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Chapter 5 Air and Fuel Introduction T Alrayyes Introduction

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Chapter 5 Air and Fuel Introduction T Alrayyes Introduction Chapter 5 Air and Fuel introduction T Alrayyes Introduction • This chapter describes intake systems of engines-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 engine 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. • Throttle. • Intake valves. • Fuel injectors or a carburettor to add fuel. • The intake manifold has historically been manufactured from aluminum or cast iron, but use of composite plastic materials is gaining popularity Inlet manifold 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 cylinder. • The inside diameter of the runners are controlled by two factors: • Large enough to have high volumetric efficiency 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 torque figures due to change in runners length and diameter Throttle • The amount of air in SI Engine is controlled using a throttle valve((butterfly 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 cylinder head (CI engines and modern two-stroke 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 fuel injection 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. • Water cooling. 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, vacuum is created as a result of increase in volume as the piston 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 cam and fixed in one position • That explains the high volumetric efficiency at certain speed and it gets lower as the speed increase or decrease.
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