What exactly makes all the noise in a diesel motor?

[Spooled-up]

I was talking to a co-worker the other day and the subject turned to diesels. The question came up about what makes a diesel sound the way it does. I had to admit that I really didn't know.



So what is the source of all that sweet noise?

 

[ToolManTimTaylor]

Power Elves Running rampant within our intakes #ad








??? Pre ignition from being ignited via compression ???

 

[BarryG]

Could be wrong here but I think it is literally the fuel lighting, igniting, exploding, burning whatever the proper term is for it.

 

[Jeff H]

High compression motors will make more noise. Fuel is shot into the cyclinder and it knocks. In a way your engine is running wide open even at idle. howitworks.com

Look up diesel engine. Theres an animation of one running. Hope the link works. Hope you get a better idea.

 

[parcher]

It's the sudden spike in cylinder pressure, the reason the '03s are quieter is they use "pilot injection" where a smaller fuel charge is quickly followed by the main charge, this "ramps up" cylinder pressure with much less of a spike.

 

[rbattelle]

parcher's right. The sudden rise in cylinder pressure literally has a "hammer" effect on the top of the piston. Using pilot injection lowers the slope of the pressure rise and decreases the "hammer" effect.



On a related topic, knock is a problem in diesels, despite popular opinion. If the rate of pressure rise during combustion is too high, the engine is knocking. This can be heard by a marked increase in the amount of noise the engine is producing. It's possible to measure knock in a diesel, and very experienced diesel mechanics can tell by ear when a diesel is knocking.

 

[WMeyers]

Now a related (sort of) question -- I saw a post somewhere that diesel compression is higher than on a gasoline engine. What is the compression on a Cummins 24 valve and what would it be on a typical big block gasoline engine?

 

[WebCrush]

24v CTD has a comp ration of 16. 3:1, a typical gasser would be in the 10-11:1 ratio

 

[Timinator]

The P7100 pump for the 12 valvers is quite noisy too. Probably due to the mechanical change in pressure from lift pump pressures to injection pressures.

 

[Spooled-up]

So am I to understand that when a gasser knocks after shutdown, it is the same thing as when a diesel is running? I think when a gasser knocks it is the result of a hot spot in the cylinder igniting some leftover fuel/air mixture. I believe this is very bad for the gas engine due to metal to metal contact in the valve train that is not normally there during normal combustion.



Our 12 and 24 valve engines are so loud just due to the controlled explosions in the cylinders? When I listen to any straight six diesel engine it sounds almost like there is some metal to metal contact although through some very thick cast iron parts that are made to take such abuse.



When I hear powerstrokes, they sound a little quieter, possibly due to the smaller explosions and more of them. I think they sound terrible with straight pipes too.

 

[WebCrush]

thats why my dad always called it 'dieseling' when a older car would continue to stutter run even after we shut it down.



the heat of the engine would combust fuel which would continue to be delivered (via vaccum) as long as the combustion kept up.



Its a vicious cycle.

 

[illflem]

Originally posted by Timinator

The P7100 pump for the 12 valvers is quite noisy too. Probably due to the mechanical change in pressure from lift pump pressures to injection pressures.

If any of you have ever been next to an injection pump on a test stand without the rest of the engine noise you'd jump back five feet when it starts spinning. It's quite normal for them to sound like they're going to start flying apart at any second.

Most of the rattle like noise you hear from your engine is coming from the injection pump. There are moving parts with up to 20,000 psi of force concentrated in a area less than 1/8 square inch.

 

[rbattelle]

Beware that 20000 psi is a pressure. The force of that pressure on 1/8 inch is actually 2500 lb. Certainly that'll make some noise (if it's applied suddenly)!!



I believe that a lot of the higher-frequency noise you hear is the injection pump. Lower frequencies is the combustion process described earlier. The noise you hear is definetly NOT metal-to-metal contact. Such contact making that much noise would quickly become fatal to the engine.

 

[Chipstien]

Curious, how loud is the VP-44?

 

[boonsur]

About Compression Ratios:

It was mentioned earlier, but to simplify why a diesel makes more power:



A good way to make a lot of power in an engine is to raise the compression ratio. However, gasoline will ignite on its own (before spark) when it's really squeezed. The way to increase compression (power) is to use higher octane fuel (which isn't as volitile). I'm not sure of the exact numbers, but I believe an engine rated for normal pump fuel is around 9-10:1 compression. With super high octane (I believe the highest obtainable octane is something like 120), compressions can be increased into the 12-13:1 ratio. (Alcohol even higher?).



The beauty of the diesel concept is, there is no fuel present during compression. The air can be compressed to a super potent level, then a violent explosion is set off wth the introduction of the fuel. (Compression ratios are around 17:1). To acheive the high pressures necessary (to push the fuel into this explosion), single pop injectors have long been used. When told to, they basically blow their wad once and wait until the next time. This gives a single, violent (hammering) explosion, driving the piston down hard (that's why the components need to be so big). The "new", High-Pressure-Common-Rail systems have a resevoir of pressurized fuel ready to go at all times. The injectors are controlled to spurt a little fuel to get things rolling, then dump the rest once the fire is good and hot. This makes the hammer much smoother (quieter), and burns cleaner, which appeals to a lot of the general public. By simply adjusting the timing/volume of the injection, the new engines can be made to "knock" just like the old ones.

 

[Deezul1]

illflem is right, I was at a pump shop that had a P pump running and it sounded just like my truck without exhaust noise. My friends 1st gen is very quite once you get rolling and you cant hardly hear the "Diesel" so I think that early pump isnt near so noisy

 

[David Oxland]

Think of the noise this way. Injection commences and carries on over a period of time. If combustion does not commence immediately fuel accumulates all the while vaporising and heating; When combustion finally does occur all the fuel burns at once creating the sound. In cold operations or at startup commencement of combustion is usually delayed because of lack of heat for good vaporization and combustion. Towing guys will notice that when they finally get their engine good and hot after a sustained hard pull things become remarkably quiet. If you were to bypass your charge air cooler and let good hot air in you would be surprised at how smoothly the beast would become under load.

 

[Peterbilt379CJ]

Wow!!!!!!! Tons of responses on this one.



The noise is from... ... ... ... .



#1-high compression in the cylinder

#2-injection pump (sounds like broken chainsaw)

#3- and I am surprised that nobody picked up on this, a lot of the noise is transfered into the geartrain in the front of the engine and the gears actually emit most of the noise. This is why cars use timing chains and more and more timming belts. Belts and chains absorb sounds and vibrations, gears dont. Cummins 6BTA5. 9 is all gear in the front end as are 99. 9% of all diesels. I would say that about 60% of all engine noise is tranfered through the geartrain.



Now, I know you are all saying, "Then why do the newer diesels sound quieter than the old ones"



Well, to answer the question... ... ... .....



The newer diesels have many new features that eliminate major sources of noise one of the big ones is the injection pump. another is putting in a pre-combustion chamber, plus diffrent timming and injectors.



If any of you have any questions just ask me.

 

[rbattelle]

Peterbilt,



When you refer to a "precombustion chamber", do you mean divided chamber designs? Or were you referring to pilot injection? I don't believe there are any commercial divided-chamber designs out there, because of the high cost of manufacture.



I'm not convinced the geartrain is contributing to 60% of the noise of a diesel. The geartrain normally makes a "whining" noise. The "knocking" noise, which in my opinion is most of the noise people think of when they think diesel, is definetly caused by combustion.



When you refer to high compression being the number one cause of noise, are you referring to compression ratio or maximum pressure? Air doesn't make any noise when you compress it, unless you compress it so rapidly that you create a pressure wave. During the compression stroke, the pressure rise is much too slow to create a very loud noise. During combustion, however, the sudden pressure rise certainly does create a lot of noise. In fact, the amount of noise is a direct function rate of pressure rise (which is a strong function of delay angle) and maximum pressure. Pilot injection effectively reduces rate of pressure rise, thereby reducing a significant fraction of the noise.



This is a great discussion.

 

[Marco]

Me Me, I know it!

The answer is

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