PTSQ Series - 4: The art and future of PTSQ

How will the PTSQ evolve and survive the oncoming challenges?

If you remember part 1 of this series (a long time ago, time to refresh you memory) here is what I had to say about a specific category of car:

• VROOM VROOM: Everything you hear coming from the powertrain has been extensively worked on: flare-up speed and max RPM, idle/fast-idle and off/part/full-throttle characters, i.e. loudness and linearity of the overall level and the engine orders over the whole RPM range and pops and bangs. The limiting factors in that case can be related to money but also to the compromises that have to be made with other significant attributes, such as dynamics, weight, consumption, depollution as well as regulation such as Pass-by-noise putting a cap on how loud your exhaust noise can be.

First of all, let’s recap what the OEM expects the customer to expect when buying a sports car, oftentimes involving slight compromises, depending on the character that will be chosen for the car, broken down by engine RPM and throttle inputs:

• Effortless and thrilling flare-up when starting the engine as well as a fruity/powerful idle noise → Sets up expectations of performance.

• Rewarding max engine speed → You want to know you are using the engine peak power and feel rewarded for doing so.

• Capable engine at mid-throttle/mid-RPM → Your car is so capable you don’t even need its full power to accelerate or maintain speed, also the situation you will encounter most of the time.

• Linear increase in loudness vs. RPM → You don’t want to feel like you peaked midway through the engine speed and whatever happens after is not worth chasing.

• Rewarding off-throttle behavior, i.e. pops and bangs and off-throttle character → You want to know that, even if you lift-off for a sec, all the power is still waiting at the tip of your foot.

Now, the sports car category exists on a multi-dimensional spectrum, based on price, segment, performance, refinement but there are a few categories that stand out:

• A very capable, track-oriented performance coupe/sedan.

• A track-derived, refined, powerful coupe/sedan.

• A luxurious, refined, powerful coupe/sedan GT.

• A luxurious, refined, powerful SUV.

While these categories have a lot of interlap between them, fuzzy barriers and will create much debates between enthusiasts, from a PTSQ point of view there will be marked differences:

• The track-oriented car will be the loudest, uncompromising, have a moderate to high amount of vibrations, have the most drama and will prioritize on rewarding the driver at high RPMs (think Lotus Exige).

• The more refined car will be better isolated, not excessive when not pushed hard, rewarding at all RPMs (think Jaguar F-Type SVR).

• The luxurious car will retain a feeling of powerfulness, but will be even more tamed, less dramatic, more focused on mid-RPM/throttle, even more refined and isolated (think S-Class Coupe AMG 63s).

• The powerful SUV will usually sit between the SUV equivalent of a track-derived and luxurious car (think Range Rover SVA/SVR).

Obviously, other factors differentiate these categories even further, including other NVH aspects, such as road noise or operational sound quality, that will have some impact on the PTSQ.

One variable that remains the same for all these categories, is the tendency for OEMS to respect the linearity between engine speed and loudness, if only because it feels unnatural not to.

Thanks to the simulation capabilities, the know-how of all the teams involved and the physics at play, the first prototypes will often come out having a relatively linear increase in loudness, but for the following:

• For turbocharged engines, overall noise levels will increase once the max boost pressure as been reached (often happening around 3000 RPM or lower on modern car) because of the increase in combustion noise, intake and exhaust noise and turbocharger noise itself. Different engines will have turbochargers optimized for a different range of RPMs, hence there might also be a decrease in noise once the engine goes above the max boost range.

• For supercharged engines, overall noise levels will increase dramatically towards the higher end of the RPM range, for similar reasons to the turbocharged engines, albeit in that case supercharger whine will be added to overall noise.

• For all engines, there is an “extinction regime”, meaning that at a certain RPM the mechanical and combustion-induced forces can effectively cancel out each other or be out of phase, leading to quieter operation. This is often engineered to target commonly used RPMs such as cruising speeds and achieved with a mix of engine components design, calibration and counterweights or balancers.

• For all engines/intake/exhaust systems there will be resonances that will cause the overall level to increase at certain RPMs because of engine orders exciting these.

Most of the remaining work will revolve around fixing error states or fine-tuning the PTSQ (in the best case).

There are a lot of knobs to turn, but as the program moves forward, there will be fewer of them, less freedom to do so, more costs associated and more compromises to accept.

We can have a look at a few examples, inspired by real life events.

• Too boomy at a certain engine speed. Needs a different exhaust, but changes the character of the car at different engine speed.

• Not enough character at high speed, needs changes in the air intake but degrades the experience at different throttle condition.

• One engine order is way too dominant overall some rpm range, drowning all the other frequencies, might need a different engine cal, different exhaust, different down pipes etc..

• A conjunction of different parameters, such as low temperature, last minute engine cal change for depollution reasons, modal alignment leads to a redesign of some of the exhaust bushes, cascading a whole series of tests and validation for different departments.

• Another department, dynamics for ex, asking for an increase in powertrain bush stiffness, leading to an error state at idle, necessitating a different cal strategy, again leading to a cascade of changes in different attributes.

As we can see, the real problem in achieving the intended PTSQ is essentially a case of identifying a bunch of different issues that can arise during development and working together with a lot of different teams to fix them.

In the past decades, fine tuning the PTSQ would involve the following:

• Modifying the internal of the exhaust mufflers, i.e. baffles, chambers, perforations, resonators, glass wool, in order to impact overall level, engine orders and frequencies - Mostly airborne noise; covers a wide range of frequencies 20-4000Hz.

• Modifying the air intake system, i.e. length, volume, material, resonators, quarter-wave tubes, porosity, slots or perforations - Mostly airborne noise; covers mostly the mid to high frequencies 400-8000Hz.

• Modifying the cabin acoustic pack, mainly to change what frequencies get to enter the cabin and impact the engine noise character - Mostly airborne noise;covers mostly high frequencies 800-8000Hz.

• Modifying some powertrain/gearbox/exhaust bush stiffness to change the vibrations coming into the cabin - mostly structure-borne noise; covers mostly low to mid frequencies 20-300Hz.

Nowadays, as we have covered in a previous post, a lot of the fine tuning is made through the use of active sound design, greatly simplifying the overall process, at the cost of “authenticity”. Arguably, if the customer can’t tell the difference between speaker generated noise and “naturally” generated noise, or more generally doesn’t complain, it is a no-brainer for the OEM.

What does that mean for the future of sports cars ?

As the regulations for lower consumption, less environmental impact and quieter cars inexorably lead to downsizing (lower displacement, fewer cylinders), turbocharging/supercharging, hybridization/electrification, combined with higher customers expectations about refinement, meaning better acoustically insulated cabins, we can see a time when most if not all of the PTSQ customers will hear in the cabin will be speaker generated, for better or for worse.

One interesting recent development has been Mercedes-Benz’s flip-flopping decision about killing their V8 engine or not, which I suspect is strongly correlated to their understanding of how future legislation will evolve, customer demand for high displacement engine and the aura associated with these.

Even though a ban on ICE vehicles is looming on the horizon in a lot of countries, I wouldn’t be surprised to see some exception seeing the light of the day, authorizing the sale of a limited number per year. Doing so would allow some historical high performance car OEMs such as Ferrari or Bugatti to keep on selling very desirable ICE cars, in the process increasing their price and margins even further, while adapting to the new electrification challenges.

While there is no doubt in my mind that today’s best electric cars are becoming the new synonymous for hyper car, there will always be a lot of demand for all the drama and emotions that comes with having a good old engine.