Study Of Variable Valve Timing Engineering Essay

Variable Valve Timing is a mechanism in which the lift, duration or timing of either the intake or exhaust valves or both can be altered in several 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 range of the engine operating spectrum. In this paper, the effect of VVTs on gasoline as well as diesel engines were discussed and also compared to their series counterparts. For a diesel engine, it was observed that with the introduction of a VVT, higher fuel-line and combustion pressures were achieved with inadvertently led to a better engine performance along with lower smoke density and higher torque at low speeds. Minute variations in CO, NOx and PM emissions were observed. However at full-load operation, NOx emissions had increased marginally. In the case of the gasoline engine, it was observed that there wasn’t any significant improvement in the CO and NOx emissions. However, the engine torque had increased particularly at low speeds and at partial load. This was complimented by a decrease in Brake Specific Fuel Consumption (bsfc).

INTRODUCTION

The valves in 4-Stroke IC Engines are mostly spring operated [1]. The opening and closing of these valves is governed by the crankshaft which is connected 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 useful especially at high speed operation where a car requires more air in order to deliver the required 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 open for a longer duration, there would be a case of incomplete combustion inside the combustion chamber at low speeds since the pressure required 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 adopted VVT engine systems.

The popular VVT mechanisms are classified 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 Shaft Combined Profile [2].

TYPES OF VVTs

Purely Mechanical

In this type of VVT, the valve timing is operated by mechanical means. It is further subdivided into:

Oscillating Cam

This 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 section 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. BMW’s Valvetronic, Toyota Valvematic( both conventional) and Nissan VVEL( eccentric and connecting rod) are variations of this method. The Valvetronic is very dependable however the lift duration is dependent on amount of lift. Hence its only used on intake valves. The Valvematic and the VVEL are more compact while delivering the same performance as the Valvetronic [2].

Eccentric Cam Drive

The Rover Company is the only company 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 it’s expensive as each valve requires its own controller [2].

Three-Dimensional Cam Lobe

This system comprises of an axially elongated 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 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 rectified which is why it’s not used commercially [2].

Two Shaft Combined Profile

In this system, two closely arranged camshafts are used. The angular positions of the camshaft with respect 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-mechanical

Camless 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 engine efficiency [2].

Hydraulic

Another 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 energy 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 INSTALLED

VVT 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.

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Figure 1 – Optimal EO Vs Engine Speed[3]

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Figure 2 – Optimal IC Vs Engine Speed[3]

Figures 3,4,5 and 6 compare Torque produced, bsfc, CO emissions and NOx emissions respectively for a normal engine and an engine with VVT installed at full load.

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Figure 3 – Torque Vs Engine Speed at full load

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Figure 4 – bsfc vs Engine Speed at full load[3]

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Figure 5 – CO emissions Vs Engine Speed at full load [3]

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Figure 6 – NOx emissions Vs Engine Speed at full load [3]

Its 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.

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Figure 7 – Torque produced vs Engine Speed at part load[3]

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Figure 8 – bsfc vs Engine Speed at part load [3]

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Figure 9 – CO emissions Vs Engine Speed at part load [3]

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Figure 10 – Nox emissions Vs Engine Speed at part load [3]

At 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 INSTALLED

There are a number of factors which have hampered the research of VVT in diesel engines. Two of those factors are the complexity of VVT technology 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 Dead 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 closing

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Figure 11 – Torque Vs Engine Speed at full load with inlet valve closing control [4]

It 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.

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Figure 12 – Torque Vs Engine Speed at full load with EO control [4]

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Figure 13 – Torque Vs Engine Speed at full load by controlling EO & IC

At part load is was observed that by closing the inlet valve before Bottom Dead Centre (BDC), the bsfc had reduced [4].

Figure 14 shows the effect of IC timing on effective compression ratio after the installation of VVT.

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Figure 14 – Effect Of IC Timing On Effective Compression Ratio [4]

It is clear from figure 14 that by advancing the IC after BDC the compression ratio increases inside the combustion chamber.

CONCLUSION

The need for Variable Valve Timing has been discussed in detail. The various types of VVTs popularly have been discussed briefly. Most of modern 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 noticed that though there wasn’t 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.