Originally Posted By: RamFan
I'm trying to figure out why you'd want a big and slow gas engine, even diesel for that matter.
90s GM
I'm trying to figure out why you'd want a big and slow gas engine, even diesel for that matter.
90s GM
Big, slow, gasoline engines?
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Originally Posted By: RamFan
I'm trying to figure out why you'd want a big and slow gas engine, even diesel for that matter.
All a matter of how one likes their engine to respond. I myself prefer low rev engine; I bought my SOHC Saturn over a comparable Civic for that. I wouldn't say my tdi is low reving, but it's torque is certainly biased low.
I'm trying to figure out why you'd want a big and slow gas engine, even diesel for that matter.
All a matter of how one likes their engine to respond. I myself prefer low rev engine; I bought my SOHC Saturn over a comparable Civic for that. I wouldn't say my tdi is low reving, but it's torque is certainly biased low.
More btu in a gallon of diesel and more torque.
Originally Posted By: A_Harman
In the case of very large bore engines, gasoline engines have classically had detonation problems when the bore size goes above 6". Flame propagation occurs at the same rate in gasoline combustion, regardless of the cylinder size, so there is more time for end gases to heat up to autoignition temperature in a large bore engine. Automotive engines that we are accustomed to talking about are in the range of 3-4.25" bores.
The diesel combustion process does not have a limit on cylinder bore size because flame initiates in many different locations in the cylinder at the same time.
Spot on.
Also diesel injection/ignition processes usually take a fair amount of crank time
In the case of very large bore engines, gasoline engines have classically had detonation problems when the bore size goes above 6". Flame propagation occurs at the same rate in gasoline combustion, regardless of the cylinder size, so there is more time for end gases to heat up to autoignition temperature in a large bore engine. Automotive engines that we are accustomed to talking about are in the range of 3-4.25" bores.
The diesel combustion process does not have a limit on cylinder bore size because flame initiates in many different locations in the cylinder at the same time.
Spot on.
Also diesel injection/ignition processes usually take a fair amount of crank time
Originally Posted By: A_Harman
In the case of very large bore engines, gasoline engines have classically had detonation problems when the bore size goes above 6".
+1
This is the answer.
In the case of very large bore engines, gasoline engines have classically had detonation problems when the bore size goes above 6".
+1
This is the answer.
Originally Posted By: circuitsmith
Originally Posted By: A_Harman
In the case of very large bore engines, gasoline engines have classically had detonation problems when the bore size goes above 6".
+1
This is the answer.
Exactly. Interesting that it used to be considered any bore over 4 inches!
In any case, typical aircraft engines have bores North of 5 inches and detonation is a big problem with any compression ratio over about 8.5 to 1. That's why the dual sparkplugs are on opposite sides of the head (flame front speed) , and the octane minimum for the "high compression" versions is 100.
Originally Posted By: A_Harman
In the case of very large bore engines, gasoline engines have classically had detonation problems when the bore size goes above 6".
+1
This is the answer.
Exactly. Interesting that it used to be considered any bore over 4 inches!
In any case, typical aircraft engines have bores North of 5 inches and detonation is a big problem with any compression ratio over about 8.5 to 1. That's why the dual sparkplugs are on opposite sides of the head (flame front speed) , and the octane minimum for the "high compression" versions is 100.
Originally Posted By: supton
What causes the large bores to have detonation problems? Just more surface area, so more chances to have a hotspot, be it from carbon buildup or some uneven cooling?
As the gas mixture starts to burn, around the source of ignition, the expanding gas puts pressure on the yet unburned gas causing possible detonation.
The longer the combustion process takes (ie. the larger the volume. Because the 'burn' speed is constant) The more time there is for detonation to occur.
What causes the large bores to have detonation problems? Just more surface area, so more chances to have a hotspot, be it from carbon buildup or some uneven cooling?
As the gas mixture starts to burn, around the source of ignition, the expanding gas puts pressure on the yet unburned gas causing possible detonation.
The longer the combustion process takes (ie. the larger the volume. Because the 'burn' speed is constant) The more time there is for detonation to occur.
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Originally Posted By: expat
Originally Posted By: supton
What causes the large bores to have detonation problems? Just more surface area, so more chances to have a hotspot, be it from carbon buildup or some uneven cooling?
As the gas mixture starts to burn, around the source of ignition, the expanding gas puts pressure on the yet unburned gas causing possible detonation.
The longer the combustion process takes (ie. the larger the volume. Because the 'burn' speed is constant) The more time there is for detonation to occur.
Which is why big-bore gas engines were, back in the day at least, equipped with dual ignition. Two distributors, slightly out of phase with each other so that the flame-front propagation could be controlled/manipulated by ignition occurring at two sources in the chamber. This worked to curb detonation.
Originally Posted By: supton
What causes the large bores to have detonation problems? Just more surface area, so more chances to have a hotspot, be it from carbon buildup or some uneven cooling?
As the gas mixture starts to burn, around the source of ignition, the expanding gas puts pressure on the yet unburned gas causing possible detonation.
The longer the combustion process takes (ie. the larger the volume. Because the 'burn' speed is constant) The more time there is for detonation to occur.
Which is why big-bore gas engines were, back in the day at least, equipped with dual ignition. Two distributors, slightly out of phase with each other so that the flame-front propagation could be controlled/manipulated by ignition occurring at two sources in the chamber. This worked to curb detonation.
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Originally Posted By: supton
Ok, the why for the detonation makes sense. But not the cure. If you light off in two places, won't you have the same issue when the two waves hit each other? Or are the two waves intended to hit each other while pressures are still low?
The one kind of runs into the back of the other so that the pressure spread across the cylinder is uniform IIRC.
Ok, the why for the detonation makes sense. But not the cure. If you light off in two places, won't you have the same issue when the two waves hit each other? Or are the two waves intended to hit each other while pressures are still low?
The one kind of runs into the back of the other so that the pressure spread across the cylinder is uniform IIRC.
As motors get bigger, so do the vibrations, hence why you see semis using inline 6 motors nowadays, more natural balance there.
Originally Posted By: OVERKILL
Originally Posted By: supton
Ok, the why for the detonation makes sense. But not the cure. If you light off in two places, won't you have the same issue when the two waves hit each other? Or are the two waves intended to hit each other while pressures are still low?
The one kind of runs into the back of the other so that the pressure spread across the cylinder is uniform IIRC.
Detonation is end gas autoignition.
The flame travels across the chamber at about 50 fps in a large quiescent chamber, while the pressure travels at the speed of sound. (which increases as temperature rises, coincidentally)
The gas away from the flame is getting compressed rapidly, which makes it hotter, much like the gas in a diesel engine. When it gets hot enough, it burns, in a general, rather than flame front pattern, and gives massive pressure rise (knocking - in a diesel, the idle knock is typically fuel that is vaporising suddenly burning as a mass rather than a front).
Dual ignitions reduce the distance that the flame has to travel, so reduces end gas propensity to autoignite.
"Closed Chambers", those kidney shaped chambers that were soo good in the 60s and 70s :
* reduce the flame travel distance to the kidney shape;
* the last bit of piston travel "squish"es the air/fuel into the kidney shape, creating turbulence, and increasing flame speed.
* the bit that's not the kidney shape "quench"es the gasses away from the plug so that they can't autoignite.
* Have all those areas for unburned hydrocarbons to sit, waiting to come out the tailpipe on the exhaust stroke.
Originally Posted By: supton
Ok, the why for the detonation makes sense. But not the cure. If you light off in two places, won't you have the same issue when the two waves hit each other? Or are the two waves intended to hit each other while pressures are still low?
The one kind of runs into the back of the other so that the pressure spread across the cylinder is uniform IIRC.
Detonation is end gas autoignition.
The flame travels across the chamber at about 50 fps in a large quiescent chamber, while the pressure travels at the speed of sound. (which increases as temperature rises, coincidentally)
The gas away from the flame is getting compressed rapidly, which makes it hotter, much like the gas in a diesel engine. When it gets hot enough, it burns, in a general, rather than flame front pattern, and gives massive pressure rise (knocking - in a diesel, the idle knock is typically fuel that is vaporising suddenly burning as a mass rather than a front).
Dual ignitions reduce the distance that the flame has to travel, so reduces end gas propensity to autoignite.
"Closed Chambers", those kidney shaped chambers that were soo good in the 60s and 70s :
* reduce the flame travel distance to the kidney shape;
* the last bit of piston travel "squish"es the air/fuel into the kidney shape, creating turbulence, and increasing flame speed.
* the bit that's not the kidney shape "quench"es the gasses away from the plug so that they can't autoignite.
* Have all those areas for unburned hydrocarbons to sit, waiting to come out the tailpipe on the exhaust stroke.
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