Cooling

A cooling system is needed to keep the engine or motors within their operational temperature range.

Of all chemical energy burned in the engine, the largest fraction of the energy goes into heating the engine and the exhaust. Please refer to chapter 12 of the textbook Internal Combustion Engine Fundamentals by Heywood for reference and detailed information on engine specific heat transfer, this page will be kept as a FSAE specific reference page.

EV Cooling follows much of the same principals with the total heat rejection on a smaller scale, however there is generally no OEM radiator to compare to. Additionally, EVs have the option to use oil as a cooling liquid instead of water.

Theory

Cooling systems are designed to dissipate the unwanted thermal energy. Systems are often designed around heat exchangers (HEXs), allowing for efficient exchange of heat from a hot fluid to a cooler one. There are three main methods of heat transfer;

• Conduction: Heat transfer over a temperature differential without motion between the materials
• Convection: Heat transfer over a temperature differential with fluid motion between the materials
• Radiation: Heat transfer in the form of electromagnetic radiation absorbed and emitted by bodies

A heat exchanger (HEX) is normally designed to facilitate the heat transfer between two fluids using conduction and convection. The most common HEX in FSAE/FS is a water to air cooler, meaning it transfers heat between the hot water and the cooler air. These HEXs have a few distinct characteristics, full metal construction, thin metal fins in the streamwise direction, water inlets on top and bottom, with air inlets on the front and back. The metal construction is advantageous because of its high conductive heat transfer coefficient and its low specific heat. These factors allow the metal rapidly conduct heat from the hotter internal water channels to the cooler metal fin tips. Aluminum is nearly universally used in automotive radiators. While materials such as copper may notionally provide more cooling due to higher thermal conductivity, the thermal resistance of aluminum sheet metal is so low that the heat transfer from the water circuit to the air is limited by the convective coefficients. Aluminum also is light and strong as fins or sheets. The fin structure is to maximize convection heat transfer by increasing the surface area the air flows over. The convection heat transfer coefficient is also a function of the airspeed passing the fin, allowing for 'forced convection' where a fan and or vehicle speed is used to impart an inlet speed to the system. Radiator performance varies widely based upon water channel sizes, fin spacing, and a plethora of other characteristics.

For a more detailed understanding of theory, any heat transfer textbook such as Incropera and Dewitt will serve. Kays and London's heat exchanger text is invaluable as a practical reference. It includes heat transfer coefficients for various fin and tube geometries. A car radiator is considered a cross-flow radiator with both fluids unmixed. It is possible to measure the geometry of your radiator's fins, determine what type of fin geometry you are using, and then look up the appropriate set of coefficients for a rather robust heat exchanger model.

System Design

The amount of thermal power that the engine puts into the water has to be the same as the thermal power the radiator dissipates at steady state. FSAE rules state that the system must be cooled using water only(IC and EV) or oil(EV). The coolant can not contain additives. Additionally the system must be able to pass a 45 degree tilt test with no leaks.

There are a couple of things that influence how much power the cooling system has. Because the heat generation is not a tunable parameter in the cooling system design, the parameters that can be easily adjusted are:

• Radiator Choice (Type, Size, Number)
• Radiator Packaging
• Radiator Angle
• Radiator Ducting
• Fan Sizing
• Fan Control

Cooling Power Determination

Combustion

A rule of thumb is 1/3 of the power you put into the engine in the form of fuel flow turns into heat. 1/3 becomes the power that turns the wheels and the last third comes out as exhaust enthalpy. This ratio means that the power the radiator has to dissipate is approximately the same as the power sent to the wheels. The actual cooling load can range between 60 percent to 20 percent of the fuel LHV depending on the engine and driving conditions: a throttled engine will be much less thermally efficient.

Keep in mind though that you don't drive full throttle all the time, so you should verify any cooling assumptions made during design with physical tests.

Finding average power output of the engine is one way to determine an approximate necessary cooling power for the engine. This can be accomplished by measuring fuel consumption or engine torque/vehicle velocity (P = FxV).

EV

The main heat generating components are the motor and motor controller. Conservative rules of thumb put the ballpark efficiencies at about 90% for both components. The heat generated by this powertrain would be about 15 kW when running at max power. A better estimate is 30kW average draw for the traction system which yields a ballpark heat generation of ~6kW. This is likely higher than most teams will see.

A more thorough method is to use data from previous years, or that generated by a lap sim, coupled with the efficiencies given on the motor/motor controller datasheets if available to model the generation over a lap.

EV teams also need to consider cooling the battery. This is generally best considered as a battery design problem, as it requires a different approach to that of the motor.

Radiator Choice

The radiator for most teams is outside of the scope of custom design and production. Therefore, many teams have limited to no ability to tune radiator parameters, and the choice of which radiator to run becomes the main design choice. Radiator selection can be done by size: length, width, and thickness. Radiator dimensions are not created equal as lower temperature water will be cooled less than high temperature water. The focus should be on getting more mass flow of hot water. On a downflow radiator, this means radiator width will often be more impactful than radiator length. A longer radiator forces water to travel further through the radiator, causing a greater ΔT between the inlet and outlet, and a lower average water temperature. The width of a radiator is analogous to the diameter of a pipe, the larger the width, the more water can flow through the radiator at a time. The thickness of the radiator will increase the massflow of the water through the radiator and the amount of time a quantity of air is in contact with the radiator, but can negatively impact the massflow of air through the radiator.

The two types of radiator are cross flow and down flow. The difference in performance is insignificant. A downflow radiator has water enter the top and go to the bottom in one pass. Crossflow radiators force water through the radiator sideways and can include multiple passes. Downflow radiators are most common in FSAE as they are usually taller and skinnier and will fit on the side of the vehicle without increasing frontal cross sectional area.

Fin density is an important part of radiator design, but is not tunable, and not worth the time to specify if buying an off-the-shelf solution.

You can also purchase radiators by specifying the approximate engine power or by contacting a radiator supplier for assistance.

Radiator Placement

Side
The most common radiator placement in FSAE is on the side of the vehicle, usually within the sidepod. This is a space that is usually free of other systems, somewhat unobstructed airflow, and for most bike engines it is a favorable routing position.

The sidepod design allows the team to guide air into the radiator. A properly constructed duct will dramatically increase cooling power and can reduce drag.

Rear
A few teams rear-mount the radiator. Rear mounting is usually to take advantage of diffuser airflow, free up space on the side for aero devices, or to tune weight distribution. The disadvantages can be the reduced free air flow, yaw inertia, and possible routing complications.

Multiple
Common FS/FSAE electric mounting scheme is to run two symmetric radiators.

Fluid Flow Rates

Tuning radiator performance is largely balancing the flow rates of the water and the air passing through the radiator. For analysis, heat exchangers use a term called capacity rate which is the mass flow rate times the specific heat of the fluid. An over-simplified way to think about this is as if the fluid are conveyer belts for "heat units", the specific heat capacity is how many "heat units" can fit on the belt at a time and the mass flow rate is how fast the belt is going. Heat capacity rate is defined as:

C = ṁ * c_p

The maximum possible heat transfer by a heat exchanger will be limited by the smallest heat capacity rate as seen by the equation:

Q_max = C_min * (t_hot,inlet - t_cold,inlet)

The limiting factor in our analogy will be whichever conveyer belt is filled up first. The ratio of flow rates is called Capacity-Rate Ratio and is the lower flow rate C_min over C_max. The effectiveness of the heat exchanger will be a function of this ratio. A few assumptions will be important in the flow rate discussion here: (1) The engine is designed such that the water inlet temperature to the radiator (t_hot,inlet) will be the same temperature regardless of the water flow rate and (2) The inlet air temperature will be independent of the air flow rate. The above equation tells us two things: heat transfer is limited by one of the mass flow rates, and that increasing the lowest heat capacity rate will increase our heat transfer rate. We can discuss the impact of changes to a single fluid flow rate interchangeably with capacity rate if we assume the heat capacity of the fluid stays constant, but not when comparing the capacity rates of two fluids.

Water

Tuning water mass flow rate is about finding the right balance: if you have 0 water flow rate, you'll get close to 0 cooling no matter what else you do. Hot water will stay in the engine, cold water will stay in the radiator. On the other end, there is a practical limit of flow rate due to pressure rating of the radiator and pump specifications. Between the extremes, an increase in water flow rate will increase heat transfer until it is no longer the limiting factor. If you have a properly sized water pump and swap to a bigger one, you'll see limited gains.

Practically speaking: Upgrading a water pump can be a serious investment in time and resources. The water pumps on the stock engine are designed to pump coolant for an unrestricted engine running at significantly higher power for long periods of time specifically on sport bikes like the YZ450 or CBR600RR. Therefore, it is unlikely but certainly possible that upgrading the water pump is a good step for your team.

Note: The stock CBR 600RR water pump generates 30-60 Lpm. With a few assumptions, we can calculate the maximum ballpark capacity rate for a CBR:

• Volumetric flow rate of 60 LPM
• 1kg/L density of water
• Temp near 100C
• c_p of 4.2157 KJ/kg*k

This yields a maximum heat capacity rate of 4.2157 kW/k

Air

The specific heat capacity (c_p) of air is 1.006 between 0 and 35C so you'll need about 4 times the mass flow rate of air to reach heat capacity rate parity with water, or, assuming ~1.2g/L density of air, more than 3300 times the volumetric flow rate to reach parity.

Sufficient air flow for adequate cooling is easy to achieve, but making a simple mistake that costs cooling performance is equally easy. The first decision that will affect the radiator performance is location of the radiator, this will dictate how much clean air the radiator will see during driving operation. If packaged behind the car, it is possible that the radiator will see increased air temperature because of the engine. The main ways the team can achieve adequate airflow once the radiator has been located on the vehicle are angling the radiator, utilizing fans, and adding a duct or shroud.

Angle of radiator
Angling a radiator will allow a larger radiator area to fit in a similar frontal cross sectional area, effectively fitting a larger radiator in the same space. Yes, this will sacrifice fore/aft space, but this space should be free of obstruction anyways in order to allow for free air-flow. You can angle it to about 45° relative to the airflow both vertically and horizontally without any significant loss in cooling power. Tilting it in the vertical and one of the horizontal axes will allow a radiator of double surface area in the same frontal area, or to halve the frontal area with the same radiator.

Fans
Because air capacity rate is likely to be a limiting factor, the vehicle will need a way to keep air flowing through the radiator when the vehicle is not moving. Fans attached to the back of the radiator are instrumental in vehicle operation. Fans should be sized for their airflow ratings, and attached such that all the air drawn by the fan is pulled through the radiator, this is done using a shroud. It may be necessary due to electrical draw constraints to only run the fans when needed. This can be accomplished by using the ECU to control a relay based on temperature or speed or by having the driver control them according to a light in the cockpit.

Because the margin between running temperature (about 90C) and disaster (100C) is so tight (check these numbers), the fans might have to run fairly aggressively especially on a hot day when chaining autocross laps. A tight seal between fan shroud and the radiator is key to avoid leakage. Spacing between the fan and radiator is also key: you want a uniform pressure difference across the radiator to achieve a uniform flow rate.

Ducting/Shrouding
A shroud seals the fan to the radiator ensuring all of the air flow generated by the fan goes into raising the mass flow through the radiator. A duct channels air from the environment through the radiator and back to the environment. A properly designed duct can produce higher heat transfer rates will less air flow or power used due to clever exchanges of air velocity to static pressure.

Further reading on ducting:

• https://www.racetechmag.com/2017/08/willem-toet-explains-air-ducts/
• http://www.glasairproject.com/GlasairI/AirSig/CoolingSystems/cooling2/CoolingSystems2.htm

Routing

Coolant routing is largely a packaging concern. There are minor theoretical gains to be had in cooling performance if you minimize head losses. However, these are likely not big enough to gain points in competition, or seconds off in a dynamic event. Like most other fluid systems, routing can be done with hard lines or soft lines. The soft lines are often silicone similar to those seen in aftermarket applications, as they do not need to withstand high pressures or a caustic fluid like gasoline. Hard lines are often connected by short sections of silicone tubing.

Keeping routing short and straight will reduce weight. Reducing bends will drastically reduce the difficulty to bleed the system. In addition, any routing section that is not monotonic will trap air bubbles. It is recommended that the fill point of the system be the highest point to aid in complete filling.

Hosing

Coolant hosing comes in 2 types: hard lines and soft lines. Hard lines are often aluminum due to their low cost and ease of bending. Soft lines are often silicone or rubber hosing, but the use of water as coolant allows for lower spec lines than a production or high performance industry automobile. The stock motorcycle coolant lines are pre-formed rubber lines and are more than capable of being used for FSAE.

Hard lines weigh less per foot of distance and are often cheaper to purchase. However, they require more planning to execute. Soft lines can be routed around much more complicated geometry without planning the routing before hand.

Filling and Bleeding

For the coolant system to run correctly, the system needs to be purged of air. A bleeder valve should be placed on the highest point in the system, often connected to the filler neck. You may have to turn the engine over a few times to flush it through the engine.

DON'T TAKE THE RADIATOR CAP OFF WHEN IT'S HOT. If one was to hypothetically remove the rad cap when the system is dangerously hot, they could cover it with a heavy cloth to shield themselves from the coolant that will come out. Do not do this. Under no circumstances is it advisable to attempt to service the system until it has cooled down to a safe level.

Catch Can

According to rules T.5.6, catch cans must meet the following criteria:

1. Must have a minimum capacity of 10% of the fluid being contained or 0.9 liter, whichever is greater
2. Capable of containing boiling water without deformation
3. Located rearwards of the firewall below the driver’s shoulder level
4. Positively retained, using no tie wraps or tape
5. Must vent through a hose with a minimum internal diameter of 3 mm down to the bottom levels of the Chassis.

There are commercially available solutions for automotive catch cans. There are many ways to make lighter or more package-able catch cans than what is sold off the shelf.

Simulation/Analysis

The system can be modeled parametrically to analyze how sensitive your setup will be to changes in each of these parameters. You will need to have the correct (or close enough) values for your mass flow rates for water and air, coefficient for thermal conductivity, area and heat input. In order to verify the model, tests will have to be performed, either on a dyno or on a vehicle. You must gather temperature data before and after the radiator.

Thermal Resistance Circuit

The easiest way to model heat transfer is with a resistance based electrical analogy. A thermal mass can be looked at as a single capacitor, a heat transfer boundary can be looked at as a resistor, and a heat source, unsurprisingly, as a DC source. This allows a number of elegant simplifications to heat transfer analysis.

Thermal resistance is how the temperature difference required over a boundary to conduct a certain amount of heat (°C/W). Thermal capacitance is a heat capacitance, the mass of a object times the specific heat (J/°C).

An alternative but almost identical analytical lens is to use conductances instead of resistances. Thermal conductance is the inverse of thermal resistance (W/°C). For conductive heat transfer, the conductance is k/t. For convective heat transfer, the conductance is hA.

Battery Thermal Analysis

Teams should ensure their accumulator does not enter thermal runaway. Important in this endeavor will be analyzing the internal heat generation of the cells, and the heat rejection of the accumulator. Often some information about heat generation can be found from the manufacturer, but as the DCIR of a cell is highly temperature dependent, often the manufacturer's data is insufficient for analyzing a large range of temperatures.

Data and Data collection

see Data Acquisition

The least amount of data needed for verification of coolant performance is the temperature of the engine coolant (ECT) taken from the engine itself. Stock motorcycle engines will provide an ECT sensor. To characterize radiator performance, the team will need to add temperature sensors before and after the radiator. It is recommended to have a pressure data and if possible flow data on vehicle.

Other applications

Brake cooling

Brakes convert the kinetic energy of the car into heat. The heat will either go into the brake pads or the brake rotors. Different pad materials need different temperatures to reach peak stopping power. If the brakes are too hot, they will lose braking performance known as brake fade.

The rotors are spinning disks of metal, there will be some natural convection as the car is driven around. This can be modeled in software to give an approximate look at thermal performance in the brakes. However, unless the model has been verified against existing data, there is only one way to truly determine thermal behavior of the brakes and that is to physically test them. To find the temperature, the team can use an IR thermometer to periodically take rotor temperature, or the team can employ the use of brake temperature paint or stickers that will change color depending on how hot the system gets.

Due to the unique nature of every car, it cannot be universally recommended to use or not to use ducting to force a higher rate of convection to the brakes. Most teams do not find this necessary as the wheel well is open enough to facilitate sufficient passive cooling.

Oil cooling

Many engines have an OEM oil/water heat exchanger. From my experience with the 600rr, these are very compact and well integrated with the original powertrain system. Most teams use what is close to the manufacturer's recommended engine oil and the water coolant should be a perfectly fine substitute for the antifreeze solution on the OEM bikes.

Driver cooling

lmao