Why do diesel engines sound like that?

—pebbles or rocks a-tumbling sort of noise–

Only thing I can think of is pieces of carbon exploding within the cylinders due to the sper high compression.

But that would mean a brand new diesel engine with no carbon build up would make no distinctive sound at all --for a while at least. Doubt that is true.

So what is that sound exactly?

Lots of compression and direct injection of fuel into the cylinder, with some real knocking and pinging from pre-ignition.

A diesel engine, as you might know ,has a different technology that does not use an electrical spark to ignite the fuel under compression in the engine; instead a controlled quantity of diesel is injected through a very high compression mechanical pump coupled to the engine that turns along with the engine constantly. The diesel engine noise that one hears is the detonation of the fuel injected rigth into the engine cylinder. The fuel ignites under great compression some time during the compression stroke…not from any spark. The timing is always just a bit less than perfect (pinpointing it w/ a spark would make it more accurate, like gas…but diesel ignites under compression only)

Also, many diesel engine suffer from pre-ignition of the fuel, just as a poorly tuned gasoline engine will knock and ping when the fuel pre-ignites. In basic diesel engines, it happens alot, and high compression engines are more vulnerable.

That knocking and pinging is high compression engine, with less than perfect timing causing pre-ignition.

Knocking and pinging from pre-ignition is supposedly seriously harmful to gasoline engines. ------damage to pistons and rings and valves I suppose. Usually due to lower than required octane gas, carbon deposit build up in the upper cylinders or timing probs.

But even when a gasoline engine has a pinging problem, it is usually very short duration type—a few seconds while accelerating hard ------nothing at all like the constant pre-ignition a diesel goes through.

So how is a diesel engine made differently from a gasoline engine to prevent internal damage?-----from the humongous and constant amount of preignition pinging it has.

I know that diesel engines way outlast gasoline engines because of the way they are built. Just don’t know what that “way” is. Different alloys in the metals used perhaps? Thicker metal used?

Diesel engines loaf along at minimum RPMs, and their design (to tolerate high compression) is inherently more resistant to detonation.

Diesels are low-rpm workhorses, with no ignition system to break, with internal engine parts designed to tolerate high compression, with less wear/friction as a result of lower rpms, and all associated parts (belts, pulleys, etc) experience hundreds of thousands, if not milllions, fewer revolutions than their gasoline counterparts (along with hundreds of thousands/millions less revs by pistons, cranks, valve train, etc!).

Commercial engines (diesels are the boon of big industry) and are of a higher grade (more robust parts) versus non-commercial consumer-use gasoline engines.

But even diesels that find their way into common autos do less spinning and are designed to be tolerant of big compression.

Diesel engines loaf along at minimum RPMs, and their design (to tolerate high compression) is inherently more resistant to detonation.

Diesels are low-rpm workhorses, with no ignition system to break, with internal engine parts designed to tolerate high compression, with less wear/friction as a result of lower rpms, and all associated parts (belts, pulleys, etc) experience hundreds of thousands, if not milllions, fewer revolutions than their gasoline counterparts (along with hundreds of thousands/millions less revs by pistons, cranks, valve train, etc!).

Commercial engines (diesels are the boon of big industry) and are of a higher grade (more robust parts) versus non-commercial consumer-use gasoline engines.

But even diesels that find their way into common autos do less spinning and are designed to be tolerant of big compression.

The sound that diesels make.
It is just the nature of the beast. Diesels sound like diesel, not ICE’s.
[Detonation, valves, rpm, load, and other factors.]

With no spark system, how do you shut off a diesel engine? Cut off the fuel?

Yes. Either that or an air shutter.

About the longevity of the diesel vs gas engine, I would suspect that some of it is because the gas engine ‘explodes’ it’s fuel near TDC which is the smallest volumn, so high forces/stresses. Diesel inject during the power stroke, much slower and less forces/stresses.

As others have mentioned, diesels are generally built for commercial/industrial use, so they are often built with higher quality parts than your typical gasoline engine. Also, due to the extremely high compression ratios and larger working stresses, diesels are built strong and heavy. Everything is bigger and heavier, so it tends to last longer. Also, as previously mentioned, they turn fewer RPMs, so suffer less wear from that aspect as well.

In a gasoline engine, you typically want a nice even burn of the charge (the fuel air mixture in the cylinder), starting from the spark and propogating outward in the cylinder. Detonation is when the entire contents of the cylinder suddenly explodes because it has been compressed and has reached ignition temperature. Low octane gasoline is often the culprit, as it will ignite at a lower temperature than higher octane gasoline. Pre-ignition is when, for whatever reason, the mixture ignites before it is supposed to. It does not necessarily explode (detonate); it may smoothly burn outward across the cylinder from the ignition source. One common cause is carbon build up in the cylinder, which can glow red hot and ignite the charge before the spark occurs. Both detonation and pre-ignition are bad news for gasoline engines, as they impose heavy stresses on the internal components.

Like Philster wrote, detonation is the way that diesels operate. Often, pre-ignition occurs as well. This is a lot of what you are hearing. Much of the rest of it can be attributed to the large gears which drive the accessories (oil pump, fuel pump, etc), to the valvetrain, and to piston slap (when the skirt of the piston hits the cylinder wall).

In a diesel engine, fuel is injected into the cylinder over the length of the power stroke, not all at once near top dead center. Ideally you get even burning over the entire stroke, and if this happens there is no knocking. The “marbles in a tin can” sound is usually most pronounced when a diesel engine is idling. I believe that, when the engine is turning really slowly, too much fuel may be injected too quickly near top dead center which can lead to an explosion (detonation). This is probably more pronounced with older mechanical fuel injection - I’d imagine that finer control is possible with electronically controlled fuel injection.

The sound of a diesel at idle is called diesel clatter.

Most modern common-rail diesels with electronically controlled injectors often utilize an ultra-minute spray of fuel preceeding the power spray: this minute injection creates a small pressure wave front which plows the way clear for the main spray to commence combustion in a quiet manner…or words to that effect.
(this is paraphrased from a half-remembered VW TDI press release)

Diesels are ICE’s surely? They combust internally.

The other reason diesels last longer is that diesel fuel is quite lubricating. So the cylinder walls are being lubed all the time. Gas engines suffer more piston/cylinder wear because the gas isn’t lubricating.