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Polaris got it right with Patriot!

Insider look at the new Polaris Patriot 850 engine
2019 Polaris Patriot 850
A first impression might be that the new Polaris 850 Patriot engine is a hasty response to Ski-Doo’s release of their 850 twin last year, but that’s not the truth. Reality is that they both have been working on 850s for years. Polaris was already working on theirs before the Ski-Doo motor was released. So this is NOT a tit-for-tat situation and certainly not a copycat scenario. The Polaris 850 Patriot engine is a brand new design.

A new engine design can usually take four to six years to complete, starting with component designs all the way through tooling, testing in the field and finally production. This engine is so new that there are no interchangeable components with the trusted former 800 Liberty engine. The Liberty 800 has been changed and updated at least four times over numerous chassis and each time durability and power improved. That is a great testament to the original design!

However, eventually you can only do so much within the constraints of the design you started off with, and if you want to move on to higher performance levels, while also improving durability, it is time to start fresh with a new engine that incorporates all the lessons learned over twenty-five years of Liberty engine development.

■ Applying what was learned
The most critical parts in 2-stroke engines are cranks and pistons. Both live in highly stressed environments, and when performance requires bigger bores, longer strokes and higher RPM, you have to deal with increased component stresses. A good example of bumping up against this wall was the 1000cc twin cylinder engines introduced by Ski-Doo and put in the Mach Z in 2005 and Arctic Cat with their Thundercat that hit the snow several years earlier. There was the 900 Fusion from Polaris just 13 years ago as well. Many of these sleds ran into crank vibration problems, and as a result, clutch and belt durability problems that could not be solved at the time, and they were all quickly dropped in favor of the more reliable, smaller 800 twins.

The new approach today is much more cautious and precise with smaller incremental changes that may eventually lead to 900cc engines once any possible problems are worked out at the 850 level. It is interesting that both Ski-Doo and Polaris chose to increase the stroke to gain displacement. Ski-Doo kept their 82mm piston and increased stroke from 75 to 80mm, while Polaris kept their 85mm bore and went up from 70 to a 74mm stroke.

A good crank working environment requires a sturdy case that does not twist and this is greatly enhanced if the engine also has a mono-block cylinder design instead of separate cylinders. The new twin mono-block cylinder assembly on the new Polaris Patriot 850 went through a design process using a computational fluid dynamics program to optimize water flow around the cylinder, exhaust port, power valve and through the one piece head and around the combustion chambers. The new crank features a larger clutch taper, and higher capacity rod bearings running on stronger redesigned pins.

Rod bearings are also critical in engines with longer strokes, as inertia load goes up. Beefier big end rods are preferred to keep the rod end round under the high load and prevent the rod eye from deforming and “pinching” the bearings. A lot of development went into the new rod design on the Patriot and the piston pins and bearings were also made stronger.
2019 Polaris Patriot 850
2019 Polaris Patriot 850
New, new, new A new throttle body, with contact-less throttle position sensor, plus new V-force reeds in the cage, and more.
Piston design is equally important as bores increase. With bigger bores the distance heat has to travel from the center of the dome, past the rings and into the skirts to be cooled increases. The 85mm piston in the Patriot engine is a complete redesign from the Liberty engine as problems with overheating and collapsing of the dome started to occur on the old engine. The new piston dome is 3mm thicker and there is 25% more skirt area to transfer heat. Polaris went to a single-ring design on the new Patriot pistons. Logic would suggest that two rings would give a better seal, but that is only the case if the rings move freely, which often happens with smaller pistons that have shorter heat paths. With larger pistons, two rings may impede the heat flow to the skirt, and as a result, the ringland overheats. This in turn may cause the ringland to break or distort. When the ringland distorts it pinches the rings and prevents them from sealing. The reasoning behind this decision could be that one ring-groove will not distort as easily, and one ring that always works is more reliable than two that may pinch. The new reed cages are also wider and redesigned to streamline the airflow.

The earliest snowmobiles often had an industrial based 2-stroke engine with mounting bolts at the bottom of the crank case. This was not good for vibration isolation and as sleds matured, engine mounting plates with rubber pads arranged in several angled configurations became standard. Automotive practice shows that ideal mounts should be on the sides, but with an angle upward that intersects some distance above the crankshaft center line and closer to the engine’s center of gravity. This works great in a car with a straight drive shaft, but sleds with CVT belt transmissions take the load off at a 90 degree angle to the crankshaft. This induces a twisting load that distorts the belts during critical acceleration, and as a result, reduces belt life.

Several remedies to decrease twisting loads and increase belt life have been tried over the years, from torque straps to direct links between the engine and secondary shaft. The problem with direct links and no engine movement is that with no give in the system, the belt slips under sudden loads and the crankshaft gets a sharp load spike that may break shafts, bearings and pins. This sharp load is hard on clutch components and belts. The ideal situation is good vibration isolation but with a configuration that allows the engine to move straight back as much as an eighth of an inch during sudden loads. This not only protects the cranks and clutches but is easier on the belt offering better grip instead of spinning loose.

Polaris obviously took a good look at the mounting problems and built new strategic rubber isolation locations directly into the new Patriot crank-cases. The mounting location in the back on the clutch side is directly in line with the belt pull, and the front mounting on the mag side is also aligned with the belt pull. This reduces twist but allows some movement to cushion belt pull. The rear mount on the mag side is high up, allowing the mounts to intersect above the crankshaft centerline and closer to the engine center of gravity.

The new mounting system is an example of a good design with the right compromises between vibration isolation and reduced twist under load, yet compliant enough to protect cranks, clutches and belts from “spike” loads.

■ More power!
Polaris claims higher maximum electrical power at idle and low RPM with a new power-boosting regulator. The flywheel and stator have similar output to the 800, but the output is managed differently through capacitors. This allows the current flow to be evened out and makes more power boost at lower RPM. Basically the electrical system uses wattage more efficiently. The flywheel incorporates a plastic fan and the starter housing incorporates a generous amount of ventilation slots to keep electrical components cooler. More consistent wattage is needed for the larger, improved, and faster acting processor for the new co-planar exhaust valve control integrator in the power valve servo motor.

On the intake side, the 850 Patriot engine breathes through a new lightweight throttle body with improved, more reliable contact-less throttle position sensor. Fuel delivery is still with two electronic fuel injectors positioned in the back ports. Redesigned V-Force reeds with carbon fiber petal seating against a rubber coated cage surface increases petal life. The new Polaris Patriot 850 engine incorporates many new design features aimed at providing more consistent power combined with greater durability. Polaris backs the new Patriot engine with a four-year warranty too.
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