Why is the sound of a diesel different?
Discussion
You'd be surprised just how different the occurances inside are.
Modern diesel engines operate at far higher internal pressures, which may have some effect, but the main thing that causes the rattling you hear at idle is "diesel knock" - as the combustion is started purely by compression, rather than a spark at a specific point, it's quite hard to control it as precisely as you can in a petrol engine. As a result, combustion happens in several places at once, at least some significant portion of the flame front will be going the opposite way to the rest, the two fronts collide and cause a large vibration inside the engine.
Modern diesel engines operate at far higher internal pressures, which may have some effect, but the main thing that causes the rattling you hear at idle is "diesel knock" - as the combustion is started purely by compression, rather than a spark at a specific point, it's quite hard to control it as precisely as you can in a petrol engine. As a result, combustion happens in several places at once, at least some significant portion of the flame front will be going the opposite way to the rest, the two fronts collide and cause a large vibration inside the engine.
McSam said:
You'd be surprised just how different the occurances inside are.
Modern diesel engines operate at far higher internal pressures, which may have some effect, but the main thing that causes the rattling you hear at idle is "diesel knock" - as the combustion is started purely by compression, rather than a spark at a specific point, it's quite hard to control it as precisely as you can in a petrol engine. As a result, combustion happens in several places at once, at least some significant portion of the flame front will be going the opposite way to the rest, the two fronts collide and cause a large vibration inside the engine.
Excellent, thanks very much for that. Makes sense. I knew about the difference between the way diesel & petrol is used for combustion, I didn't know about not being able to control it as easily Modern diesel engines operate at far higher internal pressures, which may have some effect, but the main thing that causes the rattling you hear at idle is "diesel knock" - as the combustion is started purely by compression, rather than a spark at a specific point, it's quite hard to control it as precisely as you can in a petrol engine. As a result, combustion happens in several places at once, at least some significant portion of the flame front will be going the opposite way to the rest, the two fronts collide and cause a large vibration inside the engine.
So that also causes the exhaust note to be different? I've heard diesels with an aftermarket exhaust and the noise is quite a weird one..
It's entirely due to the rate of rise in cylinder pressure. On a CI engine, the combustion event results in a cylinder pressure characteristic that has a rapid increase in cylinder pressure over a very small change in chamber volume. This "diesel knock" results in a sharp short "Crrack" kind of noise, that is transmitted to the engine structure. In conjunction, a diesel engine operates at a higher peak cylinder pressure than for a SI engine. This requires a mechanically stiffer system, and one that as a side effect transmits the "pressure wave" from the combustion event with less damping etc.
Modern CR diesel engines have lower static compression ratios that ever, and use multiple pilot injection events to manage the noise signature to try to limit the characteristic diesel knocking noise. However, these pilot events are less efficient than one single main injection events, so modern diesel EMS systems switch between various modes depending on temps and loads etc.
It's easy to hear the difference, just start your diesel from cold with the bonnet up. After a fe wminutes running, the engine will suddenly get a LOT quiter as it switches to a pilot injection strategy (normally around 50degC coolant temp)
Modern CR diesel engines have lower static compression ratios that ever, and use multiple pilot injection events to manage the noise signature to try to limit the characteristic diesel knocking noise. However, these pilot events are less efficient than one single main injection events, so modern diesel EMS systems switch between various modes depending on temps and loads etc.
It's easy to hear the difference, just start your diesel from cold with the bonnet up. After a fe wminutes running, the engine will suddenly get a LOT quiter as it switches to a pilot injection strategy (normally around 50degC coolant temp)
Have a listen to the idle of the direct injection petrol 330i 272bhp likewise the direct injection e60 M5 v10 at idle. It is a common comment that they sound diesly... Why mega high injector pressure v
The old indirect injection diesels wow they made a noise.
A drag racing car will melt the spark plugs and be creating te explosion by compression at it hits full power over the finish line. They have a high risk of runaway engine with a petrol but thankfully they have fuel cut off
The old indirect injection diesels wow they made a noise.
A drag racing car will melt the spark plugs and be creating te explosion by compression at it hits full power over the finish line. They have a high risk of runaway engine with a petrol but thankfully they have fuel cut off
The biggest driving force in the constant increase in common rail pressure is the requirement to be able to inject ever increasing volumes of fuel in smaller and smaller time frames. Modern CI EMS systems now use up to 7 pilot injection events, 1 main, and upto 5 post events. These all need to occur in approx 80 degrees of crnak angle. As diesel engine power outputs climb, the speed of peak power increases also (over 5krpm these days) Hence the time availible for those 80degCA is small.
Huge boost pressures and resultant massive cylinder filling means you need to get a lot of fuel into each chamber, accurately and in a short period of time. This requires either 1) injection nozzles with big holes using a lower rail pressure or 2) small holes and high pressure. Unfortunately 1) cannot be used as it would make the low load control of fuelling impossible (and hence lead to poor tailpipe emissions). This means small holes and high pressure is the only viable method for accurate and precise fuel delivery in a modern CR diesel over the whole load/speed range required.
The secondary benefit is that the "small holes / high pressure" solution also leads to excellent atomisation of the injected fuel mass, which leads to better mixing with the air charge, and results in a faster cleaner burn. (more power potential, better cold starting and emissions)
Huge boost pressures and resultant massive cylinder filling means you need to get a lot of fuel into each chamber, accurately and in a short period of time. This requires either 1) injection nozzles with big holes using a lower rail pressure or 2) small holes and high pressure. Unfortunately 1) cannot be used as it would make the low load control of fuelling impossible (and hence lead to poor tailpipe emissions). This means small holes and high pressure is the only viable method for accurate and precise fuel delivery in a modern CR diesel over the whole load/speed range required.
The secondary benefit is that the "small holes / high pressure" solution also leads to excellent atomisation of the injected fuel mass, which leads to better mixing with the air charge, and results in a faster cleaner burn. (more power potential, better cold starting and emissions)
M5 V10 is not a GDI engine (it's port injected (PFI)).
They sound nasty at idle due to the aggressive cam lift required for the high specific output (i.e. it's all valvetrain noise)
GDi engines (Gasoline Direct Injection) engines can sound "diesely" because pretty much noise = effiency. In order to loose the least heat to the combustion chamber walls, the highest efficiency is gained by burning the charge at the smallest cylinder volume in the shortest possible time. So deisgners aiming to increase engine efficency will also be designing a combustion system that as a side effect has a rapid rate of rise of cylinder pressure.
Modern engines also are much more mass and stress optimised. They have less excess material in the head and block etc to damp out directly transmitted combustion noise.
They sound nasty at idle due to the aggressive cam lift required for the high specific output (i.e. it's all valvetrain noise)
GDi engines (Gasoline Direct Injection) engines can sound "diesely" because pretty much noise = effiency. In order to loose the least heat to the combustion chamber walls, the highest efficiency is gained by burning the charge at the smallest cylinder volume in the shortest possible time. So deisgners aiming to increase engine efficency will also be designing a combustion system that as a side effect has a rapid rate of rise of cylinder pressure.
Modern engines also are much more mass and stress optimised. They have less excess material in the head and block etc to damp out directly transmitted combustion noise.
Edited by anonymous-user on Tuesday 3rd April 19:48
Max_Torque said:
M5 V10 is not a GDI engine (it's port injected (PFI)).
They sound nasty at idle due to the aggressive cam lift required for the high specific output (i.e. it's all valvetrain noise)
They sound nasty at idle due to the aggressive cam lift required for the high specific output (i.e. it's all valvetrain noise)
I'll take that error but the 330i 272bhp is direct injection the older 256bhp 330i while much more fuel thirsty is vastly better sounding
Max_Torque said:
The biggest driving force in the constant increase in common rail pressure is the requirement to be able to inject ever increasing volumes of fuel in smaller and smaller time frames. Modern CI EMS systems now use up to 7 pilot injection events, 1 main, and upto 5 post events. These all need to occur in approx 80 degrees of crnak angle. As diesel engine power outputs climb, the speed of peak power increases also (over 5krpm these days) Hence the time availible for those 80degCA is small.
Huge boost pressures and resultant massive cylinder filling means you need to get a lot of fuel into each chamber, accurately and in a short period of time. This requires either 1) injection nozzles with big holes using a lower rail pressure or 2) small holes and high pressure. Unfortunately 1) cannot be used as it would make the low load control of fuelling impossible (and hence lead to poor tailpipe emissions). This means small holes and high pressure is the only viable method for accurate and precise fuel delivery in a modern CR diesel over the whole load/speed range required.
The secondary benefit is that the "small holes / high pressure" solution also leads to excellent atomisation of the injected fuel mass, which leads to better mixing with the air charge, and results in a faster cleaner burn. (more power potential, better cold starting and emissions)
There will get to a time when the pressure can't really go much higher, surely we can't be far from that? I know that even our 1.4 diesel Mazda2s run at something like 20,000psi (Hopefully I've remembered that right..)Huge boost pressures and resultant massive cylinder filling means you need to get a lot of fuel into each chamber, accurately and in a short period of time. This requires either 1) injection nozzles with big holes using a lower rail pressure or 2) small holes and high pressure. Unfortunately 1) cannot be used as it would make the low load control of fuelling impossible (and hence lead to poor tailpipe emissions). This means small holes and high pressure is the only viable method for accurate and precise fuel delivery in a modern CR diesel over the whole load/speed range required.
The secondary benefit is that the "small holes / high pressure" solution also leads to excellent atomisation of the injected fuel mass, which leads to better mixing with the air charge, and results in a faster cleaner burn. (more power potential, better cold starting and emissions)
JonnyFive said:
There will get to a time when the pressure can't really go much higher, surely we can't be far from that? I know that even our 1.4 diesel Mazda2s run at something like 20,000psi (Hopefully I've remembered that right..)
The question should be what is the theoretical maximum pressure. Then what's fhe max we have now. It's likely the technology or materials are not viable currently to take it up to that theoretical maximum but at some point in the future it will and likely to be progressive. Didn't the pump duse VAG offering allow the potential for vastly higher pressure than common rail but then they switched to common rail die to industry standard and the cost for then in r&d
ATTAK Z said:
Welshbeef said:
ATTAK Z said:
Why does the VW 3 cylinder engine in a Polo (60ps) sound like a diesel?
Do you mean at idle or steady state revs or when accelerating or decelerating in gear ie on load or off load. Isn't the question familiarity?
I mean diesel was invented 30+ years after petrol engines existed and until fuel costs became significant diesels were just not used in cars.
When I grew up diesel cars I just never saw any. Had the situation been the other way round only diesels and then only really in the last 15 years petrol Tom off like diesel has in the Uk would we still think petrol sounds good? Would we think heck we don't like this new driving characteristics of the engine ? I don't know
Or it could be the case petrol sounds better.
Out of interest when petrols are converted to Gas if they were nice sounding do they sound the same worse or better with gas?
I mean diesel was invented 30+ years after petrol engines existed and until fuel costs became significant diesels were just not used in cars.
When I grew up diesel cars I just never saw any. Had the situation been the other way round only diesels and then only really in the last 15 years petrol Tom off like diesel has in the Uk would we still think petrol sounds good? Would we think heck we don't like this new driving characteristics of the engine ? I don't know
Or it could be the case petrol sounds better.
Out of interest when petrols are converted to Gas if they were nice sounding do they sound the same worse or better with gas?
JonnyFive said:
ATTAK Z said:
Welshbeef said:
ATTAK Z said:
Why does the VW 3 cylinder engine in a Polo (60ps) sound like a diesel?
Do you mean at idle or steady state revs or when accelerating or decelerating in gear ie on load or off load. JonnyFive said:
So that also causes the exhaust note to be different? I've heard diesels with an aftermarket exhaust and the noise is quite a weird one..
Yeah, they sound absolutely dreadful! They sound like a jackhammer splitting concrete, but apparently are popular in Ireland though.With regard to the theory that perhaps we enjoy the sound of petrol more because we are more familiar with it, well I don't put any stock in that. Diesels sound terrible because they are clattery as a result of their engines, whereas petrols on the other hand (generally) produce much a smoother, more rounded tone with more of the frequencies that are pleasing to the ear (bass and low mids with treble accents) instead of piercing higher frequencies overlaid on a white noise of bass you get from a diesel.
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