiple quantities of everything, and getting all the components to work together. Carburettor jetting, valve inspection and replacement, single component failure (leading to expensive investigatory mechanics) and exhaust issues will lead to more expensive repairs than in other configurations. The four cylinder will use more fuel, chew more chains and smooth down more tyres than engines blessed with less.
Currently, there are no north-south layouts available for four cylinder bikes. The Vee four, however is a development of the multi-cylinder which still has wide acceptance for motorcycle power units. This is more like a set of Vee twins than an inline four, and has many of the advantages of the Vee twin in a larger capacity. The Honda VFR and Yamaha Vee Max models are good examples. Overheating and the drawbacks associated with inlet/exhaust manifold placement are apparent in Vee fours.
Another notable mention is the old British Ariel Square Four, which, as its name suggests has two front and two rear cylinders in a square layout on a common crankcase. The problems caused by attempting to air-cool two cylinders behind two others led to a short lifespan and unique maintenance issues.
BMW has also incorporated four cylinders into an engine design, horizontally opposed like a VW engine. The "boxer" engine does not suffer the problems of the Ariel due to liquid-cooling, and is known as one of the smoothest, most reliable engines ever to grace a motorcycle frame. Well balanced and powerful, the BMW Boxer engine is a popular choice for the serious mile-muncher.
Six cylinder bikes were not a market favorite - the engines were too wide for serious riding and the expense involved with maintenance and servicing could not justify its size in the face of minimal gains in power and torque compared to powerful, modern four cylinder bikes. The Honda inline six-cylinder is thought of more as a novelty these days than a choice for the daily rider. Some prototypes of Vee six bikes have been built (notably the MV Augusta 6 and the legendary Laverda 1978 model V6 Racer), but usually they have been quietly consigned to the circular file. The four cylinder has too much future development potential to bother with much more in the way of cylinders.
Finally, the Wankel Rotary of Mazda fame has also been incorporated into motorcycles with some success. Although the Suzuki Rotary and older Norton bikes have worked on the concept, they only produced heavy, uneconomical, evil-handling walruses of bikes in the earlier designs. It wasn't until the nineties when the Norton F1 JPS Special race-bike was built that the power to weight benefits and the unlimited revving capacity of a rotary were realized. The bike has been immortalized by winning the 1992 Isle of Mann TT, and the 1989 British F1 title. The Norton project was cut down mid-stride as the company folded, and the enormous fuel usage of the engine has probably put an end to all further development of a promising design in these days of escalating petrol prices.
The Motorcycle engine in all its configurations is still undergoing research and development far beyond that of automobile engines. The current crop of four-stroke (!) motocross bikes proves that yesterdays impossible is tomorrows commuter, and the potential for further gains in power, economy and rider experience is as great as any period in biking history.
