Intake, Exhaust, and In-cylinder Flow. Section 4

Transcription

1 Intake, Exhaust, and In-cylinder Flow Section 4 1

2 2 Valve Flow At WOT the most significant gas flow restriction in an IC engine is the flow through the intake and exhaust valves = k k v o k P P Flow chokes when 1) 2( 1 1) 2( = + = k k o o v f k k o v o f cr k T P R k A c k A c c m ρ, mass flow rate independent of P v = + k k o v k o v o v o f P P P P k A c c m ρ P o = upstream stagnation pressure, P v = valve static (for subsonic =P cyl ) ρ o = stagnation density, c o = krt o stagnation speed of sound, A v = valve area, c f = flow coefficient o P o m ρ,, P v P cyl The mass flow rate through the valve is given by:

3 Valve Flow Minimum areas: low lift - A v = A 1 = πdl A 1 high lift - A v = A 2 = πd 2 /4 l d A 2 d Low lift High lift flow coefficient (c f ) = effective flow area (Af ) actual valve area (A ) v 2 discharge coefficient (c d ) = effective flow area (Af ) actual valve area (A ) v 1 3

4 4 Flow Coefficient Measurement Set: A v Measure: m i, T i, P i Calculate: c f Flow coefficient (c f ) Nondimensional valve lift (l/d) = k k o v k o v o v o f P P P P k A c m c ρ

5 Valve Sizing In order to avoid choked flow the intake valves are sized based on: ( U ) p 2 max Av 1.3b ci where Av is the average valve area, b is the cylinder bore, U is average p piston velocity at max engine speed, c i speed of sound of gas in intake port. Exhaust valves can be smaller since the speed of sound of the exhaust gas expelled is significantly larger. Since there is only so much room available for valves it is common to have multiple intake and exhaust valves per cylinder. This increases valve area to piston area ratio permitting higher engine speeds. 5

6 Valve Sizing Heads are often wedge-shaped or domed, this permits A v /A p up to 0.5. Permits more than two valves per cylinder Limited to two valves per cylinder 6

7 Valve Sizing Double overhead cams per cylinder bank are used to accommodate multiple valves, one cam for each pair of intake and exhaust valves 7

8 Valve Opening and Closing In thermo cycles it is assumed the valves open and close instantaneously In reality a cam is used to progressively open and close the valves, the lobes are contoured so that the valve lands gently on the seat. EVO Valve displacement (l) TC IVO EVC 180 o BC IVC Duration CA Valve starts to open Valve completely closed 8

9 Valve Overlap In real engines in order to ensure that the valve is fully open during a stroke, for high volumetric efficiency, the valves are open for longer than 180 o. The exhaust valve opens before BC and closes after TC The intake valve opens before TC and closes after BC. At TC there is a period of time called valve overlap where both the intake and exhaust valves are open. EVO 4 e i IVO EVC IVC 5 1 TC 180 o BC BC TC BC CA 9

10 Valve overlap When the intake valve opens btc the cylinder pressure is at roughly P e Part throttle (P i < P e ): residual gas flows into the intake port. During intake stroke the residual gas is first returned to the cylinder then fresh gas is introduced. Residual gas reduces part load performance. WOT (P i = P e ): some fresh gas can flow out the exhaust valve scavenging residual (increases power but reduces fuel efficiency and increases emissions) Supercharged (P i > P e ): fresh gas can flow out the exhaust valve P i P e P i P e P i P e Throttled P i < P e WOT P i = P e Supercharged P i > P e 10

11 Engine Operating Conditions Conventional engines operate at low rpms, with idle and part load fuel economy being most important. High performance engines operate at high rpms, with WOT torque (i.e., volumetric efficiency) being most important. WOT bmep sfc Engine load Engine speed: Idle rpm Economy rpm Performance rpm 11

12 Valve Timing Conventional Performance EVO EVO e i IVO EVC IVC e i IVO EVC IVC TC 180 o BC TC 180 o rpm intake duration: 230 o = 38.4 rpm 230 o = 15.4 rpm 230 o = 7.7 ms, 285 o = 9.5 ms 12

13 Valve Overlap Overlap 15 o 65 o At high engine speeds less time available for fresh gas intake so need more crank angles to get high volumetric efficiency large valve overlap At low engine speed and part throttle want to minimized valve overlap Variable Valve Timing (VVT) used to obtain optimum performance over a wide range of engine speeds and load Variable valve lift: high speed want high lift to increase air mass flow rate, low speed want low lift to minimize overlap effects 13

14 Honda Variable valve Timing and lift Electronic Control (VTEC) Intake valve pair has three cam lobes, two that operate the valves at low-rpm, and a third that takes over at high rpm (4500 rpm). Rocker Camshaft Follower First introduced in N.A Honda NSX model. 14

15 VTEC Intake Valve Operation During low-rpm operation, the two rocker arms riding the low-rpm lobes open the intake valves. During high-rpm operation a pin locks the three rocker arms and the valves are opened by the larger center cam lobe. High rpm lobe has longer duration and higher lift raises max to 8000 rpm giving higher peak power (good for racing) no benefit below 4500 rpm 15

16 Honda DOHC 3VTEC 16

17 Latest VTEC systems no pin pin 1 pin 2 med-cam high-cam low-lift med-lift med-lift high-lift Stage 1 (low speed): left valve left rocker arm driven by the low-lift left cam. Right valve right rocker arm driven by the medium-lift right cam Stage 2 (medium speed): left and right valve right rocker arm driven by the medium-lift right cam Stage 3 (high speed): left and right valve middle rocker arm driven by the high-lift right cam i-vtec (2001): VTEC + continuously variable camshaft phasing for benefit even at lower speeds 17

18 VVT - Cam Phasing Shifts the phase angles of the camshaft, does not change the valve open duration. Most systems provide inlet, two-stage discrete phasing (0 o and 30 o ), others provide continuous phasing (0 o - 30 o ) At low speed, 0 o phasing is used so as to minimize valve overlap to minimize residual gas backup into intake (good idle performance) At high speeds, max phasing so as to increase valve overlap high-speed exhaust gas inertia pulls in fresh gas purging residual gas out of cylinder (improves volumetric efficiency) 18

19 BMW Double VANOS and Valvetronic Double VANOS system provides continuous phasing for both the intake (max range 40 o ) and exhaust valves (max range 25 o ) Cap moves towards or away from the cam based on engine speed and gas pedal position by varying hydraulic pressure in the two chambers Valvetronic also permits continuously variable intake valve lift, from ~0 to 10 mm, on the intake camshaft. This eliminates the need for a throttle valve reducing pumping losses (10% improvement in power and fuel economy). 19

20 Delphi cam phasing system LP HP 20

21 Toyota s VVTL VVTL uses cam phasing and two cam profiles for duration At low rpm: long duration cam not engaged, short duration cam runs on roller follower to reduce friction At high rpm: long duration cam engaged by sliding pin and locking follower height also increases the lift (for Honda VTEC, both the duration and lift are implemented by the cam lobes) 21

22 Solenoid Activated Valves Needs a large alternator to supply high current, also gently seating the valve is difficult, needs sophisticated electronics 22

23 Intake and Exhaust Processes in 4-Stroke Cycle EVO P, L v P = cylinder pressure L v = valve displacement 1 Exhaust P L v, exh L v, int Pp o o Intake P o TC 1 st crank shaft rev: nd crank shaft rev: 4 BC WOT Part throttle e i 3 2 TC 1 4 BC 23

24 Valve Float The valve spring normally keeps the top of the valve stem in contact with the cam lobe At very high engine speeds, and thus high camshaft speeds, it is difficult to maintain contact between the cam lobe and the top of the valve stem as a result the valves stay open longer than desired and slam into valve seat. Spring 24

25 Intake and Exhaust System for Single Cylinder Engine P Air cleaner P o, T o Cylinder P o Muffler 25

26 Intake and Exhaust Manifold The intake manifold is a system designed to deliver air to the engine from a plenum to multiple cylinders through pipes called runners. Velocity magnitude (m/s) Exhaust manifold used to duct the exhaust gases from each cylinder to a point of expulsion such as the tail pipe. 26

27 Manifold Pressure 3000 rpm 6000 rpm 27

28 Supercharger and Turbocharger These devices are used to increase the power of an IC engine by raising the intake pressure and thus allowing more fuel to be burned per cycle. Allows the use of a 4 cylinder instead of 6 cylinder engines cost effective and weight reduction Superchargers are compressors that are mechanically driven by the engine crankshaft and thus represents a parasitic load. Compressor P int > P atm P atm W in 28

29 Positive Displacement Compressors Positive displacement compressors: piston, Roots, and screw Most common is the Roots compressor pushes air forward without pressurizing it internally. P 1 P 2 Pressurization occurs in the manifold when the air flow rate supplied is larger than that ingested by the cylinders. Produces constant flow rate independent of boost pressure (P 2 ) 29

30 Performance of Positive Displacement Compressors s/c o = rotor tip Mach# ~ pump speed Screw Roots η c η c = compressor efficiency = ratio of isentropic work and actual work Extra energy goes to heat up air leading to a reduction in density 30

31 Dynamic Compressors Dynamic compressor has a rotating element that adds tangential velocity to the flow which is converted to pressure in a diffuser. Most common is the radial (or centrifugal) type Produces a constant boost pressure independent of the mass flow rate 31

32 To the left of surge line the flow is unstable (boundary layer separation and flow reversal) To the right of 65% line the compressor becomes very inefficient: a) air is heated excessively b) takes excess power from the crank shaft Mass flow rate (Pounds of air per minute) 32

33 Turbochargers couple a compressor with a turbine driven by the exhaust gas. The compressor pressure is proportional to the engine speed Aftercooler Compressor also raises the gas temperature, so after-coolers are used after the compressor to drop the temperature and thus increase the air density. 33

34 The peak pressure in the exhaust system is only slightly greater than atmospheric small P across turbine In order to produce enough power to run compressor the turbine speed must be very fast (100k-200k rev/min) long term reliability an issue Takes time for turbine to spool up to speed, so when the throttle is opened suddenly there is a delay in achieving peak power - turbo lag EXHAUST FLOW INTAKE AIR 34

35 Waste gate valve used to bypass exhaust gas flow from the turbine It is used as a full-load boost limiter and in new engines used to control the boost level by controlling the amount of bypass using proportional control to improve drivability Engine WASTE GATE Proportional valve Exhaust P atm AIR P atm WASTE GATE Turbine Compressor 35

36 Turbo Lag Reduction: Twin Turbo Two turbochargers: Smaller turbo for low rpm low load and a larger one for high load Smaller turbo gets up to speed faster so reduction in turbo lag Supercharger/turbo: Supercharger used at low speed to eliminate turbo lag At higher rpm turbo charger used exclusively to eliminate parasitic load 2006 Volkswagen Golf GT 1.4 L GDI uses twin turbo: rpm roots blower >3500 rpm turbocharger 36

37 BMW 2.0L I4 turbo diesel surpasses 100 hp/l (75 kw/l) 2008 BMW 4.4L V8 valley mounted twin turbo 37

38 Turbo Lag Reduction: Variable Geometry Turbo (VGT) Variable guide vanes direct the flow of exhaust gas from the engine in exactly the direction required on to the turbine wheel of the turbocharger. Good response and high torque at low engine speeds as well as superior output and high performance at high engine speeds VGT used on diesel engines with exhaust temps ( C) not normally used in SI engine due to high exhaust temp (950 C) Guide vane 2006 Porche 911 Variable Turbine Geometry uses temperature-resistant materials 38

39 Low engine rpm (low exhaust flow velocity): Vanes are partially closed accelerating the exhaust gas flow. The exhaust flow hits the turbine blades at right angle. Both make the turbine spin faster High engine rpm (high exhaust flow velocity): The vanes are fully opened to take advantage of the high exhaust flow. This also releases the exhaust pressure in the turbocharger, saving the need for waste gate. 39

40 Variable Geometry Turbo Holset VGT 40

41 Volumetric Efficiency Recall volumetric efficiency is defined as: η v = ρ ρ V P cyl Vd V m a a, cyl d, a cyl, a = = P a, ovd ρa, ovd o d T T o Volumetric efficiency is affected by : i) Fuel evaporation ii) Mixture temperature iii) Pressure drop in the intake system iv) Gasdynamic effects v) Valve timing Note: mean piston speed proportional to air flow velocity or engine speed N = ( S / 2) U p 41

42 Factors affecting η v Fuel evaporation: In naturally aspirated engines (no supercharging) the volumetric efficiency will always be less than 100% because fuel is added and the fuel vapour will displace incoming air. The earlier the fuel is added in the intake system the lower the volumetric efficiency because more of the fuel evaporates before entering the cylinder. In Diesels and GDIs the fuel is added directly into the cylinder after the intake stroke so get higher volumetric efficiency. 42

43 Factors affecting η v Heat transfer: All intake systems are hotter than ambient air, e.g., injection system and throttle bodies are purposely heated to enhance fuel evaporation. Therefore, the density of the air entering the cylinder is lower than ambient air density. Greatest problem at lower engine speeds more time for air to be heated. Use cold air intake P cyl m f m a 43

44 Factors affecting η v Fluid friction: The air flows through a duct fitted with an air filter, throttle and intake valve Air moving through any flow passage or past a flow restriction undergoes a pressure drop The pressure at the cylinder is thus lower than atmospheric pressure Greatest problem at higher engine speeds when the air flow velocity is high 44

45 Pressure losses over the length of the intake system P o = atmospheric pressure P air = pressure losses in air cleaner P o P Air cleaner P u = intake losses upstream of throttle P air P thr = loss across throttle P valve = loss across intake valve P u P throttle P valve WOT Part throttle Cylinder Muffler Extreme case of flow restriction is when the flow chokes at the intake valve as engine speed increases flow velocity remains the same have less fill 45 time.

46 Factors affecting η v Residual gas: Residual gas takes up cylinder volume that would otherwise contain air Recall the residual fraction given by f 1 = P e P r ( / ) 1/ k 4 As (P e /P 4 ~ P e /P i ) increases, or r decreases the fraction of cylinder volume occupied by residual gas increases and thus volumetric efficiency decreases. 4 e i TC 1 BC 46

47 Factors affecting η v Opening intake valve before TC (valve overlap): The longer the valve overlap, more exhaust gases rush into the intake port. Greatest problem at idle (part throttle and lower engine speeds) low intake pressure and more time for exhaust gases to back up. EO e i TC IO EC IC BC 47

48 Factors affecting η v Closing the intake valve after BC (backflow): P, L v When piston reaches BC still have P across the intake valve, mixture continues to flow into cylinder, close the intake valve after BC. p o P L v, exh L v, int As the piston changes direction the mixture is compressed, when the pressure equals the intake manifold pressure the flow into the cylinder stops. Best time to close the intake valve is when the manifold and cylinder pressures are equal, close the valve too early and don t get full charge, too late and air flows back into the intake port. At high engine speeds larger P across intake valve because of higher flow velocity, so ideally want to close valve later after BC (60 o abc). At low engine speeds smaller P across the intake valve so ideally want to close the intake valve earlier after BC (40 o abc). 48

49 Factors affecting η v RAM Effect: As the intake valve closes at higher engine speeds, the inertia of the air in 2 the intake system increases the pressure in the intake port, P + ρu = P s allowing more air to be injected This effect becomes progressively more important at higher engine speeds. To take advantage of ram effect close intake valve after BC. P cyl 49

50 Intake tuning: Factors affecting η v When the intake valve opens the air suddenly rushes into the cylinder and an expansion wave propagates back to the intake manifold at the local speed of sound relative to the flow velocity. When the expansion wave reaches the manifold it reflects back towards to intake valve as a compression wave. The time it takes for the round trip depends on the length of the runner (L) and the flow velocity. If the timing is appropriate the compression wave arrives at the inlet at the end of the intake process raising the pressure above the nominal inlet pressure allowing more air to be injected. t wave = 2L 2 2π tvalve c 3 N L c N For fixed runner length the intake is tuned for one engine speed. 50

51 Since L~1/N : high engine speed use short runners, low engine speeds use long runners Audi V6 Adjustable runner length Similarly the exhaust system can be tuned to get a lower pressure at the exhaust valve increasing the exhaust flow velocity. 51

52 Factors affecting η v as a function of engine speeds Fuel vapour pressure 52

53 In-Cylinder Fluid flow Three parameters are used to characterize large-scale in-cylinder fluid motion: swirl, squish, and tumble. Swirl is the rotational flow about the cylinder axis. Swirl is used to: i) promote rapid combustion in SI engines ii) rapidly mix fuel and air in gasoline direct injection engines iii) rapidly mix fuel and air in CI engines The swirl is generated during air induction into the cylinder by either: i) tangentially directing the flow into the cylinder, or ii) pre-swirling the incoming flow by the use of helical ports. 53

54 Cylinder Swirl and its Generation Swirl motion Tangential injection Helical port Contoured valve 54

55 Swirl Theory Swirl can be simply modelled as solid body rotation, i.e., cylinder of gas rotating at angular velocity, ω. Tangential flow velocity is v = ω r The swirl ratio, R s, is defined as the ratio of the gas angular velocity and the crank shaft angular velocity, i.e., ω R s = 2πN where N is the engine speed (revolutions per second) ω is the air solid-body angular velocity (rad/s) Most production engines have R s in the range of

56 Swirl Theory The angular momentum, Γ, and moment of inertia, I, of a rotating volume of gas is: MB Γ = Iω I = rdm for a cylinder I = 8 where M is the total gas mass B is the cylinder bore 2 During the cycle some swirl decays due to friction, but most of it persists through the compression, combustion and expansion processes. Neglecting friction, angular momentum Iω is conserved, I decreases ω increases 56

57 Engine Swirl Many engines have a wedge shape cylinder head cavity or a bowl in the piston where the gas ends up at TC. During the compression process as the piston approaches TC more of the air enters the cavity and the air cylinder moment of inertia decreases and the angular velocity (and thus the swirl) increases. 57

58 Squish and Tumble Squish is the radial flow occurring at the end of the compression stroke in which the compressed gases flow into the piston or cylinder head cavity. As the piston reaches TC the squish motion generates a secondary flow called tumble, where rotation occurs about a circumferential axis near the outer edge of the cavity. 58

59 Intake Flow The intake process governs many important aspects of the flow within the cylinder. The gas issues from the valve opening as a conical jet with radial and axial velocities that are about ten times the mean piston velocity. The jet separates from the valve producing shear layers with large velocity gradients which generate turbulence. The jet is deflected by the cylinder wall down towards the piston and up towards the cylinder head producing recirculation zones. Additional turbulence is generated by the velocity gradient at the wall in the boundary layer. Shear layers Large vortices become unstable and eventually break down into turbulent motion 59

60 Turbulent Flow Turbulent flow is characterized by its transient and random nature that is superimposed on a steady mean flow. Steady flow Turbulent flows are always dissipative, viscous shear stresses result in an increase in the internal energy at the expense of its kinetic energy. So energy is required to generate turbulence, if no energy is supplied turbulence decays. The source of energy for turbulent velocity fluctuations is shear in the mean flow, e.g., jets and boundary layers. 60

61 Statistical Approach to Turbulence The fluid velocity measured at a point in a specific direction: U x (t) U x (t 1 ) u (t 2 ) U x mean velocity (steady) t 1 Reynolds decomposition for statistically steady flow: where U ( t) = U t 2 + u'( t) 1 t2 U = U(t)dt mean velocity 1 Δt t u' is the fluctuating component It is common practice to define the turbulent fluctuation intensity, u t, in terms of the root-mean-square of the fluctuations: t u t t2 = u' rms = u' where u' = t 1 t ( u'( t) ) 2 dt 61

62 Turbulence Measurements in Engines The following shows the velocity measurement at a point in the cylinder over time for a two-stroke engine (cycle has 360 CA) TC + Instantaneous Cycle i BC Measurement point BC TC BC TC BC Individual cycle mean CA In engines the flow is statistically periodic (the flow pattern changes with crank angle) not steady. The instantaneous velocity measured at a specific crank angle θ in a particular cycle i is: U ( θ, i) = U ( θ ) + u'( θ, i) 62

63 Turbulence Measurements in Engines There are both cycle-by-cycle variations in the mean flow at any point in the cycle as well as turbulent fluctuations about that specific cycle s mean flow. u Uˆ Individual cycle mean Instantaneous U EA Ensemble average CA Flows that are statistically periodic are treated using ensemble average: 1 n U EA( θ ) = U ( θ, i) n where n is the number of cycles averaged. i 63

64 Turbulence Measurements in Engines The difference between the mean velocity in a particular cycle and the ensemble average is defined as the cycle-by-cycle variation in mean velocity: Uˆ ( θ, i) = U ( θ, i) U EA ( θ ) If the cycle-by-cycle variations are small then the cycle mean is equal to the ensemble average. Thus, the instantaneous velocity can be split into three components: U ( θ, i) = U ( ) ˆ EA θ + U ( θ, i) + u'( θ, i) The turbulent intensity is determined by ensemble averaging: u t 1 ( ) = ' n θ u n i= 1 2 ( θ, i)

65 Turbulence Measurements in Engines At the end of compression when the piston is at TC, the turbulence fluctuating intensity is about one-half the mean piston speed: 1 u t = U P 2 The two data sets shown with red lines are for individual cycle turbulence intensity. The rest of the points are for ensemble averaged, which means they include cycle-by-cycle variations in the mean velocity, making it larger by up to 2 times. 65

66 Turbulence Length-Scales Turbulent flow is comprised of unsteady eddies (vortices) with a multitude of length-scales and time-scales (turnover time). The largest eddies in the flow are limited in size by the enclosure with characteristic length-scale of L (e.g., large eddy associated with swirl). The integral scale l represents the largest turbulent eddy, determined by the fluctuating velocity frequency. Most of the turbulent KE is contained in the large eddies that breakdown into smaller size eddies via inviscid mechanisms. The turbulent KE cascades from the larger structures to the smaller structures where it is converted to thermal energy via viscous effects. What scale eddy is required to dissipate energy? 66

67 Length-Scales of Turbulence Reynolds number (Re) of an eddy with circulation velocity u' and size L is: Re = ρu' L µ = 2 2 ρu' L L µ u' L 3 L 3 inertia force per unit volume viscous force per unit volume Viscous forces are only important in the smallest scale where the Re 1 The eddy size at which the flow KE is dissipated by viscous effects is known as the Kolmogorov scale, and the eddy dimension is η. There is one more length-scale between the integral and Kolmogorov scales known as the Taylor microscale which represents the distance over which viscous effects can be felt, or the mean spacing between dissipative eddies. 67

68 Length-Scales of Turbulence The scales are: Integral (l), Taylor micro (l), Kolmogorov (η) η l λ λ η Gas flow through intake valve l 68

69 The Length-Scales of Turbulence Dimensional analysis leads to the following relationships between the scales: l = C L λ = l η = l 1 15 Cλ 1 2 Re 1 2 t ( C ) Re where C 1, C λ, and C η are numbers unique to the flow. η The turbulent Reynolds number is based on the integral scale and the turbulent fluctuation intensity u t l Re t = υ If the integral scale can be determined, so can all the other scales. t As the engine speed increases the Re increases, so the smaller scales of turbulence decrease in size. 69

70 Two-Stroke Engine In-Cylinder Flow Most common two-stroke engines are crankcase-scavenged Another class of two-stroke engine uses a separate compressor to deliver air into the cylinder to scavenge the combustion products, fuel is injected directly into the cylinder. AIR PROD AIR 70

71 Scavanging Performance Delivery ratio, D r D r = mass of delivered air per cycle displaced volume ambient density Trapping efficiency, Γ Γ = mass of delivered air retained mass of delivered air Scavenging efficiency, e s e s = mass of delivered air retained mass of trapped cylinder charge If the cylinder volume is completely filled with air the delivery ratio is ρ given by: a Vbc Vbc r D = = = 1 V V r 1 > r ρ a d d 71

72 Scavenging Models A. Perfect scavanging no mixing, air displaces the products out the exhaust if extra air is delivered (D r > r/(r-1) ) it is not retained B. Short circuiting the air initially displaces all the products within the path of the short circuit and then flows into and out of the cylinder C. Perfect mixing the air that enters the cylinder mixes instantaneously with the products, so immediately the gas leaving includes both air and products Scavenging efficiency Trapping efficiency Delivery ratio Delivery ratio 72