Kul-14.4100 Internal Combustion Engine Technology Gas Exchange, 2015 Topics Gas exchange in four stroke engines Volumetric efficiency Valves and valve flow Two stroke engine scavenging Camshaft and intake systems 2 1
Gas Exchange in 4-Stroke Engines Exhaust and intake strokes = gas exchange NON SUPERCHARGED = NATURALLY ASPIRATED ENGINES Gas exchange starts at exhaust valve opening. A high pressure pulse (blow down pulse) enters the exhaust channel and exhaust manifold and cylinder pressure starts rapidly to fall down. There is an over pressure in the cylinder over the whole exhaust stroke while piston pushes residual gases away from the cylinder. 3 Gas Exchange in 4-Stroke Engines The exhaust valve is opened before the bottom dead center in order to have the valves open enough when piston starts to move up. The exhaust valve is closed after the top dead centerin order not to choke the flow too early. Heywood Fig. 6-1 (d). The piston needs to make work on the gases during gas exchange. Especially in SI engines with stoichiometric combustion the gas exchange work is remarkable at part load = pumping losses. The intake air flow is throttled and there is a remarkable underpressure in the intake manifold. 4 2
5 Valve Overlap During valve overlap period one must take care of valve and piston clearance. Valves may not collide with the piston. Some pistons have so called valve pockets if compression ratio is high and large overlap perod. Turbocahrged engine typically habe larger overlap than naturally aspirated engine to ease the operation of compressor at low engine speed. Negative overlap tells about special valve timing, where exhaust valves are closed before top dead center and intake valves are opened after top dead center. This is made deliberately to leave some of the residual gases in the cylinder. This is also called Internal Exhaust Gas Recirculation, Internal EGR). This is made to reduce combustion temperatures to reduce NOx formation. 6 3
Turbocharged Engines During intake stroke there is over pressure in the intake manifold or scavenging air receiver. At high load and high charge air pressure the gas work towards piston might be positive. At part load however, there may be some backflow during gas exchange. 7 8 4
Volumetric Efficiency ma Volymetric efficiency (Heywood 6-9) η v= ρvd If the fuel air mixture i.e. charge is made in the intake manifold or in the intake channel, the volume fraction of the fuel (on molar basis) takes part of the cylinder volume. That reduced volumetric efficiency. (Kvasistatic effect) Gaseous fuels may lead up to 30% reduction of the volumetric efficiency. On the contrary, the liquid fuel evaporation may reduce the charge temperature which then increases the volumetric efficiency. Residual gases left in the cylinder also reduce the volumetric efficiency 9 Fuel vapor reduces volumetric efficiency 10 5
Volumetric Efficiency Volymetric efficiency (Heywood 6-9) Charge heat up, B Flow friction (~ v^2) in manifolds and channels, C Flow choke (speed of sound), especially in intake valves, D Flow inertia, conservation of momentum, E Backflow at low load and speed, F Tuning, G. The usage of pressure pulses. Variable lenght intake piping. 11 12 6
Volumetric Efficiency The restricted (small) valve opening periods increase the low speed area volumetric efficiency and thus increase the low speed torque, as well. Large opening periods do increase the high speed volumetric efficiency and torque, but too large period make the low speed torque remarkably low. The valve lift increase makes the volumetric efficiency higher to some extent. Typically there is no increase of the flow cross section area over the lift 0.25*D, where D is the inner seat diameter. 13 Volumetric Efficiency 14 7
Typical Pressure Pulses 15 Volumetric Efficiency There are high pressure pulses in the intake and exhaust manifold proceeding with the speed of sound. That is why the pulses look quite different at different speeds on crank angle basis. p1, intake 150 mm before cylinder p2, exhaust 200 mm after cylinder p3, intake 700 mm before cylinder 16 8
Volumetric efficiency Prechamber diesel engine volumetric efficiency at Wide Open Throttle (WOT). The double pulse due to intake tuning. The SI engine volumetric efficiency is less due to the throttling of air flow,charge heat up, fuel evaporation and the large amount of residual gases in cylinder. 17 Tuning Intake manifold lenght and the volumetric efficiency, 2.3 l SI-engine 18 9
Poppet Valve Geometry 19 Inlet and Exhaust Valve Design 20 10
Intake Valve Cd vs. Valve Curtain Area 21 Valve flow reference area 1. Inner Seat Diameter bases area, pi*d**2/4 2. Valve Curtain area pi*d*l 3. Real, physical cross section Case one is the most suitable. The discharge coefficient start fron zero and strong dependet on L/D ratio. (Reference area is constant.) In Europe Cd is often replaced by the symbol μσ (myysigma). 22 11
Exhaust valve flow 23 Cd with Valve Curtain Area as Ref. 24 12
25 Myy sigma and flow equations 26 13
27 28 14
Residual gas fraction 29 Exhat pulse mass flow 30 15
16 31 Real cross section and flow equations ) sin 2 2 2 ( cos β β π v v v m L w D L A + = [ ] 2 1 2 2 ) tan ( w w L D A v m m + = β π ) ( 4 2 2 s p m D D A = π 2 1 1) ( 0 1 0 2 1 0 0 1 1 2 ) ( = γ γ γ γ γ p p p p RT p A C m T T R D & 1) 2( 1) ( 2 1 2 1 0 0 1 2 ) ( + + = γ γ γ γ γ RT p A C m R D & 32 Two-Stroke Engines
33 Two stroke scavenging period 34 17
Scavenging pump alternatives 35 Two stroke scavenging Delivery ratio Λ (6.20) is fresh air mass delivered divided by a reference mass a. displacement * density at ambient conditions or b. displacement * density at intake conditions or c. cylinder charge i.e. the mass of the cylinder contents. Scavenging efficiency η sc is the new fresh charge in the cylinder divided by the cylinder charge. 36 18
Two stroke scavenging Trapping efficiency η tr is the ratio of the new fresh air trapped in the cylinder to the air delivered. Purity (does not include residual gas air) Charging efficiency η ch is the ratio of the new fresh air trapped in the cylinder to the displacement * density at intake conditions 37 Ideal scavenging models η = Λ and η = 1 sc η = 1 and η = Λ sc tr tr 1 for Λ 1 for Λ > 1 dm ar m = 1 ar dmad mtr m m m = 1 exp m η = 1 e sc ar tr Λ 1 ηsc = (1 e Λ Λ ) ad tr 38 19
Ideal scavenging models 39 Real scavenging 40 20
Intake Port Configurations 41 Intake port flow 42 21
Cam 43 Cam 44 22
Intake systems 45 Variable lenght intake runner 46 23
Charge Cycle 47 Two-Stroke 48 24
Two-Stroke 49 Two-Stroke 50 25
Two-Stroke 51 Variable valve actuation 52 26
Miller cycle Miller cycle, early intake valve closing Atkinsson cycle, late intake valve closing Increased thermal efficiency of the engine process Lower compression temperatures and lower peak temperature during combustion Reduced NOx formation To keep the same in-cylinder air amount and torque, we do have to increase charge air pressure. 53 Variable valve actuation 54 27
Variable valve actuation 55 Variable valve actuation 56 28
Variable valve actuation 57 Variable valve actuation, valvetronic 58 29
Valve actuators 59 References References: Internal Combustion Engine Handbook by Richard van Basshuysen and Fred Schäfer Internal Combustion Engine Fundamentals by Heywood Thermodynamic der Verbrennungskraftsmaschinen by Pischinger 60 30