Peak Pressure Inside the Internal Combustion Engine 2

[razarafique]

I have to design a connecting rod with minimum weight. For this I selected the Honda GX25 engine. I want to know the peak gas pressure on the piston so that I can calculate force on the piston and hence maximum force on the connecting rod. Does anybody know how I calculate the peak pressure or how pressure varies between TDC and BDC.

 

[gruntguru]

Perhaps 100 bar. Don't forget to consider inertial forces also - both axial and transverse.

je suis charlie

 

[GregLocock]

You are looking for a cylinder pressure diagram. They can be measured. They can be calculated but I don't know how accurately.

Cheers

Greg Locock


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[razarafique]

@GregLocock Yes exactly I need pressure diagram at 7000 rpm. Can you tell me how it can be calculated?

 

[GregLocock]

No. You can google as well as I can. However, I would point out that an accurate calculation would be more time consuming than designing a conrod.

Cheers

Greg Locock


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[pontiacjack]

Be advised: that inertial forces on the rod (of total reciprocating mass- piston, etc.) will typically exceed any combustion-related forces at the engine speed you're concerned about. One quick example is an engine I'm building- with a 3.62" stroke, the acceleration at TDC at 10,000 RPM is 8,400 g's.

 

[GregLocock]

pj and gg, yes exactly. Gas forces are pretty benign in high speed spark ignition engines.

Cheers

Greg Locock


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[uaetech]

Remember doing something like this a long time ago...in school. There was a book that looked very good at that time. High Speed Combustion Engine Design (?) by P M Heldt. If you are an engineering student, try that.

 

[IRstuff]

There are lots of examples of cylinder pressure graphs:

https://www.google.com/search?q=cyl...2&ved=0ahUKEwjaru_flsfQAhVBIGMKHWObAKIQsAQIGg

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[SwinnyGG]

A cylinder pressure graph is nice, but remember that inertial forces on the piston increase with the square of RPM, while peak cylinder pressure is NOT a function of speed- meaning it is going to vary within a comparatively small range.

Point is, at peak engine speed on any modern gasoline engine, inertial forces on the connecting rod far exceed forces due to cylinder pressure.

 

[durablack2]

Here is a good start to conn rod forces :

http://emweb.unl.edu/Mechanics-Page...ion and Compression in Connecting Rods VI.htm

As many said, inertial forces are most important in conn rod design. Conn rod forces are largest when the piston reaches tdc on exhaust stroke (all masses are going up, and then come to an abrupt stop, causing extreme tensile forces).

 

[Screwman1]

The Bosch book contains info about this subject- your university library should have access to it.

 

[TugboatEng]

100 BAR seems high for peak pressure on a gasoline engine. I would put it closer to 600 psi with a MEP of around 150 psi. I worked with a diesel engine that had cylinder relief valves once, I wish I could remember what they were set for but it certainly wasn't more than 1400 psi. That engine had a particularly high BMEP for its age at 250 psi.

 

[durablack2]

100 Bar is not uncommon nowadays. 130 bar peak is reached by some manufactures in gasoline engines (with turbo and direct injection). BMEP's in the range of 25 bar is common now as well. Downsizing is pushing the limits of engines today which leads to this.

I wrote the above without looking at what a GX25 is.. Soon to find out that it is a lawnmower or weedeater engine. So considering that, I don't think there are many lawnmower engines that make these kinds of peak pressures, if so I want one...

 

[Lou Scannon]

BMEP's in the range of 25 bar is common now as well.

I'm out of touch lately with the light duty end of the engine business. Could you kindly provide examples with links to tech articles?

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[durablack2]

I'm having some difficulty finding publicly available documents, but there engine dyno test reports and data performed at EPA in the link below (your tax dollars being put to some interesting work for once )

https://www.epa.gov/regulations-emi...greenhouse-gas-ghg-technology#engine-mapping.

Here you can see the F-150 2.3L EcoBoost making ~23 bar bmep

https://www.epa.gov/sites/productio...ine-tier2-fuel-mapping-process-2016-06-20.pdf

There is also some raw test data in the "Test Data" section of the first link.

Of course these may not be the highest on the market nowadays, but you can form your own conclusions about how turbocharged and naturally aspirated engines are running in terms of peak torque, bsfc, bte in today's world. In addition to the vehicles in the articles in the links, calculting bmep from GM's Cadillac ATS rated at 295 lb-ft torque equates to bmep of 25 bar; Ford Focus RS rated at 350 ft-lb equates to nearly 26 bar.

 

[cerial]

In the link below is a basic general page giving a general formula for finding forces on the bearings during combustion. You can use it to easily get in the ballpark of the forces at X compression at X ratio kind of deal and even get an idea of expected power output.

http://mb-soft.com/public2/engine.html

Best of luck reducing your rotating mass on those connecting rods.