Study Of Variable Valve Timing Engineering Essay

Study Of Variable Valve Timing locomotiveering EssayVariable Valve Timing is a tool in which the lift, du dimensionn or timing of either the wasting disease or run over valves or both croup be altered in some(prenominal) combinations during engine operation. This is not possible for an engine with fixed intake and exhaust valves. An engine with VVT installed helps in improving engine performance over a wider outrank of the engine operating spectrum. In this paper, the effect of VVTs on gasoline as well as diesel engines were discussed and also comp atomic number 18d to their series counter transgresss. For a diesel engine, it was ascertained that with the introduction of a VVT, higher(prenominal) go off-line and blaze pressures were achieved with inadvertently led to a better engine performance along with let down smoke density and higher torque at low securenesss. Minute variations in CO, NOx and PM emissions were observed. However at integral- commitment operation, NO x emissions had change magnitude marginally. In the slip-up of the gasoline engine, it was observed that there wasnt any significant improvement in the CO and NOx emissions. However, the engine torque had increased particularly at low recreates and at incomplete load. This was complimented by a decrease in Brake Specific Fuel Consumption (bsfc).INTRODUCTIONThe valves in 4-Stroke IC Engines argon mostly spring operated 1. The coal scuttle and closing of these valves is governed by the crankshaft which is machine-accessible to the valves through an intermediary camshaft which is attached to every valve through a cam. The amount of fuel that enters the combustion chamber is dependent upon valve timing. The camshaft is connected to the crankshaft by using any one of gears, chains or belts. In conventional engines without VVT, the cam lobe shape and position as well as it profile are tuned for a particular engine speed and this usually compromises high-end torque or low-end power. However, with the introduction of VVT, the cam characteristics, e.g. timing, can be altered. Thus, this allows for better engine efficiency and power at variable engine speeds 2.This is workful especially at high speed operation where a car requires to a greater extent air in order to verbalize the undeniable power. If lesser amount of air were to be entered at high speeds, there would a considerable decrease in performance. However, if the valves were to be kept candid for a longer duration, there would be a case of incomplete combustion inside the combustion chamber at low speeds since the pressure involve to attain proper combustion of fuel is insufficient. This again results in reduced engine performance as well as an increase in fuel emissions 2.Modern emission regulations set by various governments have forced several manufacturers to adopt VVT in their engines. Most of the current car manufacturers have espouse VVT engine systems.The popular VVT mechanisms are classifie d into purely mechanical, electro-mechanical and hydraulic. Purely mechanical VVT systems are further classified into Oscillating Cam, Eccentric Cam Drive, Three-Dimensional Cam Lobe and Two sleep together Combined Profile 2.TYPES OF VVTsPurely MechanicalIn this type of VVT, the valve timing is operated by mechanical means. It is further subdivided intoOscillating CamThis method involves using either a conventional cam lobe or an eccentric and connecting rod which generates oscillatory motion on part cam lobe which is connected to the follower and thus, the valve. The part lobe is made up of a dent each of base circle and lobe flank. The position of the part cam lobe can be varied depending on whether zero lift and zero position are required in which case the part cam lobe is completely base circle. The exact opposite situation is maximum duration at full lift in which case it is fully flank. BMWs Valvetronic, Toyota Valvematic( both conventional) and Nissan VVEL( eccentric and co nnecting rod) are variations of this method. The Valvetronic is very dependable however the lift duration is dependent on amount of lift. Hence its only use on intake valves. The Valvematic and the VVEL are more compact while delivering the same performance as the Valvetronic 2.Eccentric Cam DriveThe Rover Company is the only smart set in history to have ever used this version of VVT. In this method, an eccentric disc mechanism is used which can vary the angular speed of the cam. The lesser the angular speed, the greater the duration of lift. The disadvantage of this method is that its expensive as each valve requires its own controller 2.Three-Dimensional Cam LobeThis system comprises of an axially lengthen cam lobe which has shorter duration profile and a greater duration profile at either ends. Thus, valve lift duration can be varied by tilting the cam lobe axially. However, such(prenominal) a system requires the follower to tilt in various directions as the lobe flanks of the cam lobe are not parallel to the axis of rotation of the camshaft. This problem has never been rectify which is why its not used commercially 2.Two Shaft Combined ProfileIn this system, two closely arranged camshafts are used. The angular positions of the camshaft with think of to the crankshaft can be arranged by using a phasing mechanism. A follower connects both camshafts and is operated by the two lobes simultaneously. The two lobes are used for the intake and exhaust valves respectively. The disadvantage of this system is that the settings on one lobe may affect the other thus tampering the entire system. It has yet to be used commercially 2.Electro-mechanicalCamless engines use this type of VVT in which the valve opening and closing is achieved by using electro-magnets. The disadvantages of this systems are deceleration of the valve is hard to achieve, springs used in the valves have to adjusted to the smallest of margins and finally, the use of electromagnets reduces the en gine efficiency 2.HydraulicAnother system developed as an alternative to cam engines. In this system, the pressure of the liquid is used to activate valves. However, this system has its disadvantages. The verve required to operate a highly active hydraulic system under different circumstances is very high. The viscosity of the hydraulic fluid can vary over different temperatures. The use of springs deters high engine speed generation. The adequate operation of this system requires the aid of powerful computers and accurate sensors 2.GASOLINE ENGINES WITH VVT INSTALLEDVVT was installed in a gasoline engine and it was observed that for optimal performance, the inlet valve the inlet valve closing(IC) and the exhaust valve(EC) opening periods have to be optimised. The maximum temperature and pressure obtained in the combustion chamber is a result of IC optimisation. Optimal timing for each valve is dependent on engine speed. This characteristic is shown for both IC and EO at part load as well as full load through figures 1 and 2.CUsersUSERDesktopoptimal EO vs engine speed.png habitus 1 Optimal EO Vs Engine pep pill3CUsersUSERDesktopOptimal IC vs engine speed.pngFigure 2 Optimal IC Vs Engine Speed3Figures 3,4,5 and 6 compare tortuousness produced, bsfc, CO emissions and NOx emissions respectively for a blueprint engine and an engine with VVT installed at full load.CUsersUSERDesktopTorque vs engine speed, full load petrol.pngFigure 3 Torque Vs Engine Speed at full loadCUsersUSERDesktopbsfc vs engine speed,full load petrol.pngFigure 4 bsfc vs Engine Speed at full load3CUsersUSERDesktopCO emissions vs engine speed, full load petrol.pngFigure 5 CO emissions Vs Engine Speed at full load 3CUsersUSERDesktopCO emissions vs engine speed, full load petrol.pngFigure 6 NOx emissions Vs Engine Speed at full load 3Its observed that VVT has little or no effect on the CO or NOx emissions. However, the torque produced has increased by 6% accompanied by a bsfc decrease of 2% 3.Similarly at part load, the torque produced, bsfc, CO and NOx emissions were compared for both engines and the results are indicated in figures 7,8,9 and 10 respectively.CUsersUSERDesktopTorque vs engine speed, part load petrol.pngFigure 7 Torque produced vs Engine Speed at part load3CUsersUSERDesktopbsfc vs engine speed, part load petrol.pngFigure 8 bsfc vs Engine Speed at part load 3CUsersUSERDesktopCO emissions vs engine speed, part load petrol.pngFigure 9 CO emissions Vs Engine Speed at part load 3CUsersUSERDesktopNOx emissions vs engine speed, part load petrol.pngFigure 10 Nox emissions Vs Engine Speed at part load 3At partial load, its observed that the torque increases by 4% and 3% when the engine is rotating at 2000rpm and 5000rpm respectively complemented by a bsfc decrease of 6% and 14%.The main advantage of the VVT for a gasoline engine is maximum torque can be achieved at a lower speed of rotation 3.DIESEL ENGINES WITH VVT INSTALLEDThere are a number of factors which have hampered the research of VVT in diesel engines. Two of those factors are the complexity of VVT engineering and the rapid evolution of diesel engine technology. Due to the lean nature of air fuel mixture, diesel engines emit much less CO as compared to their gasoline counterparts. This is because of the absence of unburnt hydrocarbons in the mixture. However, due to higher temperatures involved in diesel engines owing to higher compression ratios, NOx are much more prominent as compared to gasoline engines 4.Since diesel engines operate under higher compression ratios, the gap between the valves and the top of the piston at Top groundless Centre (TDC) is very minimal. Therefore, the VVT has to be designed such that there is no contact between the valves and the piston.Figure 11 shows the variation of torque with engine speed by only varying the inlet valve closingCUsersUSERDesktoptorque vs speed, IC diesel.pngFigure 11 Torque Vs Engine Speed at full load with in let valve closing control 4It is observed particularly at low engine speeds that the maximum torque increases at low speeds. In this case, there was a 6% increase in torque at 1000rpm and 8% increase at 1600rpm.A similar pattern is observed by controlling the exhaust valve opening (EO) as well as the combined control of EO and IC. This is indicated in figures 12 13 repectively.CUsersUSERDesktopTorque vs engine speed EO diesel.pngFigure 12 Torque Vs Engine Speed at full load with EO control 4CUsersUSERDesktopTorque vs engine speed EO,IC- diesel.pngFigure 13 Torque Vs Engine Speed at full load by controlling EO ICAt part load is was observed that by closing the inlet valve before Bottom lifeless Centre (BDC), the bsfc had reduced 4.Figure 14 shows the effect of IC timing on effective compression ratio after the installation of VVT.CUsersUSERDesktopCompression ratio IC- diesel.pngFigure 14 Effect Of IC Timing On Effective Compression Ratio 4It is clear from figure 14 that by adv ancing the IC after BDC the compression ratio increases inside the combustion chamber.CONCLUSIONThe need for Variable Valve Timing has been discussed in detail. The various types of VVTs popularly have been discussed briefly. Most of mod vehicles use VVTs mainly due to legislation. The effect of using VVT in a gasoline as well as a diesel engine was analysed and explained accordingly. It was sight that though there wasnt any appreciable decrease in emissions, the maximum torque produced was attained at a lower speed compared to their series counterparts. Also, the bsfc in both cases had been reduced.