Two-stroke engines had been around since the dawn of internal combustion, regularly trading dominance with competing four-strokes. German engineers devoted great effort to the development of high-performance two-stroke engines. As early as the 1930s, DKW achieved impressive results, winning races throughout Europe with a supercharged five-piston twin-cylinder engine – you read that right: two pairs of pistons shared Siamese cylinders at the top, and the oversized fifth piston drove the compressor. At the time, many two-stroke engines relied on supercharging to develop their race-winning potential, but when supercharging was banned, a two-stroke did not compete for top laurels until the efforts of MZ in the early 1960s.
Credit for developing the modern high-output two-stroke engine to its full potential generally goes to the MZ engineers of the former East Germany. MZ is the abbreviation for Motorenwerke Zschopau, the company named after their hometown, located about a dozen kilometers from the Czech border. In 1961, MZ took the world by surprise when his 125 Grand Prix motorcycle almost won the World Championship.
Walter Kaaden was MZ's chief project engineer, and his intuitions proved fundamental when it came to extracting ever-increasing power from two-stroke designs. Walter Kaaden and MZ made breakthroughs with their resonant expansion chamber exhaust systems and the time/area relationship in two-stroke volumetric efficiency. After finishing the 1961 World Championship with a solid second place, MZ factory rider Ernst Degner fled East Germany and took Walter Kaaden's designs with him, delivering them to Suzuki. From then on, two-strokes began their domination of all forms of motorcycle sport: track, road racing, motocross, ISDT, enduro and trials. Only endurance remained out of reach, even if Yamaha's TZ700 gave the competition quite a scare at the Bol d'Or.
With the exception of Honda, Japanese factories focused on the evolution of two-stroke technology, achieving exceptional levels of specific power. Suzuki, Yamaha and Kawasaki have all constantly improved their engines, developing and refining every type of intake system possible, from traditional port to rotary valve, and using reed valves in the cylinder port and in the crankcase. The end results were astonishing, especially after Honda joined the fray.
The European two-stroke renaissance
After years of Japanese domination, a sort of resonant two-stroke comeback wave revitalized the European motorcycle industry. Highly competitive two-stroke engines appeared, mainly from top German and Dutch technicians like Jan Thiel, Jan Witteveen and Georg Moeller. These men further refined the two-stroke and achieved winning results on the track, particularly in the small displacement GP categories. The potential for continued progress of the two-stroke, both in Europe and Japan, sparked a kind of euphoric enthusiasm for further research and experimentation. And given the results that four-stroke engines were able to achieve by adopting fuel injection, a huge amount of time and money was spent equipping two-stroke engines with the latest fuel injection systems. electronic management.
Limits quickly emerged. Two-strokes required very powerful computers to handle the three-dimensional fuel metering programs needed to cope with the two-stroke engine's rev range, which is much wider than that of a four-stroke.
Thus, the most successful early fuel-injected two-stroke engines were the Mercury outboard marine engines. Outboard motors, however, offer only moderate specific power, low peak rpm, and spend most of their life at constant rpm – reactive throttle transitions are negligible. This is hardly the case in motorcycle or road racing. Piaggio attempted to solve the problem by placing the fuel injector not directly into the combustion chamber, but pointed upwards inside a transfer port. Although this prevented the injector from overheating, the fuel metering problems were still there and poor fuel atomization made the problems worse.
Aprilia's innovative solution
Ivano Beggio was the son of the founder of Aprilia. He was passionate about motorcycles and invested enormous personal and financial effort into building the former bike shop into such an accomplished motorcycle powerhouse that he was able to wrest 125 and 250 GP titles from the mighty Japanese factories. When his technical team, led by Jan Witteween, suggested that Aprilia could develop a fuel-injected two-stroke engine, Ivano Beggio immediately underwrote a substantial R&D budget. Named the Aprilia Ditech System, it was inspired by research that the Australian Orbital Engine Corporation had carried out into direct fuel injection systems for two-stroke engines.
In fact, Aprilia and Orbital have joined forces to develop a new generation of low- to mid-displacement two-stroke engines expected to deliver higher specific power than any four-stroke engine in the class, while using less fuel and polluting less. In 1999, Aprilia announced that this new generation of direct injection two-stroke would easily meet current Euro 1 emissions regulations and reduce fuel consumption and emissions by 40 and 80% respectively compared to a two-stroke unit. traditional engine of similar displacement and level of performance.
The Ditech system under the microscope
Aprilia's Ditech system used a very powerful Siemens computer to manage the fuel system. Another key point was a fuel pump operating at a pressure of 6 bar, higher than competing systems at the time, which were fed at 3 bar. Gasoline was fed to a rail connected to the fuel injector, which in turn was located at the top of the combustion chamber. An electronic pressure regulator modulated the fuel charge, operating the fuel injector for a precise interval to deliver a measured amount of fuel, based on engine speed and throttle position.
While the fuel injector was located above the piston, it did not deliver the charge directly into the combustion chamber but rather into the final injector, where the high pressure fuel mixed with a stream of compressed air at 5 bars. This ensured a micro-atomized fuel charge, creating the perfect conditions for stratified charge combustion. Such a load allows the engine to operate in lean combustion conditions, which significantly reduces fuel consumption. A small air pump driven by an eccentric on the crankshaft flywheel supplied the compressed air.
Note that the fuel and oil were not mixed. An electronically controlled pump was required to supply lubricating oil to the intake port in the traditional manner for two-stroke engines. A throttle position sensor manages the quantity of lubricant.
Aprilia never produced the Ditech system on its own, but instead formed a partnership with Orbital and Siemens – named Synerject regarding the two-wheel field at the time. Ditech seems to "think" like a direct fuel injection system for four-stroke engines: perhaps it was software limitations that prevented the system from accomplishing its mission.
Fifteen years later, KTM tried and succeeded in making the fuel-injected two-stroke work, although not in direct injection form, while still delivering superb performance.
