It’s only a matter of time before fan-cooled two-stroke snowmobile engines are due for extinction. With more stringent Environmental Protection Agency (EPA) standards looming that require individual model compliance instead of a fleet average, the dirtiest of the remaining two-strokes have little lifespan remaining.
Fortunately, there is a new example of an entry-level snowmobile engine that delivers comparable performance to a lightweight fan — if measured in speed and peak power output. Its other performance attributes are worthy of fanfare and special attention: nearly 30 miles per gallon, quiet operation and claims of the lowest emissions yet from a snowmobile moved by internal combustion.
The “ACE” moniker on the new Rotax 600 ACE engine is yet another acronym, this time for Advanced Combustion Efficiency. It’s a new engine family that is available in MX Z, Renegade, Grand Touring, Tundra, Expedition and Skandic platforms.
The 600 ACE is a high-tech, 600cc, parallel twin, dual-overhead cam four-stroke with the manufacturer’s 60 hp claim coming at 7250 rpm. Though the Rotax 550F fan-cooled engine remains in the lineup, the ACE is destined to replace it completely either by mandate or consumer choice.
Machines with the 600 ACE engine benefit from a silky-smooth acceleration curve, great fuel mileage, low emissions and incredibly low sound levels, with similar performance to the fan in terms of peak power. Here’s a closer look at the inner workings and design of the new powerplant.
Ski-Doo 600 ACE Efficiency Benefits
With a 74mm bore and 69.7mm stroke, the slightly over-square Rotax 600 ACE is a well-mannered compromise of quick-revving, short stroke and smooth-running “square” design.
The 600 ACE is a parallel twin with a 180-degree firing order position on the forged crank. While this basic architecture has been around for decades, modern design combined with better science and materials have resurrected this dated engine design to many new applications, including ATVs and motorcycles. Generally speaking, the modern parallel twins have several advantages for efficiency and packaging.
Efficiency was the chief target of the 600 ACE, and if the claims are true the result of the effort is the most fuel efficient production snowmobile engine. The 29 mpg advertised spec provides a travel range of more than 300 miles in most Ski-Doo models with the 600 ACE.
The intake tract has a near vertical orientation and there are shallow intake and exhaust valve angles (11 and 13 degrees, respectively) for efficiently moving the fuel-charged air.
The combustion chamber is hemispherical instead of a more typical elliptical dome, which in part allows the high 12:1 compression ratio. Rotax engineers said many high-performance four-stroke engines can’t run compression that high because the fuel charge is less concentrated to the spark, which causes detonation at the edge of the piston.
Burning less fuel means more than simply delivering less. It means metering the fuel precisely for a better, more complete burn that minimizes loss. As a result of its design, Ski-Doo claims its Rotax 600 ACE has high volumetric efficiency because of the combustion chamber shape and airflow. Intake pulses in the plenum are timed to pack in more air at certain rpm that result in efficiency greater than 100 percent at certain engine speeds.
Fuel arrives in the chamber from a single 48mm throttle body, and the engine is safe to burn 87 octane fuel. The ECU gives the fuel metering instructions from a monitoring system with four sensors. Individual sensors monitor coolant temperature, oil pressure and crankshaft speed, and there is a fourth called a Temperature and Manifold Absolute Pressure (TMAP) sensor. The TMAP sensor provides the ECU information relating to inlet manifold vacuum and barometric pressure, along with the temperature of the air in the inlet manifold.
Additional mileage gains come from reduced friction in the valve train and less parasitic systems to drive the water pump and oil pumps (there are three; one primary and two scavenge pumps).
If the 29 mpg claim is true, by the way, the 600 ACE is the not just the mileage and range king, it also is an emissions champion. Current emissions standards from the EPA are a maximum of 75 grams of hydrocarbons (HC) and 200 grams of carbon monoxide (CO) per kilowatt hour. The 600 ACE produces just 8 grams of HC and 90 grams of CO, according to Ski-Doo.
Ski-Doo 600 ACE Packaging Benefits
Rather than performance-minded riders, the 600 ACE engine is aimed at riders who want maximum fuel economy and minimal maintenance. With that goal in mind, the engine was built with several features that require little or no maintenance.
Like other modern four-stroke snowmobile engines, it’s a dry sump design. The oil reservoir is integrated in the engine block and part of the flat-bottom design for lower mounting. Combined with the square/short stroke design, the engine is able to be shorter in overall height for a low center of gravity when strapped to the snowmobile bulkhead.
During the presentation of the 600 ACE, Ski-Doo officials said that the engine is only 13 pounds heavier than the 550 fan engine, due to the clever packaging and lightweight parts. The cylinder head cover is magnesium, and an aluminum magneto housing was used to minimize weight. The design also eliminates parts by including the housings for both the water pump and the oil filter – and the filter is easy to access. Utility model snowmobiles have an additional oil cooler due to the expected heavier duty cycle.
There are additional benefits to the 600 ACE valve and cam chain. Automotive and high-performance motorcycle-style valves have a cam acting directly on a bucket. Where it differs, though, is the bucket resides over a hydraulic lifter instead of a valve shim, eliminating the need for valve service — they are self-adjusting. Likewise, the cam chain uses slippery surfaces to guide the chain with less friction and it does not require a manual adjuster. Material choice and size aim to strike the right balance between durability and light weight.
The best packaging benefit is the way machines we tested with the 600 ACE behaved. As reported on snowgoer.com in a first-ride report, “The Rotax 600 ACE powerplant brings zero intimidation from the silky smooth 2800 rpm clutch engagement all the way up to full throttle. These characteristics make it an excellent sled for beginners, teens and rental shops, or someone who’s looking to take a quiet and smooth snowmobile ride.”
We’re used to four-stroke machines feeling heavier, leaving no mystery as to the location of the engine in the chassis. But the 600 ACE, though not a powerhouse, offers a torquey feel of an electric golf cart.
“Riding, steering and driving the 2011 Ski-Doo MX Z 600 ACE TNT feels the same as any other short-track sled in the REV-XP chassis,” we said. Read the rest of that online review by logging onto snowgoer.com.
— Tim Erickson
