Study Of Variable Valve Timing Anatomist Essay

Variable Valve Timing is a mechanism where the lift, duration or timing of either the intake or exhaust valves or both can be transformed in several mixtures during engine procedure. This is not easy for an engine with fixed consumption and exhaust valves. An engine with VVT installed helps in improving engine unit performance over a wider selection of the engine operating spectrum. In this particular paper, the result of VVTs on gasoline as well as diesel machines were talked about and also compared to their series counterparts. For your diesel engine unit, it was witnessed that with the advantages 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 rates of speed. Minute variations in CO, NOx and PM emissions were observed. However at full-load operation, NOx emissions experienced increased marginally. In the case of the gasoline engine, it was discovered that there wasn't any significant improvement in the CO and NOx emissions. However, the engine motor torque possessed increased especially at low rates of speed and at incomplete load. This was complimented by way of a reduction in Brake Specific Energy Ingestion (bsfc).


The valves in 4-Heart stroke IC Motors are mostly planting season operated [1]. The starting and closing of the valves is governed by the crankshaft which is linked to the valves through an intermediary camshaft which is attached to every valve through a cam. The amount of fuel that gets into the combustion chamber would depend after valve timing. The camshaft is connected to the crankshaft by using anybody of gears, chains or belts. In regular motors without VVT, the cam lobe condition and position as well as it profile are tuned for a particular engine speed which usually compromises high-end torque or low-end vitality. However, with the benefits of VVT, the cam characteristics, e. g. timing, can be transformed. Thus, this allows for better engine efficiency and electric power at variable engine speeds [2].

This is useful especially at broadband operation in which a car requires more air to be able to deliver the required power. If lesser amount of air were to be joined at high rates of speed, there would a significant decrease in performance. However, if the valves were to be kept open for an extended duration, there will be a case of imperfect combustion inside the combustion chamber at low speeds because the pressure necessary to attain proper combustion of gas is insufficient. This again results reduced engine unit performance as well as a rise in petrol emissions [2].

Modern emission polices establish by various government authorities have obligated several manufacturers to adopt VVT in their engines. A lot of the current car manufacturers have implemented VVT engine motor systems.

The popular VVT mechanisms are categorised into purely mechanised, electro-mechanical and hydraulic. Purely mechanised VVT systems are further categorised into Oscillating Cam, Eccentric Cam Drive, Three-Dimensional Cam Lobe and Two Shaft Combined Account [2].


Purely Mechanical

In this kind of VVT, the valve timing is run by mechanical means. It really is further subdivided into

Oscillating Cam

This method includes using either a regular cam lobe or an eccentric and connecting rod which creates oscillatory movement 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 up and zero position will be required in which particular case the part cam lobe is completely base circle. The exact contrary situation is maximum period at full lift in which case it is totally flank. BMW's Valvetronic, Toyota Valvematic( both conventional) and Nissan VVEL( eccentric and connecting pole) are variations of the method. The Valvetronic is very reliable however the lift up duration would depend on amount of lift. Hence its only applied to 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 one company ever sold to have ever used this version of VVT. In this method, an eccentric disc mechanism is utilized which may differ the angular rate of the cam. The less the angular velocity, the greater the length of lift. The disadvantage of this method is that it is expensive as each valve requires its controller [2].

Three-Dimensional Cam Lobe

This system comprises of an axially elongated cam lobe which includes shorter duration profile and a greater duration account at either ends. Thus, valve lift period can be mixed by tilting the cam lobe axially. However, such a system requires the follower to tilt in various guidelines as the lobe flanks of the cam lobe are not parallel to the axis of rotation of the camshaft. This issue hasn't been rectified which explains why it's not used commercially [2].

Two Shaft Combined Profile

In this technique, two closely set up camshafts are used. The angular positions of the camshaft with respect to the crankshaft can be organized by utilizing a phasing device. A follower connects both camshafts which is operated by the two lobes simultaneously. Both lobes are used for the intake and exhaust valves respectively. The disadvantage of this system is that the settings on one lobe may influence the other thus tampering the entire system. It offers yet to be utilized commercially [2].


Camless engines use this type of VVT in which the valve starting and final is achieved by using electro-magnets. The disadvantages of the systems are : deceleration of the valve is hard to achieve, springs used in the valves have to changed to the smallest of margins and lastly, the use of electromagnets reduces the engine motor efficiency [2].


Another system developed as an alternative to cam machines. In this technique, the pressure of the liquid is utilized to trigger valves. However, this technique has its disadvantages. The energy required to operate an extremely dynamic hydraulic system under different circumstances is very high. The viscosity of the hydraulic fluid may differ over different temperatures. The usage of springs deters high engine speed generation. The adequate operation of this system requires aid from powerful pcs and accurate receptors [2].


VVT was installed in a gasoline engine and it was noticed that for optimal performance, the inlet valve the inlet valve final(IC) and the exhaust valve(EC) starting periods need to be optimised. The utmost temp and pressure obtained in the combustion chamber is because IC optimisation. Best timing for every valve would depend on engine acceleration. This feature is shown for both IC and EO at part load as well as full insert through figures 1 and 2.

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Figure 1 - Ideal EO Vs Engine motor 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 standard engine motor and an engine motor with VVT installed at full insert.

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Figure 3 - Torque Vs Engine unit Speed at full load

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Figure 4 - bsfc vs Engine unit Acceleration at full insert[3]

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Figure 5 - CO emissions Vs Engine Speed at full weight [3]

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

Its discovered 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 loss of 2% [3].

Similarly at part insert, the torque produced, bsfc, CO and NOx emissions were likened for both machines and the email address details are indicated in results 7, 8, 9 and 10 respectively.

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Figure 7 - Torque produced vs Engine Speed at part insert[3]

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Figure 8 - bsfc vs Engine Acceleration at part load [3]

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Figure 9 - CO emissions Vs Engine Rate at part insert [3]

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Figure 10 - Nox emissions Vs Engine unit Acceleration at part fill [3]

At partial load, its observed that the torque increases by 4% and 3% when the engine unit is revolving at 2000rpm and 5000rpm respectively complemented by way of a bsfc loss of 6% and 14%.

The main advantage of the VVT for a gasoline engine unit is maximum torque can be achieved at less speed of rotation [3].


There are a number of factors which have hampered the research of VVT in diesel motors. Two of those factors will be the complexity of VVT technology and the speedy evolution of diesel engine technology. Because of the lean mother nature of air energy mixture, diesel motors emit significantly less CO as compared to their gas counterparts. This is because of the absence of unburnt hydrocarbons in the combination. However, due to raised temperatures involved with diesel engines due to higher compression ratios, NOx are a lot more prominent when compared with gasoline motors [4].

Since diesel engines operate under higher compression ratios, the gap between the valves and the top of the piston at Top Deceased Centre (TDC) is very little. Therefore, the VVT must be designed such that there is absolutely no contact between your valves and the piston.

Figure 11 shows the variation of torque with engine unit acceleration by only differing the inlet valve closing

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Figure 11 - Torque Vs Engine Quickness at full fill with inlet valve closing control [4]

It is witnessed specifically at low engine speeds that the utmost torque enhances at low speeds. In cases like this, there was a 6% upsurge in torque at 1000rpm and 8% increase at 1600rpm.

A similar routine is witnessed by handling the exhaust valve beginning (EO) as well as the combined control of EO and IC. This is indicated in numbers 12 & 13 repectively.

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Figure 12 - Torque Vs Engine motor Acceleration at full weight with EO control [4]

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Figure 13 - Torque Vs Engine motor Speed at full insert by managing EO & IC

At part insert is was observed that by shutting the inlet valve before Bottom level Deceased Centre (BDC), the bsfc possessed reduced [4].

Figure 14 shows the effect of IC timing on effective compression proportion after the installing VVT.

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Figure 14 - AFTEREFFECT OF IC Timing On Effective Compression Percentage [4]

It is clear from body 14 that by evolving the IC after BDC the compression percentage enhances inside the combustion chamber.


The need for Adjustable Valve Timing has been mentioned in detail. The various types of VVTs popularly have been reviewed briefly. The majority of modern vehicles use VVTs due mainly to legislation. The result of using VVT in a gasoline as well as a diesel engine unit was analysed and discussed accordingly. It had been noticed that though there wasn't any appreciable decrease in emissions, the utmost torque produced was accomplished at a lower speed in comparison to their series counterparts. Also, the bsfc in both conditions had been reduced.

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