Intake, Exhaust, and In-cylinder Flow

Section 4

Cycle Exhaust ProcessIn the Otto and Diesel cycles the exhaust process is modeled asconstant volume heat extraction, it does not consider the actual gas flow.PiPeProductsAirQoutActual exhaustOtto exhaust process 4-1BC

Ideal Four-Stroke Cycle Intake and ExhaustThe intake & exhaust processes areoften shown as a constant pressure processes, e.g., 56 and 61 For WOT states 1 and 5 are the same.

Process 5-6 indicates a decrease in specific volume which is incorrect sinceas the cylinder volume decreases so does the mass specific volume remains the same!

This inconsistency results from treating an open system problem with a closedsystem model Pressure, PSpecific volume, vEV closesIV opensIV closesEV opensWOT

Unthrottled: Pi = Pe = 1 atmThrottled: Pi < PeSupercharged: Pi > PeIdeal Intake and Exhaust StrokesValves operate instantaneously, intake and exhaust process are adiabatic and constant pressure.1EV closesIV opensEV closesIV opensEV closesIV opens66EV opensIV closes (state1)EV opensIV closes

Blowdown At the end of the power stroke when the exhaust valve opensthe cylinder pressure is much higher than the exhaust manifold pressure which is typically at 1 atm (P4 > Pe), so the cylinder gas flows out through the exhaust valve and the pressure drops to Pe.

Displacement Remaining gas is pushed out of the cylinder by the piston moving to TC.Actual Exhaust Stroke (4 6)State 4 (BC)

State 5 (BC)

State 6 (TC)

Pi TiPeThe actual exhaust process consists of two phases: i) Blowdown ii) Displacement Blowdown Displacement Products

During blowdown the gas remaining in the cylinder undergoes expansion which can be modelled as isentropic (neglecting heat transfer) State 5 at the end of blowdown is a fictitious state corresponding to no actual piston locationExhaust Blowdown5 (Otto/Diesel)

Residual GasThe gas remaining in the clearance volume when the piston reaches TC (pt 6) called the residual gas mixes with intake gas (fuel-air for SI and air for CI)

The residual gas temperature T6 is equal to T5

The Residual gas fraction f is defined as the ratio of the mass of residual gas to the mass of the fuel-air (assume ideal gas Pv = RT)Typically values of f are in the range 3% to 12%, lower in Diesels (larger r)

Exhaust Manifold Gas Temperature (T7)The gas leaving the cylinder during blowdown has kinetic energy which is converted into thermal energy when the exhaust gas comes to rest in the exhaust gas manifold gas temperature in the exhaust manifold ishigher than T5.5Fluid leaving the cylinder earliest has higher velocity so temperature higherwhen it stagnates in the manifold (T7a > T7b > T7c).

When the intake valve opens the fresh gas with mass mi mixes with the hotter residual gas with mass mR so the gas temperature at the end of the intake stroke T1 will be greater than the inlet temperature Ti.

Applying conservation of mass:Intake Stroke 6 1Applying conservation of energy (open system):Ti

Recall m6 = m1f and assuming ideal gas P6v6 = RT6 and h = cpTIn terms of inlet and exhaust conditions P1 = Pi , P6 = Pe , T6 = TeIntake Gas Temperature (T1)

Volumetric efficiency

Pumping work

Pumping mep

Recall:

Net pmep

Heat input 23Qin = miqin = (m1-m6) qin = m1(1-f) qinNote: qin is the heat added per kg of inducted gas

Otto Cycle Analysis with Intake and ExhaustThe equations on the following slide are solved iteratively for the following initial conditions:

Tiinlet fuel-air mixture temperaturePeexhaust pressurercompression ratioPiinlet pressurekspecific heat ratioqinheat added per unit mass of gas inducted

In the following equations T1 depends on the residual gas fraction f and and temperature Te. These values are assumed to start the calculation and then improved over the subsequent iterations.

Otto Cycle Analysis with Intake and Exhauste,i -1: Intake stroke1 - 2: Isentropic compression stroke2 - 3: Constant volume heat addition3 - 4: Isentropic expansion stroke5 - e: Constant pressure adiabatic exhaust stroke4 - 5: Isentropic blowdown

Computer Program for Cycle Calculations The Ferguson textbook has a companion web site at:

http://he-cda.wiley.com/WileyCDA/HigherEdTitle.rdr?productCd=0471356174

that has a useful program that does the calculation for you:

Four Stroke Gas Otto Cycle Applet

Valve FlowThe most significant gas flow restriction in an IC engine is the flow through the intake and exhaust valves

The mass flow rate through the valve is given by:Po = upstream stagnation pressure, Pv = valve static (or cylinder pressure)ro = stagnation density, co = stagnation speed of sound, Av = valve areacf = flow coefficient

If the flow is choked (Po/Pcyl > 1.9) flow only depends on upstream pressurePvPcyl

Minimum areas: low lift - Av = A1= pdl high lift - Av= A2= pd2/4A1dValve FlowPcylA2l

Flow Coefficient MeasurementSet: Av

Measure: mi, Ti, Pi

Calculate: cfFlow coefficient (cf)Nondimensional valve lift (l/d)

Valve Opening and ClosingIn 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 land gently on the seat. CAValve displacement (l)Valve startsto openValve completelyclosedDuration

Valve SizingIn order to avoid choked flow the intake valves are sized based on:where is the average valve area, b is the cylinder bore, is average piston velocity, ci speed of sound of gas in intake port.

Exhaust valves can be smaller since the speed of sound of the exhaustgas 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 increasesvalve area to piston area ratio permitting higher engine speeds.

Heads are often wedge-shaped or domed, this permits Av/Ap up to 0.5.

Double overhead cams per cylinder bank are used to accommodatemultiple valves, one cam for each pair of intake and exhaust valves

Valve Sizing

Valve OverlapIn 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 180o. The exhaust valve opens before BC and closes after TC and 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.TCBCBCCA

When the intake valve opens bTC the cylinder pressure is at roughly Pe

Part throttle (Pi < Pe): 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 (Pi = Pe): some fresh gas can flow out the exhaust valve reducing fuel efficiency and increasing emissions.

Supercharged (Pi > Pe): fresh gas can flow out the exhaust valveValve overlapPiThrottledPi < PePiSuperchargedPi > PePePe

Engine Operating ConditionsConventional 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.Engine speed:Idle - 1000 rpmEconomy - 2500 rpmPerformance - 4000 rpmWOT bmepsfcEngine load

@1000 rpm intake duration: 230o = 38.4 ms @2500 rpm 230o = 15.4 ms @5000 rpm 230o = 7.7 ms, 285o = 9.5 msValve TimingieBCTCEVOIVOIVCEVC180oieBCTCEVOIVOIVCEVC180oConventionalPerformance

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 valve overlap is minimized by reducing the number of CA the intake valve stays open.

Variable Valve Timing used to obtain optimum performance over wide arange of engine speeds and loadValve TimingOverlap15o65o

Honda Variable valve Timing and lift Electronic Control (VTEC) Each pair of valves has three cam lobes, two that operate the valves at low-rpm, and a third that takes over at high rpm (4500 rpm). During low-rpm operation, the two rocker arms riding the low-rpm lobes push directly on the top of the valves. At high rpm a pin locks the three rocker arms and the valves follow the larger center cam lobe.First introduced in 1991 Honda NSX model.

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

Intake and Exhaust Processes in 4-Stroke Cycle1st crank shaft rev: 1 - 3 22nd crank shaft rev: 4

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.Valve FloatSpring

MufflerAir cleanerIntake and Exhaust System for Single Cylinder EngineCylinderP

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

Manifold Pressure3000 rpm6000 rpm

Volumetric EfficiencyRecall the volumetric efficiency is defined as:Volumetric efficiency is affected by :Fuel evaporationMixture temperaturePressure drop in the intake systemGasdynamic effectsor engine speedNote: mean piston speed proportionalto air flow velocity

Factors affecting hvHeat transfer:

All intake systems are hotter than ambient air, so the density of the airentering the cylinder is lower than ambient air density.

Injection system and throttle bodies are purposely heated to enhance fuel evaporation.

Greatest problem at lower engine speeds more time for air to be heated.Pcyl

Factors affecting hvFluid friction:

The air flows through a duct through an air filter, throttle and intake valve

Air moving through any flow passage or past a flow restriction undergoes apressure 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

Factors affecting hvFuel 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 volumetricefficiency because more of the fuel evaporates before entering the cylinder.

In Diesels fuel is added directly into the cylinder so get a higher efficiency.

Residual gas:

Recall the residual fraction given by

As (Pe/Pi) increases, or r decreases the fraction of cylinder volume occupied by residual gas increases and thus volumetric efficiency decreases.

Opening intake valve before TC (valve overlap):

The longer the valve overlap, more exhaust gases are pushed into the intake port.

Greatest problem at lower engine speeds when there is more time for exhaust gases to back up.Factors affecting hv

MufflerWOT

Part throttlePo = atmospheric pressure

DPair = pressure losses in air cleaner

DPu = intake losses upstream of throttle

DPthr = loss across throttle

DPvalve = loss across intake valveAir cleanerPressure losses over the length of the intake systemCylinderPExtreme case of flow restriction is when the flow chokes at the intake valveas engine speed increases flow velocity remains the same have less fill time.PoDPairDPuDPvalveDPthrottle

Factors affecting hvClosing the intake valve after BC (backflow):

When the piston reaches BC there is still a pressure difference across the intake valve and the fuel-air mixture continues to flow into the cylinder, therefore close the intake valve after BC.

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 dont get full load, too late and air flows back into the intake port.

At high engine speeds larger pressure drop across intake valve because of higher flow velocity, so ideally want to close valve later after BC (60o aBC).

At low engine speeds smaller pressure drop across the intake valve so ideally want to close the intake valve earlier after BC (40o aBC).

RAM Effect:

As the intake valve closes at higher engine speeds, the inertia of the air in the intake system increases the pressure in the intake port, called the ram effect.

This effect becomes progressively more important at higher engine speeds.

To take advantage of ram effect close intake valve after BC.Factors affecting hv

Intake and exhaust tuning:

When the intake valve opens the air suddenly rushes into the cylinder and anexpansion 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 tointake valve as a compression wave. The time it takes for the round trip depends on the length of the runner 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.

For fixed runner length the intake is tuned for one engine speed (flow velocity).

Similarly the exhaust system can be tuned to get a lower pressure at the exhaust valve increasing the exhaust flow velocity.Factors affecting hv

Factors affecting hv as a function of engine speedsFuel vapour pressure

In-Cylinder Fluid flowThree 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 enginesii) rapidly mix fuel and air in gasoline direct injection enginesiii) 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.

Helical portTangential injectionSwirl motionContoured valveCylinder Swirl and its Generation

Swirl TheorySwirl can be simply modelled as solid body rotation, i.e., cylinder of gas rotating at angular velocity, w.

Tangential flow velocity is v = w r

The swirl ratio, Rs, is defined as the ratio of the gas angular velocity andthe crank shaft angular velocity, i.e.,where N is the engine speed (revolutions per second)w is the air solid-body angular velocity (rad/s)

During the cycle some swirl decays due to friction, but most of it persiststhrough the compression, combustion and expansion processes.

Neglecting friction, angular momentum Iw is conserved, I decreases w increasesSwirl Theorywhere M is the total gas massB is the cylinder boreThe angular momentum, G, and moment of inertia, I, of a rotating volume of gas is:

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

Engine Swirl

Squish and TumbleSquish 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.

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 wallin the boundary layer. Large vortices become unstableand eventually break down into turbulent motionShear layers

Turbulent Flow Turbulent flow is characterized by its transient and random nature.

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.

A common source of energy for turbulent velocity fluctuations is shear in the mean flow, e.g., jets and boundary layers.Steady flow

Turbulence The fluid velocity measured at a point in a specific direction:t1Ux(t1)Ux mean velocity (steady)tUx(t)Reynolds decomposition for statistically steady flow:It is common practice to define the turbulent fluctuation intensity, ut, in terms of the root-mean-square of the fluctuations:u(t2)t2

Turbulence Measurements in EnginesIn engines the flow is statistically periodic (the flow pattern changes with crank angle) not steady.

The instantaneous velocity measured at a specific crank angle q in a particular cycle i is:Cycle iCAThe following shows the velocity measurement at a point in the cylinder over time for a two-stroke engine (cycle has 360 CA)+InstantaneousIndividual cycle meanMeasurementpointBCTCBCTCBCBCTC

where n is the number of cycles averaged. Flows that are statistically periodic are treated using ensemble average:Turbulence Measurements in Engines There are both cycle-by-cycle variations in the mean flow at any point in thecycle as well as turbulent fluctuations about that specific cycles mean flow.InstantaneousIndividual cycle meanEnsemble averageUEAuCA

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: Thus, the instantaneous velocity can be split into three components: If the cycle-by-cycle variations are small then the cycle mean is equal to the ensemble average.

The turbulent intensity is determined by ensemble averaging:

Turbulence Measurements in EnginesAt the end of compression when the piston is at TC, the turbulence fluctuating intensity is about one-half the mean piston speed:The two data sets shown with red lines are for individual cycle turbulence intensity.

The rest of the points are forensemble averaged, which meansthey include cycle-by-cyclevariations in the mean velocity,making it larger by up to 2 times.

Turbulence Length-Scales Turbulent flow is comprised of eddies (vorticies) with a multitude of length scales and vorticities (measure of angular velocity).

The largest eddies in the flow are limited in size by the enclosure withcharacteristic length-scale of L (e.g., large eddy associated with swirl).

The integral scale l represents the largest turbulent vortex size, determined by the fluctuating velocity frequency.

Superimposed on the large scale flow is a range of eddies of smaller and smaller size.

Most of the KE of the flow is contained in the large eddies.

The KE is converted to thermal energy via viscous effects.

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 isknown as the Kolmogorov scale, and the eddy dimension is h.

There is one more length-scale between the integral and Kolmogorov scalesknown as the Taylor microscale which represents the distance over which viscous effects can be felt, or the mean spacing between dissipative eddies. The Reynolds number (Re#) is the ratio of inertia to viscous forcesThe Length-Scales of Turbulence The Re# of an eddy with circulation velocity u' and size L is:

The Length-Scales of TurbulenceThe scales are: Integral (l), Taylor micro (l), Kolmogorov (h)llhlhlGas flow through intake valve

The Length-Scales of TurbulenceDimensional analysis leads to the following relationships between the scales:where C1, Cl, and Ch are numbers unique to the flow.

The turbulent Reynolds number is based on the integral scale and the turbulent velocity If the integral scale can be determined so can all the other scales.

As the engine speed increases the Re# increases so the smaller scalesof turbulence decrease in size.

PRODAIRTwo-Stroke Engine In-Cylinder FlowMost common two-stroke engines are crankcase-scavanged

Another class of two-stroke engine uses a separate compressor to deliver air into the cylinder to scavange the combustion products, fuel is Injected directly into the cylinder.AIR

Scavanging PerformanceDelivery ratio, Dr

Trapping efficiency, G

Scavanging efficiency, es If the cylinder volume is completely filled with air the delivery ratio is

given by:

A. Perfect scavanging no mixing, air displaces the products out the exhaust(if extra air is delivered (delivery ratio > 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 first air to enter the cylinder mixes instantaneouslywith the products and the gas leaving is almost all residual (for larger delivery ratio most of gas leaving is air)Scavanging Models