Peak Cylinder Pressures 1

[ip308]

Hi guys,


Just a quick question,

was wodnering what kind of figures would be reasonable to assume for peak and mean pressures of the power strokes in a high BMEP engine

A bit of searching, ive find figures of around 50 bar for peak pressures, is this sensible?


Thanks

 

[ivymike]

oh, no, definitely more like 200bar. Of course, it kinda depends on whether you're talking about a lawnmower or a truck, whether that's a continuous power level or a recreational one, what fuel, etc.

 

[GregLocock]

or SI vs CI, or .....

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ip308]

Hi, sorry forgot to mention.

Its an SI engine, 1.4 turbocharged.

Basically, trying to do some calculations for the conrod and trying to get some figures to do so.


thanks

 

[JSteve2]

K - rough rough estimate. Depends on timing, exact compression ratios, etc. say 5 psi boost for SI engine, 8.5 compression ratio, that's a baseline 170 psi. Post-combustion, maybe a factor of 5 increase to 850 psi. So 60 bar give or take. Can be much higher (mainly through a higher than 5:1 increase) depending on many factors. Of course on a CI rather than SI engine the peak cylinder pressure will be much higher.

 

[Lou Scannon]

I've done performance development of high BMEP (~19 Bar) turbocharged heavy duty natural gas engines. Our design point for peak cylinder pressure was in the 1500-2000 psi range. Incipient knock would bring pressures into the 2000-2500 psi range. In heavy knock I've seen well over 3000 psi.
High BMEP heavy duty diesel engines, with their steel piston crowns, are operated in the 2500 psi range. They do not have the same exposure to knock, of course.

JSteve your analysis does not take into account adiabatic temperature increase during compression.

 

[JSteve2]

hemi,
It doesn't take into account much of anything, but that should be included in my SWAG 5:1 pressure ratio due to combustion. I would be really surprised to see 15:1 as your numbers suggest on the high end (although only 10:1 at the lower end). I suspect our numbers are not actually in conflict, because I was thinking gasoline. What is the compression ratio and turbo boost on your natural gas engine?

A diesel could be much higher than 2500 psi: e.g. 50 psi boost plus a 16:1 compression ratio gets you to 1035 psi. My 5:1 spark estimate is probably 4:1 or even lower in a diesel, so 4000 psi (280-ish bar) is a number I would believe on the high end, although a typical operating point would be much lower.

 

[JSteve2]

... and just to avoid endless fruitless debate on the topic you are quite correct about adiabatic heat during compression. Rather than stacking up a series of fudge factors to make it work (adiabatic heat during compression factor, heat losses to environment, etc.) I just baked it all into a single fudge factor that should be pretty close subject to timing (injection or ignition, and valves).

 

[ivymike]

4000psi PCP on a diesel? Bologna. Not even close, except perhaps on an experimental rig.

2500psi is about typical for 2007 and earlier (but recent) on-highway trucks (7L to 15L). Go back before 2002 or so, and 2500 would be the exception (2250 would be more common). In the near future, 3000psi might be doable for continuous ratings (it's currently done only for recreational low-life-expectancy ratings, AFAIK). I expect that some manufacturers will be upping the pressure a bit in the future, based on all the fancy head and block materials they've been advertising lately.

4000psi? Haven't even heard that being floated as a research goal.

I seem to recall seeing cylinder pressure traces for a marine gasoline (pleasurecraft) engine with peaks at about 110bar, for a naturally aspirated configuration. It's been a while, though. Wouldn't expect much higher given typical gasoline engine piston geometry, turbocharged or not. In fact, I'd expect a bit lower from the turbo version, due to higher temps and their impact on the strength of Al.

 

[turbomotor]

I tested an experimental opposed-piston two stroke diesel with 10 atmospheres air box pressure up to about 3500 psi firing pressure a few years ago. I had problems with pre-ignition of the small amount of oil that was being carried above the top fire ring at those high firing pressures.

DV

 

[turbomotor]

More germaine to your original question - I've also looked at data for modern common rail automotive (and truck) diesel engines, and I think 150 to about 180 bar (2200 to 2600 psi) is about tops for anything in production. Near-stoichiometric homogeneous charge SI engines with turbocharging may run about 70 to 90 bar. Turbocharged F1 engines from the 1980s ran as high as 100 bar PFP with a "gasoline" fuel that was primarily toluene. I don't know what the turbocharged alcohol engines are running for PFP.

I'd like to know what the PFP is for the engine in the Audi R10 turbodiesel Le Mans car. Does anyone (who has seen actual data) care to comment?

DV

 

[tbuelna]

I would agree with ivymike. Most current, heavy-duty, commercial diesel (CI) engines are designed for peak cycle pressures of 2500 psi or less. And lighter duty automotive diesels are usually designed for peak cycle pressures of 1800 psi or less. The benefit of being able to run at higher peak cycle pressures is improved thermal efficiency. A peak cycle pressure of 2500 psi would yield a mean brake 4-cycle pressure of maybe 300 psi in a well developed engine.

However, I actually did work on an engine development program about 15 years ago with Army TACOM and Detroit Diesel, called TRC. The 2-stroke, opposed piston reciprocator was designed for operation at peak cycle pressures approaching 4000 psi. The engine cycle used very high manifold pressures (>100 psia) and low compression ratio (12:1). Of course, it was obviously an SOB to start.

We never made it quite as high as 4000 psi during firing runs, due to limits with our piston ring design. But at operation with peak cycle pressures above 2500 psi, the BTE and SFC numbers were truly impressive. And when it was running at those high firing pressures, it shook the whole building that the test cell was in. Scared the hell out of me to be in the control room 15 ft. away while it ran!

 

[BrianPetersen]

MAybe *older* light-duty diesels were designed for 1800 psi (approx 120 bar) peak cylinder pressure but the newer designs, e.g. the higher-powered versions of current VW, BMW, Mercedes turbodiesels, are all in the 150 to 200 bar range.

For JSteve2, your "fudge factor" for adiabatic compression is easily eliminated. With air as the working fluid, (P2/P1) = (V1/V2)^1.4

V1/V2 is also known as the compression ratio, and P1 is whatever the pressure is in the cylinder at the end of the intake stroke. Assume this to be equal to intake manifold pressure.

 

[Lou Scannon]

In answer to JSteve, I can't share specific design details such as compression ratio, I'll just say north of 10:1. Boost in the 40-45psia range.

 

[JSteve2]

hemi,
Understood. Just with the rough numbers provided my SWAG would put you at 45 * 10 * 5 = 1800 psi, or the center of your 1500-2000 psi range. So, as a SWAG, that seems OK to me.

 

[Lou Scannon]

In answer to part of the OP that hasn't been answered yet, the mean pressure in the power stroke will be the BMEP*4*(1/mechanical efficiency), since the power stroke is 1/4 of the cycle.

 

[TDIMeister]

I was in a development program for a V12/V16 locomotive Diesel engine, in production since about 2005. PCP in that was 220 bar. For HD engines, 250 bar is already production-ready.

In automotive CI engines, 200 bar is more-or-less state-of-the-art. For forced-aspirated SI engines, it's about 120 bar, although we have in-house turbocharged DISI designs for 140 bar PCP with BMEPs approaching 27 bar. (the twincharged 1.4L VW TSI engine has a peak BMEP of 21.7 bar).

 

[ivymike]

on which HD engines?

 

[NANAPATIL]

Hi Guys,,,
how to increase strength of Intake Swirl in CI engine?
valve shrowding is not possible because valve rotate

Nana Patil

 

[TDIMeister]

I am aware that MAN has a new engine family in final development operating at a PCP of 250 bar. Remember, I said production-ready, not in-production.

Cheers