THROTTLE BODY

Introduction Throttle body regulates the amount of air entering the engine depending on the demands of the engine. At higher speeds or while accelerating when more fuel is burned is to yield more power throttle is opened up to increase the amount of air entering the engine while in other conditions throttle remains closed to a degree preventing unrestricted flow of air.

FSAE Constraints Throttle must be actuated manually, use of electronic throttle is prohibited. The throttle actuation system must use two return springs located at throttle body. Positive pedal stop. Throttle must be placed upstream of the restrictor and no throttling can be done downstream to the throttle. Also the restrictor and the throttle body are to be separate i.e. a single body incorporating both isnt allowed.

Design Options Barrel Valve: It consists of a barrel like valve with a through hole, when at default position it completely blocks the flow of air through the pipe but as it turns, the mouth of the hole is exposed and allows for the air to flow. However, it is used more in rally racing than FSAE (with only two positions ON and OFF). Also, it does not allow for easy installation of the TPS. The air flow through this valve is also less regular. It allows a faster throttle response but at the expense of throttle control Fluid Flow through a barrel valve: Fig. Fluid Flow through a closed barrel valve Fig. Flow through a more open barrel valve

Fig. Flow through a wide open valve or WOT condition Slider: It works more like a gateway than a valve. It is composed of a slider that slips in and out of the pipe. However, there are several problems associated with it such as sticking. After giving throttle when we pull back on the valve the slide might get stuck with the plate leading to an open valve or throttling when we are actually not throttling. TPS placement is also an issue with this design of the valve.When the driver would let off the gas at the end of the straight, the high vacuum would suck the slider against its mating surface hard enough for it to stick at partial (but disturbing) throttle positions in the braking zone. Hence we wont be using this design in our car.

Butterfly Valve: Well be using this for throttling. It consists of a butterfly valve that regulates the flow of air into the engine. The valve completely cuts off the supply to the engine but as it turns it presents lesser and lesser area to the incoming air and hence lesser restriction. The butterfly does obstruct airflow even at WOT. Turbulence is bad as the air enters the restrictor. However, turbulence is a problem with barrel valves and sliders too, because while they don't obstruct the center of flow, it is difficult to make the sides of the walls perfectly smooth, and in transients a 90% throttle opening on a slider or barrel is causing a lot of turbulence too.A bigger throttle body ensures more power, but there is a restrictor of 20 mm downstream. Thus the increased size of a throttle body has an impact only to an extent. A 38mm valve produces much more power than a 32mm one and only slightly less than a 50mm or 48 mm valve. The diameter of the valve would be of 38mm and with actual testing we will improve on this with the results of the prototype. It will be positioned at a distance of about 15 cm above the restrictor but this too would be varied during testing to give optimum results.

RESTRICTOR

IntroductionIn automobile racing restrictor plate or air restrictor are placed in intake manifold to restrict the amount of air flowing to the engine thereby limiting its power. This type of system are incorporated for insurance purpose by limiting top speed and thus increase safety and provide equal level of competition.

FSAE Constraints 1. A single circular restrictor (20 mm for Gasoline and 19 mm for E-85) must be placed between the throttle and the engine and engine airflow must pass through the restrictor.2. If more than one engine is used, the intake air for all engines must pass through the one restrictor.3. It must be located to facilitate measurement during the inspection process.

Objective1. To maximize the volumetric air flow rate through the restrictor. 2. To keep the weight of system as minimum as possible.3. Minimize the pressure drop across it.4. Choose the material so as to reduce the weight as well as withstand the flow pressures.5. Design the restrictor such that no choking takes place.

Design Options1. Flat plate: It is a thin, square aluminum piece with holes that restricts airflow from the carburetor to the engine.

Pros: Simple design and easy to manufacture.Cons: Large pressure drop, low air flow rate, creates turbulence

2. Venturi type (Converging diverging nozzle): It is a tube or pipe which employs a temporary restriction or narrowing in its length to reduce the pressure and increase the velocity of a fluid or gas passing through it.

This phenomenon of simultaneous pressure reduction and velocity increase is known as the venturi effect.

A Diagram of a CD Nozzle

This is what a CD nozzle looks like, an hourglass shaped tube.

Pros: Pressure recovery takes place in divergent section, less turbulences.Cons: Manufacturing is a bit difficult (than flat plate).So we have decided to use venture type restrictor in our intake manifold.

Preliminary calculationIf we keep on increasing pressure difference between inlet and outlet of a CD nozzle, a critical pressure difference will come at which mass flow rate becomes constant. This is called choked flow.The maximum allowable mass flow rate of gas through a cross-section can be computed by setting the cross-sectional area to be same as the throat area. =M(dot)= mass flow rate, kg/sC= discharge coefficient, dimensionlessA= discharge hole cross-sectional area, mk= Cp/Cv of the gasCp= specific heat of the gas at constant pressureCv= specific heat of the gas at constant volume= real gas density at P and T, kg/mP= absolute upstream pressure of the gas, PaM= the gas molecular mass, kg/R= Universal gas law constant = 8314.5 (Nm) / (kmoleK)T= absolute upstream temperature of the gas, K

All the above parameters are known for 20 mm restrictor. On plugging all values choked mass flow rate comes out to be about 0.075 Kg/s.For this flow rate we get maximum possible rpm of around 15000 rpm for 500cc Enfield engine (ideally). For our rpm range (6000 rpm) flow rate comes out to be 0.03 kg/s which would be ideal for our case.

CFD AnalysisIdeally in a CD nozzle for minimum pressure difference, converging angle lies between 7-10 degrees and converging angle lies between 18-22 degrees.The objective of cfd analysis is to analyze the air flow in the restrictor, verify above dimension and come up with a design which will minimize the pressure drop across it.2D shape of restrictor was modeled (since it is axially symmetric) in ANSYS 13 workbench and meshed with inflation at surface. Then it was simulated in fluent with mass flow rate of 0.03 kg/s. Pressure drop across restrictor was observed for different combination of above angles.

Contour of Absolute pressurePlot of pressure drop (bar) vs half diverging angle (degree) at constant half convergent angle is shown. Hence it can be easily verified that the fluent result matches with that of ideal range of CD angles.

Material Al 6061

Fabrication

Three methods of fabrication are available: Metal spinningPros: Complex shape (cone & cylinder) can be easily made out of a metal disk.Cons: This method is applicable for thin metal and thus lack strength.

CNCPros: Very accurate geometry can be madeCons: Expensive

Lathe MachineIt is cheap and easy to use. Our restrictor will be made on lathe.

TestingThe restrictor will be integrated on the intake system prototype (made of pvc and rubber pipes) and tested on the engine to get the required output.

PLENUM

IntroductionAfter throttle and restrictor air comes to the plenum which is a chamber that distributes air to various cylinders. The main purpose of the plenum is to equalize the air flow to the various cylinders, but its volume and shape, as well as the shape of the bell mouths, which are the opening to the runners, are also important.

Objective1. To generate a close to laminar flow. 2. To keep the weight of system as minimum as possible.3. Minimize the pressure drop across it.4. Choose the material so as to reduce the weight as well as withstand the flow pressures.

AnalysisTwo designs were modeled on ANSYS workbench and meshed with surface inflation. The objective of the cfd analysis was to compare the two available shape of plenum (one is with tapered and other is without tapered) for less turbulence and minimum pressure drop. Mass flow rate was set to 0.03 kg/s and turbulences and pressure drop inside the plenum was analyzed.

ResultPressure contour is show below for both the plenum designs:

Without tapered

TaperedIt can be easily seen that pressure drop across the tapered one is less than that of without tapered.

Contour of velocity is shown below:

Without tapered

TaperedAgain tapered plenum is better than other as it can be easily seen that flow is more turbulent in case of without tapered.The prototype of tapered plenum will be tested on engine to get the required volume and shape of the plenum.