Numerical Investigation of Influence of Injection Timing and Knock on Dual Fuel Engine

Transcription

1 Injection Timing and Knock on Dual Fuel Engine Mario Sremec* Department of Internal Combustion Engines and Motor Vehicles Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Croatia I. Taritaš, M. Sjerić, D. Kozarac Faculty of Mechanical Engineering and Naval Architecture University of Zagreb, Croatia

2 PRESENTATION OUTLINE 1) Introduction and Motivation 2) Diesel Natural Gas Dual Fuel Engines 3) Influence of Injection Timing on Dual Fuel Combustion 4) Knock Phenomena in Dual Fuel Engine 5) Conclusion Injection Timing and Knock on Dual Fuel Engine 2

3 Introduction, motivation (source: Injection Timing and Knock on Dual Fuel Engine 3

4 Dual Fuel Engines Direct injected: both of fuels are injected Directly in cylinder; Dual fuel or diesel dual fuel or DDF engine - Diesel engine configured to use CNG Primary fuel: Natural gas(cng) Secondary fuel: Diesel Engine operate with both of fuels simultaneously Diesel fuel is used for start of combustion, then burns rest air-cng mixture Technologies/combustion/hpdi; 2015 Port injected: Gas injection in intake pipe, diesel directly in cylinder like a pilot fuel Simply converting to monofuel (only diesel fuel) Injection Timing and Knock on Dual Fuel Engine 5

5 Dual Fuel Engines Port injected dual fuel Injection Timing and Knock on Dual Fuel Engine 6

6 Simulated engine Injection Timing and Knock on Dual Fuel Engine 7

7 Fuel slip Valve overlap: advantage by diesel engine better cylinder scavenging disadvantage by port injected dual fuel CNG slip in exhaust pipe through exhaust valve 50 CA Injection Timing and Knock on Dual Fuel Engine 8

8 Simulation model Fuel injector Software: AVL Boost v2013 Injection Timing and Knock on Dual Fuel Engine 9

9 Injection Timing Case Name n (min -1 ) p intake (bar) λ (air excess ratio) EGR (%) Simulated cases with continuous fuel injection Case Name n (min -1 ) p intake (bar) λ (air excess ratio) EGR (%) Injection duration ( CA) Start of injection (SOI) ( CA) relative to FTDC , 180 0, 100, 200, 300, 400, , 180 0, 100, 200, 300, 400, 500 Simulated cases with intermittent fuel injection Injection profiles with injection duration = 90 CA Injection profiles with injection duration = 180 CA Injection Timing and Knock on Dual Fuel Engine 10

10 Injection Timing Detail of simulation model Naturally aspirated condition Charged condition Fuel slip through exhaust valve at continuous fuel injection. Injection Timing and Knock on Dual Fuel Engine 11

11 Injection Timing The best injection timing decreases of fuel slip up to 50 % at 1000 rpm Injection duration = 180 CA, p intake = 1 bar Charged engine - similar behaviour Reduction in fuel slip 43 %, compared to continuous injection Injection duration = 90 CA, p intake = 1 bar Injection Timing and Knock on Dual Fuel Engine 12

12 Injection Timing SOI = Start of Injection EOI = End of Injection SOI + Injection duration = EOI SOI + inj. Dur. = EOI 220 CA CA = 400 CA Injection duration = 180 CA, p intake = 1 bar SOI + inj. Dur. = EOI 320 CA + 90 CA = 410 CA EOI more important point in decreasing of fuel slip then SOI the critical point for EOI is around 410 CA (50 CA ATDC) Injection duration = 90 CA, p intake = 1 bar Injection Timing and Knock on Dual Fuel Engine 13

13 Injection Timing Injection duration (ID)= 180 CA, p intake = 1 bar Result of backflow of charge from cylinder to the intake pipe caused by late intake valve closing Fuel concentrations in intake pipe in dependence of crank angle Injection Timing and Knock on Dual Fuel Engine 14

14 Knock Original engine compression ratio 20.5 Naturally aspirated not prone to knock with only diesel fuel Too high compression ratio for knock free operation with dual fuel Simulation parameters: Excess air ratio λ = 1,5 Combustion duration = 40 CA Calculation with 2 zones (burned and unburned) Vibe model and integrated knock calculation was used Kozarac, D., Tomic, R., Taritas, I., Chen, J. et al., "A Model for Prediction of Knock in the Cycle Simulation by Detail Characterization of Fuel and Temperature Stratification," SAE Int. J. Engines 8(4): , 2015, doi: / Kozarac, D., Schuemie, A., Ofner, H., and Tatschl, R. Modeling of natural gas engine with the emphasis on prediction of knock presented at 6th European Congress on Computational Methods in Applied Sciences and Engineering (ECCOMAS 2012), Austria September 10-14, Injection Timing and Knock on Dual Fuel Engine 15

15 Knock simulation results SOC Start of Combustion Original engine knock intensity larger than 0 in all ranges of speeds Not acceptable Knock intensity, compression ratio = 20.5, p intake = 1 bar Advanced SOC results with higher knock intensity Knock intensity for compression ratio = 19, p intake = 1 bar. Injection Timing and Knock on Dual Fuel Engine 16

16 Knock simulation results No tendency to knock with SOC 0 CA (TDC), or later Acceptable for naturally aspirated conditions Knock intensity for compression ratio = 18, p intake = 1 bar Knock intensity for compression ratio = 17, p intake = 1 bar Injection Timing and Knock on Dual Fuel Engine 17

17 Knock simulation results - Charged conditions Elevated intake pressure increases knock intensity values! Knock intensity for compression ratio = 17, p intake = 2.2 bar Can be accepted because engines operate at low speed with λ higher than 1.5. Knock intensity for compression ratio = 16, p intake = 2.2 bar Injection Timing and Knock on Dual Fuel Engine 18

18 Conclusions The fuel slip from cylinder due to valve overlap is highest at low engine speed (1000 rpm) At higher engine speeds (2000 rpm, 3000 rpm) fuel slip becomes very low. Appropriate intermittent injection timing can reduce fuel slip up to 33% compared to the continuous injection at defined excess air ratio, while inappropriate intermittent injection timing can result with increase of fuel slip up to 62% in charged conditions. When diesel engine is converted into the conventional dual fuel engine, a risk of knock combustion exists, especially in charging conditions. The advanced start of combustion increases the tendency to knock, while longer combustion duration decreases the tendency to knock. Compression ratio 16 allows knock free operation in charged engine, if the start of combustion is at the top dead center or later. Earlier start of combustion increases the risk of knock occurrence. Naturally aspirated engine can operate with compression ratio of up to 18 without the risk of knock, with start of combustion also at top dead center or later. Injection Timing and Knock on Dual Fuel Engine 20

19 Thank You for attendance! The study was performed within the FMENA project Experimental Research, Optimization and Characterization of piston engine operation with DUal-Fuel COmbustion - DUFCOROC IP funded by the Croatian Science Foundation. This help is gratefully appreciated. Injection Timing and Knock on Dual Fuel Engine 21