Aprilia’s Fuel-Injected Two-Stroke Concept

Admin · March 22, 2022

The heart of Aprilia’s fuel-injected two-stroke incorporates a fuel injector above the combustion chamber, as well as a provision for injecting compressed air. (Aprilia/)

The credit for developing the modern high-output two-stroke engine to its full potential usually goes to MZ engineers in the former East Germany. (MZ is an abbreviation of Motorenwerke Zschopau, the firm named after their home city, located about a dozen miles from the Czech border.) In 1961, MZ took the world by surprise when its 125 Grand Prix bike nearly scooped up the world championship.

Two-stroke engines had been around since the dawn of internal combustion, regularly trading dominance with the competing four strokes. German engineers devoted great efforts to developing high-performance two-stroke engines. As early as the 1930s, DKW achieved some impressive results, winning races all around Europe with a wild supercharged five-piston twin-cylinder engine (yes, you read that correctly; two pairs of pistons shared cylinders siamesed at the top, and the outsized 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 didn’t vie for the top laurels until MZ’s efforts in the early 1960s.

Walter Kaaden was MZ’s chief project engineer, and his intuitions proved fundamental when it came to extracting ever increasing output from two-stroke designs. Kaaden and MZ made breakthroughs with their resonant expansion-chamber exhaust systems and the time/area relationship in the two-stroke’s volumetric efficiency. After ending the 1961 World Championship a strong second place, MZ factory rider Ernst Degner fled East Germany and took Kaaden’s drawings with him, delivering them to Suzuki. (See Mat Oxley’s 2010 page-turner Stealing Speed for the whole story.) From then on, two-strokes began their domination of all forms of motorcycling sport: track, roadrace, motocross, ISDT, enduro, and trials. Only endurance remained out of reach, although Yamaha’s TZ700 gave the competition a solid scare at the 24 Hours of Bol d’Or.

The whole concept was designed so Aprilia could offer 50cc two-stroke scooter engines in a time of tightening emissions standards. Of course, there was potential to apply the solution to larger engines as well. (Aprilia/)

With the exception of Honda, the Japanese factories focused on further evolving two-stroke technology, reaching outstanding levels of specific power. Suzuki, Yamaha, and Kawasaki all constantly improved their engines, developing and refining every possible kind of induction system from the traditional port induction to rotary valve, and using reed valves in both the cylinder port and in the crankcase. The final results were astonishing, especially after Honda joined the fray.

Europe’s Two-Stroke Renaissance

After years of Japanese dominance, a sort of resonant two-stroke return wave revitalized Europe’s motorcycle industry. Highly competitive two-stroke engines appeared, mostly 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 lower-displacement GP classes. The two-stroke’s continuously progressing potential, both in Europe and in Japan, triggered a sort 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 massive amount of time and money was devoted to fitting two-strokes with the latest electronically managed fuel-injection systems.

Limitations soon appeared. Two-strokes needed very powerful ECUs in order to manage the three-dimensional fuel-metering programs necessary to deal with the two-stroke’s range of engine speeds, which is much wider than a four-stroke’s. Remember, back in 1975 the legendary Cray-1 supercomputer was a 5.5-ton monster featuring a 64-bit, 80-MHz processor with 8.39 megabytes of memory. Today a good quality 64-bit desktop easily offers 300-plus gigabytes of memory.

Those computer monitors should give you a clue: The ECU and fuel-injection mapping capacity of the day just wasn’t sophisticated enough to fully exploit the system’s advantages. (Aprilia/)

The most successful early fuel-injected two-strokes were Mercury outboard marine engines. Outboards, though, offer only moderate specific power, low peak revs, and spend most of their life at constant rpm; responsive throttle transitions are negotiable. This is hardly the case in either motorcycle racing or streetbike use. Piaggio tried to end-run the problem by placing the fuel injector not directly in the combustion chamber, but aimed upward inside a transfer port. While this prevented injector overheating, the fuel-metering problems were still there, and poor fuel atomization compounded the problems.

Aprilia’s Innovative Solution

Ivano Beggio was the son of Aprilia’s founder. He was passionate about motorcycles, and invested enormous personal and financial effort to grow the former bicycle shop into a motorcycling powerhouse so accomplished that it was able to snatch 125 and 250 GP titles from the mighty Japanese factories. When his technical team, led by Jan Witteween, suggested that Aprilia might develop a fuel-injected two-stroke engine, Beggio immediately underwrote a substantial R&D budget. Named Aprilia Ditech System, it was inspired by the research that Australia’s Orbital Engine Corporation had conducted on direct fuel-injection systems for two-stroke engines.

Actually, Aprilia and Orbital joined forces to develop a new generation of low- to medium-displacement two-stroke engines expected to deliver higher specific output than any four-stroke engine in the class, all while using less fuel and polluting less in the bargain. In 1999 Aprilia announced that this new generation of direct-injection two-strokes would easily meet the then-current Euro 1 emission regulations, and would cut fuel consumption and emissions by 40 and 80 percent respectively compared to a traditional two-stroke unit of similar displacement and performance level.

How Ditech Works

Aprilia’s Ditech system exploited a very powerful Siemens ECU to manage the fuel system. Another key point was a fuel pump operating at a six-bar (87 psi) pressure, higher than anything else at the time. The gasoline was routed to a rail connected to the fuel injector, which in turn was located atop the combustion chamber. An electronic pressure regulator modulated the fuel charge, actuating the fuel injector for a precise interval to deliver a measured amount of fuel, depending on engine rpm and throttle position.

This schematic shows how the system works. Note the small air compressor at the bottom of the illustration, whose output helped atomize fuel for extra-lean combustion. (Aprilia/)

While the fuel injector was located above the piston and bore, it didn’t deliver the charge directly into the combustion chamber but rather to the final injector, where the high-pressure fuel mixed with a flow of air compressed at five bar (73 psi). This ensured a micro-atomized fuel charge, creating the perfect conditions for stratified-charge combustion. Such a charge allows the engine to run under safe lean-combustion conditions, resulting in drastically reduced fuel consumption. A small air pump driven by an eccentric on the crankshaft flywheel provided the compressed air.

Note that the fuel and oil were not mixed. An electronically managed pump delivered lubricating oil to the inlet port in the traditional two-stroke fashion; there was no “four-stroke-style” direct oiling of the crank or rod bearings. A throttle-position sensor managed lubricant quantity.

Aprilia never produced the Ditech system by itself, but rather formed a partnership with Orbital and Siemens named Synerject LLC. Ditech seems to “think” like a direct fuel-injection system for four stroke engines; it may have been software limits that prevented the system from accomplishing its mission.

Fifteen years later KTM tried and succeeded, though not in direct-injection form, but its electronic port-injection enduro two-stroke singles, which as expected, deliver superb performance.

Seen from the left side, Ditech was an overall tidy package. (Aprilia/)