Formula Student Race Car Exhaust Noise Testing

Rimac FS Alpe Adria is an annual Formula Student racing event held in Northern Croatia. Formula Student competitions are held every year in more countries and on some of the most iconic race tracks in the world like Silverstone, Hockenheimring, Red Bull Ring, and Circuit de Barcelona - Catalunya. From 26th till 29th of August it all took place on the karting race track St. Rauš in Novi Marof near the city of Varaždin. 

It was early Thursday morning when we - the Dewesoft team - packed up our show car, a Ford Ranger, with technical equipment and some promo material. After the around 140 km drive from Trbovlje in Slovenia, we arrived at the event of FS Alpe Adria. And immediately, we were blown away by the atmosphere - the students there were all loaded with pure engineering and racing energy. 

This year at Novi Marof 37 teams from 14 countries took part in the competition. Hundreds of young people, and a lot of tents. Every team had a tent - a place to get their cars ready for the racing disciplines. At these pits, the teams presented themselves with signs and banners. Other tents were reserved for officials and spectators; inspections, mechanical or electrical services, food stalls, etc.

The main sponsor of the event is the Croatian company RIMAC Automobili which makes electric hypercars and associated technologies such as batteries or powertrains. Also, Dewesoft is a sponsor of the event - and at Novi Marof we not only did the exhaust noise checks on all racing cars but were even selected to do the final check-ups and testing of all the cars before the race. 

The Formula Student competition

Formula Student (FS) is the biggest engineering competition in the world. There are over 500 teams and more than 40 competitions worldwide. The organizer of the competitions is usually a national engineering association with the help of some of the biggest names in the automotive industry such as Porsche, Daimler, Audi, Škoda, and so on.

FS is an engineering design competition promoting clever problem-solving. Student teams all over the world design, build and test a prototype racing car based on a series of rules made to ensure on-track safety. At the events, the motor vehicles are then driven by the students themselves.

Taking part in the competition, the students gain valuable knowledge about practical work, turn theory into practice, develop their engineering and problem-solving skills and learn to work together as a team. However, the competition is also about networking. Every event is an opportunity to share experiences, and meet like-minded people, and potential future employers.

Entering the competition

Every team that wants to compete in the desired formula student events has to qualify for participation. First, there is a quiz. All events are taking place during the summer while the quiz is done in February. The quiz questions derive from the formula student rule book; some are on problems from the engineering field and some are of a business nature. The best-scoring teams are eligible for the next step.

Because that’s not all. The next hurdle to enter the competition is the vehicle status video (VSV). Every team has to produce a video of their vehicle traveling 30 m with 180° cornering while driving at least 10 km/h. The VSV video must be submitted at least 6-8 weeks before any FS competition.

Then, when the event starts, every team has to go through a variety of mechanical inspections and tests before going on the training grounds - and before joining the dynamic events. Every part of the technical inspection is ‘awarded’ with a pass sticker put on the nose of the vehicle. At the end, when your race car goes to the track, all stickers must be there.

There are 3 main categories of FS cars: combustion vehicles (CV), electric vehicles (EV), and driverless vehicles (DV). The competition consists of various static and dynamic disciplines and it is governed by several judges who are often renowned experts in their respective areas of the automotive industry. 

The teams will compete in more disciplines. Each of these carries a certain amount of points, totaling 1000 points available in each category of cars. Naturally, the team gathering the most points is the winner!

The series of disciplines are falling into two different parts: static events and dynamic events.

Static events

Designing an FS car requires an interdisciplinary approach and teamwork. Besides technical understanding, the students must also possess economic and social skills:

• Business plan presentation (BPP)
  In BPP the task is to evaluate the team’s ability to develop a comprehensive business model which demonstrates their product – a prototype race car. 

• Cost and manufacturing
  The objective of this event is to evaluate the manufacturing process and the cost associated with building a race car for every team.

• Engineering design
  In this event judges - usually, judges are engineers from the automotive industry - evaluate the engineering process, efforts, and solutions that are implemented in the design of Formula Student race cars.

Dynamic events

In this part of the competition, the teams put their „beasts“ on the track to show what they are made of. The cars are put to their limits across several disciplines to evaluate different features:

• Skidpad
  An event where cars drive in two circles which form a figure 8.

• Acceleration
  The team has to go as fast as possible in a straight line for 75 m.

• Autocross
  Autocross is some kind of qualification for the endurance event, but here you drive only one lap and the time of one lap is measured. It is driven on the same track as the endurance event.

• Endurance
  The goal is to travel the distance of 22 km - in this case, 22 laps - as fast as possible. At half the distance, the drivers must be switched and the car has to be turned off.

• Efficiency
  This is measured during the endurance event - everybody starts the endurance event with a full fuel tank.

From the spectator’s point of view, the endurance drive is the main event. Here all the vehicle’s strengths and flaws will show - and up to 4 cars may be on the track at the same time. Acceleration, speed, handling, dynamics, fuel economy, efficiency, reliability, and driver skills are tested to the limits. The winner of this event is awarded a maximum of 300 points - the highest score achievable in a single discipline.

Technical inspection 

The inspection is done in an order of different steps - if you don’t pass one, you can’t move on:

• Pre-Inspection
  Here every team is checked if it has the correct helmets, safety gear, fire extinguishers, and wet and dry tires. Here the teams are checked for the direct safety of the drivers.

• Mechanical Inspection
  Scrutineers check ready-to-race vehicles if they are built according to the rules.

• Tilt test
  The vehicle is placed on the table at an angle of 60°. Scrutineers check the vehicle for fluid leaks and they check if all wheels are still in contact with the table.

• Vehicle weighing
  Vehicles are measured in ready-to-race conditions.

• Noise test
  Here the teams turn on the vehicles for the first time in the competition. It is the first step for every team if they want to compete in the dynamics event. Scrutineers measure the noise that comes from the exhaust system.

• Brake test
  A team passes the brake test if its race car stops by locking all 4 wheels at the end of the acceleration run.

Dewesoft was invited to the event as a sponsor and also to do the official noise testing of all cars. Furthermore, as our team members have experience from past years of Formula Student, we were also given the task to check the safety features of all cars - all in strict accordance with the FS rulebook.

At the event, all cars are started up for the first time to do the noise test - participants are not allowed to turn on the engines before this. When the noise test has been successfully completed all other checks are done - and must be passed that the scrutineers are satisfied with the structural and safety features of the car.

The exhaust noise test

Race cars are loud, very loud - even the FS ones. Noise from student-made formula cars can exceed 120dB(C). Noise is part of the fun, but noise control is a concern for all parties involved in motorsports. Loud noise is damaging and may impair hearing. Scientists recommend no more than 15 minutes of unprotected exposure to sounds that are 100 decibels or higher. 

Luka Pavlović, the Rimac FS Alpe Adria 2021 organizer, says: “One of the key components during the scrutineering phase of the event is the noise test. Combustion vehicles are noise-limited, the same as your conventional road car or bike. Teams usually design their exhaust to be as close to the limit as possible in order to extract maximum performance from their engines.”

The karting track at Novi Marof Croatia is a great place for such noisy racing events. Due to its isolation, the noise is not really an issue for local residents. The areas around the track are not populated, and not even covered by unspoiled forests or pastures - wildlife is not so fond of noise either. 

However, the issue here is the concern for the people on and around the track, participants, officials, and spectators. To pass the technical inspection for the formula student competition, all internal combustion engine cars need to undertake an exhaust noise measurement procedure. That’s why the unified Formula Student rules state that the noise levels generated by combustion cars must be within certain limits:

• CV 3.2 Maximum Sound Level

• CV 3.2.1 The maximum sound level test speed for a given engine will be the engine speed that corresponds to an average piston speed of 15.25 m/s. The calculated speed will be rounded to the nearest 500 rpm. The maximum permitted sound level up to this calculated speed is 110 dB(C), fast weighing.

• CV 3.2.2 The idle test speed for a given engine will be up to the team and determined by their calibrated idle speed. If the idle speed varies then the vehicle will be tested across the range of idle speeds determined by the team. At idle the maximum permitted sound level is 103 dB(C), fast weighing.

Check out Formula Student race car exhaust noise testing with Dewesoft at Novi Marof

RPM calculation

The test was performed using a sound level meter. Usually, each team needs to have a dash display or a computer connected to show the engine RPM during the test, but the DewesoftX software offers us a variety of features and gadgets.

One useful feature, in this case, was the FFT analyzer plugin which exposed to us the first - the most dominant - harmonic of the frequency produced in sound made by the combustion process within the engine. With a little “magic” in the math module, we could calculate the RPM of the engine on the base of sound pressure coming from the exhaust.

To verify the RPMs presented by the engine management software of each team, we created a formula to detect engine RPM directly from the microphone signal - without the need for any additional sensors.

We defined a simple formula within the standard DewesoftX software feature, Dewesoft Math, which allows users to set up analysis for their measurement tasks. First, we used a function called MAXPOS - which returns the x-axis position of the maximum amplitude detected within the measured frequency range. As the input to the function, we used the FFT analysis spectra from the measurement microphone. 

The output of the function is then the frequency at which maximum amplitude occurs. As we wanted output in the form of RPMs instead of Hz, we multiplied by 60 to achieve the desired units. 

As the number of cylinders proportionally multiplies the location of the first harmonic, it was necessary to correct for the varying number of cylinders - the engines used in the race cars are different in design, some use a single cylinder, and others even 4.

The number of cylinders was defined as user input, allowing us to input the right value for each of the tested cars before the measurement. Since the Formula Student requires all of the cars to use 4-stroke engines it was also necessary to correct for the occurrence of the exhaust cycle, which happens once every two revolutions - meaning the overall formula had to be multiplied by the value of 2.

In short, the Array function MAXPOS returns the last position of the MAX value which was used to get the exact frequency of the rotating engine - the position on FFT is frequency. This was divided by the number of cylinders, multiplied by two, and converted from Hz to RPM.

As the test requires testing at idle speed and upper RPM limit, we defined a formula to automatize the determination of the upper RPM limit. This was calculated and rounded to 500 RPMs with the help of the engine’s stroke - defined in mm. This was defined as user input and entered into the software for every team based on the stroke length that was supplied to us by the racing teams.

Such a detection method has its limitations. It is based on the hypothesis that the 1st harmonic will always be the most prominent on the measured spectrum. In some cases, the exhaust design features resonators, and blind exhaust pipe sections added to the main pipe to drive the vibration down. In such cases, there might be higher harmonics having higher amplitudes which results in an inaccurate reading of the engine speed.

However, the RPM detection is not required to be performed as part of the noise scrutineering procedure - it was an optional part of the measurement. Because of this and also due to the rare occurrence of incorrect reading, we deemed this method suitable. Based on the feedback from the race team members, showing results during the measurement itself was a useful feature.

Noise test setup

The noise test is performed as a static test - all cars were brought to us. We had put up our tent to have a roof that kept the equipment safe from bad weather. For the measurements, we didn’t need a tent. Results from such noise tests can be misleading if you don’t take care to avoid reflections from the surroundings. During our testing, the exhaust outlets were facing away from the tent to reduce sound reflection to a minimum.

As the exhaust outlet of any car is in a unique position the microphone must be set specifically to adhere to the FS rules: 

• IN 10.1.2 The vehicle must be compliant at all engine speeds up to the maximum test speed, see CV 3.2.1. 

• IN 10.1.4 Measurements will be made with a free-field microphone placed free from obstructions at the exhaust outlet level, 0.5 m from the end of the exhaust outlet, at an angle of 45° with the outlet in the horizontal plane. 

For the measurements we used:

Hardware:

• A Dewesoft SIRIUSi with ACC amplifier,

• A G.R.A.S. IEPE microphone G.R.A.S. 146AE - half-inch free field rugged microphone IP67 rated with TEDS mounted on a tripod stand,

• A G.R.A.S. 42AG multi-functional calibrator - calibrated by an internationally accredited acoustic calibration laboratory), and

• A laptop computer.

Software:

• DewesoftX RC2021.5 DAQ software

• DewesoftX plugins:

  • Sound Level Meter

  • FFT Analyzer

  • Math module

The complete setup was not only following FS rules but also international standards. The measurement chain - the DAQ system, the microphone, and the SLM software used for analysis - complied with IEC 61672 Class 1 requirements. The microphone was compliant with the 61094 standard for measurement microphones and the level calibrator with IEC 60942 - and it had a valid internationally accredited calibration certificate. Before each measurement, the calibrator was used to ensure that the microphone sensitivity was accurately set for the current environmental conditions.

The noise measurements

We set up the measurement system and put the microphone on an adjustable tripod - the decibel meter was ready. The teams then brought their cars to our inspection station and performed an engine warmup. All vehicles must be compliant at all engine speeds up to the maximum test speed. 

As the cars are different in design it’s sometimes tricky to place the microphone in the right position. It has to be 0.5 meters from the exhaust outlet and at a horizontal angle of 45°. However, the angle and direction of the outlets vary, and in some cases, there is more than one outlet - these all need to be tested separately, and the highest noise reading counts. Any active tuning or throttling device on the exhaust must be compliant in all positions.  

The teams are aiming to get the maximum power out of the engine - that means that they don’t want to “choke” the engines too much. Our measurements showed that quite a few teams were exceeding or on the limit of the allowed noise level.

For reducing the noise level, we noticed a few techniques used by the teams. One was the so-called “dB killer” - a fitting mounted into the exhaust outlet and reducing its diameter making the gas pressure waves bounce and return to the pipe. Others are an extension of the exhaust pipe or turning the direction of the exhaust outlet into free air.

In our case, the FS Rimac Alpe Adria was the last competition of the 2021 season and almost every team had been competing in at least one of the previous competitions. That’s why the noise levels were pretty much within the requirements. 

When it comes to the measured noise values, the intuitive thought might be that to some extent a small difference in the dB levels is neglectable and that it is sensible to allow some tolerance on top of the prescribed limits. 

But in reality, it is important to follow the limits rather strictly - even a difference as little as 1dB in value means a big increase in the measured sound pressure levels. The dB scale is logarithmic and not linear - if the noise level exceeds the required max of 110dB(C) to 111dB(C) it’s not just 0.91% too loud but roughly 10%.

Conclusion

We were happy to spend the weekend at the race track in Novi Marof. We had great fun - and the noise tests at the Rimac FS Alpe Adria 2021 were performed in strict accordance with the specified FS procedure and the equipment used was well-suited.

The organizer Luka Pavlović, said:

We invited Dewesoft to help us with the measuring since they are known worldwide for their expertise in sound engineering. As expected they did their job perfectly and professionally, ensuring everything was according to the official Formula Student Rulebook. They even helped some of the teams and gave them some useful tips, after all this is a student competition, it's all about learning and growing.  

A key to our success was to follow the procedure. However, our tests were somewhat different from those conducted at events in the past. We introduced a new level of objectiveness by displaying all measurement outputs directly on a large screen - giving the team members immediate insight into their measured noise levels as well as the test procedure.

One of the drivers of the UNI Maribor GPE team, the technical leader Adam Grah said: “I have been in this competition for over three years and I have been to more than six competitions now and I have to say this year’s noise test was the most objective and the most impressive I’ve ever seen, especially as we could see the values directly from the scrutineers themselves”.

In conclusion, we gave each team the measured data along with the installer of our latest software version on a USB stick. The teams can now do further analysis of the acquired data and get vital information for further improvements or modifications of their exhaust and engine design.

Dewesoft has for some years been supporting the Slovenian FS racing teams from the universities of Ljubljana and Maribor. The leader of Aerodynamics at the UNI Maribor GPE team Jakob Razdevšek concluded our collaboration: “Our development process was made much easier by using Dewesoft measurement equipment”.

This year’s event in Croatia was a blast! All our sensors and equipment complied with international standards for measuring sound. Our measurements were accurate, repeatable, and useful. A job well done!

The teams at Rimac FS Alpe Adria

Combustion

Electric