wanting your input on my "Optimized Otto Cycle Engine"

[RGRv6guy]

The future of the IC engine thread was good, and while this
seems much more short-sighted than that topic, I believe it
would be an incremental gain in efficiency and easily
adaptable to current engines with cam and boltons. It mostly
centers on delaying exhaust valve opening as much as
possible and increasing exhaust port flow to possibly near
100% of intake flow. I am also considering using Larry
Widmer-style sonic exhaust porting or Monty Campbell ASC
(wave charging) exhaust devices but for now please
concentrate on the possible incremental gains thru
conventional means as I need to more fully understand the
effects of delayed Exh. Valve opening and increased exhaust
flow.

Also suggestions for accurate Dyno Sim programs would be
helpful, I am considering the latest Dynomation as they
claim to use actual engine dynamic models "filling &
emptying" which I believe would apply here. "My" theories
actually seem to pan out on Dyno 2000 as far as the retarded
exhaust opening goes when I input increased exhaust flow.

TIA and I'll have many more points to add, but the main
thing is increasing thermal efficiency by combining
technologies and theory (hopefully it's good theory)!

 

[SBBlue]

Just what numbers are you talking about when you mention delaying exhaust valve opening as long as possible?

Beyond Bottom Dead Center? I'm not sure that would work very well.

What crankshaft angle do you think the exhaust valves open at present?

 

[RGRv6guy]

thx SBBlue. Currently, the stock cam hits .050 at around
30* BBDC and the actual off seat would be around 60* BBDC
which effectively kills the latter 1/3 (as far as crank degrees are concerned)
of the power stroke. I am hoping to hold the valve closed much
closer to BDC but not past it by any means. Hitting nearly an
even flow ratio with the intake would make this possible and
get the maximum return from the power stroke.

 

[kenre]

By the time the crankshaft has moved through 90 degrees of rotation most of the effective work has been done. Also by delaying the opening of the exhaust you reduce the time for the cylinder to exhale, meaning that the piston has to push harder to remove the exhaust.

Ken

 

[RGRv6guy]

"By the time the crankshaft has moved through 90 degrees of rotation most of the effective work has been done."
************************************************************
Thanx, this part was helpful, but I have emperical (2nd hand)
evidence that states the PDR (point of diminishing returns)
is actually much closer to BDC. Not to say that your
statement is not true, but with proper exhaust flow ratios,
you can dig deeper into the power stroke and extract more
usable thermal efficiency. I'm absolutley sure though that
the first 90* utilizes much more of the available energy
than the last 90* by far, but common EGT's around 1200* F
tells me that there is a good bit more usable energy to
extract. This is why turbos work, as one example. I'd like
to use more of this energy with as little external paraphenalia
as possible, wouldn't that be nice!



Also by delaying the opening of the exhaust you reduce the time for the cylinder to exhale, meaning that the piston has to push harder to remove the exhaust. Ken.
************************************************************
I hope to offset this with exhaust flow ratios much closer
to 100% of the intake flow, and this flow # has been hit
using a nominally larger exhaust valve (only .100" larger)
while maintaining excellent velocity. Stock was 1.45" and
the BV exhaust was 1.55" to hit 211 CFM, while unported
intakes go around 210 to 220 CFM with similar lifts. Also,
the lower EGT will still be around 800* F (or more, poss.
lots more) so it will have plenty of excess energy to escape
the cylinder. I suspect I will have to open the exhaust
valve 5 to 15 degrees BBDC (seat timing) which will still
hopefully initiate blowdown soon enough to get the job done
without extra pumping losses. The ASC system I mentioned
in the first post actually claims to be a totally optimized
scavenge device which further helps the situation, but if I
can get powerstroke and blowdown right, the OOC can be used
as a stand alone system. Then other add-ons would also be
starting at a much higher baseline Tq/HP.

 

[SMOKEY44211]

I think that your line of reasoning has some merit at for low cycling speeds (under 2,000 rpm) to reduce BSFC. The exhaust gas temps. you are referencing suggest that you are investigating WOT performance. I fail to see where intake flow capability enters into it. Once the air fuel charge is entraped the intake performance is out of the equation. Worth mentioning is that as the piston descends not only is the working pressure diminished, the mechanical leverage on the crank decreases also. The engine simulation software that I have tried give pretty good feed back on power trends but I haven't run across one that gives BSFC predictions. In conclusion I think there is very little if anything to be gained by going from say 30deg. BBC seat timing to 15deg. BBC. I do think there is a significant gain available by raising the compression ratio (that yields an increase in the expansion ratio) and incorperate some improvements of detonation control such as reverse flow cooling and/or ceramic coatings on valves and piston.-----Phil

 

[patprimmer]

RGRv6guy

It all very much depends on speed of operation, air flow capacity of the exhaust and to a small extent on rod length to stroke ratio.

The last 10 deg of rotation produces very little piston motion and does give the exhaust gas a head start to reduce pressure before the piston has to move up and push it out. Even 20 degrees before BDC is a very small fraction of the stroke. The exact optimum will be decided by the best compromise of many parameters, and I am sure, designers of modern engines spend much time anguishing over these matters when deciding on their exact cam profile.

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.

 

[GregLocock]

The Lotus single cylider sim /does/ predict sfc.

Of course, you are rather at the mercy of the model they use to geneate those numbers, but at least it is a start.

http://www.lesoft.co.uk

Cheers

Greg Locock

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

 

[kenre]

Your idea may be somewhat useful at a constant or more likely a low speed engine.

Might be something in looking at the cam timing of some recent engines with variable cam timing and see what they are doing with regards to delaying exhaust opening.

 

[thundair]

I have spent some time working with asymmetrical cams to delay the exaust opening
The results were that the leverage of the stroke was so small, and so little pressure that it wasn't worth pursuing for our project...

There was even thought of a window port at BDC that was valved from a rocker and pushrod from the cam. got real messy

Regards

 

[RGRv6guy]

Lots of interesting thoughts so far, especially
that the powerstroke is basically over @ 10* BBDC...

Here's some highlights of the rest of my combo (very
generally). I'm not afraid to share specifics, but
many of them are still in limbo. I can comment on what
I am leaning towards if it helps the discussion.

1. Long rods (approaching 1.9:1 r/s ratio)
2. Tight quench pistons with mirror-image dish
3. More centrally located spark plug and greatly increased
spark energy to minimize spark advance to reduce "negative torque"
4. Coatings for thermal management
5. Coatings for friction reduction
6. Formula One style engine assembly (as much as possible)
7. The ASC exhaust mentioned above, which will supposedly
augment the exhaust flow enough to make the delayed exh.
valve opening feasible. But I wish to get this working as
well as possible prior to the ASC install to get a good idea
of just how well it actually works.

 

[RGRv6guy]

Quoting SMOKEY44211:
"I think that your line of reasoning has some merit at for low cycling speeds (under 2,000 rpm) to reduce BSFC. The exhaust gas temps. you are referencing suggest that you are investigating WOT performance."
**************************************************************
Excellent, I am hoping to do just that, increase fuel
efficiency but certain elements are hopefully going to
have a dual benefit of mileage and HP increases. The
engine I am working with is called Split Port, and it
has a definite split-personality of WOT HP and efficiency.
So does the ASC device mentioned.



"I fail to see where intake flow capability enters into it. Once the air fuel charge is entraped the intake performance is out of the equation."
*************************************************************
I'm strictly using this intake flow reference to indicate
that the exhaust flow ratios, which are nearly equal, should
make the pumping losses nearly zero, even with reduced exh.
valve timing. Good observation, Smokey!



Also, thanks to Greg and Pat for the comments.

Thundair, I was wondering if you could elaborate
with some specs such as rod ratio and seat timing
where the thing just seemed to actually quit working...
My calculations show that longer rods gain almost a
5* advantage with a 1.6-ish ratio increased to 1.9 range.
This assume rod angle on the crank, with a fixed crank
angle of 90*, but that is just from memory. I do not have
my crib notes here to look at...

 

[thundair]

Sorry for this delayed response but I just got back to review some old threads.

The rod ratios were 1.7 to 1.8 and we were only looking at dwell time at TDC. The increase is very small and a longer rod would have helped but there is this nagging weight problem. Just when the rod is as long as you want it is to heavy..

I do not have the specs for the A symetrical cam but you could pursue a engine that it actually worked on was the 400M Ford engine..

Regards

 

[crysta1c1ear]

I agree with the idea of long rods for what you are trying to achieve, but I am sceptical of the actual rewards to be achieved. With short rods, the rod motion near BDC might partly resemble the motion of a pendulum, where the bottom is rotating but the top is stationary.

I think there are three things which will combine to make any reward from late valve closing minimal.
1. Limited piston travel
2. Rod almost perpendicular to crank motion
3. Cylinder pressure has dropped.

Do you know the expression <I>"Half of two thirds of naff all is ...? Naff all!"</I> ?
I would argue that late exhaust valve closing would only give you a fraction of a fraction of a small amount.

I am with you on the spark stuff though! At ignition the heat of combustion has to ignite all the stuff around it. For illustrative purposes only, imagine a 1 mm radius sphere growing 1mm to become a 2 mm radius sphere. The burning stuff would have to ignite 7 or 8 times its own volume. Now imagine a flame front has hit a piston and is growing sideways a bit like an expaning cylinder. For a 10 mm radius cylinder to grow to an 11mm radius cylinder, any burning stuff on the periphery would only have to ignite 1.1 times its own volume.

So I'm in favour of igniting as much stuff as possible as quickly as possible to kick off the difficult bit of combustion. In that case, any advantage obtained can profit from (a) long piston travel, (b) when the rod and crank are perpendicular, and (c) before the pressure drops. That is more likely to be something times something times something rather than the nearly nothing times nearly nothing times very little which I suppose applies to the valve closing idea.

 

[thundair]

CUT AND PASTE FROM VICTORY LIBRARY

Connecting Rod vs. Stroke Analysis The ratio between the connecting rod length and the stroke length of a motor greatly affects the way it performs, and how long it lasts. This ratio (normally represented by “n”) can be calculated as follows:

Ratio “n” = Rod Length ÷ Stroke
The rod’s length is measured (for this purpose) from the center of the piston-pin opening to the center of the big-end bore, not overall. There is a small range of ratios for most conventional piston engines: the rod is between roughly 1.4 and 2.2 times the stroke length. It’s not possible for the rod to be the same length as the stroke, and rods much longer than twice the stroke make the motor very tall, and are not practical for most purposes (although used for racing).
The rod angle must not encourage excessive friction at the cylinder wall and piston skirt. A greater angle (smaller value of “n”) will occur by installing a shorter rod or by increasing the stroke. A reduced angle (larger value of “n”) will occur with a longer rod or a shorter stroke.
If the rod length is decreased, or the stroke is increased, the “n” ratio value becomes smaller. This has several effects. The most obvious is the mechanical effect. Motors with low values of “n” (proportionately short rods or long strokes) typically exhibit the following characteristics (compared to high “n” motors):
» physically shorter top-to-bottom & left-to-right (more oil pan, header, and air cleaner clearance)
» lower block weight (400 vs. 440, for example)
» higher level of vibration
» shorter pistons, measured from the pin center to the bottom of the skirt
» greater wear on piston skirts and cylinder walls
» slightly higher operating temperature & oil temperature due to friction
There are also differences in how the motor breathes:
» intake vacuum rises sooner ATDC, allowing bigger carburetors or intake port runner & plenum volumes to be used without loss of response
» on the negative side, a small or badly designed port will “run out of breath” sooner
» piston motion away from BDC is slower, trapping a higher percentage of cylinder volume, making the motor less sensitive to late intake valve closing (hot cams)
Spark advance is also affected:
» earlier timing (more advance) is required, as the chamber volume is larger (piston is farther from TDC) at the same point of rotation
» the motor may also be less knock-sensitive, as the chamber volume increases more rapidly ATDC, lowering combustion pressure (this is useful for nitrous & supercharged motors)

--------------------------------------------------------------------------------
Effects of Long Rods
Pro:
» Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
» The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
» For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

Con:
» They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.
» Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.
Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.

--------------------------------------------------------------------------------
Effects of Short Rods
Pro:
» Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
» The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
» Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.

Con:
» Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
» Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.

--------------------------------------------------------------------------------

Rod Ratio vs. Intake Efficiency
An “n” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design. Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results. There will be instances where the choice of stroke or rod has not been made, but the intake pieces (carburetor, manifold, and head) have been selected. Some discretion exists here for making the rod and/or stroke choice compatible with the existing intake. The “n” value can be used to compensate for less-than-perfect match of intake parts to motor size & speed. The reverse is also possible: the lower end is done, but there are still choices for the top end. Again, the “n” value can be used as a correction factor to better “match” the intake to the lower end.
The comments in the following table are not fixed rules, but general tendencies, and may be helpful in limiting the range of choices to those more likely to produce acceptable results. Rather than specify which variable will be changed in the lower end, “n” values will be used. Low “n” numbers (1.45 - 1.75) are produced by short rods in relation to the stroke. High “n” numbers (1.75 - 2.1) are produced by long rods in relation to the stroke.
Best Combinations of “n” Values & Intake Characteristics
“n” = 1.45 - 1.75 more compatible with: “n” = 1.75 - 2.1 more compatible with:
Large intake port volume vs. motor size
(”J” head on 273) Small intake port volume vs. motor size
(stock 452 head on 498” RB stroker)
Single-plane or 360° intake manifolds
(Edelbrock Victor, Torker & Torker II, TM7. Holley Strip Dominator. Offenhauser Equa-Flow, Port-O-Sonic. Weiand X-Celerator, Team G) Dual-plane 180° intake manifolds
(Edelbrock: LD340, CH4B, DP4B, Performer & Performer RPM, Streetmaster, SP2P. Holley Street Dominator. Weiand Stealth, Action Plus)
Large carburetor vs. engine size
(273 with 750cfm) Small carburetor vs. engine size
(440 with 600cfm)
Moderate engine speed
(pick-up, RV, towing) High engine speed
(peak power more important)
Tall axle ratio
(2.76, 2.93, 3.23, 3.55 and/or with tall tires) Short axle ratio
(3.91, 4.10, etc. and/or with 25 or 26” tires)


Planning a 383 Motor This engine is generally overlooked in selecting a high-performance project. The motor has an excellent bore to stroke ratio: 1.26-1 (similar to 327” SBC, better than 340). The short stroke allows high RPM without destructive piston speed (7100 RPM = 4000 ft./min., the accepted “safe” limit for piston stress). The large bore permits big valves (2.14” intake, 1.81” exhaust).
A potential method of increasing peak power is to substitute the longer 440 6.768” (LY) rods for the original “B” 6.358” rods on the original crank. This has the following effects:
» Increases the rod ratio (“n”) from 1.884-1 to 2.005-1
» Reduces the piston compression distance to about 1.525” for a useful weight savings
» Slightly reduces piston acceleration

 

[RGRv6guy]

OK guys, I'll be trying a crucial part fo my OOC engine,
in a more conventional engine, I'll be testing the exhaust
system that the inventor claims it takes advantage of the
delayed exhaust opening. First thing, I'll be using a more
conventional cam with more overlap (part of his criteria)
and then testing the exhaust system against a good normal
and properly sized exhaust system. I'll be shooting for
around 300 BHP and the system I have is just made for that,
then I will try the specialized system. If it indeed helps,
I can switch to the purpose built engine with the OOC mods.