absorbs any engine imbalance. The original opposed-piston SPDE is inherently balanced and does not require the mass. For a single-cylinder engine, depending upon whether an active or passive balancing system is employed, the mass penalty may range between 3% and 10%. Sunpower Inc., of Athens, Ohio, successfully designed, built, and tested a passive balancing unit for a 1 kW free-piston Stirling at NASA Lewis. MTI has also demonstrated a passive balancer for a 3 kW free-piston Stirling engine. One of the first tests to be conducted at NASA is to determine the amount of power consumed in a balancing device for a single-cylinder engine which replaces the large ballast mass.
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