With all the talk about “timing rattle” I thought I put something out here to get everyone on the same page. My explanation is very brief and huge papers can be written on this subject. The constant talk of timing rattle is taking up too much time and space on these pages and needs to be better explained.
Before I get started there are a couple of terms that need to be defined.
Injection timing: The time measured in degrees when the main injection BEGINS. This is usually stated as BTDC(before top dead center) or ATDC (after top dead center).
Ignition timing: The time measured in degrees when the fuel self ignites stated at BTDC or ATDC.
Ignition delay: The time difference between Injection and Ignition. Diesel fuel needs to evaporate after it is injected before it can self ignite. This delay varies with temperature in the cylinder.
To explain the diesel cycle as it applies to our engines, I am going to use some numbers that I pulled out of my Bu**, for explanation purposes only.
Lets take a typical truck cruising down the road unloaded on a typical day with the ideal timing map installed. Fuel is injected at lets say 6deg BTDC and continues to flow throughout the pulse duration. The engine is at operating temp of 200F and fuel starts to evaporate and by 5deg BTDC it ignites. When it ignites, the rest of the fuel ( 1 degree worth) in the cylinder also flashes causing a rapid increase in cylinder pressure. There is a knock associated with this rapid increase in cylinder pressure but it is hardly noticed.
Now lets see what happens when this same truck is parked overnight and fired up in the morning.
The driver takes the truck up on the same road, same speed, same load, but the engine is not up to temp. The fuel is injected at the same 6deg BTDC and continues to inject throughout the pulse duration. The engine temp is 60F and the fuel starts to evaporate but slowly because the cylinder is so cold. By 1deg BTDC the fuel ignites and all the fuel in the cylinder, 5 deg worth (6deg- 1deg) flashes over. This rapid increase in pressure causes the cylinder to ring loud enough to be heard in the cab! This is what everyone here has been calling timing rattle.
Ignition delay is what is causing the noise. The time between when the fuel is injected and the fuel actually ignites.
So how do you get rid of the noise? Inject later in the stroke when the air in the cylinder is hotter. This is what is done from the factory. Another way is to use pre-injection. A small amount of fuel that is injected early and allowed to flash but is such a small amount that it doesn't make noise. Just before the pre-injection has finished burning the main injection is turned on creating a smooth transition and much less noise. I think advancing pre-injection with the main would work with our engines but I have never seen an adjustment for pre-injection.
The issue is, as far as I know, there is no cold timing adjustment in the 6.7 computer. Even EFI doesn't have a temperature dependent timing adjustment as far as I know.
So you are stuck with an engine that rattles when cold or gets good mileage when warm.
So does rattle hurt my engine? From theory I would say no at small throttle. Even though there is a pressure spike associated with the rattle there is so little fuel and total pressure that I feel it is highly unlikely to cause damage. Empirically it think it is pretty obvious that nobody I know has ever blown a head gasket at part throttle. But...what about full throttle on a cold engine!!!
There have been many cases of 6.7 engines blowing head gaskets when not allowed to warm up. So lets look at what is happening.
By looking at a H&S timing map, you see at WOT your timing could be as much as 32 deg! So on a cold engine you inject at 32BTDC and have ignition at lets say 15BTDC and you have 15DEG of fueling going off at once! Oh, did I mention you are also at full boost when all this is occurring!
Well this explains the blowing head gaskets on cold engines.
I was never satisfied with the excuse that the head studs needed to be heated to reach full clamping force. Just didn't make sense to me.
So, light throttle till warm and enjoy the sound of MPG!
Any questions:confused013:
Credits,
I have read so many papers on diesel injection that there is no way I could remember them all but one does stand out,
Michael Patton at KillerBeePerformance provided much to my understanding and with the exception of being a Dmax fan, he is a pretty smart guy.
Before I get started there are a couple of terms that need to be defined.
Injection timing: The time measured in degrees when the main injection BEGINS. This is usually stated as BTDC(before top dead center) or ATDC (after top dead center).
Ignition timing: The time measured in degrees when the fuel self ignites stated at BTDC or ATDC.
Ignition delay: The time difference between Injection and Ignition. Diesel fuel needs to evaporate after it is injected before it can self ignite. This delay varies with temperature in the cylinder.
To explain the diesel cycle as it applies to our engines, I am going to use some numbers that I pulled out of my Bu**, for explanation purposes only.
Lets take a typical truck cruising down the road unloaded on a typical day with the ideal timing map installed. Fuel is injected at lets say 6deg BTDC and continues to flow throughout the pulse duration. The engine is at operating temp of 200F and fuel starts to evaporate and by 5deg BTDC it ignites. When it ignites, the rest of the fuel ( 1 degree worth) in the cylinder also flashes causing a rapid increase in cylinder pressure. There is a knock associated with this rapid increase in cylinder pressure but it is hardly noticed.
Now lets see what happens when this same truck is parked overnight and fired up in the morning.
The driver takes the truck up on the same road, same speed, same load, but the engine is not up to temp. The fuel is injected at the same 6deg BTDC and continues to inject throughout the pulse duration. The engine temp is 60F and the fuel starts to evaporate but slowly because the cylinder is so cold. By 1deg BTDC the fuel ignites and all the fuel in the cylinder, 5 deg worth (6deg- 1deg) flashes over. This rapid increase in pressure causes the cylinder to ring loud enough to be heard in the cab! This is what everyone here has been calling timing rattle.
Ignition delay is what is causing the noise. The time between when the fuel is injected and the fuel actually ignites.
So how do you get rid of the noise? Inject later in the stroke when the air in the cylinder is hotter. This is what is done from the factory. Another way is to use pre-injection. A small amount of fuel that is injected early and allowed to flash but is such a small amount that it doesn't make noise. Just before the pre-injection has finished burning the main injection is turned on creating a smooth transition and much less noise. I think advancing pre-injection with the main would work with our engines but I have never seen an adjustment for pre-injection.
The issue is, as far as I know, there is no cold timing adjustment in the 6.7 computer. Even EFI doesn't have a temperature dependent timing adjustment as far as I know.
So you are stuck with an engine that rattles when cold or gets good mileage when warm.
So does rattle hurt my engine? From theory I would say no at small throttle. Even though there is a pressure spike associated with the rattle there is so little fuel and total pressure that I feel it is highly unlikely to cause damage. Empirically it think it is pretty obvious that nobody I know has ever blown a head gasket at part throttle. But...what about full throttle on a cold engine!!!
There have been many cases of 6.7 engines blowing head gaskets when not allowed to warm up. So lets look at what is happening.
By looking at a H&S timing map, you see at WOT your timing could be as much as 32 deg! So on a cold engine you inject at 32BTDC and have ignition at lets say 15BTDC and you have 15DEG of fueling going off at once! Oh, did I mention you are also at full boost when all this is occurring!
Well this explains the blowing head gaskets on cold engines.
I was never satisfied with the excuse that the head studs needed to be heated to reach full clamping force. Just didn't make sense to me.
So, light throttle till warm and enjoy the sound of MPG!
Any questions:confused013:
Credits,
I have read so many papers on diesel injection that there is no way I could remember them all but one does stand out,
Michael Patton at KillerBeePerformance provided much to my understanding and with the exception of being a Dmax fan, he is a pretty smart guy.
