If you missed yesterday's post, it was about diesel engines and how urea selective catalytic reduction systems (SCR) can reduce nitric oxide emissions from the engine's exhaust very effectively. The trouble is that SCRs require the user of the car to be responsible for keeping the on-board urea tank replenished. The EPA plans to issue guidelines for urea SCR systems that could be as drastic as requiring a vehicle to stop running if drivers don't keep the tank filled. Manufacturers are opposing this measure, citing safety issues. While the need to fill an additional tank is clearly a drawback of the urea SCR systems, they allow diesel engine manufacturers to meet the strictest worldwide emissions regulations without fuel economy penalty and at an acceptable price.