The Effect Of A Twisted Tape Biology Essay

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In this paper, the effect of a twisted tape and a shrouded valve on the swirl characteristics of a single cylinder, air cooled engine were investigated. A steady flow analysis of swirl enhancement techniques was conducted on a steady flow bench with a swirl adaptor to obtain a global view of the intake flow. The variations of various non-dimensional parameters such as Coefficient of Discharge, Flow Coefficient, Swirl Coefficient and Swirl Ratio for different valve lifts were studied. A higher swirl ratio and swirl coefficient was obtained with the diesel head inserted with a shroud and twisted tape.

The investigation of in-cylinder flow motion has marked its presence since the inception of new engine technology and development. Many developments have occurred over a course of time in the automobile sector such as significant advancement in burn rate enhancement through head and port optimization, spark location and valve timing. Before the advent of complex computational simulation, engineers created methods to speed up the development of new, advanced engine designs from those of high performance designs through similarity and parametric studies. This method, most commonly used by the diesel industry, provided a first order assessment of a new design. The small engine community, due to other constraints, has not kept pace with the developments of the automotive industry, but certain techniques can be adapted to small engine development. Through a greater understanding of in cylinder fluid flow and techniques developed in the automotive industry, the small engine community can by-pass ineffective technology to meet the burn rate and emission requirements of tomorrow. By optimizing combustion with enhanced in-cylinder flow motions and by means of charge stratification achieved through gasoline direct injection technique, the lean operating combustion limit can be significantly extended and the emissions of pollutants can be reduced [1]. The in-cylinder flow is a decisive factor for combustion in the engine, which in turn provides significant effects on the engine performance. In-cylinder fluid flows govern the flame propagation rate in the spark ignition (SI) engines and control the air-fuel mixing in the diesel engines [2]. In IC engines, induction of air is of greater importance than that of fuel even though air and fuel are vital substances in the combustion. In an IC engine, the air enters the combustion chamber through the intake port with high velocity during the intake stroke. Therefore, a good understanding of the evolution process of fluid motion in internal combustion engines is critical to develop engine designs with the most attractive operating and emission characteristics [3].

EXPERIMENTAL SETUP

The steady flow experimentation of this investigation was conducted on a flow bench outfitted with a swirl adapter to measure the frequency of the flow of the head and swirl adaptor attachment. A swirl test rig similar to [4] was made except that a vane method was used for measuring the swirl instead of an impulse swirl meter. An Acrylic tube of outer diameter 99.1cm, inner diameter 92cm and length 1ft was selected as the cylinder for the Diesel engine. This transparent material was selected since visualization of flow and the rotation of the paddle wheel inside the cylinder is necessary. An Aluminium block of 35mm thickness and 15x15cm was used as an intermediate to fasten the acrylic tube to the engine cylinder.

The whole setup was supported on a stand along with an air box on the top which was connected to the intake side of the blower. An orifice plate was fixed to the inlet side of the air box to measure the mass flow rate. A blower of 16000 rpm, 600W, and 3.3m3/min discharge rate is used to induct air through the intake valve in order to simulate the actual engine operating condition A Bosch air flow meter was placed in between the blower and the air box to measure the mass flow rate of the air inducted into the blower. Similarly, the blower was clamped to the intake side of the cylinder head with a flow straightener in between to ensure laminar flow to the engine head

To measure the pressure drop in the engine a Manifold Absolute Pressure (MAP) sensor was also used. The rotation of the paddle wheel gives a measure the swirl inside the cylinder. The paddle wheel used for the setup was developed using Rapid prototyping technique. A mechanism for moving the paddle wheel within the engine cylinder volume was also manufactured in order to analyse swirl at different positions in the cylinder.

The engine valve lifts were adjusted with the help a mechanism using the stock rocker arms and bolts and were measured using a calibrated Dial Gauge.

Table 1: Specification of diesel engine

Engine Parameters Value

Bore (mm) 85.0

Stroke (mm) 76.6

Displacement (cc) 436

Number of Cylinder 1

Maximum intake valve open (mm) 10.4

Intake valve diameter (mm) 38.1

Intake Valve Stem diameter (mm) 7

CALCULATION PARAMETERS

When characterizing the flow through an engine, non-dimensional parameters such as volumetric efficiency, discharge coefficient, flow coefficient, swirl coefficient and swirl ration provide a description of the breathing performance. The use of non-dimensional parameters allows experimenters to remove the size effects from the data and compare various design based on geometry [5].

Discharge Coefficient

The discharge coefficient, Cd is defined to be the measured mass flow rate over the ideal mass flow rate as defined by an isentropic nozzle:

(1)

Where the characteristic area, Aci is

(2)

Dvalve is the inner seat diameter of the valve and Li is the valve lift. The characteristic velocity vc, is defined by a compressible flow velocity equation

vc (3)

The discharge coefficient is used to highlight low lift valve lift performance due to the selection of the characteristic area to reflect the minimum area between the valve and seat lips

Flow Coefficient

The flow coefficient, Cf, has a similar definition to Cd, but highlights higher valve lift performance:

(4)

The characteristic velocity follows equation outlined before but the area defined differently:

(5)

This area is constant across all valve lifts and is based off of the inner seat diameter of the valve. When used in conjunction with Cd, the static flow parameters allow for a description of the flow restriction in the system. These parameters are commonly plotted in conjunction with non-dimensional valve lift based off of the inner seat diameter.

(6)

Swirl Coefficient

Swirl Coefficient is defined which essentially compares the flow's angular momentum with its axial momentum. For the paddle wheel, the swirl coefficient Cs is defined by

(7)

Where is the paddle wheel angular velocity (=2πNp, where Np is the rotational speed)

Swirl Ratio

Swirl ratio is defined as the ratio of circumferential air speed in the cylinder to the axial speed of the air flow in the cylinder.

(8)

Circumferential air speed in the cylinder is calculated using,

π--n (9)

Axial speed of the air flow in the cylinder is calculated using,

(10)

Theoretical volumetric flow rate across the system is calculated using orifice meter equation as,

(11)

=100000 N/ m2

= 288.7 K

Fig 1: MAP and MAF sensor mounting

An Agilent Data Acquisition unit, 34970A measured the test voltage of MAP and MAF sensor, ambient temperature, and frequency with real-time output through an Agilent data logger. The test parameters were recorded for 60 seconds per valve lift for seven blower speeds so as to obtain a repeatable time-averaged voltage. The valve lifts set were 3.5mm, 4.5mm, 5.5mm, 6.5mm, 8mm, 9mm, and 10.4mm. The flow bench and test voltage data were post-processed through Microsoft Excel to determine the non-dimensional parameters and swirl ratio.

Fig 2: Mechanism for moving Paddle wheel

Fig 3: Valve Lift Mechanism

Fig 4: Experimental Setup

The Rpm of the Paddle wheel is found from frequency by

The line count of the flow meter used was 22.

RESULTS AND DISCUSSIONS

The data acquired from the flow meter, paddle wheel and mass flow rate sensor was used to determine the discharge, flow and swirl coefficient and swirl ratio. The non-dimensional parameters are discussed in the following

Fig 5: Mass Flow Rate With/Without Flow Meter

The swirl adapter was utilized for all the flow bench tests to measure the frequency while providing a negligible pressure drop across the instrument. Any significant pressure loss the instrument would be reflected in a change in mass flow rate. Since the difference in mass flow rate from the tests with and without the adapter is within the range of error, the adapter was left in place for all steady flow tests.

Fig 6: Cd Vs. L/D for Diesel and other modification

The coefficients of discharge (Cd) of the flow bench test flow from this experiment result are shown in Fig 6. The Cd investigation is based on difference pressure and valve lift per diameter. Coefficient of discharge in this experiment results show that, increasing the pressure and valve lift from 0L/D until 0.28L/D or 0mm until 10.4mm for intake valve lift can decrease the coefficient of discharge in the intake Cd.

Fig 7: Cf Vs. L/D for Diesel and other modification

Furthermore, the graphs depict the general trend of coefficient of discharge for intake from 0L/D until 0.28L/D is decreasing and after 0.28L/D is stable or horizontal. The experimental results showed that, increasing the valve lift can decrease the coefficient of discharge in intake manifold, but after the maximum valve lift, the coefficient of discharge is stable and does not decrease significantly.

The Flow coefficients (Cf) of the flow bench test flow from this experiment are also simultaneously shown in Fig 7. The Cf investigation is based on difference pressure and valve lift per diameter. Flow coefficient calculated from the experimental results showed that, increasing the pressure and valve lift from 0L/D until 0.28L/D or 0mm until 10.4mm for intake valve lift can increase the flow coefficient in the intake Cf. The graphs also depict the general trend of flow coefficient for intake from 0L/D until0.28L/D is increasing and after 0.28L/D is stable or horizontal. The experimental results showed that, increasing the valve lift and test pressure can increase the flow coefficient in intake manifold, but after the maximum valve lift, the flow coefficient is stable and does not show significant increase. However, in the modified diesel engine there is a slight change in the trend after 0.21L/D. The discharge and flow coefficients are not stable as in the previous two cases and dips significantly until 0.23L/D.The Flow coefficient for the Diesel engine fitted with shroud has almost the same value as that of the normal diesel engine, but at higher valve lifts it drops behind the normal diesel engine. In the Diesel engine fitted with twisted tape the Flow coefficient is greater than the other two engine head configurations up to 0.21L/D and dips significantly. The height of the shroud allowed the shroud to stay within the port during maximum valve lift, but not obstruct the intake runner during low lift.

Fig 9: Swirl Coefficient Vs. L/D

Fig 9 shows the steady-state swirl measurements of the diesel engine. The swirl coefficient increases with increasing valve lifts, reflecting the decreasing impact of the flow restriction between the valve head and seat. It is evident from the graph of the Diesel (Twisted Tape) head that the swirl coefficient becomes greater than the Diesel head at 0.15L/D ratio. The swirl coefficient for the Diesel (Shroud) head becomes more than the diesel head at L/D ratio 0.17L/D.The swirl coefficient obtained is maximum for the Diesel(Twisted Tape) and its peak value is at 0.21L/D or 8mm valve lift. The effect of introduction of twisted tape and shroud in valve is clearly evident from the fig 9.The introduction of both twisted tape and shroud proved to increase the swirl coefficient at higher valve lifts(>0.18L/D),but at lower valve lifts it provided a considerably less value.The Swirl ratio curves measured on the steady flow bench are presented in fig 10 as a function of valve lift/Diameter. The results shows that swirl ratio for original diesel head increases as the valve lift increases up to 0.15L/D, afterwards it falls and then increases .But the general trend of the curve is increasing. The maximum swirl ratio of 7.87 is obtained at 0.27L/D or 10.4mm valve lift fir the diesel head inserted with a twisted tape. It was also found that the modified diesel head (twisted Tape) had more swirl ratio than diesel head for higher valve lifts (>0.15 L/D) and had lesser swirl ratio for lower valve lifts. On the other hand, the swirl ratio for the modified diesel (shroud) also follows a similar trend as that of the twisted tape. But the overall swirl ratio obtained for the Diesel (shroud) head is less than the Diesel head inserted with a twisted tape. It can be also seen from the fig 10 that the diesel head fitted with a shroud provided almost the same swirl ratio as that that of the normal diesel head at lower valve lifts, but at higher valve lifts(?0.15L/D) provided a significantly improved swirl ratio making it more efficient than the twisted tape.

Fig 10: Swirl Ratio Vs. L/D

Fig 11: Valve Lift Vs. Crank Angle (Diesel)

Fig 12: Swirl Ratio Vs. Crank Angle After Intake (Diesel)

CONCLUSION

A steady flow analysis was performed on a Piaggio Ape Diesel head to determine a global perspective of the in-cylinder flow. The flow and swirl coefficient provided information about the flow rate in the cylinder by the intake manifold and to analyse and compare the effects of introduction of various modifications to port and valve. The use of Swirl in diesel is to primarily promote more rapid mixing between the inducted air and the injected fuel.

The major conclusions that can be drawn are

(1)The experimental results showed that, the mass flow rate and flow coefficient in the intake port of four stroke diesel engine provided the best values in the maximum valve lift per diameter 0.27L/D. It also seen that, increasing the valve lift can increase the air flow, decrease the coefficient of discharge and increase the Flow coefficient in intake manifold system.

(2)The increase in swirl coefficient with increasing valve lifts reflects the decreasing impact of the flow restriction between the valve head and seat. It was found that the swirl coefficient obtained for the diesel (shroud) and diesel (Twisted tape) is greater than the original head more L/D ratio>0.15,but at lower valve lifts is significantly lesser.

(3)The maximum swirl ratio of 7.87 is obtained at 10.4mm valve lift for Diesel head inserted with a twisted tape. The diesel head fitted with shroud proved to be more effective than the head inserted with a twisted tape. The effect was more prominent at lower valve lifts. However at higher valve lifts the head inserted with a twisted tape had more swirl ratio.

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