Diesel engines have a reputation for being a louder engine than others.
With this guide, as a guy who owns all of his 3 cars with diesel engines, I’ll help explain why a diesel can be this way, how new diesel engines behave, and how they can keep their voices a little lower.
Why are Diesel Engines so Noisy?
Diesel fuel molecules are bigger than gasoline molecules which are partly to blame. Because diesel engines use compression to ignite the fuel, the larger molecules make larger explosions.
Therefore, you get the distinctive clattering sound that diesel engines produce. Gasoline engines use spark plugs to ignite fuel instead of the simpler, but harsher method of compression.
The smaller, mechanical details play their part too. A diesel engine is far from being light and airy, given the parts it’s composed of. The following parts are some of the biggest reasons that help a diesel produce its loudness.
- The valvetrain: This is what handles intake and exhaust valves. Parts like this are more heavy-duty than gasoline engines, which heightens the diesel’s clattering. This is particularly noticeable in trucks as parts involving the valvetrain are larger and bulkier.
- The injectors: As its name suggests, these fire fuel into the cylinders. Like the valves, these too are part of the engine’s overhead. Therefore, these will play a part in a diesel engine’s sound.
- The turbocharger: A fan that almost every diesel engine has equipped. It’s a small turbine that shoves compressed air into the engine’s combustion chamber. Turbos spin at very high RPMs (I’m talking tens of thousands of RPMs) and, on some engines, can contribute to a diesel’s loudness. A turbo typically produces a type of whining noise when an engine is being worked hard.
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Are New Diesel Engines Less Noisy?
Compared to diesel engines of old, new diesel enignes are less noisy. A key point as to why that is can be thanks to common rail injection. This means fuel is injected at higher pressures inside the combustion chamber.
This method involves more controlled combusting of diesel fuel, unlike the older injection methods. Those earlier versions were heavy-handed and, more-or-less, dumped the fuel in rather than optimizing it.
With the more modern form of injection, the process involves a small bit of fuel being combusted to get the chamber warm for the main charge of fuel. This creates a smoother, cleaner sound.
Variable-geometry turbochargers, or VGTs, have a hand in making newer diesels more refined as well. These turbos control exhaust flow which increases engine efficiency and decreases noise.
Additionally, modern diesels find themselves in engine bays with better sound attenuation. With greater developments in refining cars, newer and better ways of padding specific parts of the car have been introduced.
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Diesel Engine vs Gasoline Engine. How Noisy Are They?
Modern gasoline engines can be very low in volume. You’ll find lots of gasoline cars managing around 32-40 decibels at low speed and about 50-60 at higher speed. Diesel engines are about 10% noisier.
However, performance cars typically reach around 90 decibels. This is because their focus on delivering more power, be it through tuning or large engines, also generates more sound.
Diesels generally prove louder, due to their mechanical nature, despite efforts of silencing them. An extreme example would be a standard diesel truck which, approximately, makes a rattling 100 decibels.
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That said, the difference between gasoline and diesel in terms of noise has gotten smaller. With improvements to noise reduction, a modern diesel engine comes close to being as quiet as a gasoline engine.
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Which Diesel Engines are the Noisiest?
On the road, diesel trucks are the loudest. The noise they produce makes them some of the loudest vehicles on the road. One of the noisiest diesel engines is a popular V6.
The larger engines and less attention to muffling their rumbling roar result in a mighty racket. Even when idle, these diesels make their presence felt.
Vans, specifically older models, are also known for their harsh clattering. Even when idle, these will make a racket you or I will never forget!
Other noisy diesel-powered vehicles include locomotives. Diesel trains use enormous 12 or 16 cylinder engines that are around 28-32 liters in size. Anyone who’s been to a train station will likely know they’re some of the loudest vehicles in public transport.
As for cars, old and large diesel-engined ones are almost always louder in volume than the modern ones. Pick-up trucks, SUVs, and big family cars are the usual suspects here when it comes to which types of car. The diesel engines used are normally large six-cylinders (sometimes inline-six, or more commonly a V6).
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Which Diesel Engines are the Quietest?
Luxury cars, such as the BMW 7-Series (Diesel 3.0) and Mercedes S-Class feature some of the quietest diesels out there, including 2.0 L B47 I4 turbo, 3.0 L B57 I6 turbo, 2.9 L I6 turbo. Other executive saloons, such as the 523d and Audi A5 also possess some very refined diesel powertrains.
This is thanks to German quality and their pursuit of superior soundproofing and improved engineering towards diesel power. Some of them are the quietest diesel engines ever made as well as some of the most efficient.
A core reason for their reputation as silent machines is their habit of being low revving cars. This fact is most true when on the motorway at high speeds.
A lot of sound monitoring in cars is actually conducted on motorways and as a result, these luxurious diesels are some of the calmest cars around.
It should be said most modern diesel cars these days, be it the traditionally noisy SUVs or pick-ups, are still very gentle sounding themselves. However, it cannot be understated that German saloons feature diesel engines that when compared to old ones, are as silent as a library.
When is a Diesel Engine At Its Noisiest?
Like any gasoline engine, diesel engines are at their loudest at high RPMs. This means that a diesel would be making more sound at lower gears than it would at higher speeds, where the RPMs would be lower.
Another way a diesel produces the most noise can be when a truck or a large van goes up a hill. This will lead to the engine being worked more and the turbo spinning at very high speeds. So you get a loud, deep sound that can only be produced by that of a diesel engine under high strain.
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Can you do Something to Reduce Diesel Engine Noise?
To reduce diesel engine noise, you can add coverings, material that dampens sound and additives such as synthetic oil to help curb noise. These methods can save you up to 10 or even 15 decibels.
Below is a list of the various ways you can make your diesel behave in a quieter manner.
- Sound dampening material can be one of the most effective ways. Placing this material behind the firewall, under the bonnet, and the sides of the engine bay will work well. You might find sound-deadening mats for your car’s interior will do the job too. These mats can be bought online or at stores dealing in parts and accessories for vehicles.
- Synthetic oil can be a big improvement over your typical mineral oil. They are better at lubrication, possess a protective coating for moving parts, and lessen noise and overall friction. However, I’d highly suggest making sure the oil used is recommended by the engine’s manufacturer.
- If synthetic oil is a bit pricey for you, then there are also other oil-based additives. These, too can reduce mechanical friction and lead to quieter driving. Like synthetic oil, though, it’s best to check whether these additives are recommended by your engine’s manufacturer.
- Underbody coverings can both lower road and diesel engine noise. Using underbody coatings that are more than 100 microns thick will eliminate some of the clattering. Not only that, but this type of coating will help keep the rust away. It also minimizes potential damage from scraping on a stone or a high curb.
- Valve spacing can pipe the engine down. If the valves aren’t aligned properly, then you might have annoying rattling sounds. A mechanic can adjust the valves and space them at the correct distances.
- Sound deadening foam can be installed behind the inner door panels. Like the floor mats I mentioned, these will ensure a gentler-sounding cabin. Look for foam with good review scores. You’ll find such foam available online, which is the easier option, but auto shops sell it too.
I hope you found this guide helpful. At the end, when it comes to a diesel engine’s sound, it all comes down to its mechanical nature that can be put in one word as compression. This action is practically the main source, so being noisy is in a diesel engine’s nature.
Diesel engines and noise may often go hand in hand, but there are ways in helping reduce that unmistakable clattering. Any of the solutions I mentioned are useful ideas towards curbing it and turning a diesel engine into something a little less noisy.