Low pressure EGR system and examining method for efficiency of low EGR cooler

A low pressure EGR system, which includes an intake line through which an intake air is supplied into an engine, an exhaust line through which an exhaust gas of the engine passes, a low pressure EGR line that is diverged from the exhaust line and connected to the intake line, a low pressure EGR valve that is disposed on the low pressure EGR line, and a low pressure EGR cooler that cools the exhaust gas flowing through the low pressure EGR valve, the system comprising:

• pressure sensors disposed at a downstream side and an upstream side of the low pressure EGR valve;
• a temperature sensor that is disposed at a downstream side of the low pressure EGR cooler to detect a temperature at the downstream side of the low pressure EGR cooler;
• an ECU
• wherein the ECU sets a threshold of a difference between a model temperature and a detected temperature downstream of the low pressure EGR cooler, measures a pressure difference between the upstream side and the downstream side of the low pressure EGR valve by using the pressure sensors, calculates a mass flow rate of the exhaust gas passing through the low pressure EGR cooler based on the measured pressure difference calculates an efficiency of the low pressure EGR cooler based on the calculated mass flow rate, calculates the model temperature downstream of the low pressure EGR cooler based on the calculated efficiency;
• and compares the model temperature downstream of the low pressure EGR cooler with the detected temperature downstream of the low pressure EGR cooler,
• wherein the mass flow rate of the exhaust gas is calculated according to a first equation of
• =ρexhQexh=C·AFlap√{square root over (2ρexhΔP)}=f(ΔP,valve position)√{square root over (2ρexhΔP)} Eq. (1)
• wherein, the is a mass flow rate of the exhaust gas flowing in the low pressure EGR line,
• wherein the ρexh is a density of the exhaust gas,
• wherein the Qexh is a volume flow rate of the exhaust gas,
• wherein the C is a coefficient,
• wherein the AFlaP is a cross-section of the low pressure EGR valve,
• wherein the ΔP is the pressure difference between the upstream and the downstream of the low pressure EGR valve,
• wherein the efficiency of the low pressure EGR cooler is calculated according to a second equation of
• η=f(,Tcoolant−Tin), Eq. (2)
• wherein the Tcoolant is a coolant temperature flowing in or flowing out of the low pressure EGR cooler, and
• wherein the Tin is an exhaust gas temperature of an upstream side of the low pressure EGR cooler,
• and
• wherein the calculating of the model temperature downstream of the low pressure EGR cooler is performed by substituting the second equation into a third equation of
• Tout=Tin−η(Tin−Tcoolant), Eq. (3)
• wherein the Tout is the model temperature of the downstream side of the low pressure EGR cooler, and
• wherein the η is the efficiency of the low pressure EGR cooler.