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Fordz (Automotive)
28 Aug 12 18:05
Can someone help me with Peak cylinder pressure in SI and CI engines.
From some engine data I have it appears that in a SI engine peak cylinder pressure can vary from 10 degrees after TDC to 27 degrees after TDC. The factors that seem to affect where peak pressure occurs are the throttle position or how full the cylinder is and I suppose if it is full throttle what the compression ratio is. I suppose ignition timing could also affect where peak pressure occurs.

1. What I want to have help with is, under normal highway driving conditions where would cylinder peak pressure occur for the crankshaft position in a SI engine.
2. Where does peak pressure from the crank position occur in a CI engine over the varying fuel injection rates from normal highway fuel settings to full fuel loading or as we say full throttle.
Thanks Regards Fordz
hemi (Automotive)
28 Aug 12 20:07
There are myriad factors that influence the location of peak pressure. Speaking from a performance development point of view, the target for peak pressure angle in an Otto engine is determined by, in part:
  • engine out NOX target
  • available engine strength (assuming development on an existing engine platform) - this is because, the more advanced the peak pressure angle, the higher the peak pressure, everything else being equal
  • target engine longevity/reliability
  • target performance/efficiency (of course)
  • maximum permissible exhaust temperatures (i.e. as constrained by exhaust valve, exhaust manifold, turbine/turbine housing if applicable, & catalyst brick material limits)
The above is a partial list of the constraints that the performance engineer is striving to simultaneously maintain within targeted values. The angle of peak pressure is an intermediate variable that is useful to target and monitor in the effort to meet these constraints. Underlying the peak pressure location is the flame speed (itself resultant of many intertwined factors). In order to achieve all the targeted parameters simultaneously, the performance engineer manipulates some or all of the following independent variables, as well as others not listed:
  • compression ratio
  • allowed fuel quality (e.g. regular or premium)
  • spark timing
  • EGR rate (can be internal or external)
  • valve events
  • engine cooling parameters
  • intake manifold mixture temperature
  • spark plug characteristics
  • air/fuel ratio
  • air mass flow into the engine
There is an analogous but different complex answer for diesel cycle engines which I will endeavour to provide soon if someone doesn't beat me to it.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

Fordz (Automotive)
28 Aug 12 20:28
Thanks. But what would be a realistic crank angle peak pressure point for most ordinary car engines with say 14.5% throttle setting or normal cruising speed.
patprimmer (Publican)
29 Aug 12 4:39
This is starting to sound very much like a homework question in that you want a specific answer rather than an understanding of the situation. Why do you ask?

Regards
Pat
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Fordz (Automotive)
29 Aug 12 8:26
I have designed an engine that has been tested independently by a leading University. Detroit sent me data they wanted me to use. It showed that at 14.2% throttle setting they were achieving peak pressure at 15 deg after TDC. They then set the throttle at 100% and it gave a peak cylinder pressure at 27 deg on the crank. The same engine.
We got a 45% efficiency gain over the Detroit test engine when the peak pressure was at 15 deg after TDC. But at 27 deg after TDC our efficiency gain dropped to 30% improvement on the Detroit test engine.

I understand what others have tried to teach me, but I deal with this stuff every day. I simply don't know where an idea peak cylinder pressure for a diesel would be under varying fuel settings and a target peak cylinder pressure for crank angle. Same with an SI engine. Where is the ideal cylinder pressure peak for the crank position. Regards
SomptingGuy (Automotive)
29 Aug 12 8:29
Why are you using a throttle on you diesel?

- Steve

140Airpower (Automotive)
29 Aug 12 13:22
Fordz, with the figures you are giving, you should be telling everyone else where peak pressure should occur. If I'm understanding what you are saying (looking at different interpretations) you are breaking efficiency barriers.
Fordz (Automotive)
29 Aug 12 14:30
Yes it does sound like a lot of Bologna. But it is what it is. I think I need to rephrase my question. I will keep it as broad as I can. Can anyone tell me if CI engines in general under all the varying specs and running conditions achieve cylinder peak pressure earlier or later for crank angle than a SI engine. I am not concerned with things like BMEP IMEP BSFC Throttle Pos Fuel AIRMASS BSHC BSCO BSNOX ELS_IMEPC CA50_EAM ELS_IMEPM ELS_NMEP ELS_PMEP FMEP SPINDT LHV BSCO BSCO2 BSFC BSHC BSNOX BSCNOX and other ditty things.

Heavy CI engine at very low RPM or small high speed diesels. Is there a difference where peak cylinder pressure occurs for crank angle and in general is it earlier or later on the crank angle.
Thanks for all your comments to date. It all helps. Regards
ivymike (Mechanical)
29 Aug 12 14:49
on a particular "heavy" engine at 900rpm, peak cylinder pressure was virtually unchanged vs load based on some indimeter data that I have. The (electronic) timing was retarded significantly at idle, maintained constant (and much advanced vs. idle) at 25, 50, 75% load, and was progressively more retarded at 90, 100, 110% load until at 110% the timing was roughly equivalent to idle.

Cylinder pressure measurements painted a different picture:
Load %, timing of peak
10%: 3-9deg ATDC (broad, flat "peak")
25%: 8-15deg ATDC (broad, flat "peak")
50%: 14-15deg ATDC
75%: 9-11deg ATDC
100%: 10-12deg ATDC
110%: 10-11deg ATDC

NOx, smoke, and PCP limits had to be met, so without those limits perhaps better efficiency could have been achieved with more advance, faster cams, and larger-orifice injectors in the higher-load cases (boost varies significantly with load). The engine achieved about 195 g/kW-hr fuel consumption at 100% load and was at about 300 g/kW-hr at 10% load (a 35% improvement if you will).

Fordz (Automotive)
29 Aug 12 16:59
Thanks IvyMike,
That is so helpful to me. I appreciate your well thought out and detailed comments.
Best Regards
hemi (Automotive)
29 Aug 12 19:27
The limited amount of cylinder pressure data I've seen for fairly late model, emissions compliant diesel engines suggests that peak cylinder pressure is severely reduced and significantly retarded from what would be optimum, from a thermal efficiency and power density point of view, in order to limit engine out NOX. But maybe things have moved on since my last exposure.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

Fordz (Automotive)
29 Aug 12 19:50
Thanks Hemi. This is what I was hoping someone could help me with. Regards
140Airpower (Automotive)
3 Sep 12 8:46
I think the NOX problem with Diesels is a hard barrier to what you would otherwise like to do with respect to peak pressures. BTW, water injection is powerful in reducing NOX in Diesels.
Fordz (Automotive)
3 Sep 12 17:46
Not a lot of interest in water injection from my experience. Fortunately we have found a way to move peak cylinder pressure to a more advanced crank angle without the NOX problem. Thanks for your comment. Regards
mattsooty (Automotive)
3 Sep 12 18:20
I don't believe there is any wizardry involved nowadays, with Common Rail Direct Injection diesel engines. The use of pilot injection events reduces both NOx & combustion noise and is established technology. My understanding is that rate shaped injection is the current focus but fuel metering remains a challenge.

MS

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