In some competitions the rules clearly state that all participating cars must install a restrictor valve structure in the intake system of the engine. The intake air volume of the engine is considerably affected due to the existence of such a valve. Indeed, a small interface diameter through which gas flows can lead to considerable flow resistance and loss. In this study, a four-cylinder engine for FSC racing is analyzed using a combined method based on numerical simulation and experiments. The analysis reveals that the main factors affecting the intake air volume are the intake manifold and the volume of the resonance chamber. The influence of such factors is assessed using a single variable method and an optimal model and parameters are obtained accordingly. Comparison of different results show that the maximum torque for the optimized system is increased from the original 42.6 N·m to 46.9 N·m, thus demonstrating an increase of 10.6%. These findings provide a theoretical basis for the design of the intake system and the improvement of engine performance.
Formula Student China (FSC) follows the requirement of the rules, that is, Zhang et al. [
Domestic and foreign studies on air intake systems mainly focused on ordinary car engines whose speed is generally lower than 6000 rpm [
The quality of the intake system has a substantial impact on engine performance. The best layout for the intake system uses the front overhead arrangement based on the race rules and the overall layout of the car [
Related literature indicates that the intake restrictor valve belongs to the venturi tube [
The wave and inertial effects of the gas can be used to generate a large value pressure wave at a specific speed stage to increase the intake air volume. The calculation of the intake manifold length is based on the fluctuation effect.
In the equation above, q is the number of fluctuations, taking 4.5, c is the speed of sound under normal temperature conditions, which is 345 m/s, n is the engine speed, taking 8500 r/min, and L is calculated as 0.2706 m.
The total tube length is finally determined to be 270 mm. The calculated intake manifold length L includes the engine intake port length. Therefore, the CBR600 intake port length is 100 mm, the original engine throttle length is 75 mm and the final intake manifold length is
The volume of the resonance chamber also affects the intake air volume. If the volume is substantially large, then the pressure fluctuation in the resonance chamber will be reduced, the resonance effect will be weakened and the throttle response time will be prolonged. If the volume is substantially small, then the pressure fluctuation in the resonance chamber will increase, which cannot meet the high-load air intake requirements of the engine. The volume of the resonance chamber is generally four to seven times the total displacement of the engine. The displacement of the CBR600 engine is 0.6 L. Therefore, the volume of the resonance chamber is initially set to be five times the displacement, which is 3 L.
The structural parameters of the engine with a restrictor valve based on the above calculation data are shown in
Structure name | Numerical value |
---|---|
Intake (mm) | 270 |
Inner diameter of the inlet of the restrictor valve (mm) | 28 |
Tapered angle (°) | 22 |
Length (mm) | 21.5 |
Taper angle (°) | 14 |
Length (mm) | 213.5 |
Regulator chamber (L) | 3 |
Intake manifold (mm) | 95 |
The above calculation results reveal that the UG three-dimensional modelling software is used to establish a three-dimensional model of the intake system, as shown in
The competition rules require that the engine displacement must not exceed 610 mL. Therefore, the engine displacement used is 599 mL, and some of the engine’s structural parameters are shown in
Structure name | Basic parameters |
---|---|
Displacement (mL) | 599 |
Cooling form | Forced water cooling |
Compression ratio | 12.2:1 |
Diameter of inlet/exhaust valve (mm) | 27.5/23 |
Number of valves | DOHC, 4 valves per cylinder |
Bore × Stroke (mm) | 67 × 42.5 |
Intake valve advance/retard angle | 22° (advance)/43° (retard) |
Exhaust valve advance/retard angle | 40° (advance)/5° (retard) |
Electronic control and ignition system | Motec M84/Bosch Ignition Module |
GT-Power, which is a software that simulates and calculates the working process of the engine, performs a one-dimensional simulation analysis on the flow of the intake port. The simulation software uses numerical simulation methods to solve the mass, momentum and energy conservation equations to address the problem of unstable and compressible gas flows in the intake system. The flow state of gas in the air duct is remarkably complex [
The fluid state in the tube is similar in the same section, which conforms to the ideal gas state equation.
The state of the working fluid in the cylinder is uniform. That is to say, the pressure, temperature and concentration of all points in the cylinder are equal everywhere at the same instant.
The fresh charge entering the cylinder through the system boundary and the residual exhaust gas in the cylinder achieve instantaneous and complete mixing during the intake period.
The flow in the engine intake and exhaust pipes is simplified to a one-dimensional unsteady flow, that is, the flow parameters only change with the x axial coordinate and time (t). The finite volume method is used to analyse the following governing equations.
In the above formula, u is the air velocity, ρ is the gas density, p is the gas pressure, F is the pipe section area, f is the frictional resistance, D is the equivalent diameter, a is the gas flow acceleration, k is the heat transfer coefficient, q is the radiant energy.
The engine combustion model selects the Wiebe model, and its heat release law is the load Wiebe function. The relationship is presented as follows:
In the above formula, Q is the total energy of combustion, a0 is the angle of burning start, m is the shape factor and Δac is the burning duration.
This study chooses the Woschni model as the in-cylinder heat transfer model in the absence of the in-cylinder eddy current data. The thermal conductivity of the fluid to the wall is calculated by Colburn’s similarity theory. The calculation formula is as follows:
In the above formula, Cf is the friction factor, Cp is the gas specific heat capacity, Ueff is the effective velocity outside the boundary layer, Pr is the Planck number.
A one-dimensional GT-Power simulation model of the CBR600 engine, which is based on the introduction of the restricted flow intake system, is established. The model includes basic parameters, such as intake environment, air filter, throttle valve, resonance chamber, engine intake port, intake and exhaust valves, cylinder, crankcase and intake and exhaust pipes. The GT-Power model is shown in
In the engine model, the intake and exhaust ports are the boundaries of the model, and the boundary conditions are all set to standard atmospheric temperature and pressure (298 K and 100 kPa, respectively) [
The intake system with a restrictor valve shown in
The curve of the above figure reveals that the analysis results of the simulation model and the external characteristic data of the bench test can be compared and analysed. Comparing the torque and power of all other corresponding operating conditions, the maximum error is approximately 4.8%, which is lower than the 5% error range required by the project. Therefore, the GT-Power model simulation modelling is reasonable, and the analysis results demonstrate a certain degree of credibility.
The analysis of the influence of resonance chamber volume change on the dynamic performance of the engine is also studied by taking the full load characteristic simulation as a research method. When the volume of the resonance chamber is taken as a single variable, the length of the intake manifold is 95 mm to maintain a constant, and the variable is set at four to seven times the engine displacement, that is, the volume of the resonance chamber is 2.4, 3.0, 3.6 and 4.2 L. The torque (Ttq)–speed (n) and power (Pe)–speed (n) curves obtained by model simulation are respectively shown in
Assuming that the length of the intake manifold is 95 mm and kept as a fixed constant, the length of 20 mm is set as the gradient of change, and the main analysis manifold length parameters are 55, 75, 115 and 135 mm. The analysis results from intake manifold length variation of the engine under full load are used to examine the law of its influence on engine power. When the length of the intake manifold is taken as a single variable, the volume of the resonance chamber remains unchanged at 3 L, and the change curve is obtained as shown in
The analysis in
Intake manifold length/mm | Resonance chamber volume/L | |
---|---|---|
Case 1 | 95 | 3 |
Case 2 | 115 | 3 |
Case 3 | 95 | 3.6 |
Case 4 | 115 | 3.6 |
The data of Pe–n and Ttq–n are sorted into characteristic curves according to the case requirements in the above representation, as shown in
Considering the overall layout design of the vehicle, the above analysis results indicate that the optimised intake manifold length is set to 115 mm, the volume of the surge tank is 3.6 L and the Pe–n and Ttq–n characteristic curves are shown in
Combined with the previous comparative analysis data, the optimised intake system model is designed using UG as shown in
Structure name | Original data | Optimized data |
---|---|---|
Intake (mm) | 270 | 270 |
Inner diameter of the inlet of the restrictor valve (mm) | 28 | 28 |
Tapered angle (°) | 22 | 22 |
Length (mm) | 21.5 | 21.5 |
Taper angle (°) | 14 | 14 |
Length (mm) | 213.5 | 213.5 |
Regulator chamber (L) | 3 | 3.6 |
Intake manifold (mm) | 95 | 115 |
The analysed air is considered to be compressible fluid, and the gas is easily converted from laminar to turbulent flow in the actual intake process. Therefore, the simulation analysis using the turbulence model is performed as the analysis model [
Completing the flow field analysis of the intake limit flow through CFD analysis software is necessary to analyse the airflow movement of the intake system between the original and the optimised intake systems effectively. The velocity cloud and the gas flow diagrams of the intake system before and after optimisation are respectively shown in
Comparing
This study takes the intake system of the FSC engine as the research object. A research method combining simulation and experiment is used to analyse the influence of the intake restrictor valve structure on the engine performance to explore the engine performance completely. Using the principle of a single variable, the influence on the engine is analysed by changing the parameters of the intake manifold length and resonance chamber volume, and the following conclusions are obtained.
Increasing the length of the intake manifold when the intake manifold is within 115 mm can improve the engine performance at medium and high speeds. However, the intake resistance is increased when the length of the intake manifold exceeds 115 mm. This phenomenon affects the increase in the intake air volume, which results in a decrease in power torque. The power and torque of the resonance chamber increase with volume, whereas those with volume exceeding 4.2 L relatively decrease to 3.6 L. Combining the analysis results and comparing them with the original intake system, the change form of the external characteristic curve reveals that the maximum torque of the engine increased from the original 42.6 N·m to 46.9 N·m, thus showing an increase of 10.6%.
The paper carries on CFD simulation analysis to the original intake system and the optimized system. By analyzing the velocity vector cloud diagram and mass flow diagram of the original intake model and the optimized intake model, it can be seen that the optimized intake model changes the mass flow from 4.4895 kg/s increased to 5.8562 kg/s, which is an increase of 30.44%. Therefore, the analysis method that combines experiment and simulation has certain promotion and helpful significance for research on engine performance optimization.