Modellazione del sistema di raffreddamento della monoposto di Formula SAE

U M R Eniversity of Odena and Eggio Miliad e "E F" Epartment of Ngineering Nzo Errarib ' D M Eachelor S Eegree in Echanical Ngineering

Modelling of the cooling system of the Formula SAE single-seater car UniMoRe

Supervisor Candidate Dr. Alberto Muscio Luca Zannini 975 A. A. 2017/2018

You never know what you can do until you try. — William Cobbett

Preface

This paper aims to describe the modelling of the cooling system of the M19-L Formula SAE single-seater car of the University of Modena and Reggio Emilia (Unimore). The need to develop and characterise this system arose from the need to validate the theoretical data provided by the manufacturers of radiators, fans and water pumps used on the 2018/2019 single-seater, thus allowing in-depth knowledge of the components used during the vehicle design phase.

The working environment was set up in the Unimore laboratories, where a flushing bench was set up for the radiator, which is essential for obtaining experimental data and comparing it with theoretical data. To this end, the Thermal Management division of the Formula Student project created a test circuit in which water, heated by a boiler to simulate the operating conditions of the vehicle, passes through the radiator to be cooled. This system, consisting of a boiler and radiator, was equipped with sensors for detecting temperature, pressure and flow rate, with the data acquired digitally using LabVIEW, data acquisition and analysis software from National Instruments. The data was then collected in an Excel file by the Formula Student team students for analysis and comparison.

This thesis begins with an introduction to the Formula SAE competition and the tests it includes. It then illustrates the technologies, research and calculation procedures that Team Unimore has developed over the years for the cooling system. This is followed by a description of the physical construction of the flow bench, followed by an analysis of the data collection methodology and the main characteristics of the instruments and software used. The three main working environments will be analysed in detail: the hot flow bench, the engine test bench and the cold flow bench. In particular, the data collected from these environments will be examined, with a focus on parameters such as the number of revolutions, the pump flow rate, the water temperature and the fan speed. Finally, the results obtained, together with any limitations, will be presented in graphs to allow experimental validation of the data by comparing them with theoretical data.

Summary

• Preface ........................................................................................................................... 5
• Chapter 1 Introduction to Formula SAE .............................................................. 8
• Chapter 2 State of the art ...................................................................................... 14
• 2.1 Methodology .................................................................................................. 14
• 2.2 Architecture .................................................................................................... 14
• 2.3 Historical overview ........................................................................................ 16
• 2.3.1 Logarithmic mean difference method....................................................... 16
• 2.3.2 NTU efficiency procedure ........................................................................ 19
• Chapter 3: Instruments and method ..................................................................... 21
• Construction of the bench............................................................................... 21
• Instrument characteristics ............................................................................... 25
• Heating elements ...................................................................................... 25
• Pump ......................................................................................................... 26
• Sensors ...................................................................................................... 26
• Radiator .................................................................................................... 29
• Fans ........................................................................................................... 31
• Flow regulator .......................................................................................... 32
• Software.......................................................................................................... 32
• Chapter 4: Data acquisition ................................................................................... 35
• Hot bench ....................................................................................................... 35
• Measurements ........................................................................................... 35
• Results ...................................................................................................... 36
• Engine bench .................................................................................................. 39
• Measurements ........................................................................................... 39
• Results ...................................................................................................... 40
• Cold bench ...................................................................................................... 41
• Engine cooling duct leaks ......................................................................... 42
• Pump characterisation............................................................................... 44
• Fan characterisation .................................................................................. 46
• Conclusions .................................................................................................................. 51
• Hot bench .............................................................................................................. 51
• Engine bench ........................................................................................................ 51
• Cold bench ............................................................................................................ 52
• Bibliography ................................................................................................................ 53
• Acknowledgements ..................................................................................................... 54
• Appendix ...................................................................................................................... 54
• Index of figures ........................................................................................................... 56
• Index of tables ............................................................................................................. 58

Introduction to the Formula SAE

Chapter 1: Introduction to the Formula SAE

The Formula SAE (Society of Automotive Engineers), organised in Italy by the ATA (Associazione Tecnica dell'Automobile), is a competition that began in 1981 with the aim of bringing university students closer to the automotive sector. It offers the opportunity to design and build a single-seater car from scratch to compete in a series of tests against other universities in an international event. It is a multidisciplinary experience that allows students to apply the technical knowledge acquired during their studies and understand the importance of teamwork.

Today, the competition is widespread throughout the world, with a dozen events organised annually by SAE in collaboration with national associations of automotive engineers and technicians. The aim is to create a prototype open-wheel racing car with a motorcycle-derived engine, as shown in Figure 1.1.

Figure 1.1: Circuit de Catalunya

Engineering students from around the world compete in the design and construction of single-seater cars, following strict regulations established by the US central body, which ensures safety standards but leaves plenty of room for technical innovation. The same car can participate in all events in the series in the twelve months following the first event, provided it complies with the regulations. Teams typically take eight to nine months to design, build and tune their cars, and there are currently over 140 universities competing globally.

Each event consists of a series of static and dynamic tests, each of which contributes to the overall score and final ranking, which is determined by technical judgements and results on the track.

The static tests evaluate technical and economic aspects of the design and include:

• Engineering Design: Evaluation of design choices through the 'Design Report' and 'Design Spec Sheet', which include drawings and technical specifications of the car. The judges evaluate these elements, the team's ability to answer technical questions and the car's compliance with the regulations.
• Cost Analysis: Analysis of the car's production costs, presented in the "Cost Report", to compare the cost of each component with the available budget, in order to optimise the production techniques and processes essential for engineering design.
• Business Plan Presentation: An economic discussion in which the car is proposed as a prototype for limited production of 1,000 units, intended for non-professional drivers, with a cost of less than $25,000, low running costs and high reliability.

Once it has passed the compliance check, the car undergoes dynamic testing on the track, which assesses agility, speed and driveability, and includes:

• Acceleration: Acceleration test on a 75 m straight, with two batteries for different drivers, each with two attempts.
• Skid-Pad: Test measuring cornering grip on an '8' shaped track with two circles 12.5 m in diameter.
• Autocross: Sprint test on a circuit that evaluates the car's handling.
• Endurance & Fuel Economy: Endurance test over 22 km on the autocross track, with a driver change halfway through the race during a pit stop lasting no more than 39 minutes. The fuel economy ranking is based on the average fuel consumption per kilometre during the test.

Figure 1.2: Endurance test.

The maximum total score is 1000 points, divided as specified in the regulations.

In terms of technical specifications, the regulations require a four-stroke engine with a maximum displacement of 710 cc, usually derived from a motorcycle. The MMR (MoRe Modena Racing) team uses a Suzuki GSXR 750 K7 four-cylinder engine, modified to achieve a displacement of 708 cc without altering the compression ratio of 12.5:1, with longitudinal layout and bevel gear transmission, without first and sixth gears. This year's races will be held at Silverstone (England), Catalonia (Spain) and Red Bull Ring (Austria).

A crucial aspect of the regulations is the intake system, which must include a reduction in the cross-sectional area of the duct upstream of the throttle valve, with a maximum diameter of 20 mm for petrol engines and 19 mm for ethanol engines. This significantly limits power, which usually does not exceed 100 hp in Formula SAE engines. However, the regulations allow considerable freedom in the design of the cooling system, specifying only that the coolant must be water, without additives to improve its performance.

Optimising the cooling system is essential for tackling the various tests. Correct characterisation of the pump, fans and radiator allows for informed component selection, reducing weight and improving space distribution, with the advantage of lowering the centre of gravity. These improvements are essential in the Skid-Pad race to increase cornering grip and improve cooling efficiency during the Endurance race.

Figure 1.3: 2018 car telemetry at the Varano de Melelgari track.

From the results of past competitions, the team has recorded a positive trend in the Acceleration race and a negative trend in Endurance. As shown by the telemetry from the Varano de Melegari circuit in 2018, the water temperature (blue line) was not constant: the activation of the fans above 90 °C lowered the coolant temperature to around 70 °C, due to oversized radiators causing fluctuations in the water temperature and heat dissipated by the engine and partly by the oil (green line, partially overlapping the blue line).

Optimal design of the cooling system, integrated into the overall design principles of the car, is therefore crucial.

Figure 1.4: rear view of car without fairings.

Capitolo 2: State of the art

The design and optimisation of the cooling system for a Formula SAE car is a challenge faced by numerous universities around the world, including several Italian ones, such as the University of Padua [1], and others in Europe, such as the universities of Leeds [2] and Worcester [3] in the United Kingdom. Universities outside Europe, such as the University of New South Wales [4] in Australia and the California Polytechnic State University [5] in San Luis, are also working on this topic. Starting this year, the University of Modena and Reggio Emilia has also decided to tackle this problem. The following is a retrospective look at the technologies adopted to date.

2.1 Methodology

In previous years, as part of the Formula Student Unimore project, cooling systems were assembled using an exclusively empirical approach. Empirical research, from the Greek ἐμπειρία (empeirìa), meaning 'experience,' is based on direct or indirect observations of phenomena. Therefore, the choice of cooling system was made by trial and error, seeking the best possible compromise, without the support of a structured engineering study to justify these choices.

Starting in 2018, however, calculations based on one-dimensional models were introduced to support and justify design decisions. This year, experimental tests are planned to validate the models developed previously and expand the design with three-dimensional models, with the aim of achieving an increasingly accurate and detailed configuration of the car.

2.2 Architecture

In previous seasons, the MMR Team adopted a cooling scheme with radiators arranged in series, as shown in Figure 2.1. However, this configuration has some disadvantages: since both radiating masses share the same water flow rate, the total dissipated heat is limited. In addition, the scheme involved oil spillage, which reduced the flow rate of the first radiator and increased it in the second. As a result, the first radiator did not contribute effectively to cooling, while the second had to cool a higher flow rate at a high temperature, handling the entire thermal load of the engine.

Figure 2.1: 2018 radiator render.

To overcome these limitations, we redesigned the radiator layout, choosing a parallel configuration, as shown in Figure 2.2. This new layout allows both radiators to exchange the same amount of heat, thus operating at maximum efficiency. Each radiator now handles half the water flow rate compared to the previous configuration, allowing the use of smaller diameter pipes. This results in a reduction in the overall weight of the system, as it requires less water and less radiating material. The reduced flow rate per radiator also translates into a lower temperature difference.

Finally, experience has taught us to position the radiating masses closer to the ground, thus improving aerodynamics and lowering the car's centre of gravity.

Figure 2.2: CAD extrapolation of 2019 radiators.

2.3 Historical background

The two most common methods for calculating the dimensions of a car cooling system are illustrated below. These approaches were used to determine the design parameters and then compare them with the experimental data collected in order to assess any deviation between the theoretical and actual results.

2.3.1 Logarithmic mean difference method

To estimate the performance of a heat exchanger, it is necessary to correlate the heat transfer rate with parameters such as the mass flow rate of the fluid, the inlet and outlet temperatures of the fluids, the overall heat transfer coefficient and the total heat transfer area. The logarithmic mean difference method is particularly useful for sizing a heat exchanger capable of reaching predetermined inlet and outlet temperatures when the flow rates of the fluids involved are known. In our case, the inlet and outlet temperatures of both fluids are known, while the overall heat transfer coefficient, UA, of the heat exchanger must be determined.

The value of the overall heat transfer coefficient (UA) is obtained from the following equation:

= ∆

where Q represents the heat transferred by the heat exchanger, while ∆T represents the logarithmic mean temperature difference.

The heat exchanged is defined by the equation: Q = m ∙ c ∙ ∆T, where m represents the mass flow rate of water.