The regulation proposes an 11 year time frame for compliance, mandating 50% of the engine fleet meet NOx targets within 6 years, and 100% of the fleet meeting NOx targets in 11 years. Owners/operators of these engines will have two approaches to meet compliance: a Per-Unit approach, and an average approach. It is important for owners/operators of these engines to complete a detailed fleet analysis, business plan and implementation plan promptly to determine the scope of their compliance responsibilities.