Scandinavian Airlines was faced with a decision to purchase new Boeing 737 aircraft and had to choose between a standard engine or a more expensive "green" engine. The green engine, a two-stage dual annular combustion engine, produced significantly lower NOx emissions but would cost the airline an additional $2 billion. As an airline known for its environmental leadership and operating in a culture that values environmental protection, SAS had to weigh the financial costs against the benefits of upholding its green reputation.