# Any guessy figure for combustion chamber pressure ?

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Hello.

At the explosion/expansion cycle of a plain internal combustion engine -no turbos/high performance/fancy fuels-  what pressures ranges are typical ?

60-100psi.

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Mechanical compression of a typical gasoline engine is about 10:1.
Combustion chamber temperatures for gasoline engines can reach about 2000o.

An educated guess would use gas laws to calculate the added increase in pressure due to combustion.

( keep in mind diesel engines have a higher mechanical compression for auto-ignition, and turbo/superchargers add to the mechanical compression )

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I thought the explosion would cause near 3000 psi...  🤔   (Subject is not about compression ratios nor temperature)

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4 hours ago, Externet said:

Hello.

At the explosion/expansion cycle of a plain internal combustion engine -no turbos/high performance/fancy fuels-  what pressures ranges are typical ?

Peak pressure of the order of 50bar. BMEP (brake mean effective pressure) of the order of 10bar . I think.

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Just some ballpark figures to give an indication how deep this rabbit hole goes:

For a 10:1 (isentropic) compression ratio of inlet air @ 300 K, 100 kPa, compression stroke ends at 754 K, 2,511 kPa. (~ 350 psig)

Theoretical adiabatic, isochoric, stoichiometric flame temperature of isooctane ~ 2,900 +754 - 300 ~ 3,354 K

By Pressure Law, Peak Theoretical Combustion Pressure = 2,511 * 3,354 / 754 = 11,170 kPa (~1,600 psig)

And yet car engines appear able to run for quite a while without the spontaneous catastrophic disassembly such figures may suggest. This observation is consistent with actual instantaneous P,T values not exceeding 50% of the above by very much.

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26 minutes ago, sethoflagos said:

Just some ballpark figures to give an indication how deep this rabbit hole goes:

For a 10:1 (isentropic) compression ratio of inlet air @ 300 K, 100 kPa, compression stroke ends at 754 K, 2,511 kPa. (~ 350 psig)

Theoretical adiabatic, isochoric, stoichiometric flame temperature of isooctane ~ 2,900 +754 - 300 ~ 3,354 K

By Pressure Law, Peak Theoretical Combustion Pressure = 2,511 * 3,354 / 754 = 11,170 kPa (~1,600 psig)

And yet car engines appear able to run for quite a while without the spontaneous catastrophic disassembly such figures may suggest. This observation is consistent with actual instantaneous P,T values not exceeding 50% of the above by very much.

Yup, a quick internet search reveals pressure graphs with peaks between 30 and 75bar. Latter probably highly turbocharged marine diesels.

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15 minutes ago, exchemist said:

Yup, a quick internet search reveals pressure graphs with peaks between 30 and 75bar. Latter probably highly turbocharged marine diesels.

You get very similar figures for gas turbines too (their inlet compressors typically run also at 25 bara). Whatever the technology, it's always limited by the available metallurgy.

Edited by sethoflagos
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1 hour ago, sethoflagos said:

You get very similar figures for gas turbines too (their inlet compressors typically run also at 25 bara). Whatever the technology, it's always limited by the available metallurgy.

Actually I may be getting out of date, having been out of the business for over a decade now. Take a look at this brochure for Wärtsilä's latest RT Flex:

The graph of Pmax starts at 90bar and goes up to 140! But these turbocharged 2-stroke crosshead engines are really optimised for burning residual fuel oil, with thermal efficiency >50% and, most importantly, where weight is not a constraint. So a bit different from the IC engines the OP has in mind.

I remember walking along the upper gangway of a 12 cylinder in-line engine of this type at a power station in Macau, with 4 turbos screaming (ear defenders on), and noticing the individual cylinder heads actually move fractionally up and down with each power stroke. The metal was visibly stretching with each stroke. But these engines had a stroke of ~4 metres and a bore of 90cm, so a stretch of a couple of mm perhaps should not have been a surprise. Running speed was 100 rpm and the generator they drove was like a disc, with poles all around the periphery, to give 50Hz.  Pmax has evidently gone up even further since those days.

Edited by exchemist
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18 minutes ago, exchemist said:

The graph of Pmax starts at 90bar and goes up to 140! But these turbocharged 2-stroke crosshead engines are really optimised for burning residual fuel oil, with thermal efficiency >50% and, most importantly, where weight is not a constraint. So a bit different from the IC engines the OP has in mind.

RFO is a real slow burn fuel so they're probably close to isothermal combustion through most of the power stroke. Hence they can achieve the high cylinder pressures and consequent high torque output without incurring extreme combustion temperatures. Similar to how power stations limit NOx with staged combustion.

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52 minutes ago, sethoflagos said:

RFO is a real slow burn fuel so they're probably close to isothermal combustion through most of the power stroke. Hence they can achieve the high cylinder pressures and consequent high torque output without incurring extreme combustion temperatures. Similar to how power stations limit NOx with staged combustion.

Indeed, hence the advantage for ships of the crosshead engine, which runs at very low rpm and can have a far longer stroke than a trunk-piston engine. Plenty of time for poor fuel to burn out and able to convert as much expansion into work as possible. Having said that, medium speed engines (400-900 rpm) also run happily on RFO if designed for it, as Wärtsilä in particular has always been keen to point out. They were a medium speed manufacturer for decades, until they bought out Sulzer, the designers of the original RT series, in order to give themselves a stake in the crosshead engine business. (Their big rivals are MAN B&W.  Between them these two groups designed 90% of crosshead engines worldwide, at the time I retired in 2011.)

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---> ~8 Mpa :

---> And higher pressures :

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