The last major bastion of the two-stroke engine appears to be in snowmobiles. Thanks to liquid cooling, electronic controls, fuel injection, sophisticated combustion techniques, and variable-exhaust-port technology, the latest avalanche of two-stroke "sled" powerplants aims to comply with the new U.S. EPA Phase 3 emissions regulations slated for the 2012 model year.
Can the two-strokes, with their impressive specific output, high power-to-mass ratio, and package benefits, hold their own against the four-stroke assault?
Bombardier Recreational Products (BRP) engineers believe they can. BRP’s evidence is the recently unveiled Rotax E-TEC 800R, newly equipped with direct fuel injection (DI) and slated for 2011 Ski-Doo sleds. (BRP owns Rotax, Ski-Doo, as well as Evinrude marine engines and Sea-Doo watercraft.)
The injected 800R is the latest iteration of Rotax’s Type 797 series, an 800-cm3 liquid-cooled parallel twin rated at 155 hp (115 kW).
The Phase 3 standard mandates a nominal 50% reduction in carbon monoxide (CO) and hydrocarbon (HC) emissions compared with uncontrolled levels (150 g/kW•h for HC and 400 g/kW•h for CO). The straight HC limit is 75 g/kW•h, and the corporate average CO limit is 275 g/kW•h.
For two-stroke engineers, the main HC emissions challenge is achieving a complete burn in the combustion chamber. With its DI system, Rotax’s development team employs the company’s voice-coil electromagnetic injectors to further compress the fuel from the relatively low initial pressure provided by the fuel pump and simultaneously inject it into the 800R’s cylinders at 500 psi (34 bar).
At this pressure the fuel stream vaporizes almost instantly and is then ignited by the spark plug. This creates a more complete burn as well as better throttle response when compared with the previous carbureted two-stroke, BRP engineers claim. As part of their cleaner emissions profile, the DI E-TEC engines also have greatly reduced exhaust odor during start-up.
The DI system is used in conjunction with BRP’s reed-valve induction, 3-D RAVE (Rotax Adjustable Variable Exhaust) electromechanical exhaust valve, and a powerful ECU that processes inputs from a variety of sensors—crank angle, throttle position, knock, coolant temperature, and ambient air pressure and temperature. The open-loop fuel system also cools the injectors as well as the ECU.
The 3-D name refers to three-dimensional mapping used to determine exhaust valve operation, as well as three exhaust port openings per cylinder.
The new injected 800R achieves a claimed 19 mpg (12.3 l/100 km) in Ski-Doo sleds—up to 37% better fuel efficiency than competitive units, BRP claims.
Other benefits of DI include easier starting, improved idle quality, and greater oil efficiency—264 mi/qt (450 km/L), a 15% improvement over the 2010 carbureted 800R, according to the company.
The addition of DI to its 800-cm3 engines (the 600-cm3 units are similarly equipped) proves the two-stroke is alive and, for the time being, still very well indeed.