Why do Diesel engines have that distinctive sound to them when compared to gasoline engines?
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The combustion in a diesel is caused by compression, not a spark. Their motors are more robust, heavier and lower revving
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Yet another question for an engineer
Diesel engines don’t use a spark plug, instead igniting their air-fuel mixture via pressure and heat.
With a gasoline engine, the mixture is well mixed before the ignition happens. If it gets too hot, you get pre-ignition, or knock. (The technical term for that mixture is that it’s homogenous, meaning it’s the same throughout)
Diesel engines are NOT well mixed (meaning their air-fuel is heterogeneous, or not the same throughout). In the spots that are mixing early combust (explode) before the actual power stroke. The clicking and clacking is the SAME pre-ignition/knock you hear from a gas engine, but in small pockets as the pressure is raised to combust the main body of the mix.
Those early, smaller combustions travel back through the engine mounts and drivetrain and we hear them.
On older diesels, you could also hear the mechanical fuel injectors clacking away.
Diesels use *very* high pressure fuel injectors that pulse on and off with each power stroke. This is very different than a petrol engine, which has continuous injectors at much lower pressure.
The distinctive “clack clack” sound of diesels is the fuel injectors pulsing.
If you just mean the lower “rumble” of diesels that’s because, at equal power, they spin considerably slower but with much higher torque (cylinder pressure).
Diesels create combustion by compressing fuel rather than using spark plugs like in a petrol car.
It’s why diesels sound like a wooshy-thump, compared to petrol which sounds more like thousands of tiny little explosions in short sequence.
Extra:
Diesel-type fuels are non-flammable at ambient pressures, but become incredibly volatile at extreme pressures inside a combustion chamber. The engines need to be built stronger with heavier materials, this makes them more sluggish in power delivery and consequently in noise as well.
They have a different sound, higher compression lower revs, but no one has mentioned how the majority of diesel engines has a turbo charger which creates the whoooffffffffpshhh sound.
Because they function much differently. Diesel fuel is harder to ignite than gasoline and is thicker/more oily. In order to get it to ignite you have to get it really hot in the combustion chamber. The flash point of gasoline is -49F the flash point of diesel is 150-180F. To heat it up that high and quickly they use air brought into the engine.
Quick secondary lesson. When you take air and compress it, it increases the temperature of the air. This is also why when you spray something like compressed air in a can as it expands it cools.
So diesels have much higher compression than gas engines because it raises the temperature of the air in the cylinder which allows the diesel to ignite. If you had this much compression in a gas engine it would make it hard to control the gas igniting and would cause it to ignite early harming the engine. Another important thing to note is that gasoline engines use a spark plug to ignite the gasoline. A small spark is enough to ignite it and allows them to better control when it’s ignited. Diesels don’t have spark plugs and rely solely on the increased temperature to make the diesel fuel ignite.
So in order to have the higher compression the engines have to be designed differently and operate at different speeds. Diesel engines are more robust to handle these extreme temperatures and cylinder pressure from all the compression. This is what makes them sound different than gas engines.
The TLDR: is diesels need a lot of compression to ignite their fuel. This means they are designed differently than gas engines. It’s also why you can’t just put gas in a diesel or diesel in a gas and make it run. These differences are what give them their unique sounds.
Have you heard of ‘pinging’ and ‘knocking’ in petrol engines? It is called ‘pre-ignition’, where a hot engine has some contaminants in the cylinders that ignites the petrol before the spark does. If ignition happens a bit before the piston reaches the top, you have a pressure pulse. You also have shocks caused when the flame front from the spark meets the flame front from the secondary ignition.
With a diesel engine, with no spark plug, pinging and knocking is the normal running condition! So the engine always sounds like it is knocking.
when you compress air it heats up, in a diesel engine this heat is used to ignite the air fuel mixture. in a gasoline engine its ignited with an electrical spark. with the gasoline engine the combustion propagates smoothly from a single point, the spark plug. where as with diesel there can be multiple ignition points that can collide and causing vibrations and a distinct sound.
Here’s the simple concept most of these are skipping that ELI5 wouldn’t understand, there’s a basic but interesting bit of physics that allows a diesel to operate.
If you take a volume of air the size of a beach ball, and in less than 0.04 seconds (not 4 seconds) you squeeze that air to the size of a golf ball it’s going to get super heated from all the loose air molecules getting suddenly and roughly squeezed together very tightly. Basically a lot of friction in those air molecules in that short time span make a sudden burst of heat.
That’s the basis of how diesels work, at the precise moment that air is super heated it sprays in diesel fuel in a fine mist. Think like a perfume bottle on steroids.
Diesel in liquid form is hard to light on fire where gas has fumes that are explosive.
Well if you mist diesel it actually becomes more explosive than the same amount of gas.
The knocking that an older diesel makes is basically a combination of poorly timed combustion when it’s cold, these bigger valves all rattling back and forth, the mechanical fuel injectors have to work hard to spray diesel into a chamber that is at 300-500 psi, think how it’s harder to pump up a bicycle tire with a hand pump when the pressure get up to 30-45psi. Now imagine pumping into something at 500psi 325 times a minute and that just to idle. For one cylinder most diesels are 6.
Also another reason for the “characteristic diesel sound” is the layout of the engine. Most diesels are 6 pistons and cylinders in a straight line
OOOOOO
Most car engines can be inline 4
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V8
OOOO
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V6
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If you find an older Jeep or ford truck with an inline 6 it’s going to have a similar sound minus the rattle of the diesels.
And with technology things are getting quieter and more powerful, I had a 2015 Ford F-350 for a while that was whisper quiet but could tear it’s own rear end out under a load. Some new chevys are so quiet the turbocharger whistling is all you hear from the exhaust
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