Tag: AFR

Selecting a Final Drive Ratio for AFR’s Jinx (and Why It’s Critical to Get Right)

Noah Stein on

by Matt McMurry

The Driveline Sub-system handles power delivery from the engine to the rear wheels and must design components that deliver power as quickly as possible. Acceleration performance is such a critical part of our competition that each decision can mean gaining or losing several places in the overall result.

There are many factors that influence a car’s maximum acceleration, but one of the main factors is the series of gear ratios in the transmission between the engine and the rear wheels. The transmission multiplies the engine output torque by the overall gear ratio and divides the engine rotational speed by the same amount. Increasing the overall gear ratio (by changing the primary, gear, or final drive ratios) will increase the amount of torque at the rear wheels, which will generally increase the acceleration of the car. For our team, it is too resource-intensive to change the primary or gear ratios, so we will change the final drive ratio (FDR) to meet our torque needs.

If a higher FDR will increase acceleration, why not just pick the highest ratio that will fit in the car? There are two reasons:

  1. The tires only have enough grip to transmit a certain amount of torque. If you exceed that torque value, you will begin to spin the tires, which prevents acceleration and makes the car difficult to drive.
  2. A higher FDR means the driver will have to shift gears more often. Shifting gears takes a finite amount of time (usually between 0.1 and 0.5 seconds), and the car does not accelerate during this period. This sounds like an insignificant amount of time, but 0.5 seconds of shifting is 12% of the entire Acceleration Event at competition. 

Therefore a compromise must be made between shifting time, drivability, and final drive ratio in order to maximize acceleration.

 

To find the best compromise for our car we developed a discrete-time simulation of the car accelerating. The simulation takes into account things like drag and weight transfer and uses a tire model based on empirical data from the FSAE Tire Test Consortium as well as engine data from dynamometer testing. The shift time and FDR were varied and the Acceleration Event time was recorded for each combination. The results of the parameter sweep can be seen below:

A clear minimum acceleration time for a particular shift time can be seen in Figure 2. This minimum is increasingly obvious for longer shift times. The results are comparable to those found in similar research by Ping (1).  

Previous testing has shown our average upshift time to be 0.25 seconds. For this shift time, the optimum FDR is 4.2. This will be the starting point for the car’s FDR. We plan to verify our simulations and fine tune the FDR through on-track testing. 

This is the kind of advanced analysis that Anteater Formula Racing engineers do every day and is why UCI’s Formula SAE program is so important to our engineering education. We get to solve real engineering problems and follow them through the entire engineering process from analysis and design through to manufacturing and testing. 

Matt McMurry is a Senior aerospace engineer and the Chief Engineer for Anteater Formula Racing. He is also the Lead Engineer for the Driveline sub-team, which includes Ryan Gagarin, Joseph Zhang and Patrick Hall.

Keeping it Cool: AFR’s Cooling System Upgrades for ’20

Noah Stein on

by Dillon Smith

The Engine Development team at Anteater Formula Racing is hard at work on developing cooling upgrades for this year’s car.

The Formula SAE competition demands the best from every system, but one of the biggest challenges is keeping the engine cool during the Endurance event.

This year, Wraith suffered from a seized engine during Endurance at Lincoln. This was due in part to the engine overheating during competition. The maximum safe operating temperature for our engine is 200 degrees Fahrenheit, but data taken from the ECU after the breakdown showed that coolant temps reached over 250 degrees after sitting for several minutes during the mandatory driver change, which starved the radiator of cool air.

The engine team has been running tests at UCI’s Vehicle Performance Engineering Lab to determine the specific cause of failure since the summer and are developing upgrades to keep Jinx cool. By measuring the water flow rates through the running engine at various RPMs and measuring the temperature drop from the radiator inlet and outlet, we’ve been able to measure Wraith’s overall cooling ability. Our calculations show that the previous design is insufficient for our needs, so we’re undergoing a full redesign of the cooling system to meet our current requirements. So far this year, our focus has been on reliability so that we can push the car harder and perform better in competition with less issues.

Figure 1  AFR’s Engine Development Sub-team runs tests on last year’s cooling system.
Want to support our developments and our 2020 car? Consider donating to our ZotFunder campaign here, running through 12/1/19.

How a standard water-cooled system works:

As the engine runs, it generates large amounts of heat.  In order to keep the engine temperature at 200 degrees, the excess heat is absorbed using a water-cooling system that pumps cool water through the engine block. The hot water is then passed through a thermostat which opens once the temperature of the water reaches 190 degrees. To avoid cavitation, or the creation of low-pressure pockets that interrupt water flow, the hot water is then sent to a swirl pot, which allows any steam in the system to condense back into liquid. This condensed hot water is then sent through the radiator, where heat is removed through convection, and then is sent back into the engine using a pump.

Figure 2 Block diagram for the Jinx cooling system.

The radiator performs most of our cooling by passing the engine coolant through an exposed fin setup that allows the passing air to absorb heat from the system. Increasing the temperature drop across the radiator allows our engine to run cooler and avoid overheating.

To do this:

  1. We’ll install a larger radiator as well as more powerful cooling fans to improve the airflow to the coolant.
  2. We’ll install an upgraded water pump to increase coolant mass flow through the system.
  3. We’re working with our Aerodynamics team to design a custom sidepod to maximize the amount of air that is directed to our radiator (Figure 3).

Figure 3 CAD design of Jinx cooling system.

We expect to have Jinx up and running in early 2020, stay tuned!

Dillon Smith is a fourth-year mechanical engineer on AFR’s Engine Development sub-team, which is led by Tristan Cortez and also includes Mohammed Azeem, Daniel Martinez, Mason Socha, Sangghara Kusumo, Edward Han and Dustin Ngo.