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    What Makes the New Polaris 800 H.O. Go?

    Posted by Ross Halvorson
    on Monday, December 08, 2014

     Sled engine expert, Kevin Cameron, weighs in on the new Polaris 800 H.O. engine. What makes it go and how did we get there?

    I'm impressed by the double-gated exhaust valve system on this new Polaris 800 HO, and by the Brake Specific Fuel Consumption (BSFC).

    I first worked with a variable-exhaust-timing engine in 1981, in the form of Yamaha’s TZ250-H road race engine. It had a spool-valve, which lowered the top of the single large oval exhaust port by about 6mm for an additional several hundred usable rpm on the bottom.

    Yamaha switched from the spool-type valve to an angled sliding gate, allowing the gate to be closer to the piston in its lowest position. Now, I see a similar thing in Polaris’ double gate. The same advantage is doubled because the lower gate can close the port to a greater degree, making the Exhaust Open (EO) point later, and broadening the engine’s pulling range for a quicker throttle response.

    In 2-stroke scavenging, two abstract extreme conditions are considered as models for what happens in the cylinder. The first is called ‘perfect mixing’. It assumes that as fresh charge is added to the cylinder, it mixes completely and uniformly with what is already there (mainly exhaust residuals). The scavenging of the kind of fixed-port 2-strokes I grew up with acts pretty much in this way, resulting in modest torque and power, about half of what they can be on a displacement-related basis.

     Those early engines generally had two small transfer ports and high crankcase compression ratios, above 1.5:1, resulting in high transfer velocity that mixed rapidly with cylinder contents and whose leading edge reached the exhaust port early in the transfer process. Limited by being close to ‘perfect mixing’, the classic Japanese and German 2-strokes of the 1960s had higher power using many small cylinders and high rpm.

    In 1968 GP bikes were limited by rule to 6-speed gearboxes, which forced 2-stroke engineers to re-think scavenging completely. They realized the value of slowing down the transfer jets by increasing total transfer area and reducing crankcase compression (making the crankcase bigger). Slower moving transfer jets meant a lot more fresh charge volume in the cylinder – not just two thin little streams moving so fast that they quickly mixed with the cylinder contents. Methods like Jante scavenging mapping showed that four or more slower-moving transfer streams could join to make a piston-like upflow, following the non-exhaust cylinder wall, up to the head, across it and down to the exhaust. This concept served us well, taking 30 years to double engine torque.

    The other abstraction useful in thinking about 2-stroke scavenging is called ‘perfect stratification’, and it assumes that none of the entering fresh air mixes with or is diluted by cylinder contents until the leading edge of the scavenge flow reaches the exhaust port. The scavenging of the best modern 2-strokes lies along this perfect stratification line.

     With no carburetor, the 800s injection timing has an additional control measure – the ability to delay the addition of fuel until it is added to a part of the air charge that will not reach the exhaust port at all.

     4-stroke engines in snowmobile sizes typically display BSCFs around 0.50, but carbureted 2-strokes, because of their fuel short-circuiting, have generally operated around 0.65 (30% of fuel lost to the exhaust), with exceptional examples, especially when  high compression is used, managing to squeak under 0.60. Yet in the above dyno test we saw 0.38 – 0.40 at part throttle from Polaris’ new 800 double-gated, finger-port-injected engine.

    2-strokes and 4-strokes are more similar that you’d expect. Peak torque in a 4-stroke comes at the rpm at which a negative reflected pipe wave returns to the exhaust valves during overlap, when the exhausts have not yet fully closed and the intakes are just beginning to open. This boosts torque because the low-pressure wave extracts exhaust product from above the piston (sitting motionless at TDC), and then starts the intake flow into the cylinder by propagating out through the intake valves.

    At some lower speed, it is a positive reflected wave that hits during overlap. As you’d expect, it stuffs more exhaust back into the cylinder, blows it back up the intake pipes and begins to fill the intake airbox with exhaust. When the piston does begin its downward intake stroke, much of what it pulls in is exhaust. This produces the infamous 4-stroke “flat-spot” that so many have tried and failed to tune out.

    In a 2-stroke, something similar can happen, but in reverse. In 2-strokes, torque is boosted when a returning positive wave hits the exhaust ports just as they are closing. The leading edge of the scavenge stream has by this time reached the exhaust and is sticking out into the pipe. The return positive wave crams it back into the cylinder, resulting in amazing torque. The appropriate term for this is ‘acoustic supercharging’.

    But at lower rpm than the rpm of peak torque, this wave is mis-timed, resulting in everything in the cylinder being lost to the pipe, or in the cylinders being so filled with exhaust that it takes two or three cycles before there’s enough fresh stuff air in the cylinder for it to fire. This is the 4-stroking and 8-stroking familiar from the bad old days of fixed-port engines.

    But along came variable-exhaust-timing technologies to save the day, and later came direct and indirect fuel injection. By closing down the exhaust port to the appropriate degree, fresh charge is bottled-up inside the cylinder in sufficient quantity to be ignitable on every cycle. The result: exceptional BSFC and a wider torque range.

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