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

 

[JSteve2]

"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?)

 

[TDIMeister]

Helical intake ports.

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[Lou Scannon]

"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.

 

[TDIMeister]

"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.

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[black2003cobra]

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)^(? - 1) + ?c*Qhv/[cv(1 + AFR)]

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

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

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

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

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

iMEP = iW/Vd.

Since the thermal-conversion efficiency ?t ? iW/Qin, and Qin = mf*?c*Qhv, then indicated mean-effective pressure is just

iMEP = ?t*Qin/Vd = ?t*mf*?c*Qhv/Vd

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

iMEP = 48*?*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.

 

[bobqzzi04]

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

 

[Lou Scannon]

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.

 

[761TJ]

Anyone have any experience with HCCI peak pressures?

Ed

 

[Lou Scannon]

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

 

[BrianPetersen]

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.

 

[Lou Scannon]

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.

 

[HiPerTEC]

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.

 

[tbuelna]

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.

 

[ivymike]

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

 

[HiPerTEC]

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).

 

[dgallup]

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.

 

[turbomotor]

tbuelna,

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

turbomotor

 

[EdDanzer]

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

http://www.danzcoinc.com

http://www.dehyds.com