Design and optimization of a throttle body assembly by CFD analysis

Design and optimization of a throttle body assembly by CFD analysis

J Suresh Kumar^a, V Ganesan^a*, J M Mallikarjuna^a & S Govindarajan^b

aInternal Combustion Engines Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

bUCAL Fuel Systems Ltd., Chennai 600 002, India

Received 8 November 2012; accepted 16 August 2013

Throttle body assembly plays a vital role in metering the airflow. It mainly consists of a butterfly valve to vary the flow area to control air flow rate through it. There is hardy any established procedure to design a throttle body assembly based on the engine specifications. In order to bridge the gap, this study, design and optimization of a throttle body assembly for a single-cylinder engine used in two-wheeler application has been analyzed along with the investigation of critical flow through various sub systems using computational fluid dynamics (CFD). To start with, the throttle bore and bypass passage diameters are calculated from the basic flow equations. Using CFD, best possible throttle shaft profile is arrived at, which will enhance airflow to the engine. The airflow rate for different throttle openings is predicted taking into account the distribution of main and bypass flow. It is observed that the airflow through main and the bypass passage are almost same around 12% throttle opening and the airflow through main passage takes over beyond 25% opening. The novelty of this study is that airflow through the bypass is also predicted for different screw positions. From the analysis of results, it is found that with around two turns of bypass screw opening, the required amount of air flow rate could be achieved through the bypass passage to run the accessories of the engine at idling and also to meet the required performance and emissions levels as per the design target. In addition, there is a good agreement of CFD predictions with experimental results with an error of about 6%. Finally, it is concluded that the procedure adopted in this study to design the throttle body as per engine specifications will be very useful for the engine designers and in this aspect, CFD plays an important role.

Keywords: Throttle body assembly, CFD, Main flow, Bypass flow, Wake, Experimental results

In recent years, the main concern for human society is hazardous pollutants emitted by various sources. One such source is the exhaust emissions from the automobiles. In order to control these emissions without sacrificing the performance of the engine, conventional carburetor is found to be inadequate. It is mainly because, with carburetor, it is difficult to maintain the required air-fuel ratio throughout the engine operating range. This led the automotive industry to change over to electronically controlled fuel injection systems in modern vehicles, which uses electronic control unit (ECU) and can maintain the required air-fuel ratio throughout the engine operating range. The ECU calculates the required amount of fuel to be injected based on the engine operating conditions.

Main function of a throttle body assembly is to control the air flow into the engine based on vehicle demand. Throttle body is mounted between the air cleaner and the intake manifold. It has a venturi to reduce the pressure of the air flowing through it. The

intake flow is throttled by reducing the flow area.

This is done by providing a circular shaft known as throttle shaft and is mounted with butterfly valve at the downstream of the venture. The main challenge is the change in throttle position during transient operation of the engine, which introduces additional problems as the butterfly position is frequently changed as per driver’s demand. The airflow can be considered as unaffected by the fuel flow. However, the reverse is not true and fuel flow strongly depends upon the airflow. The schematic of a throttle body assembly describing air flow path is shown in Fig. 1.

Filtered air enters into the throttle body and moves down stream. Butterfly valve (or throttle valve), restricts the amount of airflow into the engine based on accelerator position. There exists a bypass passage (Fig. 2), which is used for adjusting the airflow at idling conditions. Flow through the main and bypass passage is very important from the point of view of

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*Corresponding author (E-mail: vijaysri@iitm.ac.in) Fig. 1  Schematic of air intake system in SI engine

the performance of the throttle body is concerned.

However, challenge is to design the air intake system that will provide minimum restriction to the flow.

Therefore, it is very much essential for the engine designers to understand the air flow through the throttle body by experiments or by numerical investigations.

In the conventional throttle body, if a throttle gets struck in the open position, the driver could generally put a toe under the accelerator and lift up.

Occasionally after servicing or repair, the wire or cable between the accelerator and throttle would not be correctly reinstalled causing sudden acceleration.

The purpose of the bypass screw is to adjust the amount of air going pass the throttle plate. This screw does not control idle speed. It sets the amount of air going through the bypass port in the throttle body by changing the relationship of the throttle blade to the bypass port.

In this study, the proposed bypass system is discussed in detail; there are other methods to control the bypass airflow into the engine. In passenger car application, it is the rotary solenoid or stepper motor that is in use in large extent, which basically does the precise control of opening of bypass air. This is done to consider the additional demands on the engine such as air-condition, power steering, head lamps, etc.

Electronically controlling the air enables allowing just the right amount of air needed to maintain the designated idle speed irrespective of the ambient and/or engine conditions. This also allows the ECU to dynamically respond to changes in engine load.

In recent years new and increasing requirements in terms of emissions control, drivability, and safety have led the development for drive by wire. In DBW (drive by wire), throttle actuator is a motorized body electrically driven and controlled by an electronic system that mediates between a driver’s request and effective traction possibilities depending upon drivability, safety, and emission control constraints.

This system does not require any bypass passage.

However, in two-wheeler application, especially for port injection, owing to layout constraint and cost, simple bypass screw design is preferred over other two advanced systems.

Although the IAC (idle air control) is supposed to last the vehicle's lifetime, various reasons may cause it to fail/malfunction prematurely. The most common failure mode is partial/complete jamming of the valve (due to dirt/dust or even oil) where it cannot be

smoothly controlled. The result is an engine that idles very rough and frequently stalls. Similarly, in case of electronic throttle control (ETC), most of the drivers have no idea how much intervention is happening as the driver’s decisions are overruled. The engine state can be deducted only with failure and fault management. Most ETC, systems have sensor and controller redundancy with complex independent microprocessors.

In the proposed system, which is a combination of bypass screw with a solenoid has an advantage over passenger car system. Even though, this system consists of a solenoid, which is similar to passenger car system, even with the solenoid stuck, the bypass screw can be used to adjust and set the engine idling speed. In addition to this, this solution is cost effective, user friendly in the market for servicing for a two wheeler applications.

Alsemgeest et al.¹ have carried out simulation of time-dependent flow through throttle valve to determine flow mechanisms for various throttle plate angles and compared the results with hexahedral and tetrahedral meshes. Chen and Chen² analyzed small airflow rate at engine idling by using CFD analysis to aid throttle body design and to study tolerance effect on the estimated airflow rate. Diego³ have carried out CFD analysis across carburetor venturi for small engines. They have concluded that overall discharge coefficient can be used to correlate the mass flow rate.

Huang and Kim⁴ have carried out numerical simulation of flow around butterfly valve to investigate the physical phenomena concerned with the flow field. For their analysis, they assumed incompressible fluid flow past the butterfly valve at different valve disk angles with a uniform incoming velocity. Pursiful et al.⁵ have carried out time- efficient throttle flow data collection method by using a sonic nozzle flow bench. They have measured air flow as a function of throttle angle and pressure in a manner analogous to on-engine dynamometer throttle flow characterization. Ross et al.⁶ analyzed throttle body flow by using sonic nozzle flow bench to measure air flow as a function of throttle valve angle and pressure, in a manner analogous to an engine dynamometer throttle flow characterization. He also discussed the throttle body flow modeling considerations. Song et al.⁷ have used metamodel to analyze the butterfly valve to optimize the design to reduce the weight. They have used orthogonal array method to perform the design of experiments. Xue

et al.⁸ have carried out fluid and structural analysis of large diameter butterfly valves. They have measured the flow performance for different opening angles.

They have concluded that when the valve is closer to closing position, the flow is very turbulent. Yoshihiro et al.⁹ numerically analyzed three-dimensional flow for different throttle openings, by using k-e turbulence model and pressure boundary conditions.

Wang et al.¹⁰ have carried out here dimensional simulation of butterfly valve using a moving grid technique. They have studied the torque, flow and discharge coefficients. Heywood¹¹ and Shaw¹² had given various design factors to be considered while designing the intake system components, viz., throttle body assembly, intake manifold, cylinder head path and intake valve. The basics of CFD were explained by Versteeg and Malalasekara¹³.

The limited studies are reported on numerical simulation of flow through the throttle body, it is proposed to analyze the flow through the throttle body assembly using CFD and use it for the design and optimization of the same. The study involves the optimization of throttle shaft configuration, prediction of airflow at different throttle openings. It is hoped that such investigation will help to understand the flow structure in the throttle body assembly.

Design of Throttle Body

Figure 2 represents schematic of a throttle body with important dimensions. Major dimensions of the throttle body are the throttle bore diameter (D) and bypass passage diameter (dp), the overall length of the throttle body (L) is fixed based on the engine layout.

Diameter D1 and diameter D2 are selected based on the air-cleaner side fitment and the intake manifold side fitment respectively. Since the overall length L is known, manifold side length L3 is determined

considering the horizontal position of throttle valve at fully open condition. Next, length L1 is chosen based on the air cleaner side mounting and thereby length L2

gets automatically fixed. Other lengths (L4, L5 andL6) for bypass passage opening at the upstream and downstream sides can be arrived such that it does not exceed the overall length L.

Bore and bypass passage diameters

The throttle body assembly considered in this study is for a single-cylinder four-stroke engine with port or direct cylinder fuel injection. The schematic view of assembly of throttle body with bypass system is shown in Fig. 3. The main parameters for the throttle body are throttle bore and bypass passage diameters.

The throttle bore diameter (D) is calculated based on maximum mass flow rate of air required for the engine at wide open throttle conditions.

For the engine under consideration (displacement = 0.35 L and power 13 kW), power developed is 37.14 kW/L. At full load condition (bsfc = 330 g/kWh and air fuel ratio of 12.5), air required at full load is about 4.075 kg/kWh. Air consumption is about 42 g/s (151.2 kg/h) for 37.14 kW. The throttle bore diameter from the above airflow requirement is obtained as Flow through throttle body = Flow through engine * volumetric efficiency

Equating the flow through the throttle body to the engine flow requirement, the throttle bore diameter is calculated by Eq. (1).

t d

vol disp

V C

V N

D × ×

×

×

×

= π

2 η 4

… (1)

Fig. 2  Schematic view of assembly of throttle body

Fig. 3  Schematic view of assembly of throttle body with the bypass system

Where Cd is coefficient of discharge through throttle body, At is the area of throttle body, Vt is the air velocity through throttle body, ηvol is volumetric efficiency of the engine, Vdisp is the displacement volume of the engine and N is engine speed.

By knowing pressure at the intake side of the throttle body which is equal to atmospheric pressure (po) and engine vacuum at wide open throttle condition (p), air density at sea level (ρo), γ the adiabatic constant, velocity is calculated as,

γ γ

γ γ ρ

1 0 0

0 1

1 2

−



 



 −









= −

p p V_t p

... (2)

Then, with Vt known, considering volumetric efficiency as 100% for an ideal case, Cd as 0.9 and for a maximum engine speed of 5500 rpm, throttle bore diameter obtained is about 28 mm. However, the throttle bore diameter in actual conditions should be higher than the above value in order to account for throttle plate and shaft, which will provide additional restriction in the airflow path. For the engine considered, inlet port diameter is 29.1 mm. Therefore, final throttle bore is selected as 30 mm.

Generally, the flow rate of air required at idling is about 5-10% of the total air flow required at wide open throttle. In this case, idle air flow rate is assumed to be about 6.6% of the total air flow rate which is about 10 kg/h. Assuming flow to be incompressible, the diameter of bypass passage is obtained as,























 −









× −

=

− γ γ γ

γ ρ γ π

1

0 2

0 0

0 1

2 1

4

p p p

p p d_p m

… (3) Where m is mass flow rate though the bypass passage, A is bypass passage area, po is atmospheric pressure, ρo is density of air, γ is adiabatic constant, p is engine vacuum pressure. From Eq.(3), for a mass flow rate of 10 kg/h, the diameter of bypass passage is obtained as 3 mm.

Throttle valve opening for idling conditions

Air from the air-cleaner enters into the throttle body and move towards the main passage. At idling, since the throttle plate is almost in closed condition, all the air that is entering into the throttle body cannot move

through this small passage. Hence, air flows through the bypass air passage, which is also subjected to engine suction. The bypass screw, which is first placed in the bypass circuit, meters the air. By opening or closing the bypass screw, the amount of airflow can be varied through the bypass circuit. After this, the solenoid passage, depending upon the duty ratio as set by the ECU, once again meters the air. Finally, the bypass air enters, down stream the throttle after the throttle plate, and enters into the intake manifold. Once the throttle plate is opened, the entire suction will be in the main passage and the amount of airflow through the main passage increases. Correspondingly the airflow through bypass passage decreases.

In general, mass flow through the bypass passage is about 70% of the idling flow required for the engine and the remaining is through the main passage. Therefore, it is necessary to calculate the throttle opening required during idle conditions.The effective area of air flow is the difference of cross-sectional areas of throttle bore and the projected area of the throttle valve.

Effective area of air flow = Cross-sectional areas of throttle bore – Projected area of the throttle valve

= 2- * *

4D a b

π π

... (4) Where D is diameter of throttle bore, a and b are semi- major and semi-minor diameters of ellipse formed by throttle valve.

The major diameter 2a is equal to the bore diameter and the minor diameter depends upon the angle of inclination the throttle valve with the axis of the throttle body. However, semi-minor diameter can be calculated from the geometry of the throttle valve.

In this case, the throttle valve angle is found to be 6°

for the air flow rate required for idling at sea level conditions. However, at higher altitudes, above value of throttle valve inclination is not sufficient which has to be enhanced. For example, at 3000 m altitude, the atmospheric pressure is about 70 kPa and density of air is about 0.9095 kg/m³. Hence, to compensate the higher altitude, throttle valve inclination required at idling needs to be about 10°, which will be adjusted manually or electronically in an actual engine.

CFD Analysis Governing equations

In CFD analysis, the following governing equations are solved. Unsteady three-dimensional continuity equation is given as¹⁴:

( ) ( ) ( )

u v w 0

t x y z

ρ ρ ρ ρ

∂ ∂ ∂ ∂

+ + + =

∂ ∂ ∂ ∂ ... (5)

where t ρ

∂

∂ = rate of change of density with time, div(ρu) = net mass rate out of the element across its boundaries (convective term). Momentum equations used are as follows¹

X-momentum is,

( ^xx) ^{yx zx}

Mx

Du p

Dt x y z S

τ τ τ

ρ =^{∂ − +} +^∂ +^∂ +

∂ ∂ ∂ … (6)

Y-momentum is

( )

xy yy zy

My

Dv p

Dt x y z S

τ τ τ

ρ = ^∂ +^{∂ − +} +^∂ +

∂ ∂ ∂ ^{… (7)}

Z-momentum is

( )

xz yz zz

Mz

Dw p

Dt x y z S

τ τ τ

ρ = ^∂ +^∂ +^{∂ − +} +

∂ ∂ ∂ ^{… (8)}

where SM(x,y,z) is the source momentum per unit volume per unit time in x, y, z directions respectively.

In this study, k-ε turbulence models based on the generalized Boussineq eddy viscosity concept are employed as¹:

.

( )

( ) ^t 2 _{t ij ij}

k

k div kU div gradk E E

t

µ

ρ ρ µ ρε

σ

 

∂ + =  + −

∂  

… (9)

2

1 . 2

( )

( ) ^t 2 t ij ij

div U div grad C E E C

t _ε ^εk ^ε k

µ

ρε ε ε

ρε ε µ ρ

σ

 

∂ + =  + −

∂   … (10)

The standard k-ε model employs values for the constants that are arrived at by comprehensive data fitting for a wide range of turbulent flows and they are as¹:

1 2

0.09; _k 1.00; 1.30; 1.44; 1.92 C_µ = σ = σ_ε = C_ε = C_ε = Where,

2

eddy viscosity ( )_t k C_µ

µ ρ

= ε

Boundary conditions

In this study, three types of boundaries are involved, viz., inlet, outlet and wall. Inlet pressure boundary conditions are used to define the fluid pressure at the flow inlet. Pressure inlet boundary conditions are used when the inlet pressure is known, but the flow rate or velocity is not known. Outlet

pressure boundary conditions require the specification of static pressure at the outlet boundary. Since the outlet for the selected domain is subjected to engine vacuum, this condition is selected. Reynolds number of the flow becomes very low and turbulent fluctuations are damped considerably near the walls where wall boundary conditions are used. The laminar viscosity plays a significant role. In the present study, walls are assumed to be adiabatic with no slip condition.

In the present CFD analysis, the following have been studied: (i) optimization of throttle valve shaft configuration in order to maximise the air flow for a given throttle opening, (ii) effect of throttle opening on flow field in order to see the wake regions and velocity vectors, (iii) air flow distribution through main and bypass passages and (iv) effect of bypass screw turns on flow field.

Experimental Procedure

In order to compare the results of CFD predictions with experimental results, experiments have been conducted on a prototype throttle body, using a steady flow bench. The photograph of the experimental set-up is shown in Fig. 4. It consists of an air blower along with a flow control valve. Provisions are made to fix cylinder head, cylinder liner and the throttle body assembly. When the air blower is switched on,

Fig. 4  Photograph of experimental set-up (1- Orifice plates, 2- Test Pressure meter, 3- Cylinder head fitted with throttle body, 4- Flow meter, 5- Manometer, 6- Flow control valve)

air is sucked through the throttle body into the cylinder and flows out through the open end of the cylinder.

The flow rate of the air is adjusted by flow control valve and measured with the help of a flow meter mounted in air flow path. The air pressure is measured using an inclined manometer across the throttle valve. The required test pressure during testing is maintained by adjusting the air flow control valve with the help of inclined manometer. With this steady state experimental set-up, prototype of throttle body assembly developed in this study is tested.

Initially, the throttle opening is kept corresponding to engine idling conditions. The test pressure is set and flow through the throttle body is measured.

Subsequently, for 25, 50, 75 and 100% throttle opening positions flow rates have been measured.

In this study, the following experiments have been carried out: (i) measurement of air flow rate at different throttle openings, (ii) measurement of air flow rate at different bypass screw turns, and (iii) confirmation of engine performance and emissions for different bypass screw turns.

In this study, experiments have also been conducted with prototype throttle body of 30 mm bore and 3 mm bypass passage diameters in an actual engine at idling conditions, which is fitted with a port injection system. During the above experiments, at

different bypass screw openings, air-fuel ratio, engine speed, and hydrocarbon (HC) and carbon monoxide (CO) emissions have been measured. CO in percentage (%) and HC emissions in parts per million (ppm) are measured using a Horiba Portable analyzer.

Results and Discussion

Optimization of throttle valve shaft configuration

After designing the throttle body, optimization of valve shaft diameter is done in order to achieve maximum possible air flow rate through the given throttle bore. In this study, two types of throttle valve shaft configurations have been tried, viz., circular and rectangular shafts as shown in Fig. 5.

Figures 6 and 7 show velocity vectors and pressure contours respectively for circular and rectangular type throttle valve shaft configurations for 30 mm bore diameter. With the circular shaft, velocity at the outlet is less compared to that of rectangular shaft. This shows that loss of energy is more in case of circular shaft due to higher restriction across the flow, whereas with rectangular shaft, the flow is accelerated at the throat leading to less loss of energy. In addition,

Fig. 5 Rectangular and circular type throttle valve shaft configurations

Fig. 7  Pressure contours for circular and rectangular shaft configurations

Fig. 6  Velocity plots for circular and rectangular throttle shaft configurations

the wake region behind the shaft is more in case of circular shaft, which causes higher recirculation as compared to that of rectangular shaft. This causes additional flow restriction to the air flow downstream.

The point of separation is at the vertical diametrical plane for circular shaft, where as for rectangular shaft it occurs at the later stage down of the flow regime.

From Fig. 7, stagnation points are observed in both the cases of the shafts where the pressure is higher due to conversion of kinetic energy at the leading edge of the throttle valve. Low pressure region is observed at the point of separation in circular shaft configuration.

Minimum pressure drop is observed with the rectangular shaft configuration than that of the circular shaft configuration. Change of pressure around the throat in the rectangular shaft configuration is gradual as compared to that of the circular shaft configuration.

As expected, lower pressure is observed in the wake region in both the cases.

From the above results, it is observed that the rectangular shaft configuration streamlines the flow with lesser loss of energy as compared to that of circular shaft configuration. In CFD analysis, an additional air flow rate of 3 g/s is predicted with the rectangular shaft configuration at 100% throttle opening condition.

Therefore, rectangular configuration of valve shaft is considered as more advantageous and therefore further CFD analysis have been carried out for this configuration.

However, at idling, the effect of shaft configuration could not be evidenced. This is mainly due to the position of throttle plate at idling condition, which is almost in closed. This restricts the path of air flow into the engine, rather than the throttle shaft diameter or profile.

Effect of throttle opening on flow field

Figure 8 shows the vector plots of the air flow regime inside the throttle body under idling, 25, 50,

75 and 100% throttle opening conditions. From Fig.7, it is observed that the wake regions are created at the downstream of the throttle valve. It is also observed that, the wake decays with increase in throttle opening position. Stagnation region is observed at the upstream face of the throttle valve. As flow travels past the throttle valve edges, the velocity increases, the airflow rushes through the clearance between throttle body and throttle valve. It is observed that the velocity of air increases with increase in throttle opening. At different throttle opening positions, the air flow pattern is almost similar, i.e., the air flow passing over the leading edge of the valve accelerates downstream over the tip before heading along the wall and curving downward. The air flow past the trailing edge follows a similar pattern and the high-speed air flow remains attached to the throttle body and progressively spreading into the centre of the domain as the throttle opening increases.

Effect of throttle opening on airflow rate

Figure 9 shows variation of total air flow rate for different throttle opening positions as predicted by CFD and measured by steady state flow bench experiments. The air flow rate values shown in Fig. 8 are in normalized percentage with respect to maximum air flow rate. The trend indicates that the total air flow rate through the throttle body increases with increase in throttle opening. This is mainly due to the reduced restriction by the throttle valve when it opens more and due to increased engine suction which draws additional air. The CFD predicted values are in reasonably good agreement with those of experimental results within an error of about 5%.

Fig. 8  Velocity vector plots for different throttle opening positions

Fig. 9  Comparison of total air flow rate from CFD predictions and steady state experiments

Figure 10 shows the CFD predictions of the variation of air flow distribution through the main and bypass passages with the throttle opening positions. The air flow rate through the bypass passage is calculated by the difference in air flow rates through the main passage when bypass passage is on and off respectively. From Fig.10, it is observed that, as throttle valve opens, the air flow rate through the main passage increases and that through the bypass passage decreases and finally reaches the steady state in both the passages.

From Fig.10, it is also observed that, at about 12%

throttle valve opening (point a), the air flow rates through the main and bypass passages are equal. Further increase of the throttle opening up to 25%, increases the air flow rate through the main passage (point b), but it decreases in the bypass passage (point c). Afterwards, up to full throttle opening, the air flow rate through the main passage takes over from that of bypass passage. This is attributed to higher suction at the throat region in the main passage as it is subjected to engine vacuum directly due to

higher throttle opening. The air flow rate distributions in the throttle body in different passages can be varied as per the requirement by adjusting the bypass passage diameter and throttle valve opening at idling conditions.

Effect of bypass screw opening positions on air flow field

Figure 11 shows the enlarged view of the bypass screw in the bypass passage of the throttle body. A bypass screw and a spring control the amount of air flowing through the bypass passage. At low throttle opening conditions, viz., idling and part loads, additional amount of air flow rate required for running the accessories is provided by the bypass passage by adjusting the position (opening or closing) of bypass screw. There exists a certain clearance between the bypass passage and the tip of screw even when the screw is completely closed (no opening). The bypass screw opening is required to be adjusted may be during servicing after certain period of usage of throttle body or during engine tuning. In this study, effect of bypass screw opening on flow field is studied.

Six bypass screw openings (fully closed, 0.5, 1, and 1.5, 2 and 2.5 turns) are considered for the analysis.

The pitch of the screw is 0.75 mm. Figures 12 and 13 show the velocity and pressure plots for 0.5 and 2 turns of bypass screw opening. From Fig.12, it is observed that, at 0.5 turn of bypass screw opening, there is no air flow to the main passage from the bypass passage.

Similarly observation of the flow fields of the cases of 1, 1.5 turns of bypass screw openings revealed that, the air flow from the bypass passage gradually increases

Fig. 10  Distribution of main and bypass passage airflow rates with throttle opening position

Fig. 11  Schematic of enlarged view of the bypass passage screw

Fig. 12  Velocity and pressure plots at 0.5 turn of bypass screw opening

Fig. 13  Velocity and pressure plots at bypass screw with two turns

with the opening of bypass screw (not shown here).

From Fig.13, it is observed that, at 2 turns of bypass screw opening, there exists considerable amount of the air flow entering from bypass passage into the main passage at downstream of the main flow.

Figure 14 shows the comparison of variation of air flow rate through the bypass passage with number of turns of bypass screw opening. In Fig.13, the values of air flow rate are in normalized percentage with respect to the idling air flow rate (0.7 g/s) through the bypass passage. From Fig.14, it is observed that, the trend of bypass passage air flow rate by CFD predictions and steady state flow bench experiments are quite similar.

From Fig.14, it is observed that, at fully closed position of the bypass screw, there exists an air flow rate of about 8%, which is not sufficient to run the engine and accessories at idling conditions. This air flow rate is due to the small clearance existing between housing and bypass screw which is required to avoid metal-to-metal to contact. Therefore, here it is required to study at what opening of the bypass screw, the required amount of air

flow rate can be obtained. From Fig.13, it is found that, the required amount of air flow rate of 10 kg/h for idling is obtained at 1.5 turns. To be on safer side, it is decided to keep the number of turns of screw opening at 2 turns.

It is also observed that, the CFD predictions are in reasonably good agreement with those of experimental results with a maximum error of about 4%. After deciding about the bypass screw turns required for idling air flow rate, it is also required to check whether the bypass screw opening during idling satisfies the performance (air-fuel ratio, idling engine speed) and emissions (HC and CO) requirements. Therefore, performance and emission characteristics of the engine were tested at idling conditions which are explained in the following sections.

Effect of bypass screw opening on engine performance and emissions

So far, with the help of CFD analysis, the basic design of throttle body has been arrived. However, for fine tuning the design, experiments on an actual engine were conducted to verify the performance and emissions characteristics of the engine at idling conditions.

Figure 15 depict the variation of engine speed for different bypass screw opening. From Fig.15, it is observed that, the required engine speed of 1500 rpm during idling is achieved by one turn of the bypass screw opening itself. This design target is set based on engine idle stability test conducted, which also includes the criterion such as cold startability and recovery, hot idle restart, etc. The co-efficient of variation at the design target point was checked and was found within 0.5%.

However, it is stabilized at the bypass screw opening of two turns within a limit of about 100 rpm.

Figure 16 shows the variation of air fuel ratio (AFR) for different bypass screw openings. From Fig.16, it is seen that, with one turn bypass screw opening, the required air-fuel ratio (13.6:1) as per the design target has been achieved. However, at the set 2

Fig. 14  Comparison of variation of air flow with bypass screw position

Fig. 15  Variation of engine speed with bypass screw opening

position Fig. 16  Variation of air-fuel ratio with bypass screw opening position

turns of bypass screw opening, the air-fuel ratio satisfies the design target values.

Figure 17 depict the variation of carbon monoxide (CO) emission for different bypass screw openings.

From Fig.17, it is observed that, the CO emission as per design target at idling is achieved with one turn of bypass screw opening itself. However, at two turn of bypass screw opening which is required as discussed earlier gives about 14% lower CO emission as compared to design target value which is quite good.

Figure 18 shows the variation of hydrocarbon (HC) emissions for different bypass screw openings. From this figures, it is found that with 0.7 turn of bypass screw opening itself, the required HC emissions levels as per deign target has been achieved. However, at two turns of bypass screw opening, the HC emissions are lower by about 24% than the design target level.

Effect of throttle bore diameter on airflow and engine power developed

In order to understand the effect of throttle bore diameter on airflow to the engine, steady state airflow was measured. In addition to Ø30 mm diameter

throttle body, two different size of throttle body, viz., Ø28 mm and Ø32 mm, which is the standard practise in the industry were fabricated and measured for airflow. For the same engine, the effect of increasing the throttle bore diameter is discussed.

Figure 19 shows the comparison of airflow of different throttle bore diameters conducted on the steady state test set up. The trend of airflow is similar for all the throttle bore diameters. From Fig. 19, it can be seen that at part throttle openings, there is no additional benefit as the throttle plate restricts the airflow. The effect of increase in throttle bore diameter could be seen only after 60% throttle opening. For a 2 mm increase in throttle bore diameter, the additional air flow is about 7% at wide open throttle conditions. This gives an indication of effect of throttle bore diameters on amount of airflow into the engine.

Figure 20 shows the comparison of engine power of different throttle bore diameters conducted in actual engine. The trend of power is similar to that of airflow for all the throttle bore diameters. For a 2 mm increase

Fig. 17  Variation of CO emissions with bypass screw opening positions

Fig. 18  Variation of HC emissions with bypass screw positions

Fig. 19  Comparison of airflow of throttle bodies with different throttle bore diameters

Fig. 20  Comparison of engine power of throttle bodies with different throttle bore diameters

in throttle bore diameter, there is an increase in power of about 6% at wide open throttle conditions. However, further increase in throttle bore diameter will not yield additional benefit, in terms of performance due to restriction created by cylinder head port diameter.

Uncertainty analysis

A certain level of uncertainty in the results is always associated with experimental work. The experimental error can be classified as fixed and random error. The fixed errors are repeatable in nature and can be taken into account by calibrating the instruments, thereby minimizing the errors. For random errors, a statistical estimate is required to account of uncertainty. To quantify the magnitude of uncertainties, error estimation is done based on Gaussian method with a confidence limit of 2σ (95.5% of measured value).

The uncertainty is calculated based on following equation:

Uncertainty of the measured parameter =

∆X=2 * X 100

σ × … (11)

Where, X = mean value and standard deviation = (X X)2

σ= ^∑ N⁻

Table 1 provides the list of uncertainties in measurement results.

Finally, the analysis and experimental procedure used in this study to design the throttle body for a fuel injected SI engine will be very much useful to the designer to fix the bypass screw opening in actual engine conditions considering all auxiliary loads.

Conclusions

In this study, a procedure for designing a throttle body required for fuel injection in spark ignition engine has been successfully arrived. In addition, design verification has been done with CFD analysis for maximizing flow rate through the throttle body

and for arriving required opening of the bypass screw for driving accessories at idling conditions. Also, CFD analysis has been performed at different throttle opening positions to study the flow fields. In the above cases, the CFD predictions match reasonably well with those of experimental values with a maximum error of about 6%. From the analysis of results, the following conclusions have been drawn:

(i) The air flow rates through bypass and a main passage are equal at about 12% throttle opening position and air flow rate at main passage overtakes than that of bypass passage at about 25% throttle opening position.

(ii) It is found that 1.5 turns of bypass screw opening will meet the required air flow rate during idling of the engine alone along with the accessories during idling conditions.

(iii) In order to meet the HC and CO emissions levels comfortably, two turn of bypass screw opening position has been used.

(iv) For every 2 mm increase in throttle bore diameter, the increase in airflow is about 7% at wide open throttle conditions and at part throttle openings, the increase in airflow is by about 6%.

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Table 1  Uncertainty in measurements

Parameter % Error

Engine speed 0.5

Airflow rate 0.02

Engine power 0.75

Engine torque 0.5

Intake temperature 2.0

CO emissions 0.5

HC emissions 0.5