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Peak Cylinder Pressures

Peak Cylinder Pressures

Peak Cylinder Pressures

(OP)
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

 

RE: Peak Cylinder Pressures

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.

RE: Peak Cylinder Pressures

(OP)
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
 

RE: Peak Cylinder Pressures

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.

RE: Peak Cylinder Pressures

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.

 

RE: Peak Cylinder Pressures

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.

RE: Peak Cylinder Pressures

... 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).

RE: Peak Cylinder Pressures

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.



 

RE: Peak Cylinder Pressures

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  

RE: Peak Cylinder Pressures

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   

RE: Peak Cylinder Pressures

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!

RE: Peak Cylinder Pressures

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.

RE: Peak Cylinder Pressures

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.

RE: Peak Cylinder Pressures

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.

RE: Peak Cylinder Pressures

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.

RE: Peak Cylinder Pressures

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).

RE: Peak Cylinder Pressures

on which HD engines?
 

RE: Peak Cylinder Pressures

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

Nana Patil

RE: Peak Cylinder Pressures

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

RE: Peak Cylinder Pressures

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

Nana - that's not a light question, and it's tacked onto the end of a winding-down thread. You might want to start a new topic. (piston-bowl shape?)

RE: Peak Cylinder Pressures

Helical intake ports.

RE: Peak Cylinder Pressures

Valves are normally shrouded by the chamber and piston, so it has nothing to do with valve rotation.

Swirl in the port is VERY disrupted once the air enters the chamber, expands down the bore as the piston moves up, then is compressed into the chamber with the squish areas and profile of the chamber and the piston as they come together.

The effects as the piston approaches the head on the compression stroke is far more significant that the swirl in the port.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Peak Cylinder Pressures

"Valves are normally shrouded by the chamber and piston, so it has nothing to do with valve rotation"
Pat, I think [NANAPATIL] meant a shroud that is a feature of the intake valve itself.  I have seen this on a CFR engine, where there is an annular curtain extending upward from the inside (i.e. port) side of the valve that masks approximately 1/4 to 1/3 of the circumference.  Obviously, with such a design, valve rotation would have a significant and unpredictable impact on in-cylinder charge motion.

 

RE: Peak Cylinder Pressures

"Swirl in the port is VERY disrupted once the air enters the chamber, expands down the bore as the piston moves up, then is compressed into the chamber with the squish areas and profile of the chamber and the piston as they come together."

Which is why a suitable shape of piston bowl is also necessary.

This really should be a new thread.

RE: Peak Cylinder Pressures

Yes. We are a long way off the OP.

Also, we have done this at length before.

 

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Peak Cylinder Pressures

ip308,

It follows quite easily from the isentropic relationships, ideal gas law, and specific heats that the peak cylinder temp & pressure right after combustion for the ideal Otto cycle are given by the expressions,

T3 = T1*(V1/V2)^(γ - 1) + ηc*Qhv/[cv(1 + AFR)]

P3 = (ma + mf)*Rs*T3/V2 = ρ*Vd*(1 + 1/AFR)*Rs*T3/V2

Since V1 = Vd + Vc, V2 = Vc, and V1/V2 = rc for the ideal cycle, and Rs = cv(γ – 1), the above expressions further simplify to,

T3 = T1*rc^(γ - 1) + ηc*Qhv/[cv(1 + AFR)]

P3 = ρ*(rc – 1)*(1 + 1/AFR)*cv(γ – 1)*T3

Indicated mean-effective pressure is given by indicated work over displaced cylinder volume,

iMEP = iW/Vd.

Since the thermal-conversion efficiency ηt ≡ iW/Qin, and Qin = mf*ηc*Qhv, then indicated mean-effective pressure is just

iMEP = ηt*Qin/Vd = ηt*mf*ηc*Qhv/Vd

where for the ideal Otto cycle, ηt = 1 – 1/rc^(γ – 1).  Another useful formula that relates indicated torque T (in lb-ft) to iMEP is

iMEP = 48*π*T/Vd

where here, Vd is the displaced volume in cubic inches.  (T and Vd are either both per cylinder, or total, obviously.)

Due the fact that there is heat loss, combustion isn't really constant volume, and spark isn't at TDC, those equations should over estimate the results.  So they'll provide you with sort of a worst-case idea.  If I use a numerical-solution thermodynamic model of a more realistic cycle, I find the ratio P3/iMEP is around a factor of 5, plus or minus.  The analytical solution comes out a bit higher, of course.  I'm sure there are probably guys here that have real software at their disposal so perhaps they could give you more accurate numbers.

And don't forget about RPM when considering rod strength, of course.  Inertial forces go as rpm^2.  I hope this helps.

RE: Peak Cylinder Pressures

I have measured over 3000psi in a highly turbocharged 4 cylinder engine.

RE: Peak Cylinder Pressures

That seems reasonable, though I would expect the engine to have a short life at that condition unless it happens to be highly developed for durability at that level of stress.

RE: Peak Cylinder Pressures

Anyone have any experience with HCCI peak pressures?

Ed

RE: Peak Cylinder Pressures

What I have seen is not out of line with the discussion above.  Current production cylinder kits were used.

RE: Peak Cylinder Pressures

3000 psi is approx 200 bar ... Plausible, and comparable to modern turbodiesel peak cylinder pressure.

Regarding HCCI, nobody has much experience with those at all! From papers that I've seen, they seem to be developing these based on standard gasoline or diesel engines and the HCCI operating mode is only used at light load - not the condition that gives highest peak cylinder pressure.

RE: Peak Cylinder Pressures

BrianPetersen wrote "From papers that I've seen, they seem to be developing these based on standard gasoline or diesel engines and the HCCI operating mode is only used at light load - not the condition that gives highest peak cylinder pressure."
My experience was the opposite - HCCI was phased in somewhere above 30% load.
The combustion process is very sensitive so the control system must walk a tightrope between misfire and destructive detonation.
 

RE: Peak Cylinder Pressures

I have measured between 1800-2500 psia in the chambers.  As others have mentioned, this is the max design pressures for most commercial CI engines out there, based on thermal and structural limits.

tbuelna - was that opposed piston engine you worked on for TACOM the OPOC by chance?  I'm curious as to why there was so much shaking.  I had read that it was completely balanced, dynamically.

RE: Peak Cylinder Pressures

HiPerTEC,

It wasn't the OPOC, but it was for TACOM and it was called the TRC Engine (Turbo Rotary Compound).  It was an opposed piston, uniflow 2-cycle reciprocator coupled with a (Wankle type) compressor/expander.  

To make it capable of withstanding the high peak cycle pressures (275 bar)it was intended to operate at, each piston had two con rods. And the single cylinder test rig had a total of four gear-synchronized, counter rotating  cranks.  In theory, opposed piston engines have excellent dynamic balance, but in order to improve scavenge efficiency, we usually ran the intake and exhaust pistons up to 10 degrees out of phase.  To make matters worse, the combined reciprocating mass (30 lbs?) of each piston/conrod assembly was very high, so the the unbalance forces (shaking) due to the intake/exhaust pistons being out of phase was quite high.

The test rig was a 124 cu.in. single cylinder unit, and each of the four cranks had it's own 90 lb. flywheel.  But even with all of that flywheel mass it still shook the entire building when it ran.  

The thing that always scared me the most was starting it up.  It had a very low compression ratio (12:1, the rest of the compression/expansion work was to be performed by the Wankle unit) but utilized very high levels of manifold pressure (8 bar), so it was a bear to start.  The test rig had no starter, so we would motor it up to speed using the DC dyno, set the externally supplied intake air pressure and temperature to spec, and then begin to turn up the rail pressure control on the 25 ksi fuel injection system.  All of a sudden the thing would light off with a deafening roar and everybody in the control room would jump, even though they had been through the start-up drill dozens of times before.

Now that I look back on that program, I'm amazed we never had a serious accident in the test cell.  The high pressure common rail fuel system was state of the art for 1993, but it was still a development unit.  Being a test rig, we were constantly tearing the engine down for inspection and when we would reinstall the common rail fuel system, it would usually take  4 or 5 attempts to get all of the 25ksi fuel fittings to seal properly.  The fuel pump was engine driven, so we would have to motor up the engine to pressure check the fuel system.  I don't know if you've ever seen a leak from a 25ksi fuel system, but one second it's fine and the next instant there's a huge cloud of super-atomized fuel surrounding the engine inside the enclosed test cell.  Perfect conditions for a huge explosion.  Luckily we never had an explosion.  Maybe God takes pity on fools.

Sorry to ramble on,  but it was a great program to be involved in and I worked with a lot of sharp guys from Detroit Diesel and Sandia Labs.

RE: Peak Cylinder Pressures

that sounds a bit scary... even a regular wankel can be louder than a couple skeletons screwing on a tin roof in a hailstorm.
 

RE: Peak Cylinder Pressures

tbuelna,

Do you know if TACOM is still working on the TRC engine?  I do know they are working on the OPOC with DARPA.  I believe they are trying to clean up the emissions and get some higher performance (i.e. lower sfc) out of that 2-stroke.

Research is both scary and thrilling sometimes, but that is what makes it so fun. Although sometimes it would be calming if we could look into the future a bit to see what things that may leak, break, or blow up.  Especially when working with new engine designs, like the TRC engine.  We have a unique opposed, free-piston engine that we are developing right now for the Navy.  We have operated this engine in both 2 and 4 stroke modes, which is very unique for a free piston engine.  The really interesting thing about it is that you see the pistons/engine reciprocate as its running.  We are using propane in the engine right now to eliminate the need for heating the intake (it's throttle body injection), but propane injection has it's own set of difficulties.  We have a port injection for the next prototype so we can incorporate a liquid, heavy fuel delivery system.  We have designed the engine for 200 bar and have run pressures about 140 bar so far.  We do use spark plugs in the engine right now just because we are in the research phase and want to be able to control ignition point, but it also makes for easier starting this free piston engine (we can start with roughly 3 bar pressures on compression).  




 

RE: Peak Cylinder Pressures

I had a 10ksi fuel line split on a 700 hp engine I was running through the NATO durability cycle for TACOM.  I was sitting outside eating lunch and noticed a lot of smoke coming from the exhaust stack.  I ran inside and looked through the test cell window.  Diesel fuel was spraying straight up onto the light fixtures and dripping back down on the hot manifolds & turbo.  I have no idea why it didn't ignite.

RE: Peak Cylinder Pressures

tbuelna,

My dear, the TRC engine concept is the top of the world.

turbomotor

RE: Peak Cylinder Pressures

HiPerTEC,

Can you provide more information or a link about your engine?

I'm interested in are you extracting power out of your free piston engine and how you actuate the valves.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

RE: Peak Cylinder Pressures

tbuelna,

Why did your team chose to run the pistons out of phase rather than move the ports, or alter their shape. Seems the balance issue would be a big penalty to pay.  Is it normal for opposed piston engines to run their pistons out of phase?

Charles

RE: Peak Cylinder Pressures

Two-stroke opposed-piston engine. You want the exhaust port to open first and close first. That's only possible by having the opposing pistons slightly out of phase.

Yes, you can do it the way a normal two-stroke does it, with the pistons in phase, but why not take advantage of the ability to have unsymmetrical timing?

RE: Peak Cylinder Pressures

turbomotor,

I don't know how you lasted longer with the good doctor than I did.

Of course, now I'm working with a relatively pedestrian 12,000 hp turboshaft drivetrain.  Much more power...... but less vibration!

RE: Peak Cylinder Pressures

EdDanzer,

We started out looking at extracting power through a reciprocating generator design, but most people wanted a mechanical extraction system in order to reduce size and weight.  We designed and fabricated a mechanical conversion unit that changes reciprocating motion to rotary motion.  This unit is not a slider crank design, but uses very fast acting one-way clutches to engage and disengage the rotary power output shaft from the free piston engine.  We are in the middle of the investigation on this power extraction unit, but so far things look very promising.

The valves in our 4-stroke free piston engine are pneumatically actuated, and are independently controlled with our control system for the engine.  The valves are pneumatically controlled in for opening and closing (no springs).  We have designed very high speed pneumatic control systems for some of our rocket thruster programs.  Our actuators can run about 4000 rpm in 4-stroke mode at the moment.  We have a new design being fabricated now that will push us over 10,000 rpm in 4-stroke mode.  We went with pneumatic actuators because of the smaller size and weight compared to electro-mechanical actuators.  Plus we use the air through the system to help cool parts of the engine.

 

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