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Home , Engine block, Inlet manifold

Chapter 5 Air and Fuel introduction T Alrayyes Introduction

• This chapter describes systems of -how air and fuel are delivered into the cylinders. • The object of the intake system is to deliver the proper amount of air and fuel accurately and equally to all cylinders at the proper time in the cycle. • Flow into an engine is pulsed as the intake valves open and close, but can generally be modelled as quasi-steady state flow. • The intake system consists of an • Intake manifold. • . • Intake valves. • Fuel injectors or a carburettor to add fuel. • The intake manifold has historically been manufactured from aluminum or , but use of composite plastic materials is gaining popularity CFD analysis CFD animation for inlet manifold

• https://www.youtube.com/watch?v=XHpZY5pI800 • https://www.youtube.com/watch?v=lLpmTbqYkJs Intake manifold

• Intake manifold is designed to deliver air to the engine through pipes (runners) to each . • The inside diameter of the runners are controlled by two factors: • Large enough to have high and low flow resistance • Small enough to assure high velocity and turbulence: this enhance the ability to carry fuel droplet and increase evaporation and fuel mixing. • Active intake manifolds (Variable-length intake manifold): are used in some engines were runners can change its length and diameter for different engine speeds • At low speed: the air is directed through longer, smaller diameter runners to keep the velocity high and to assure proper mixing of air and fuel. • At high engine speeds: shorter, larger diameter runners are used, which minimizes flow resistance but still enhances proper mixing. • Variable-length intake manifold • Two and three stages Variable-length intake manifold • http://www.dailymotion.com/video/xz09fn_variable- intake-manifold-systems_auto Improvement in figures due to change in runners length and diameter Throttle • The amount of air in SI Engine is controlled using a throttle valve((). • Usually located at the upstream end.

• https://www.youtube.com/watch?v=wmrvnZT 4aDU • In the inlet manifold, the fuel injectors can be mounted : • At the inlet of the manifold (throttle body injection) (a) • By the intake valves of each cylinder (multipoint port injection) (b) • in the (CI engines and modern two- cycle and some four-stroke cycle SI automobile engines) (c). throttle body injection multipoint port injection For the throttle body injection: • The further upstream the fuel is added, the more time there is to evaporate the fuel droplets and to get proper mixing of the air and fuel vapor. • However, this also reduces engine volumetric efficiency by displacing incoming air with fuel vapor. • Early fuel addition also makes it more difficult to get good cylinder-to-cylinder AF consistency because of the asymmetry of the manifold and different lengths of the runners. • When the fuel is injected: it transfers into small droplet(atomized) due to the injector pressure and the turbulence. • https://www.youtube.com/watch?v=xwjqLgUfcq8 • The above is the most favourable condition that the fuel is at. • Better atomization allows for a more complete burn of all the fuel and helps reduce engine knock by enlarging the flame front. Problem associated with throttle position injection • Due to electrostatic forces some of the fuel will form into pools along the walls of the manifold, or may converge into larger droplets in the air • Both actions are undesirable because they create inconsistencies in the air- fuel ratio. • liquid particles will not always flow at the same velocity as the air and will not flow around corners as readily. larger droplets deviating more than smaller ones. • The third way fuel flows through the manifold is in a thin liquid film along the walls. This film occurs because gravity separates some droplets from the flow, and when other droplets strike the wall where the runner executes a corner. • Using gasoline is another problem when using throttle position injection. • Gasoline components evaporate at different temperatures and at different rates. • Different composition between vapour in the air and gasoline droplet carried by air or the liquid film on the manifold • Different composition will be carried out to the cylinder • This particularly bad for knock behaviour as one of the cylinders will have a worse knock behaviour. • Usually, when the knock number is quoted, it is based on the worse cylinder • Vaporisation rate is also affected by the throttling position as the inlet pressure will change with its position. • All the above problem are sorted when multiport is used. Volumetric efficiency of an Engine

• Obviously , the engine designer is aiming for the maximum engine efficiency • There will be a certain speed where the engine is going to get its maximum efficiency decreasing as the speed is getting higher or lower • There are few parameters that affect the engine volumetric efficiency: • Fuel used • Inlet charge temperature • Valves overlap • Fluid friction losses • Choked flow • Closing intake valve after BDC • Intake tuning • Exhaust residual • EGR

Volumetric efficiency Fuel

• Most Gasoline Engine have a lower than 100% volumetric efficiency due to gasoline vapour • The gasoline vapour displace some of the air and reduce the volumetric efficiency • Two factors that cause the fuel to affect the volumetric efficiency : 1. Injection position 2. Type of fuel Injection position/fuel delivery system

• The volumetric efficiency of carburettors or throttle injection position is far lower than that of multipoint injectors or Direct injection. • This is because no air is displaced by fuel vapour • When multipoint injectors or Direct injection is used there is no need to have a high speed and high turbulence(lower diameter and longer runner to increase friction). • The above will increase the volumetric efficiency even further because the biggest possible runner diameter will be used. • Note: Although the later the injection in the intake manifold the better the volumetric efficiency, late injection might mean that the fuel didn’t mix well and could compromise combustion quality. May be this isn’t the case in modern engines Fuel type

• Alcohol: lower AFR for Alcohol means that more fuel is needed and this will reduce Volumetric efficiency. • Alcohol on the other hand have a higher latent heat of vaporisation which will have a cooling effect that might balance the lower AFR. • Gaseous fuel: like Hydrogen and methane displace more fuel that liquid fuel. Intake charge temperature, heat transfer high temperature

• Intake runners especially metal ones are hotter than the outside surface • This result in increasing the temperature of the air getting into the cylinder, decreasing density and subsequently mass of air coming inot the cylinder • At lower speed, the intake charger take longer to pass through the runner, i.e. it get hotter. This will explain partly why are we seeing a reduction in volumetric efficiency at low speed. • . Valve overlap

• Intake valves start to open some where between 10° to 25° bTDC and should be fully open by TDC to get the maximum flow in. • Intake normally finish closing about 40° to 50° aBDC.

https://www.youtube.com/watch?v=cAlw9RpTZ4I Valve overlap

• V Valve overlap • There is usually an overlap between the exhaust valve closing and inlet valve opening at the TDC • Some exhaust is pushed back into the inlet manifold, then come back during the induction stroke causing a reduction in the volumetric efficiency. • For a fixed overlap, the problem is at its maximum when the engine is at low speed: • More time for the exhaust to get into the cylinder • The above can explain the reduction in Volumetric efficiency at low speed. Intake Valve opening bTDC

• As mentioned earlier Intake valves start to open some where between 10° to 25° bTDC and should be fully open by TDC to get the maximum flow in. • In most engines, the engine speed is set for one engine speed, with volumetric losses happening at higher or lower speed. • At lower speed, the intake valve opens too early and increase overlap duration time as mentioned earlier • At higher than design speed the intake manifold opens too late and the intake flow hasn’t been fully established at TDC Closing the intake valve After BDC

• Timing the closure affect how much air enters the cylinder • During the intake stroke, is created as a result of increase in volume as the moves toward BDC. • The lower pressure in the cylinder compare to the inlet manifold causes the air to move to the cylinder • As the piston reach BDC the vacuum is still there and air is still drawn to the cylinder. • Then the air is compressed and the pressure inside the cylinder start to increase. • That’s why closing the inlet valve at BDC will reduce volumetric efficiency as the air still can be drawn in after BDC (vacuum and subsequently pressure difference still exist) • To get the maximum volumetric efficiency, Ideally the IVC when the in- cylinder pressure is equal to the Manifold pressure • To get the maximum volumetric efficiency: the timing for the inlet valve closure is dependent on engine speed: • At higher speed the IVC later in the cycle (later after BDC), this because of greater pressure drop across the valve due to higher flow rate of air. In addition, there is less real cycle time at high speed • At lower speed IVC should be closed early after BDC (the pressure between the inlet manifold and in cylinder equalise earlier). As the pressure difference between the inlet and incylinder is lower as the engine speed is lower • In most engines the intake valve closure position is controlled by the and fixed in one position • That explains the high volumetric efficiency at certain speed and it gets lower as the speed increase or decrease. • Variable Fluid Friction Losses

• Friction causes a pressure drop when passing through the different components of the system(, runners, carburettors, and inlet valve). • Subsequently the amount of air going to the cylinder is reduced due to friction and will cause a reduction in volumetric efficiency. • Viscous drag increase with the square of velocity. as a result the volumetric efficiency decrease with speed • Method used to reduce pressure loss: • Smoother wall • Avoid corners • Elimination of carburettors • Close fitting alignment and avoid any protrusion • Using multivalves( using 2 or 3 inlet valve pre cylinder). The inlet valve is the greatest restriction to flow and multivalve is used to reduce this restriction • Question: why do engine use multivalves rather than a bigger single valve? Exhaust residual • During the exhaust stroke, not all the exhaust is pushed out of the cylinder. Some of the exhaust is trapped in the clearance volume • The amount of trapped air is dependent on , the location of the valve and overlap. • Exhaust residual affect Volume flow rate in 2 ways • Heating the income air: reducing the volumetric efficiency • Creating a vacuum: due to the cooling of the residual gas EGR • recycles,EGR, is used to reduce Nox eimissons • EGR reduce the Volumetric efficiency in two ways: 1. Increase inlet charge temperature 2. Displace some of the air with exhaust. Inlet manifold tuning

• Before 1950s, engineers believed short intake manifolds were the best to engine breathing. Then they discovered that under some conditions long intake manifolds could actually improve output, thanks to a so- called "supercharging effect". • Two and three stages Variable-length intake manifold • http://www.dailymotion.com/ video/xz09fn_variable-intake- manifold-systems_auto Improvement in torque figures due to change in runners length and diameter Intake Valves

• https://www.youtube.com/ watch?v=itE2JWqdTqE • Intake valves of most IC engines are poppet valves that are spring loaded closed and pushed open at the proper cycle time by the engine . Poppet valves • Most valves and valve seats against which they close are made of hard alloy steel or, in some rarer cases ceramic. • The Valves are connected by hydromechanical or mechanical linkage to the camshaft • Older version the engine was mounted in the with long mechanical linkage (push rods, rocker arms, ) • Recent engine: the cam is moved to the Engine block (over head cam shaft OHC). Initially mechanical linkage were moved to the cylinder head (much lower arm though ) • Modern Engines have the CAM mounted directly over the stems. • The closer the CAM shaft mounted to the stems of the valves, the greater the mechanical efficiency of the system.

• The distance which a valve opens is called valve lift (between 5 to 10 m)

• The angle of the valve is designed to give minimum flow restriction. • The Actual cross section Area is less than the flow passage area due to flow restriction. • The ratio of the actual flow area to the flow passage area is called valve discharge coefficient:

• The passage area of the flow is

• The mass and area are designed to give the maximum inlet charge to improve efficiency • Varies imperical correlation was found in literature to size intake valve. • The formula that gives the minimum valve intake area necessary for modern engine can be given in the form. Multi Valve system

• As mentioned previously the use of multi-valves is very common in modern engine. • Two or three smaller intake valves give more flow area and less flow resistance than one larger valve, as was used in older engines • Muti-valves is mainly used to increase the flow rate without compromise the charging mixing and subsequently combustion • In modern engine , both the and injectors are used in the cylinder head, that leaves very little room for a big inlet. • Using multi-valves can be better fit into a given cylinder head size with enough clearance to maintain the required structural strength; • Using multi valves allow for the use of lighter springs and reduce forces in the linkage. Lighter valves can also be opened and closed faster. • Some engines with multiple intake valves are designed so that only one intake valve per cylinder operates at low speed. As speed is increased, less time per cycle I available for air intake, and the second (and sometimes third) valve actuates, givin additional inlet flow area. • This allows for increased control of the flow of air within the cylinder at various speeds, which results in more efficient combustion. • Mass flow through the intake valve into a cylinder is shown in Fig. • Reverse flow can result when valve overlap occurs near TDC. • Reverse flow out of the cylinder will also occur at lower engine speeds as the intake valve is closing aBDC,

• VVT is used to adjust the intake and exhaust valve opening and closing. injectors

• Fuel injectors are nozzles that inject a spray of fuel into the intake air • Recap: injectors loction: • Multipoint • Throttling poition • In-cylinder Multipoint injectors:

• Most modern automobile SI engines have multipoint port fuel injectors. • In this type of system, They spray fuel into the region directly behind the intake valve • Contact with the relatively hot valve surface enhances evaporation of the fuel and helps cool the valve. • High liquid spray velocity is necessary to assure evaporation and mixing with the air. • Any backflow of hot residual exhaust gas that occurs when the intake valve opens also enhances the evaporation of fuel droplets. • How can a multipoint injector increase volumetric efficiency? • Because the injection doesn’t occur at the manifold and there is not need for mixing, the manifold runners can be as big as possible and as smooth as possible (increase volumetric efficiency). • Mixing and vaporisation happened at shorter period for that reason droplet size should be as small as possible. • It is has to be even smaller at high speed when real time is shorter

Fuel delivery systems and injectors Fuel delivery system

• Most system have a single (). The injectors act as metering device and the fuel is delivered to the injector at a high pressure • Some injectors receive the fuel at a low pressure and it increase the pressure and meter the fuel. • Some systems have a pressure pump at each injector (integrated as one piece). • The injection pressure is in the order of 200 to 300 kPa. Fuel delivery system, control • The amount of injected fuel is controlled by the EMS or ECU. • The amount of fuel injected for each cycle is controlled by injection timing (1.5 to 10 ms). • The duration of injection is determine from a feedback from engine and Fuel delivery system, control

• Measuring the oxygen content in the exhaust (Lambda sensor)is an indication of the AFR and is used to adjust Fuel delivery system. • Other feedback parameters include engine speed, temperatures, air flow rate, and throttle position. • Intake air flow can be determine through pressure drop or hot wire. • When engine start up: a richer mixer is needed. the amount needed is determine by the temperature and the switch. Injectors types

• There are so many types of injectors • Most of them operate by trapping small amount of fuel behind the nozzle orifice. • There are mainly two types: • Low pressure nozzle: injection is initiated by increasing pressure and pushing open the valve, allowing flow to occur. • On high-pressure nozzles, flow is initiated by lifting the valve needle off its seat byaction of an electric solenoid.

Throttle position injection

• They are located near the inlet of the manifold. Usually just downstream of the throttle plate. • One injector is used for all the cylinder supply, inlet manifold is used for mixing a distribution • Main advantage that is cheaper than both multipoint and incylinder injection: • Fewer injectors are needed • Coarser nozzle can be used(there is more time for mixing and vaporisation). • Control is simpler (almost constant spray at certain conditions) • Main disadvantage: • Variation in AFR between cylinders • Response time is slower Inlet cylinder injection • Some SI and all CI engine fuel injection systems have the injectors mounted in the cylinder head and inject directly into the . • This gives very constant fuel input cycle-to-cycle and cylinder-to-cylinder. • This type of system requires very precise injectors giving extremely fine droplets of fuel. • Fuel is added during the compression stroke, which allows an extremely short period of time for evaporation and mixing, less than 0.008 second at 3000 RPM. • High turbulence and swirl are also important. • Injectors that spray directly into the combustion chamber must operate with much higher pressures (10 MPa) • Better for atomization • It also injects in a much higher pressure environment (during the compression stroke)

Supercharger and • and are compressors mounted in the intake system and used to raise the pressure of the incoming air. • This results in more air and fuel entering each cylinder during each cycle and subsequently more power

• Superchargers are mechanically driven directly off the engine . • They are generally positive displacement compressors running at speeds about the same as engine speed. • Some high performance SI engine and all large CI engines are supercharged. • Some superchargers are made of two or more compressor stages. Supercharger

Advantage • The main advantage is quick response to change in throttle position Disadvantage • Driven by the engine (increase load on the engine). • Expensive • Heavy • Noisy • When the first law of thermodynamics is applied to the air flowing through a supercharger compressor,

Wse = ma(hout - hin) = macp(Tout - Tin) • This assumes that the compressor heat transfer, kinetic energy terms, and potential energy terms are negligibly small • Isentropic efficieny= Wisen/Wact = [nia(h2s - h1)]/[nia(hzA – h1)]

=[niacp(T2s – T1)]/[niacp(T2A – T1)] = (T2s – T1)/(T2A – T1) • the isentropic temperature can be calculated from: (k-l)/k T2s = T1(P2/P1) Where k =1.4 is usually used • There is also a mechanical efficiency of less than 100% between the power taken from the engine and what is delivered to the compressor:

Mechanical efficiency= (Wact)sc/Wfrom engine Aftercooler/ • Turbochargers increase the pressure in the manifold (Positive effect). • It also increase the temperature in the manifold (negative effect) due to compressive heating. • The increase in temperature reduce Volumetric efficiency and increase the probability of knock and self ignition. • To avoid this, many superchargers are equipped with aftercooler (intercooler) that reduce the air temperature to its original temperature. • The cooling can be either • Air to • Air to liquid cooling • Multi-stage compressor might have an intercooler after each stage • CI engines supercharger, mostly, don’t have an aftercooler due to the fact that there is no concern about knock inside CI engines • Aftercooler are expensive and take space inside the engine compartment. • For that reason most don’t have intercooler and instead they reduce compression ratio to avoid problem associated with knock. Turbocharger

• The compressor of a turbocharger is powered by a turbine mounted in the exhaust flow of the engine • The main advantage of turbocharger this is that none of the engine shaft output is used to drive the compressor, and only waste energy in the exhaust is used.

Disadvantage/ problems of turbocharger

Turbo lag: • It occurs with a sudden throttle change. When the throttle is quickly opened to accelerate an automobile, the turbocharger will not respond quite as quickly as a supercharger. It takes several engine revolutions to change the exhaust flow rate and to speed up the rotor of the turbine. • Turbo lag has been greatly reduced by using lightweight ceramic rotors that can withstand the high temperatures and that have very little mass inertia. Turbo lag can also be reduced by using a smaller intake manifold Back pressure • the turbine in the exhaust flow causes a more restricted flow, resulting in a slightly higher pressure at the cylinder exhaust port. This reduces the engine power output very slightly. • In total Turbocharged engines generally have lower specific fuel consumption rates. They produce more power, while the friction power lost remains about the same. Reliance on speed • Maximum pressure in an engine exhaust system is only very little above atmospheric, so there can only be a very small pressure drop through the turbine. • Because of this, it is necessary to run the turbine at very high speeds so that enough power is produced to run the compressor. • This means that the compressor starts at certain speed not before. • This was tackled by using multi-stage turbocharger

• Radial flow centrifugal compressors, turning at high speed, are generally used on automobile-size engines. • On very large engines, axial flow compressors are used because of their greater efficiency at the higher air flow rates. • The isentropic efficiency of a compressor is defined as:

Intake for CI engine

Volumetric efficiency of CI Engine is mostly higher than that of SI engines for the following reasons: 1. CI engines are operated unthrottled, with engine speed and power controlled by the amount of fuel injected during each cycle. 2. No fuel is added until late in the compression stroke, after air intake is fully completed. 3. intake system designed for very little flow restriction of the incoming air. 4. In addition, many CI engines are turbocharged, which enhances air intake even more. Combustion in CI engine

• Fuel is added late in the compression stroke, starting somewhere around 20° bTDC. • Injectors mounted in the cylinder head inject directly into the combustion chamber, where self-ignition occurs due to the high temperature of the air caused by compressive heating. • The combustion starts shortly bTDC. At this point, the fuel is still injecting, which keeps combustion occurring well into the power stroke • Fuel droplet size varies after injection: this ton and gives smoother engine operation. • Time duration of injection in a CI engine is less than that in SI engines. • This is a desirable phenomenon so that the start of combustion of all fuel particles is not simultaneous, but is spread over a short period of cycle time. • This slows the start of the pressure pulse on the piston and gives smoother engine operation. Time duration of injection in a CI engine is less than that in SI engines. Injection: • Due to high compression ratio of CI engines and late injection, the injection occur when the cylinder is at peak pressure. • For that reason a high injection pressure is required. Both due to high pressure and the fact that there is very little time to mix. • Pressure must be high enough so that fuel spray will penetrate across the entire combustion chamber. • Fuel droplet size generally decreasing with increasing pressure. • Injection pressure for CI engines must be much higher than that required for SI engines. • Injection pressure of CI engines is between 20 to 200 MPa Exam Question: • Why do we HAVE maximum torque at certain speed? • Why do volumetric efficiency decrease as the speed increase or decrease from optimum value? • How variable valve timing can be used to improve engine volumetric efficiency/Engine efficiency • How can varrible inlet manifold is used to improve efficiency? • What is the main conditions needed to get a clean combustion? • Why do some manufacturer use VVT? Explain the effect that VVT might have on engine performance? • What is the main difference between supercharger and turbocharger?