Cooling

Of all chemical energy burned in the engine, the largest fraction of the energy goes into heating the engine and the exhaust. A cooling system is needed to keep the engine within it's operational temperature range. 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.

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. 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.

must be cooled w water only(IC and EV) or oil(EV).
no additives
Must be able to pass 45 deg tilt test w no leaks

Cooling Power Determination

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).

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

Radiator Choice

The radiator for most teams is outside of the scope of custom design and production. Therefore, 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. 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.

Water Flow Rate

Tuning water mass flow rate has diminishing returns: if you have 0 water flow rate, you'll get close to 0 cooling no matter what else you do. If the water flow is too low, you won't be able to send enough heat from the engine to the radiator. If you have a properly sized water pump and swap to a bigger one, you'll see limited gains. Upgrading a water pump can be a serious investment in time and resources.
Note that the water pumps on the stock engine are designed to pump coolant for an unrestricted engine running at significantly higher power, and these engines are designed to be run hard if they are for sport bikes like the YZ450 or 600RR. 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.

It is recommended to reduce head losses in the system as much as possible to maintain designed pump performance.

Air Flow Rate

Air flow rate is one of the most important tunable parameters. Sufficient air flow 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 the following:

• Angle of radiator

You can angle it to about 30° (some even say 45°) relative to the airflow without any significant loss in cooling power. [citation needed]

• Fans

Generally, fans are ideally not running during normal operation. 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

Ducting Further Reading

• 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 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

type and sizing

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.

Data and Data collection

see Data Acquisition

Other applications

Brake cooling

• Convective
  • ducting
  • theyre spinning hot disks, they have natural convection
• Radiation

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