archive.commercialmotor.com/arti … d-imep-and
ho, ho, ho. 
ho, ho, ho.
If the peak cylinder pressure on the power stroke virtually matches the compression pressure where do we get the net work done at the flywheel from.There would be little if any left because almost all the force provided by the power stroke would be taken by the compression stroke.
If it’s supposedly ‘constant pressure’ for all how do you explain the difference in the 50 lbft per litre NA Eagle v 100 lbft + per litre of the forced induction Eagle without a corresponding doubling of cylinder pressure taking into account all else being equal piston area and leverage.
In the case of the TL 12 it’s a 43% specific torque deficit to start with and all else isn’t equal in the form of its 7% leverage deficit and its 2% piston area deficit v Eagle’s leverage advantage.
Less than 1,000 psi let alone 250 psi peak cylinder pressure for the Eagle let alone the TL12 yeah right.
It’s total bs to suggest that there is any similarity between compression pressure
and peak cylinder pressure after ignition of the resulting fuel/air charge.The latter is measured in Mpa type figures not bar.Variable dynamic compression forced induction type pressures obviously being on a different level to NA in that regard.
Reducing those loads by substituting force with leverage obviously helps.
ho, ho, ho.
Superb find.
What a wonderful era we have left behind, when the layman could take an interest in engineering, and receive an informed reply to his questions in his favourite haulage paper.
Edit- the reply he gets sounds a bit whiffy, having read it. 
If combustion pressure is no higher than compression pressure, then how does the engine self-propel?
Carryfast:
ho, ho, ho.
If the peak cylinder pressure on the power stroke virtually matches the compression pressure where do we get the net work done at the flywheel from.There would be little if any left because almost all the force provided by the power stroke would be taken by the compression stroke.
If it’s supposedly ‘constant pressure’ for all how do you explain the …Blah…
Blimey- a sentence of blinding insight, to precede the inevitable.
The above CF post appeared at about 8:30, so he’s still up in front of the beak.
I was wondering why they were calling the diesel a “constant pressure” engine, when those graphs that I posted earlier clearly show a sharp peak just after TDC. I reckon if you mirror the compression side of the graph about the TDC ordinate, it would end up sort-of parallel to the curve on the combustion side. A rough line scribed through each, both lines parallel, would give the IMEP. I think. 
[zb]
anorak:
What a wonderful era we have left behind, when the layman could take an interest in engineering, and receive an informed reply to his questions in his favourite haulage paper.
Edit- the reply he gets sounds a bit whiffy, having read it.
If power stroke pressure isn’t massively higher than the compression pressure how is there any significant net work left at the flywheel to propel a 32 tonner up a long hill and what’s the point of burning all that diesel.
Oh wait all that compression pressure is just returned to engine but crucially added to by the combustion pressure which is how a Jake Brake works by releasing the compression pressure instead.
All the net work available at the flywheel is by definition net of compression because the compression pressure had to be subtracted first from the remaining net figure available at the flywheel.
Although a significant leverage advantage in the form of stroke obviously helps in what is available at the flywheel.
While more piston area will also extract more work out of a given cylinder pressure but unlike more leverage it will obviously hammer the end bearings.
If you’ve also got a piston area v leverage deficit you’ve got the worst of all worlds of more cylinder pressure required on your head fastenings and hammering your end bearings with more force.
You’re learning but it’s a long process.
[zb]
anorak:
The above CF post appeared at about 8:30, so he’s still up in front of the beak.I was wondering why they were calling the diesel a “constant pressure” engine, when those graphs that I posted earlier clearly show a sharp peak just after TDC. I reckon if you mirror the compression side of the graph about the TDC ordinate, it would end up sort-of parallel to the curve on the combustion side. A rough line scribed through each, both lines parallel, would give the IMEP. I think.
All of the work at the flywheel is the result of the ‘difference’ between the compression pressure and combustion pressure, which includes the compression pressure.
Combined with piston area and leverage.
As for forced induction that multiplies the less pressure in for more pressure out differential thereby creating much more energy out than that used to compress the charge.
How does that make it a ‘constant pressure’ engine.
It’s actually all about a ( massive ) net combustion over pressure v compression pressure.
Carryfast:
…Again you’ve forgotten, more like don’t even know, that the BMEP figure, being based on specific torque output, also includes and is based on leverage multiplication and piston area.Not just cylinder pressure.It’s also just an average not a peak…
“Based on” means nothing. “Derived from” is correct. Have you asked those kids what a derivation is yet? It does not matter if the pressure is calculated from other variables- once it is expressed as a pressure, that is what it is- the average pressure on the piston, during the combustion stroke, net of any losses. It does not matter whether the bull has been grazing on grass, beet or feed- the result is the same.
[zb]
anorak:Carryfast:
…Again you’ve forgotten, more like don’t even know, that the BMEP figure, being based on specific torque output, also includes and is based on leverage multiplication and piston area.Not just cylinder pressure.It’s also just an average not a peak…“Based on” means nothing. “Derived from” is correct. Have you asked those kids what a derivation is yet? It does not matter if the pressure is calculated from other variables- once it is expressed as a pressure, that is what it is- the average pressure on the piston, during the combustion stroke, net of any losses. It does not matter whether the bull has been grazing on grass, beet or feed- the result is the same.
Are you prepared to deny that the 100lbft per litre doesn’t = 246.4 psi BMEP yet.Thought not.
Therefore it’s just specific torque x a constant just like power is torque x engine speed divided by a constant.
There is no ‘average pressure’ on the combustion stroke just as 10 people didn’t all die at an ‘average’ age of 75.It’s the peak pressure that matters.
What there is, is a massive pressure on the piston on the combustion stroke and an equally massive pressure on the head fastenings and end bearings.Except they don’t have the luxury of the piston area and the head area to spread the load.
You can reduce the cylinder pressure for the equivalent output by using a larger piston area or more leverage.Thereby reducing the load on your head fastenings.
Using a larger piston area won’t reduce the loads on your end bearings.
Using more leverage will.
Oh wait the TL12 has a 43% Specific torque therefore BMEP deficit.
It has a 7% Leverage deficit
and a 2% piston area deficit
v the Eagle.
That’s why the Eagle could produce more horsepower at 1,200 rpm than the TL12 could at peak power at 2,000 rpm.
The reason the Eagle went on to develop more power and torque than the original version is that money was spent on developing it and it was developed by an ex AEC man that designed the TL12 . He obviously knew that Leyland had no money to go any further with his version so he left . If the same had happened at Rolls the Eagle would have been exactly the same as the original thirsty and unreliable . I have read on another site that the tooling that was used to make the TL12 was shot and when the engines ever came in for a rebuild at main dealers they could do a better job with their better tooling. BL was skint the 1979 TL12 was exactly the same as the 1973 TL12 , could you say the same for the Eagle , i will answer that one for you … no . It had been redesigned by an AEC man .
Im 3/4 of the way through Edwards book and its a miracle the T45 ever got off the ground
Peak Cylinder Pressure and compression pressure comparison is not the easiest thing to understand. Taking a ratio of 16 :1 we would assume that in a naturally aspirated engine that the pressure at the point the inlet valve closes is at atmospheric pressure 14.7 psi so 14.7 x16 = 235.2 psi. However that is not what one will record with a diesel compression tester. Carrying out such a test one is looking to achieve figures around 400 - 450 psi. However temperature and rpm come into play. A compression test is usually carried out either to daignose which cylinder(s) are causing an issue or why an engine is hard to start. Testing with the engine running at speed and full operating temperature is not often done for safety reasons. The closest one can get is an engine which has been run but is being motored by the starter motor. This is what one is advised to do but does not pin down a cold start problem since the readings will be higher. Stone cold at an ambient single figure Centigrade one can expect to have to accept 350+ psi as the barest minimum for a worn but not worn out engine.
However as the first image shows (at an albeit higher ambient) the theoretical pressure is significantly greater. The second image of a speed/ pressure/ crank angle with a fixed injection spill point shows a much higher peak cylinder pressure and identifies how combustion develops.
Click or double click and these will be the correct way up.
Don’t ask me how to deal with the exponent 0.4 or how to use logarithms, it’s 50 years since I did this and I struggled with it then. Trying to come back without having spent a similar amount of time to back then studying the long-forgotten necessary maths, algebra and physics calculations and rules is challenging. I’ve used imperial examples since the units have some meaning to me meganewtonpascals per radianjoule is sheer gibberish to me they may be related if I multiply by .98 or they may not, but where to put the decimal point in a six figure number and visualise it in a unit which means nothing is not.
cav551:
Peak Cylinder Pressure and compression pressure comparison is not the easiest thing to understand. Taking a ratio of 16 :1 we would assume that in a naturally aspirated engine that the pressure at the point the inlet valve closes is at atmospheric pressure 14.7 psi so 14.7 x16 = 235.2 psi. However that is not what one will record with a diesel compression tester. Carrying out such a test one is looking to achieve figures around 400 - 450 psi. However temperature and rpm come into play. A compression test is usually carried out either to daignose which cylinder(s) are causing an issue or why an engine is hard to start. Testing with the engine running at speed and full operating temperature is not often done for safety reasons. The closest one can get is an engine which has been run but is being motored by the starter motor. This is what one is advised to do but does not pin down a cold start problem since the readings will be higher. Stone cold at an ambient single figure Centigrade one can expect to have to accept 350+ psi as the barest minimum for a worn but not worn out engine.However as the first image shows (at an albeit higher ambient) the theoretical pressure is significantly greater. The second image of a speed/ pressure/ crank angle with a fixed injection spill point shows a much higher peak cylinder pressure and identifies how combustion develops.
Click or double click and these will be the correct way up.
Don’t ask me how to deal with the exponent 0.4 or how to use logarithms, it’s 50 years since I did this and I struggled with it then. Trying to come back without having spent a similar amount of time to back then studying the long-forgotten necessary maths, algebra and physics calculations and rules is challenging. I’ve used imperial examples since the units have some meaning to me meganewtonpascals per radianjoule is sheer gibberish to me they may be related if I multiply by .98 or they may not, but where to put the decimal point in a six figure number and visualise it in a unit which means nothing is not.
Compression pressure isn’t the same thing as compression pressure + and including the pressure created by combustion.Which is the peak cylinder pressure on the power stroke at peak torque under load and you don’t have a tester strong enough to measure that.That will be measured in 10’s of Mpa not psi.Especially when forced induction is added to the compression side of the equation.
That’s what is then transferred to the head fastenings.
43% specific torque deficit with a 7% leverage deficit is a lot of extra pressure factoring in your 2% piston area v leverage deficit.
Your end bearings are going to get hammered by taking the full 43% torque deficit x 7% leverage deficit regardless.
ramone:
The reason the Eagle went on to develop more power and torque than the original version is that money was spent on developing it and it was developed by an ex AEC man that designed the TL12 . He obviously knew that Leyland had no money to go any further with his version so he left . If the same had happened at Rolls the Eagle would have been exactly the same as the original thirsty and unreliable . I have read on another site that the tooling that was used to make the TL12 was shot and when the engines ever came in for a rebuild at main dealers they could do a better job with their better tooling. BL was skint the 1979 TL12 was exactly the same as the 1973 TL12 , could you say the same for the Eagle , i will answer that one for you … no . It had been redesigned by an AEC man .
Im 3/4 of the way through Edwards book and its a miracle the T45 ever got off the ground
There’s no point in trying to develop something with the wrong basic architecture.
No amount of money and development would have fixed the TL12’s leverage deficit and piston area v leverage deficit.
It might be possible with modern materials.
But even then why bother going that close to the edge of the envelope when you can just make something with a much lower bore stroke ratio to substitute force with leverage.You know like the MX13 among numerous other examples of that correct design thinking.
[zb]
anorak:Carryfast:
…Again you’ve forgotten, more like don’t even know, that the BMEP figure, being based on specific torque output, also includes and is based on leverage multiplication and piston area.Not just cylinder pressure.It’s also just an average not a peak…“Based on” means nothing. “Derived from” is correct. Have you asked those kids what a derivation is yet? It does not matter if the pressure is calculated from other variables- once it is expressed as a pressure, that is what it is- the average pressure on the piston, during the combustion stroke, net of any losses. It does not matter whether the bull has been grazing on grass, beet or feed- the result is the same.
How can it be anything other than just another way of expressing specific torque.
When the figure you end up with, or start with in the case of using the 2.464 constant, is just specific torque and specific torque is a function of cylinder pressure, leverage and piston area combined.
Not just cylinder pressure.
By definition you can’t possibly derive or arrive at a cylinder pressure figure by reference to a figure derived from force x leverage and the resulting torque figure at the flywheel.
In this case the TL12 being short literally on the leverage side of the equation.Too short.
No need to use logs nowadays- the calculator does that donkey-work.
My slide rule still works. Not to twenty million significant figures may be, but good enough for many uses.
If you gentlemen are interested there is a new oil filtration system over on the Pro Forum.
Franglais:
My slide rule still works. Not to twenty million significant figures may be, but good enough for many uses.
If you gentlemen are interested there is a new oil filtration system over on the Pro Forum.
The argument here has more to do with total disarray and confusion regarding the definition of ‘BMEP’ and its use.
( The same thing as Specific torque to compare different engines ).
Torque is a function of force on the piston/con rod x leverage.
The more leverage you’ve got the less force you’ll need.
The only relevance of ‘BMEP’ in this case is the TL12’s deficit in it v the Eagle.Which is its correct/only use as a comparator of different engines not cylinder pressure.
Bearing in mind that figure by definition is ‘based’ solely on Torque output which means force x distance and therefore that the Eagle’s 7% leverage advantage and TL12’s piston area deficit v leverage, will need to be factored into matching that figure with the TL12 in terms of end bearing loads and head fastening loads.
The TL12 is always going to be under 7% more pressure on its end bearings and 2% more pressure on its head fastenings for the equivalent specific torque output/BMEP figure v the Eagle.
Carryfast:
ramone:
The reason the Eagle went on to develop more power and torque than the original version is that money was spent on developing it and it was developed by an ex AEC man that designed the TL12 . He obviously knew that Leyland had no money to go any further with his version so he left . If the same had happened at Rolls the Eagle would have been exactly the same as the original thirsty and unreliable . I have read on another site that the tooling that was used to make the TL12 was shot and when the engines ever came in for a rebuild at main dealers they could do a better job with their better tooling. BL was skint the 1979 TL12 was exactly the same as the 1973 TL12 , could you say the same for the Eagle , i will answer that one for you … no . It had been redesigned by an AEC man .
Im 3/4 of the way through Edwards book and its a miracle the T45 ever got off the groundThere’s no point in trying to develop something with the wrong basic architecture.
No amount of money and development would have fixed the TL12’s leverage deficit and piston area v leverage deficit.
It might be possible with modern materials.
But even then why bother going that close to the edge of the envelope when you can just make something with a much lower bore stroke ratio to substitute force with leverage.You know like the MX13 among numerous other examples of that correct design thinking.
You are wrong again but we will never know the true potential
cav551:
Peak Cylinder Pressure and compression pressure comparison is not the easiest thing to understand. Taking a ratio of 16 :1 we would assume that in a naturally aspirated engine that the pressure at the point the inlet valve closes is at atmospheric pressure 14.7 psi so 14.7 x16 = 235.2 psi. However that is not what one will record with a diesel compression tester. Carrying out such a test one is looking to achieve figures around 400 - 450 psi. …
Iso thermal compression would give the 235psi you expect. Adiabatic compression, when there is no time for the heat to escape, when the starter motor is spinning the engine at speed, gives the higher figure. I think. γ on the book page is the ratio of the two specific heat capacities of the gas. 1.4 sounds familiar, so your 400/235 is not far off what I would expect. Donkey’s years since I had to use thermodynamics. Never seen it done in inches and pounds! 
cav551:
Peak Cylinder Pressure and compression pressure comparison is not the easiest thing to understand. Taking a ratio of 16 :1 we would assume that in a naturally aspirated engine that the pressure at the point the inlet valve closes is at atmospheric pressure 14.7 psi so 14.7 x16 = 235.2 psi. However that is not what one will record with a diesel compression tester. Carrying out such a test one is looking to achieve figures around 400 - 450 psi. However temperature and rpm come into play. A compression test is usually carried out either to daignose which cylinder(s) are causing an issue or why an engine is hard to start. Testing with the engine running at speed and full operating temperature is not often done for safety reasons. The closest one can get is an engine which has been run but is being motored by the starter motor. This is what one is advised to do but does not pin down a cold start problem since the readings will be higher. Stone cold at an ambient single figure Centigrade one can expect to have to accept 350+ psi as the barest minimum for a worn but not worn out engine.However as the first image shows (at an albeit higher ambient) the theoretical pressure is significantly greater. The second image of a speed/ pressure/ crank angle with a fixed injection spill point shows a much higher peak cylinder pressure and identifies how combustion develops…
Carryfast:
Compression pressure isn’t the same thing as compression pressure + and including the pressure created by combustion.Which is the peak cylinder pressure on the power stroke at peak torque under load and you don’t have a tester strong enough to measure that.That will be measured in 10’s of Mpa not psi.Especially when forced induction is added to the compression side of the equation.
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At what pressure- or abstract quantity which is not pressure, but is measured in the units of pressure, for no reason that we can begin to explain- do we convert from psi to MPa?
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Is that the same non-pressure that we do not possess a pressure gauge strong enough to measure, or some other abstract value? Given that, further up the page, we read a link which showed peak combustion pressures of no more than 3xBMEP, these cut-off points must be clustered within a close range of pressures. I’ll guess at somewhere between 200psi and err… 1.0MPa.
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That same link referred to a peak combustion pressure of 75 bar, which is 7.5 MPa. Which engines have peak pressures in the 10’s of MPa? I’ll have a guess- the BMC C Series?
Carryfast:
That’s what is then transferred to the head fastenings.Chorus All together now,
43% specific torque deficit with a 7% leverage deficit is a lot of extra pressure factoring in your 2% piston area v leverage deficit.
Your end bearings are going to get hammered by taking the full 43% torque deficit x 7% leverage deficit regardless.Begin 256th verse.