Guest Contribution by David Nichols
An engine is, in essence, an air pump. Air comes in, gets mixed with fuel, goes bang, and leaves again. When talking about ways to make more power, the most obvious is to make a bigger bang. However, gasoline works best at a very specific ratio of fuel to air, which is roughly 14:1. So, if you want to make a bigger bang, you need 14 times more air than your increase in fuel to get it.
When trying to get more air, one solution is to use a bigger engine. More, larger cylinders means the pump can inhale a bigger breath, which means more fuel and more power. However, this so-called “natural aspiration” or NA for short, has a limitation in air pressure. Just like a straw, the inhale of an engine works by creating low pressure, which atmospheric pressure then fills in. So, another way to get more air into an engine is to pressurize it, or use “forced induction.”
There are a few different methods of forced induction, but today I want to talk about one in particular: turbocharging. A turbocharger is a turbine that is connected to the exhaust gas leaving the engine on one side, which then drives an impeller on the other side to create air pressure. The more and faster exhaust gas comes out of the exhaust, the more and faster the intake side compresses air. When the amount of pressure generated by the propeller is greater than atmospheric pressure, the system is making “boost” and the amount of boost can be measured in PSI.
The benefit of turbos over other forced induction methods is generating the boost out of the exhaust gasses, which is already considered “lost” energy to the system. Essentially the turbocharger is “free” (or at least very cheap) extra power compared to alternative techniques. This is important for many reasons I’ll get to below. Another benefit to turbocharging is that if the driver of the car does not ask for boost via the throttle pedal, the car behaves much like a NA car of the same engine size, in terms of fuel usage and power.
Ever since the 1960’s, automobile manufacturers and lawmakers have been pushing the automotive industry to make their cars more fuel efficient, without reducing the overall power output. (However, because vehicles have become larger and heavier over time, *more* power output is required to get the same acceleration performance!) Automobile manufacturers have learned that the best way to solve this problem has been to use smaller engines with turbochargers, which can deliver high power when asked, but fuel sipping when not heavy on the throttle. One only needs to look through the engine specifications on your typical commuter or sports car; ten years ago the top range probably had an NA V6 or V8 in the 3-4 liter range, but the equivalent model today probably has a 2-2.4L inline 4 with a turbo attached to it. As turbo technology continues to develop, so too has the efficiency of the turbocharger as well as its effect on engines.
“Efficiency” is perhaps the most important word when talking about the sound of an engine. Since noise is considered an energy loss in terms of the engine, a 100% efficient engine would also be silent. Inefficiency is also why old cars sound more “interesting” or “characterful” and why historic race cars sound so enthralling. However, no advancement in automotive technology has had such a drastic shift in the soundscape of automobiles as the turbocharger.
As Michael Hermes discussed earlier this month, there are 3 primary sources of sound in an engine – the sound of the air coming into the engine or the “intake” noise (which you hear when the car comes toward you, typically), the sound of the engine itself, with mechanical whirring or ticking types of sounds, and the sound exiting the exhaust system (which typically you hear as the car passes you). The turbocharger introduces major changes to at least two of these – the intake and the exhaust.
Cars that have strong intake sound typically have a simple flow from the intake plenum, backwards through the throttle, out through the air filter, and out into the free air the engine would suck in. Some typical cars demonstrating this behavior would be 90’s Honda Civic Type R’s, the BMW E92 M3, and even formula racecars with the huge intake above the driver’s head.
Listening to a flyby of a car like the video above, illustrates the change clearly, with the lower, deeper buzz of the intake in the first seconds of the clip contrasted to the piercing exhaust reverberating through the environment, which you can then hear overcome the sound as the car passes. Back to turbos, the intake side of a turbocharged car is often quite a bit more complex – the air comes in through a filter, into a turbocharger (whose blades behave similar to an airplane propeller), out at a 90° angle, often through some complex plumbing into an intercooler (like a radiator but without the water), through some more bends and plumbing, through a blow-off valve and then into the throttle and intake. As a result of all that extra plumbing and components, the sound from the intake has a lot tougher time making it forward through the system so the resulting “intake” sound is quieter and often “smoother” or more sinusoidal than a similar NA engine. The turbocharger itself can also introduce a whistling noise to the intake, not unlike a jet engine.
The turbo has a similar effect on exhaust systems, where the exhaust-side impeller disrupts the exhaust flow by creating an extra gate for the sound to have to flow through. When talking about cars with exciting exhaust sounds, most people gravitate to sounds where the individual “pluses” of cylinders firing are very defined, creating an exciting burble or rhythm to the engine’s sound. Unfortunately for turbos, the turbo itself acts as a sort of “revolving door” where the air has to enter (and therefore exit) the turbo in a smoother, more consistent pattern. Much like the effect on the intake, this results in a flatter, more sinusoidal tone to the exhaust, which most people would say is less exciting than an NA engine of similar design. The turbo also tends to make the exhaust quieter as a byproduct.
What’s interesting to our soundscape, to me at least, is how these turbo advancements will continue effect the overall soundscape of automobiles. As turbo systems continue to become more and more efficient, the quieter, tamer, and less exciting the average car becomes. On the one hand, I think this is fantastic for urban areas with high noise pollution problems, and quieter engines (and even electrics) solve a lot of issues. On the other, though, it becomes tough to make sporty cars feel sporty, and doubly so for race cars. We have already seen technology put in place by VW in the “Soundaktor” tactile transducer or the BMW active sound reinforcement using the cars’ speakers in attempt to put some excitement back into the tone of the cars from the inside. Even Formula 1, which this year moved to a turbo V6, has been experimenting with exhaust designs in order to make the cars louder, as in their current state they do seem quite dull compared to the NA V8’s they’ve replaced.
There are a couple other parts of turbo technology I should mention before wrapping up. Within the turbo system there are typically two parts that generate sound; the wastegate and the blow-off valve. The wastegate functions as a flow regulator for the exhaust side of the system by offering a bypass route around the turbo when certain pressure is met. On most cars sold with a turbo, the wastegate system is built into the turbo itself, but in custom or racing applications it’s often a separate system. Some people will route the air exiting via the wastegate separately from the rest of the exhaust, which creates a loud, white-noisey “woosh” when boost is achieved. Here’s a strong example:
The blow-off valve, on the other hand, works on the intake side of the turbo. The BOV is usually placed as close to the throttle as possible, and works to relieve pressure when the system is in boost and the throttle closes. Without a BOV, the pressurized air between the turbo and the now-closed throttle would be trapped and either force its way backward through the turbo (bad for the turbo’s health) or expose a weak point in the plumbing. Blow-off valves can make a variety of sounds, from the subtle “psssh” to a whistle-like or even chirping tone.
This clip has a ton of them, but 0:30, 1:05, 1:08, and 1:30 are very clear examples.
Note that not all cars have blow-off valves, particularly race cars where the engine is rebuilt anyway so turbo damage is not high concern. When the air moves backward through the turbo, it creates a very specific flutter called “compressor surge.” The Mitsubishi Lancer Evo at 1:49 in the clip above exhibits a similar sound. It can sometimes be difficult to differentiate between a compressor surge and a blow-off valve designed to “flutter” like one.
I’m not saying that turbocharged cars can’t sound cool or aggressive or interesting, as clearly that’s not the case. But as the auto industry moves further down the turbocharged route, our automotive soundscape is definitely going to be affected by it. Personally, I’m curious to see if this change in our soundscape carries forward into our media – particularly in film, and even more particularly in films set in the future. As the trend for more efficient vehicles continues, manufacturers will continue finding techniques to “liven up” the otherwise-more-efficient sound, even moving as far as synthesizing sounds to accompany the base engine tone, which opens up a world of opportunity for sci-fi productions. Broadcast media is also re-learning how to showcase quieter race cars, as the Formula 1 broadcasters determine how best to showcase the new engines. All of these changes apply even stronger as electric car technology becomes more commonplace, where nearly all of the sounds we associate with a car are gone. Where automotive media is in 5 or 10 years from now should be fascinating, and I can’t wait.
Written by David Nichols of Track Time Audio