Exercise 7 Flashcards
Exhaust gas recycling is generally more effective with spark-ignition engines then diesel. Why?
Exhaust gas reticulation is generally more efficient in SI engines because adding exhaust gas displaces some of the intake air and hence absorbs heat leading to lower temperature and hence redusing NOx. For CI engines, combustion is heterogeneous, reducing the amount of O2 will reduce the NOx, but will also create soot and reduce efficiency.
Why does CI engines have so much more PM emissions then SI engines?
CI engines have so much more PM emissions because CI do not premix and when compression occur, this mix is hence not perfectly homogeneous, leading to zones in chamber with especially fuel rich zones. As the combustion in incomplete, soot will be created from the access fuel.
Diesel engines have low CO emissions. SI engines need a catalyst to reduce CO emissions, especially for cars. Why the difference?
CO is the product of incomplete combustion of hydrocarbons and often occur due to fuel rich mixtures. SI engines often run fuel rich and experience transient conditions like cold starts, leading to more CO, while diesel always operate lean.
