Diesel Engine/Noise Question

What exactly is the source of the unique sound of a diesel engine? As does a gasoline engine, a diesel has pistons in cylinders on top of which a fuel/air mixture ignites. Can’t say for sure, but I’m fairly confident the engine blocks, pistons, rods, etc. are made of the same materials. Does the fuel/air ignition sound different when ignited by compression vs. with a spark?

From here:

“Diesel knock is a by-product of the raised compression and fuel injection process and is an acceptable result of the ignition sequence.”

Diesel engines typically run 14:1 to 25:1 compression ratios. Much higher than gas engines.

The newer engines are getting quieter, they use a series of fuel injections on each stroke. I have known several mechanics over the years who could identify the truck number when the truck was still two blocks from the shop just by the sound of the engine. Very few can do this on the newer engines.

Have you ever had an old gas-powered car that liked to run-on (i.e. diesel)? They make the same distinct diesel rattling even though they’ve still got a low compression ratio an a carburetor.

The noise seems to be inherent to compression ignition, even when it’s happening unintentionally. I’m not entirely clear on the nitty-gritty of the physics, but I believe it has something to do with the combustion starting all across the piston on a compression ignition engine versus propagating out from a central point on a spark ignition engine, which means you get a more abrupt pressure change on the diesel engine.

Kevin Cameron could make this clear to a layman in a tweet.

Sadly, I’m not Kevin Cameron.

In a diesel engine, the fuel is injected during the power stroke and burns as it’s injected, so combustion doesn’t start throughout the cylinder. Also, with a dieseling gasoline engine, the fuel is ignited by a hot spot in the combustion chamber, so it also starts combustion from a single point.

I’m guessing a dieseling gasoline engine sounds like a bogged down diesel engine because it’s spinning very slowly allowing piston slap to occur. I doubt it would sound much like a diesel if you could rev it up while it was dieseling, but unfortunately I don’t have a gasoline engine that diesels to test the theory.

I don’t believe it’s particularly related to the compression ratio. I’ve worked with methanol-fueled spark-ignited engines operating at 19:1 compression ratio, and they didn’t sound like diesel engines (until/unless they started knocking).

Two nitpicks. First, injection typically begins during the compression stroke, not the expansion/power stroke.

Second the fuel doesn’t typically burn as it’s injected. There are several steps required before combustion really gets rolling:

1. Atomization. The fuel is injected at extremely high pressures (20,000-30,000 psi). It comes out of the injector nozzle at Ludicrous Speed and immediately encounters a wall of air that’s about 20 times as dense as a normal atmosphere. Aerodynamic forces shred each jet of fuel into tiny droplets.

2. Evaporation. Each droplet evaporates as it moves through the hot air, leaving a nice rich trail of fuel vapor behind it.

3. Heat transfer and mixing. The fuel vapor heats up and mixes with surrounding air.

4. ignition delay. Even after a hydrocarbon fuel-air blend is hot enough to ignite, there’s a notable delay before it actually begins releasing signifcant amounts of energy. Experiments with rapid-compression machines in a laboratory have shown that delay to be on the order of several milliseconds.

Bottom line, a lot of fuel gets mixed and ready to burn before combustion actually begins. Moreover, a diesel injector may have half a dozen or more nozzle holes in ts tip, resulting in as many individual fuel plumes in the combustion chamber, so these processes are happening in multiple locations.

So what’s the result? You inject the fuel, nothing happens for a while…and then suddenly you get a lot of fuel burning at once, resulting in a rapid rate of pressure increase in the cylinder.

Here’s a relevant video, showing high-speed camera footage taken inside the combustion chamber (this would be in a research engine with a transparent piston crown, something like this). Note that according to the in-video caption, injection begins 20 degrees before TDC; we can’t see it because there’s no externally-supplied illumination. The first visible sign of combustion isn’t until a degree or two before TDC.

In normal spark-ignited combustion, the mixture deflagrates: the flame front progresses at subsonic velocities. A dieseling gasoline SI engine sounds that way because the mixture is detonating, i.e. the reaction front is moving through the mixture at or above the speed of sound. This results in shock waves bouncing around the combustion chamber, making the whole engine block ring; that’s the sound you hear. You get the same sound in a gasoline engine running at high RPM if it’s overheating or if the spark timing is too advanced (some/most of the mixture detonates before the spark-triggered flame front reaches it).

Machine Elf - that was a nice, concise answer. Even I understood it.

Good points but I have to inquire about this one. Why would the flame ignited by a hot carbon deposit in the combustion chamber travel faster than one ignited by a spark? It makes sense in the case of a hot engine, hot intake air (due to too much boost), or too low of an octane rating for the setup, but I don’t understand why ignition by a hot spot would cause faster flame propagation than by spark ignition.

OK, let’s step away from diesel/compression-ignited engines and talk about spark-ignited (SI) engines for a bit.

The answer to your question is that the detonation is not due to the nature of the ignition source; it has to do with the conditions of the combustible mixture.

When you ignite the mixture, the flame front propagates outward from that point. The reaction releases heat, which causes the burned part of the mixture to expand. This in turn compresses the as-yet-unburned part of the mixture, causing adiabatic heating. The very last unburned portion of the mixture ends up very compressed and very hot before the flame front arrives. Remember the ignition delay concept I described earlier, wherein mixtures that are heated/compressed to autoignition conditions experience a delay of several milliseconds before lighting off? That’s the only reason the last bit of mixture doesn’t typically detonate under normal engine operating conditions: at moderate RPM, in-cylinder turbulence results in a nice, fast flame front, deflagrating all of the mixture before any of it can autoignite/detonate. If there were no such thing as ignition delay, we’d have to run SI engines at an extremely low compression ratio to avoid ever heating/compressing the unburned part of the mixture to autoignition conditions before the flame front arrived.

Now think of operating an SI engine at very low RPM, i.e. well below idle speed. There’s a low level of in-cylinder turbulence, so if the mixture is ignited, it takes a fairly long time to burn, regardless of how it was ignited (hot spot or spark plug). The last bit of mixture gets compressed/heated to autoignition conditions, and since the flame front is so slow getting there, that bit of mixture ends up detonating. A hot spot in an overheated (or heavily coked-up) engine fitted with a carburetor is typically responsible for run-on (“dieseling”) after you turn the key off, but the same phenomenon can happen in a running manual-transmission vehicle if you screw up a launch and bog the engine down to somewhere well below idle RPM: the spark is still lighting off the mixture, but at such low RPM it takes so long to burn that you end up causing knock. I get this once in a while on my motorcycle. It’s a fair bet that I’ve also done this in my manual-transmission car, but it’s harder to hear there because of all the soundproofing.