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The Cleanfire Liberty 900 is an important engine for Polaris. Not only does it provide the power for its new Fusion sled, the centerpiece for Polaris' 50th Anniversary Celebration as a snowmobile maker, but it's also designed to meet future clean air requirements.
This means a larger twin with high torque and lower revs, with an engine management system controlling both the ignition and its semi-direct fuel injection system.
A New Architecture
The architecture is new with the throttle bodies located low in front underneath the exhaust system. These large 51mm throttle bodies deliver no fuel supply; their only function is to control airflow into the crankcase. Because of their new location, the engine can be mounted lower and tilted back 22 degrees for a better-centered mass effect. (As a comparison, the 800 Liberty twin is tilted 20 degrees forward.)
This new position for the 900 lowers the center of gravity, which reduces roll in corners, an important factor with the longer suspension travel in the new Fusion.
A centered mass also makes the machine easier to turn and moves weight away from the skis, giving a surprisingly light feel at the handlebars for a machine with this size engine. Good thing that you don't have to change jets on this unit, with throttle bodies buried way under the exhaust manifold.
This is not an engine that is easily converted to carburetors, unless the exhaust manifold is drastically refitted. When asked about this, the Polaris engineers felt that carbs would not be an option on machines in the future because of the coming clean air standards.
Here, the incoming air is fed directly into the crankcase through Polaris designed "W"double row reed bodies and picks up oil from the injectors located in the reed box. This oil-air mixture is directed toward the main rod bearing and the back of the cylinder wall. There it's met by a stream of fuel from the injectors, located in the back of the crankcase.
Location of the fuel injectors is carefully selected to fog the rear transfer ports, leaving only fresh air to enter the cylinder through the front two transfer passages and preventing the fuel mix from escaping via the exhaust ports. The fuel injection nozzles are directed to hit the crank counterweights as they swing around, and are timed to fire just after top dead center - as the piston is on its way down.
From the "winding" effect of the counterweights, the mixture is therefore forced up the back transfer ports. As a result, unburned hydrocarbon levels should be reduced to meet future EPA standards.
Appropriately Polaris named this the "Cleanfire" system, and it marks the company's return to a more efficient fuel injection system than the RXL model with injectors in the throttle body.