Dear Ralph: Can you explain, in simple terms, how the Ski-Doo DPM system with carburetors can be clean enough to be EPA-certified? If this...

Dear Ralph:
Can you explain, in simple terms, how the Ski-Doo DPM system with carburetors can be clean enough to be EPA-certified? If this is true, then why is everyone spending so much money developing expensive electronic fuel injection systems that just complicate things?
Ricky Hoffman

The Ski-Doo Digital Performance Management (DPM) found on the 2005 and 2006 PowerTek 800 engine indeed uses carburetors and is fully EPA-certified for 2006 and beyond. A carburetor, by itself, is relatively “stupid” in that it has no inputs to compensate for things like throttle position, air temperature or air pressure. Fuel is fed to the engine by virtue of a pressure differential between the float bowl and the air flow through the carb throat, or venturi. The greater the pressure difference, the more fuel delivered.
Ski-Doo electronically manipulates the float bowl pressure as a means of varying the fuel delivery, allowing them to combine smart electronics with a simple (and stupid) mechanical carburetor.
The previous versions of DPM used inputs to their on-board computer (ECU) such as engine RPM, throttle position, air temperature, engine water temperature and, on Summit models, ambient (barometric) pressure to determine the proper pressure to be applied to the float bowls for proper fueling.
The PowerTek version of DPM adds to this a knock (detonation) sensor, an important element to being able to reduce the fuel delivery far enough to become compliant. Normally, if you reduce the fuel delivery too far a 2-stroke will detonate, and this leads to engine damage if allowed for much more than an instant. The knock sensor allows the fuel delivery to be right on the edge all of the time, that is the edge of detonation and maximum power output! If the engine starts to detonate, even slightly, the sensor picks up the acoustic signature and the ECU (computer) takes the steps to correct the condition immediately. This is accomplished though both fuel delivery and ignition timing, as well as power valve positioning.
Electronic RAVE power valves are another addition to the PowerTEK version of DPM. This is the magic combination to become EPA-compliant with a carbureted 2-stroke; fuel control, ignition control, a knock sensor, and electronic power valves. Logically, any 2-stroke engine with these elements should be able to do the same; like add a knock sensor to a Cat EFI engine that already has electronic power valves and get the mapping right…
Why do it any other way? Only Ski-Doo has this level of control to do so with a carbed 2-stroke, but it shouldn’t be beyond the capabilities of the others. It is viewed as a “stop-gap” measure that should be effective until 2010, when the emissions regulations get even tighter and could render this system non-compliant. We’ll see.

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