Peak Cylinder Pressure for a Diesel Engine
Peak Cylinder Pressure for a Diesel Engine
(OP)
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
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





RE: Peak Cylinder Pressure for a Diesel Engine
- 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
RE: Peak Cylinder Pressure for a Diesel Engine
RE: Peak Cylinder Pressure for a Diesel Engine
Regards
Pat
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RE: Peak Cylinder Pressure for a Diesel 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
RE: Peak Cylinder Pressure for a Diesel Engine
- Steve
RE: Peak Cylinder Pressure for a Diesel Engine
RE: Peak Cylinder Pressure for a Diesel Engine
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
RE: Peak Cylinder Pressure for a Diesel Engine
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).
RE: Peak Cylinder Pressure for a Diesel Engine
That is so helpful to me. I appreciate your well thought out and detailed comments.
Best Regards
RE: Peak Cylinder Pressure for a Diesel Engine
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Peak Cylinder Pressure for a Diesel Engine
RE: Peak Cylinder Pressure for a Diesel Engine
RE: Peak Cylinder Pressure for a Diesel Engine
RE: Peak Cylinder Pressure for a Diesel Engine
MS