January 3, 2005

CUTTING THE CARBS




Where’d all the carburetors go? There are more injection methods available now than track lengths. Well, almost.
Think about it. There are four major types of injection in the 2005 lineup: Carburetion, Electronic Fuel Injection (EFI), Semi-Direct Injection (SDI) and Cleanfire injection.

Carburetor
Carburetors are still the most widely-used method of getting a fuel/air mixture into the engine’s combustion chamber. All of Polaris’ Edge RMKs use them, as do Ski-Doo’s 600 HO and 800 HO Summit motors, Cat’s M5 and King Cat (one version) and Yamaha’s SXViper Mountain.
The carburetor is a mechanical fuel-regulating tool that is controlled by throttle position and engine rpm. It basically meters how much gasoline enters the airstream as it’s being drawn through the airbox, carbs, reeds and into the engine’s intake ports. A wider throttle opening creates a stronger vacuum, or higher airflow, through the carburetor. This vacuum force draws fuel from the carburetor float bowl into the carburetor body through different jets, depending on throttle position.
A carburetor is a very complex piece of equipment. Different circuits control air flow and fuel delivery at different points of throttle position. For example, the pilot jet meters fuel from idle to one-quarter throttle. The needle jet and jet needle meter fuel flow from one-quarter to about three-quarter throttle. The main jet is the main fuel circuit for wide-open throttle operation. Floats in the float bowl control how much fuel comes into the bowl, which supplies fuel for the carburetor’s circuits. All of the air flow is controlled by the slide, which is linked directly to your throttle thumb via the throttle cable.

Electronic Fuel Injection
EFI is more closely related to automobile technology. Throttle bodies replace carburetors on the induction side of the engine. A butterfly valve acts as the carburetor’s slide, regulating air flow to the engine. Injectors inject a charge of fuel into the throttle bodies. The air and fuel mix as they travel into the engine. A computer monitors engine rpm, timing and throttle position and adjusts fuel delivery accordingly. The electronic system also adjusts for altitude and air temperature, so you never have to worry about changing a jet or running poorly. The biggest advancement in EFI in the snowmobile industry is Arctic Cat’s batteryless EFI system (BEFI). It’s a virtually trouble-free injection system.
Arctic Cat is the exclusive user of EFI, and uses its BEFI on the M7 and M6 mountain sleds, King Cat (optional) and its Firecat line, as well as other models.

Semi-Direct Injection
Ski-Doo and Rotax have pioneered semi-direct injection for use in snowmobiles. The main benefit is the drastic improvement of emissions. We’re talking EPA 2006 compliant here. Emissions from Rotax 2-TEC SDI engines reduce emissions by about half when compared to a non-SDI engine of similar displacement. Another environmentally-friendly (and gas card friendly) benefit is a big increase in fuel economy. Ski-Doo claims its SDI engines have about a 25 percent increase in fuel economy. And the SDI Rotax engines maintain their two-stroke performance. The SDI engines claim roughly the same horsepower numbers as their carbureted counterparts. There is little weight penalty, but the system is still relatively new, if you’re asking yourself why every engine Ski-Doo offers isn’t SDI.
Don’t confuse SDI with either EFI or direct injection. EFI injects a fuel charge into the throttle bodies outside of the engine. The EFI system still uses fuel to scavenge the exhaust on the intake stroke. Direct

injection shoots a charge of fuel directly into the combustion chamber.
The SDI technology is clean because it takes fuel out of the scavenging process. What is scavenging? At the top of a two-stroke engine’s compression stroke, the fuel and air mixture is ignited, and the combustion begins the power stroke (or combustion stroke). When the piston travels down in the cylinder far enough to open the exhaust ports, this most recent internal combustion becomes exhaust and is drawn out of the cylinder and into the exhaust pipe. The new fuel and air mixture that has entered the engine via the intake and is sitting in the crank case is now drawn up the transfer ports and into the combustion chamber. The incoming fresh fuel mixture helps push the exhaust out of the cylinder. In the process, a portion of the unburned fuel that just entered the combustion chamber is sucked out with the exhaust before the piston closes off the exhaust port. This is why traditional two-stroke engines have higher emissions levels. Unburned fuel in the exhaust accounts for increased hydrocarbons in the emissions readings.
Using two injectors per cylinder (one high-speed and one low-speed), Semi-Direct Injection injects fuel into the transfer ports at the top of the cylinder. The injectors are about as close to the combustion chamber as possible, but not actually in the combustion chamber. The fuel isn’t injected into the ports until the exhaust ports have closed. That way, no unburned fuel is being pulled out with the exhaust.
Air travels through the throttle bodies, into the cylinders and up the transfer ports. Just as it’s about to enter the combustion chamber, the fuel is injected into the transfer port.

Cleanfire
Polaris is the most recent manufacturer to step up to the plate with a new method of fuel injection. Its newest engine, the Liberty 900 twin, is the first engine in Polaris’ lineup to feature Cleanfire injection.
Cleanfire uses one injector per cylinder. The injectors are placed in the crankcase—not the cylinder. The injectors are placed much lower than the SDI setup. By placing the injector so low, Polaris engineers found that the fuel actually spreads over to the transfer ports on each side of the cylinder. This method allows the fuel and air to mix longer and spread over more of the piston when it enters the combustion chamber.
When the air enters the crankcase through the throttle bodies and W-reed cage, it meets the fuel charge at the bottom of the boost port (rather than the top) and carries it into the combustion chamber. The fuel/air mixture enters the combustion chamber through three openings (center boost port and part of each side transfer port). This spreads the mixture across the top of the piston and creates what Polaris claims to be a very efficient, very powerful combustion stroke. The down side to the Cleanfire system is that it uses raw fuel in the scavenging process. Polaris engineers acknowledge that they’re giving a little ground to emissions, but not much. The Liberty 900 passes EPA 2006 requirements. It also provides the power of a carbureted engine. The Liberty 900 has been rated at 150 hp. That’s the same figures as a carbureted Arctic Cat 900 twin.

Future?
Face it. We’re living the last days of the conventional carburetor, at least on two-stroke snowmobile engines. The industry will continue the trend toward more advanced methods of fuel injection. Carburetors are old technology. Look at the automotive industry. As soon as the EPA came down on automotive emissions and economy, the four-barrel became an antique.
So it will be with the flat-slide. Especially with the EPA’s 2010 regulations just around the
corner.

 







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