The Clean Snowmobile Challenge (CSC), which is part of the Collegiate Design Series of SAE International, was created to challenge students to reduce the environmental impact of snowmobiles while retaining the essential performance and cost limitations required to ensure a successful recreational market.
Due to the rising environmental concern pertaining to the noise and exhaust emissions of recreational snowmobiling, snowmobiles have come under increased scrutiny by the U.S. federal government.
As snowmobiles are used durning winter, local environmental impacts on air quality are greater due to the cold, dense air. This air will not disperse the exhaust emissions rapidly. Instead, the emissions are trapped, leading to locally high concentrations of pollutants. These hazards are especially of concern to ecologically sensitive areas such as Yellowstone National Park and other national parks where recreational snowmobiling is popular.
Currently, U.S. national parks are operating under a temporary winter use plan that restricts the number of snowmobiles entering the parks per day. All permitted snowmobiles are required to meet or exceed the National Parks Service Best Available Technology (BAT) standard. Thus, only the cleanest and quietest commercially available snowmobiles are allowed. As evidenced by the automotive industry, emissions standards never remain static, but are continually tightened.
Competition rules of the CSC require that teams modify a commercially available snowmobile. This base snowmobile must have been produced in a quantity of at least 500 units between the model years 2006 and 2013.
A team from Kettering University chose a commercially available 2011 Ski-Doo MXZ Sport 600 Advanced Combustion Efficiency (ACE) snowmobile to be modified for the 2013 CSC competition. This snowmobile was chosen because it is equipped with a four-stroke engine, which meets the 2012 NPS BAT requirements without modification, and it is lightweight.
Challenges in design
Since the 2013 SAE CSC used an unknown ethanol-gasoline blended fuel, ranging from 40 to 75% ethanol, an investigation was taken to determine the best use of the fuel in the engine. Of particular interest were the antiknock properties and heat of vaporization. The advantages held by E85 over regular gasoline with respect to those two properties make it an enticing fuel for charge air boosting, even with the 12:1 compression ratio of the 600 ACE engine.
Most of the advantages held by E85 are maintained down to flex fuel blends in the E40-E50 range. Boosted iterations of a GT-Power model were created to evaluate performance within the objectives of the team and the constraints of the competition.
A design strategy that behaves symbiotically with boosting is LIVC (late intake valve closure) to create Miller cycle conditions. Miller cycle operation improves pumping losses from throttling a spark-ignited (SI) engine at part load by decreasing the dynamic compression ratio and amount of retained charge, thus reducing engine output while minimizing the use of the throttle.
Further, the relative increase in expansion ratio relative to the decreased compression ratio allows more of the thermal energy to be captured during the expansion/power stroke of the engine, resulting in improved efficiency. The engine’s decreased output can be mitigated through charge boosting to provide the benefits of LIVC at part-load and increased engine output over the original engine at peak load.
To estimate the effect of intake valve timing on engine performance, a parametric sweep was conducted using the boosted model in Gamma Technologies' GT-Power. The results were then used to determine an appropriate amount of intake camshaft retard for a given intake charge boost pressure.
Based on the simulation results, it was decided that an intake charge boost pressure of 1.6 bar (232 psi) and intake cam timing retarded 20° from the engine would provide the best performance to meet the team objectives. Further, no charge air cooling would be required due to the in-cylinder charge cooling and knock resistance provided by the use of the ethanol blended fuel.
To increase charge air pressure, both supercharging and turbocharging were considered. Recreational and powersport engines such as the 600 ACE do not feature accessory belt drives; thus making it difficult to implement conventional belt-driven superchargers. Further, engine power would be lost in driving the compressor. Finally, a supercharger provides no attenuation of exhaust noise and often creates additional mid-high frequency noise that would not fit within the team objective of creating a quiet vehicle.
A turbocharger was selected for its ability to capture waste heat energy in the exhaust to drive a compressor. Further, the restriction of the turbocharger turbine housing provided significant attenuation of exhaust noise that helped simplify the design of the vehicle silencer. Two turbochargers were evaluated in a compressor matching exercise for use on the Miller cycle turbo 600 ACE—the Garrett MGT1238Z and the Garrett GT1541V. The MGT1238Z was selected because it operated in a more efficient region of the compressor map at peak boost level throughout the engine speed range used by the continuously variable transmission (CVT) equipped snowmobile.
And the winnings are…
Kettering University met its internal objectives of low chemical and noise emissions, excellent fuel efficiency, and uncompromised dynamic performance at the 2013 SAE Clean Snowmobile Challenge. This resulted in the team earning a second place finish overall.
Exhaust emissions were the lowest of any SI engine snowmobile in both laboratory and in-service tests. An E-score of 205 was achieved in the laboratory test and only 10.52 g/mile HC+CO+NOx was emitted during the in-service test.
Noise emissions were also the lowest of the combustion-powered snowmobiles. In the J192 pass-by test, the 2013 Kettering sled emitted only 73 dBA, while the competition control snowmobile emitted 88 dBA. Competition participants also ranked the snowmobile highest for sound quality in the subjective portion of the noise event.
In the fuel efficiency event, the snowmobile had the reliability to finish the 85 mi (137 km) test, with the second lowest fuel consumption, netting 18.6 mpg. All while using E66 fuel (66% ethanol), which has a lower volumetric energy density as compared with gasoline.
In both the acceleration and objective handling events, the snowmobile placed 5th. Further, the Kettering 2013 sled was the fastest of four 600 ACE-powered competitor in the acceleration event.
The snowmobile earned three special awards: the PCB award for Quietest Snowmobile the Sensors Inc. award for Lowest In-Service Emissions, and the Mahle Powertrain award for Best Engine Design.
This article is based on SAE International technical paper 2013-32-9176 by Matt Birt and Gregory W. Davis, Kettering University.Continue reading »