Selective catalytic reduction (SCR) system and engine

An engine, comprising a selective catalytic reduction system, wherein the selective catalytic reduction system comprises:

• a box body;
• a mixer;
• a baffle;
• and
• a carrier, wherein
• the mixer, the baffle, and the carrier are sequentially arranged inside the box body in a flowing direction of a gas in the box body,
• the baffle is provided with a plurality of through holes, and a distribution density of the plurality of through holes and/or an opening area of the plurality of through holes are negatively correlated with a distribution uniformity index of NH3 at the baffle, and
• the distribution uniformity index is obtained based on a formula:
• U
• vapor
• =
• 1
• -
• 1
• 2
• ⁢
• ∑
• i
• =
• 1
• n
• ❘
• "\[LeftBracketingBar]"
• m
• i
• ″
• -
• m
• mean
• ″
• ❘
• "\[RightBracketingBar]"
• ⁢
• A
• i
• Am
• mean
• ″
• wherein, m″i is an NH3 mass fraction value on each grid on a selected plane, m″mean is an average NH3 mass fraction value on the selected plane, Ai is an area of each grid on the selected plane, and A is an area of the selected plane.