Polaris 750cc Four-Stroke Engine
January 20, 2006
Filed under Snowmobile Reviews
One of the wonderful new engines we had the opportunity to test at last spring’s Rode Report was Polaris new 750cc four-stroke engine. The engine’s naturally-aspirated design is found in the FS Classic and was turbocharged in the FST SwitchBack and FST Classic.
The engine is produced by the Weber company of Germany and was originally developed for a micro car named the Matra. This engine is a parallel twin with a displacement of 749cc; a bore of 85mm, stroke of 66mm.
It is a liquid-cooled engine with a dry sump design, which means the oil is circulated through the engine and contained in a separate reservoir rather than the crankcase. This allows more freedom in positioning the engine in the chassis and makes for easier low temperature engine cranking.
A dry sump engine design also allows engineers to use the oil as a coolant because it is circulated outside the engine. The more heat produced, the more horsepower gets made. When a turbocharger is used to jam more oxygen into an engine and add more fuel, more power and more heat gets produced.
The FST engines utilize the oil to help control internal engine temperatures. The FST versions of the Weber engine have oil “squirters” that spray oil underneath the pistons to help control its operating temperatures. The oil is collected and sent out through the reservoir/cooler and then circulates back to help again.
The FS version of the engine produces considerably less heat and does not incorporate oil “squirters” as part of its cooling system. The oil is still circulated through the external reservoir/cooler, which does help remove heat from the engine.
Both the turbocharged and non-turbo version of the engine make its peak power at about 7800 rpm and the rev limit is set at 8000 rpm. The turbo version produces 135 hp while the FS version peaks at 80 hp. Clutch engagement speed is set at a rather modest 3500-4000 rpm.
A chain-driven, single overhead camshaft lifts four valves in each cylinder. The crankshaft is a 360-degree design, which means both pistons arrive at Top Dead Center (TDC) at the same time. A four-stroke engine fires each cylinder only every other revolution so the crankshaft gets a power stroke every 180 degrees of rotation from the twin cylinders.
A 360-degree crankshaft requires a lot of counterbalance on the side of the crankshaft opposite the connecting rod journals. That added mass cancels out vibration as both pistons hit TDC and BDC but it increases vibration 90 degrees to either side of TDC and BDC. To counter the vibration growth at 90 degrees from the centers, a separate balancing shaft is driven off the crankshaft. It generates centrifugal forces that offset the vibrations, which are generated perpendicular to the bores of the engine by the large counterbalance masses on the crankshaft.
Fuel delivery is provided by a Bosch electronic fuel injection system. The injector nozzles are mounted on the fuel rail behind the cylinders and inject the fuel at the rear or rider side of the cylinders. Air intake is controlled by 39mm throttle bodies on the FST engines and 43mm throttle bodies on the FS model. A throttle position sensor on the throttle bodies cues the engine management system to the power loads requested by the driver.
The turbocharged version of the engine is truly remarkable. We tested the engine at altitudes above 6,000 feet and noticed a slight turbo lag, but very little. At Midwest altitudes, there is no perceptible lag.
A turbocharger is an air compressor that uses energy the engine is already throwing away — in the form of exhaust gases — to power it. A turbocharger utilizes two turbine blades: one on each end of a common shaft.
At one end of the shaft, the blades are called the turbine and are placed in the direct flow of the exhaust gases at its outer diameter. As the exhaust gases rush past, they pull the turbine wheel with them, in turn, rotating the shaft, which is common with the compressor wheel. The compressor wheel draws air in from its center and throws it to its outer diameter, compressing it.
The compressed air then routes to the intercooler, then to the engine intake. It is a centrifugal compressor, boosting intake pressure to one bar, which is about 14 pounds per square inch (psi) above atmospheric pressure.
The turbocharger has an electronically-controlled wastegate that redirects exhaust gases around the turbine wheel to reduce its speed when cruising at part throttle in a sedate speed. The engine’s electronic management system controls the position of the wastegate based on the input of many sensors on the engine. An air-to-air intercooler with a fan-assist reduces the temperature of the compressed intake air.
Sensors include air intake temperature, boost pressure, knock sensor, crankshaft position, throttle position, atmospheric pressure, engine speed and coolant temperature. With the information from the sensors, the fuel delivery, ignition timing and boost pressure are adjusted by the engine management system controller.
The FST engine can be run on 87 octane or premium gasoline. If premium is used, ignition timing, boost and fuel delivery are automatically adjusted and an increase in power should be noticed.
The FS version of the engine is calibrated to run on 87 octane gasoline and will not offer an increase in power if premium gasoline is used.
A 50-amp alternator supplies more than adequate electrical power for the machine. The machine requires only about 20 amps to power all systems, which leaves 30 amps for accessories. Both engines are fired up with electric starters.
The FS and FST both utilize front and rear coolers and a front mounted radiator. The FS versions utilize the running board perimeter coolers to transfer coolant from front to rear, while the FST has additional coolers beneath the running boards.
The fact that the power feels so seamless on the FST comes from a diligent design of the air box, intercooler, the electronic wastegate, the clutch calibration and a wonderful job of programming the engine management system. Automotive in design, these management systems are performing more and more complex functions every year.
The on-board computer also stores self-diagnosis data for the dealer to retrieve when troubleshooting a problem with an engine. Just plug in the proper analyzer and the engine manager will release any trouble codes and other engine data.