Cylinder pressures and forces, is this right?
Discussion
Notice: Double the horsepower, with only 8 Percent Engine stress at 15 Pounds of turbo boost. Excerp from FORCED INDUCTION. Read on!
THE CHARMED LIFE OF A BOOSTED ENGINE
Turbo-boosted power is easier on an engine than you might guess from the power gains. For one thing, most of the gain in power from turbo-boosting occurs by increasing the force exerted against the crankshaft at times of subpeak stress. Under very heavy loading and higher speeds, the charge mixture in the cylinders of a modern piston engine typically ignites at 20-30 degrees before top dead center, with ignition timed to achieve peak cylinder pressure in the range of 14-18 degrees after top dead center. Only about 20 percent of the charge mixture has burned, but at this point the piston begins to accelerate hard away from the cylinder head such that even though gas volume is still increasing from combustion, the size of the combustion chamber is increasing even faster. Inevitably, cylinder pressure is dropping in this portion of the power stroke, and is a decreasing factor when considering stress on the various engine components. The average torque of a turbo engine is heavily enhanced by gains in this portion of the power stroke, but cylinder pressure and mechanical stress are considerably below the peak.
So let's analyze peak cylinder pressure. If an engine is turbo-supercharged to the extent required to deliver double the charge mixture in the combustion chamber, the cylinder pressure from compression will certainly be higher than an equivalent normal-charged engine, as will the component from boosted combustion. But how much higher? Less than you might think. Suppose normal charged cranking compression yields 185 psi, and a turbo compressor adds an additional 15 psi of boost pressure. The total compression component of cylinder pressure would be 200 psi exerted against a 4-inch piston of 12.57 square inch crown area. Multiplying the 200 psi by 12.57 indicates a total compression loading on the connecting rod of 2,514 pounds at top dead center CTDC) for the supercharged engine. But this is only 8 percent higher than the 2,324 pound loading of a similar normal-charged powerplant.
Obviously, this is small compared to combustion loading, which could easily quadruple pressure in the combustion chamber to 740 psi in the normal-charged engine and 800 psi in the boosted engine, resulting in total loading of 9,301 and 10;056 pounds for the two powerplants. Think of it: atmosphere of boost will double the horsepower but the supercharged engine's pressure is nonetheless only 8 percent higher. And 100 percent of the added load is compressive through the connecting rod against the crankshaft.
This last point is significant. Let's compare the increased pressure-based rod loading of turbocharging to the higher rod stresses that result from increasing the engine redline and thus increasing the inertial resistance of the mass in the reciprocating assembly to extremely rapid changes in piston and rod velocity. Calculating the “weakest link” tensile rod loading (when the rod bolts alone must bear the entire load of decelerating the piston-rod assembly when the crankshaft is yanking the piston to a halt toward the end of the exhaust stroke and there is no compression and combustion pressure to offset the tensile loading), we find that the loads generated by reciprocating motion increase as the square of engine rpm. If redline increases from say, 6,000 to 7,000 rpm, loading increases not 17 percent-like engine speed- but 36 percent! Compare this to the 8 percent increased rod loading from 15 psi boost.
Bottom line, considering the nature of the way turbocharging increases cylinder pressure versus the exponential nature of increased rod loading from higher engine speeds, turbocharging is clearly far easier on an engine than increasing the redline.
source http://keystoneturbollc.com/id78.html
Does this seem right to you?
THE CHARMED LIFE OF A BOOSTED ENGINE
Turbo-boosted power is easier on an engine than you might guess from the power gains. For one thing, most of the gain in power from turbo-boosting occurs by increasing the force exerted against the crankshaft at times of subpeak stress. Under very heavy loading and higher speeds, the charge mixture in the cylinders of a modern piston engine typically ignites at 20-30 degrees before top dead center, with ignition timed to achieve peak cylinder pressure in the range of 14-18 degrees after top dead center. Only about 20 percent of the charge mixture has burned, but at this point the piston begins to accelerate hard away from the cylinder head such that even though gas volume is still increasing from combustion, the size of the combustion chamber is increasing even faster. Inevitably, cylinder pressure is dropping in this portion of the power stroke, and is a decreasing factor when considering stress on the various engine components. The average torque of a turbo engine is heavily enhanced by gains in this portion of the power stroke, but cylinder pressure and mechanical stress are considerably below the peak.
So let's analyze peak cylinder pressure. If an engine is turbo-supercharged to the extent required to deliver double the charge mixture in the combustion chamber, the cylinder pressure from compression will certainly be higher than an equivalent normal-charged engine, as will the component from boosted combustion. But how much higher? Less than you might think. Suppose normal charged cranking compression yields 185 psi, and a turbo compressor adds an additional 15 psi of boost pressure. The total compression component of cylinder pressure would be 200 psi exerted against a 4-inch piston of 12.57 square inch crown area. Multiplying the 200 psi by 12.57 indicates a total compression loading on the connecting rod of 2,514 pounds at top dead center CTDC) for the supercharged engine. But this is only 8 percent higher than the 2,324 pound loading of a similar normal-charged powerplant.
Obviously, this is small compared to combustion loading, which could easily quadruple pressure in the combustion chamber to 740 psi in the normal-charged engine and 800 psi in the boosted engine, resulting in total loading of 9,301 and 10;056 pounds for the two powerplants. Think of it: atmosphere of boost will double the horsepower but the supercharged engine's pressure is nonetheless only 8 percent higher. And 100 percent of the added load is compressive through the connecting rod against the crankshaft.
This last point is significant. Let's compare the increased pressure-based rod loading of turbocharging to the higher rod stresses that result from increasing the engine redline and thus increasing the inertial resistance of the mass in the reciprocating assembly to extremely rapid changes in piston and rod velocity. Calculating the “weakest link” tensile rod loading (when the rod bolts alone must bear the entire load of decelerating the piston-rod assembly when the crankshaft is yanking the piston to a halt toward the end of the exhaust stroke and there is no compression and combustion pressure to offset the tensile loading), we find that the loads generated by reciprocating motion increase as the square of engine rpm. If redline increases from say, 6,000 to 7,000 rpm, loading increases not 17 percent-like engine speed- but 36 percent! Compare this to the 8 percent increased rod loading from 15 psi boost.
Bottom line, considering the nature of the way turbocharging increases cylinder pressure versus the exponential nature of increased rod loading from higher engine speeds, turbocharging is clearly far easier on an engine than increasing the redline.
source http://keystoneturbollc.com/id78.html
Does this seem right to you?
Edited by Luther Blisset on Tuesday 15th December 18:03
lostkiwi said:
Is this a diesel or petrol engine - they behave differently?
Dunno, suppose it's just a hypothetical engine. Don't think it matters for what I'm asking.It's this bit that gets me:
Suppose normal charged cranking compression yields 185 psi, and a turbo compressor adds an additional 15 psi of boost pressure. The total compression component of cylinder pressure would be 200 psi exerted against a 4-inch piston of 12.57 square inch crown area. Multiplying the 200 psi by 12.57 indicates a total compression loading on the connecting rod of 2,514 pounds at top dead center CTDC) for the supercharged engine. But this is only 8 percent higher than the 2,324 pound loading of a similar normal-charged powerplant.
Obviously, this is small compared to combustion loading, which could easily quadruple pressure in the combustion chamber to 740 psi in the normal-charged engine and 800 psi in the boosted engine, resulting in total loading of 9,301 and 10;056 pounds for the two powerplants. Think of it: atmosphere of boost will double the horsepower but the supercharged engine's pressure is nonetheless only 8 percent higher
Luther Blisset said:
Suppose normal charged cranking compression yields 185 psi, and a turbo compressor adds an additional 15 psi of boost pressure. The total compression component of cylinder pressure would be 200 psi exerted against a 4-inch piston of 12.57 square inch crown area. Multiplying the 200 psi by 12.57 indicates a total compression loading on the connecting rod of 2,514 pounds at top dead center CTDC) for the supercharged engine. But this is only 8 percent higher than the 2,324 pound loading of a similar normal-charged powerplant.
Does this seem right to you?
No. A turbo doesn't add 15psi to the 185psi cranking pressure. It doubles the amount of air in the combustion chamber (2x 185psi). Unless I have misunderstood the explanation.Does this seem right to you?
There is a flaw in the logic. From first principles:
Power = energy delivered per unit time, or work done per unit time if you prefer.
Work done = force x distance. That's average force if it changes over the stroke.
So to deliver 2x the power you have to deliver 2x the (average) force, given a fixed distance. Now having skim-read their blurb it seems that because a turbo engine delivers power for a longer part of the power stroke then the peak loading is not 2x but less than that. OK. However the piston loading isn't just going to be 8% more, since doubling the air per stroke doubles the fuel that can be burned, which has to be the case if you are doubling power (assuming fixed efficiency).
I don't see how you are going to double work done as F(av) x d by only multiplying peak F by 1.08. That would mean that the elevated force would have to be delivered over a power stroke 1.85 times bigger, obviously without changing the actual physical engine stroke. I don't believe that without seeing some working.
Power = energy delivered per unit time, or work done per unit time if you prefer.
Work done = force x distance. That's average force if it changes over the stroke.
So to deliver 2x the power you have to deliver 2x the (average) force, given a fixed distance. Now having skim-read their blurb it seems that because a turbo engine delivers power for a longer part of the power stroke then the peak loading is not 2x but less than that. OK. However the piston loading isn't just going to be 8% more, since doubling the air per stroke doubles the fuel that can be burned, which has to be the case if you are doubling power (assuming fixed efficiency).
I don't see how you are going to double work done as F(av) x d by only multiplying peak F by 1.08. That would mean that the elevated force would have to be delivered over a power stroke 1.85 times bigger, obviously without changing the actual physical engine stroke. I don't believe that without seeing some working.
First of all, that was hard to read due to lack of paragraphs.
As to the logic, I don't think it's right. 15psi of boost means you put about twice as much air in to the cylinder as a NA engine, yes. But the whole bit about adding that 15 to 185 is nonsensical because by the time the piston gets to that part of the cycle, the boosted air has already been shoved in. Twice the gas = twice the peak pressure.
As to the logic, I don't think it's right. 15psi of boost means you put about twice as much air in to the cylinder as a NA engine, yes. But the whole bit about adding that 15 to 185 is nonsensical because by the time the piston gets to that part of the cycle, the boosted air has already been shoved in. Twice the gas = twice the peak pressure.
Paragraphing added.
It's an excerpt from a book on turbocharging that I'm reading and it made me scratch my head when I first read it.
This is the book in question.
http://www.amazon.co.uk/Turbocharging-Performance-...
It's a bit pants tbh
It's an excerpt from a book on turbocharging that I'm reading and it made me scratch my head when I first read it.
This is the book in question.
http://www.amazon.co.uk/Turbocharging-Performance-...
It's a bit pants tbh
The bit they got right:
In general, a pressure charged engine has a wide smooth cylinder pressure trace vs a short spiky trace for an N/A engine
The bit they got wrong:
Everything else.
Failing to understand that combustion loads are thermal as well as mechanical is a major error
Also note I used the words " in general" above, it isn't a given..
In general, a pressure charged engine has a wide smooth cylinder pressure trace vs a short spiky trace for an N/A engine
The bit they got wrong:
Everything else.
Failing to understand that combustion loads are thermal as well as mechanical is a major error
Also note I used the words " in general" above, it isn't a given..
battered said:
Is the above why turbo engines need to run lower compression?
Yes and no, they run lower compression to reduce cylinder pressures leading to lower cylinder temps. All done to reduce det (pre-ignition or pinking). This is why a good intercooler set up can make so much difference on a charged engine, the cooler the charge the higher the charge pressure can be and the more advance you can use before det occurs.That paragraph you highlight is utter gibberish. The N/A cranking pressure and turbocharger boost pressure have absolutely nothing to do with each other. I am beyond astonished that anyone writing a book about engines could think they do or that you could add the notional N/A cranking pressure of 185 psi mentioned to the arbitrary 15 psi of boost pressure and get any sort of meaningful result.
The cranking pressure for a given cam duration is almost entirely dependent on the compression ratio and is explained here.
https://web.archive.org/web/20110903073621/http://...
Obviously during a cranking pressure test the engine is not running, the turbo is not boosting and whatever the turbo waste gate pressure limit is set to has zero effect on the cranking test.
As to actual peak cylinder pressures in a turbocharged SI engine, these will be heavily influenced by the ignition advance and compression ratio, both of which will normally be considerably lower than for the N/A engine. For example if a 2v N/A engine has a CR of 9.5:1 and ignition advance of 32 degrees at peak torque rpm you might want those reduced to say 8.5:1 and 20 degrees of advance when turbocharged to 1 atmosphere of boost to avoid detonation.
These two factors reduce the peak cylinder pressure just after TDC to maybe not much more than the N/A engine had. However the cylinder pressure drops much more slowly as the piston descends than in the N/A case because there's twice the mass of gas in the cylinder. The average cylinder pressure over the full cycle is obviously approximately twice that of the N/A engine because the power output is approximately double also at 1 atmosphere of boost.
In the case of diesel engines which obviously don't have a spark ignition and need a certain minimum CR just to enable them to start then the peak and average cylinder pressures scale fairly closely with the inlet manifold pressure so an engine turbocharged to 1 atmosphere of boost might have close to twice the peak cylinder pressure of a similar CR N/A engine with a similar fuel injection angle. As the desired boost pressure rises then it eventually also becomes necessary to reduce peak cylinder pressure to avoid mechanical breakage as in the SI engine and hence both CR and the fuel injection angle before TDC might need to be reduced in much the same way that CR and spark advance need to be reduced in the turbocharged SI engine.
The cranking pressure for a given cam duration is almost entirely dependent on the compression ratio and is explained here.
https://web.archive.org/web/20110903073621/http://...
Obviously during a cranking pressure test the engine is not running, the turbo is not boosting and whatever the turbo waste gate pressure limit is set to has zero effect on the cranking test.
As to actual peak cylinder pressures in a turbocharged SI engine, these will be heavily influenced by the ignition advance and compression ratio, both of which will normally be considerably lower than for the N/A engine. For example if a 2v N/A engine has a CR of 9.5:1 and ignition advance of 32 degrees at peak torque rpm you might want those reduced to say 8.5:1 and 20 degrees of advance when turbocharged to 1 atmosphere of boost to avoid detonation.
These two factors reduce the peak cylinder pressure just after TDC to maybe not much more than the N/A engine had. However the cylinder pressure drops much more slowly as the piston descends than in the N/A case because there's twice the mass of gas in the cylinder. The average cylinder pressure over the full cycle is obviously approximately twice that of the N/A engine because the power output is approximately double also at 1 atmosphere of boost.
In the case of diesel engines which obviously don't have a spark ignition and need a certain minimum CR just to enable them to start then the peak and average cylinder pressures scale fairly closely with the inlet manifold pressure so an engine turbocharged to 1 atmosphere of boost might have close to twice the peak cylinder pressure of a similar CR N/A engine with a similar fuel injection angle. As the desired boost pressure rises then it eventually also becomes necessary to reduce peak cylinder pressure to avoid mechanical breakage as in the SI engine and hence both CR and the fuel injection angle before TDC might need to be reduced in much the same way that CR and spark advance need to be reduced in the turbocharged SI engine.
Luther Blisset said:
Suppose normal charged cranking compression yields 185 psi, and a turbo compressor adds an additional 15 psi of boost pressure. The total compression component of cylinder pressure would be 200 psi
This bit is complete bullshït!If at cranking speeds, you'll have say (round numbers here) 100% VE, then if you add 15psi boost, you'll get 200% VE - again, round numbers. Compress that (which is, in effect twice the) gas by the same CR that yields 185psi whilst cranking and you'll see more than double the original compression pressure...
I remember from when I used to build turbo charged dragbikes
The compression ratio of an engine is increased with boost, at 1 bar the cr is approx doubled so your 9/1 CR engine is now functioning like a 18/1 engine so forces are dramatically increased
http://fixjunk.com/solomiata/solomiata2/BoostCompR...
The compression ratio of an engine is increased with boost, at 1 bar the cr is approx doubled so your 9/1 CR engine is now functioning like a 18/1 engine so forces are dramatically increased
http://fixjunk.com/solomiata/solomiata2/BoostCompR...
liner33 said:
I remember from when I used to build turbo charged dragbikes
The compression ratio of an engine is increased with boost, at 1 bar the cr is approx doubled so your 9/1 CR engine is now functioning like a 18/1 engine so forces are dramatically increased
http://fixjunk.com/solomiata/solomiata2/BoostCompR...
Compression ratio is mechanically fixed, so no.The compression ratio of an engine is increased with boost, at 1 bar the cr is approx doubled so your 9/1 CR engine is now functioning like a 18/1 engine so forces are dramatically increased
http://fixjunk.com/solomiata/solomiata2/BoostCompR...
witko999 said:
Compression ratio is mechanically fixed, so no.
"Static" is fixed, "effective or boost" compression ratio rises with air pressure http://www.onallcylinders.com/2013/02/15/blower-ba...
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff