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**dafodilkemmy** · 08-19-2012, 10:36 AM

Ok, well, if the combustion head (the edge of the flame) is moving faster than there will be more torque. Even if it is only 25% faster, this is something that should be considered. Your quote kind of suggests that.

Let me just clarify that it is technically impossible for all of the fuel it ignite at once (speed of light, etc), I was suggesting: increased speed = increased instantaneous torque.

--- Post Update ---

Gnius,

One clarification:

The instantaneous torque is on the shaft local to the specific currently combusting chamber. The torque has to translate through the entire shaft... since the shaft is not technically rigid, the shaft will harmonically oscillate. The danger of the oscialliations is that at some point, the material comprising the shaft will not withstand the sheering forces. (Probably should get Ricanpimp in here before I embarrass myself).

Basically, per cycle, the torque on the shaft will be lowpassed (i.e. dampened) by the inertia of the shaft itself. The overall shaft torque on average will not change a bit. The real issue is the local torques considering a chamber cross-section.

obviously engine manufacturers have designed and introduced Harmonic dampeners to help alleviate this.

I think this is more commonly known as torsional Vibration which is caused by inertia torque from pistons and connecting rods as they stop start every time per firing of the respective cylinders, where a torque pulse makes the crankshaft reach its natural harmonic resonance which then leads to failure.

also the most prone part of the crankshaft to fail due to this is the front end because the flywheel helps to partly reduce the "inertia" hence why you typically see rubber cored damper pulleys on the front end of the engine, and advanced dual mass flywheels at the rear of the engine, there is also sometimes anti resonance devices built into the sump to reduce it further.

but as is the nature of the beast these devices are also prone to failure due to what they have to do, damper pulleys break apart throwing ancillary belts causing all manner of problems and dual mass flywheels can grenade,or more commonly suffer from flywheel chatter which causes damage to the transmission system

I'm talking from what I learned at college learning to be a land-based engineer which is heavily geared towards the agricultural field.

but yeah basically Ive reiterated what you wrote.

--- Post Update ---

In both cases I assumed that the same amount of fuel was burned. I only compared the speed in which it burned.

I probably should have mentioned that.

you're talking about flame travel, it also depends on the shape of the piston crown and cylinder head, also if its direct or indirectly fuel injected. some engines have dual spark that help flame travel.
in diesels they have swirl pits/pots pre ignition chambers that allow the fuel to burn better.

I really should proof read first...

08-19-2012, 10:36 AM	#14
dafodilkemmy Join Date Oct 2005 Posts 471 Senior Member	Ok, well, if the combustion head (the edge of the flame) is moving faster than there will be more torque. Even if it is only 25% faster, this is something that should be considered. Your quote kind of suggests that. Let me just clarify that it is technically impossible for all of the fuel it ignite at once (speed of light, etc), I was suggesting: increased speed = increased instantaneous torque. --- Post Update --- Gnius, One clarification: The instantaneous torque is on the shaft local to the specific currently combusting chamber. The torque has to translate through the entire shaft... since the shaft is not technically rigid, the shaft will harmonically oscillate. The danger of the oscialliations is that at some point, the material comprising the shaft will not withstand the sheering forces. (Probably should get Ricanpimp in here before I embarrass myself). Basically, per cycle, the torque on the shaft will be lowpassed (i.e. dampened) by the inertia of the shaft itself. The overall shaft torque on average will not change a bit. The real issue is the local torques considering a chamber cross-section. obviously engine manufacturers have designed and introduced Harmonic dampeners to help alleviate this. I think this is more commonly known as torsional Vibration which is caused by inertia torque from pistons and connecting rods as they stop start every time per firing of the respective cylinders, where a torque pulse makes the crankshaft reach its natural harmonic resonance which then leads to failure. also the most prone part of the crankshaft to fail due to this is the front end because the flywheel helps to partly reduce the "inertia" hence why you typically see rubber cored damper pulleys on the front end of the engine, and advanced dual mass flywheels at the rear of the engine, there is also sometimes anti resonance devices built into the sump to reduce it further. but as is the nature of the beast these devices are also prone to failure due to what they have to do, damper pulleys break apart throwing ancillary belts causing all manner of problems and dual mass flywheels can grenade,or more commonly suffer from flywheel chatter which causes damage to the transmission system I'm talking from what I learned at college learning to be a land-based engineer which is heavily geared towards the agricultural field. but yeah basically Ive reiterated what you wrote. --- Post Update --- In both cases I assumed that the same amount of fuel was burned. I only compared the speed in which it burned. I probably should have mentioned that. you're talking about flame travel, it also depends on the shape of the piston crown and cylinder head, also if its direct or indirectly fuel injected. some engines have dual spark that help flame travel. in diesels they have swirl pits/pots pre ignition chambers that allow the fuel to burn better. I really should proof read first...
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