DavidBoren

Either way, we could literally watch the difference between 8.5dcr and 8.9dcr, that makes 91aki fuel acceptable in one and not the other. Could graph cylinder pressure versus crank position to see how pressure builds and falls in relation to the valve events. You could watch how the pressure curve changes between different engine architecture types.

Think about being able to compare the cylinder pressure graphs like dyno results between a 370ci undersquare 3.78" bore and 4.125" stroke, a square 370ci 3.9" bore and 3.9" stroke, and an oversquare 370ci with a 4.06" bore and 3.62" stroke... all with the same combustion chamber volume, static and dynamic compression ratio, blah blah blah. It would be fascinating to see how piston acceleration effects cylinder pressure. Or if/how a short/fat cylinder builds or maintains pressure compared to a tall/narrow cylinder.

Maybe I'm just a nerd...

DavidBoren

PS. Just for fun. The theoretical rod ratios of the three aforementioned 370ci engines and my predictions as to the resulting cylinder pressure "dyno" graphs...

Undersquare, 3.78 x 4.125, 6.125 rod, 1.48 ratio...
Pressure peaks earlier, drops faster.
Square, 3.9 x 3.9, 6.25 rod, 1.60 ratio...
Pressure peaks in middle, even raise/fall.
Oversquare, 4.06 x 3.62, 6.3 rod, 1.74 ratio...
Pressure peaks later, falls slower.

The longer dwell near top dead center associated with higher rod ratios has to have effect on how cylinder pressure builds and falls. The lower the rod ratio, the faster the cylinder volume expands per degree of crank rotation. The faster cylinder volume expands, the faster compression drops. Lower compression is lower pressure.

And remember, cylinder pressure (in our equation ) is a function of crank angle.

Darth_V8r

iTrader: (4)

I would actually guess the rod ratio has less to do with it, but rather just the stroke. Distance traveled will be 1/2 x stroke x sine crank angle until you get to 90 degrees. Then it will be stroke - 1/2 x stroke x sine crank angle (as91-180 degrees).

So I think you're right, but not for the rod ratio. Just the stroke length. Your short stroke engine would need a lot more timing advance to compensate I would think. Your long stroke engine would have less timing, but that's probably a good thing. If you make peak pressure with less timing, that's a faster burn

I would actually bet on the shorter stroke longer bore version making more power at the top end due to less g forces due to lower stroke. Longer stroke would have more Tq at low end due to leverage, but would reach higher internal resistance at lower rpm. At 4800 rpm, 4.125 stroke has 1350 g's acceleration at the Pistons.
3.622 stroke has same acceleration at 5120. So I predict with the same amount of air and fuel, the shorter stroke would dyno slightly higher, but would have lower torque measurement and slightly higher peak Tq rpm.

Acceleration = 2 x (PI x rpm/60)^2 x stroke / 386.1.

When you get to 6800 rpm, the 4.125 stroke has internal acceleration forces equal to 3.622 stroke at 7300.

DavidBoren

Yes, I agree that the changes in architecture would probably correlate to the power curve changes you mentioned.

But I'm talking about the cylinder pressure "curve". Piston acceleration is determined by rod ratio. And I am guessing that the faster the piston accelerates to and from top dead center, the sooner the cylinder pressure will peak. I think it's due to the increased rate at which the cylinder volume expands per degree of crank rotation.

I think a higher rod ratio, thus lower piston acceleration near top dead center, would carry higher into the rpms before peaking due to the less rapid growth in cylinder volume per degree of crank rotation.

These phenomena are totally separate from the effects on increasing friction due to longer stroke lengths, which effects mean piston speed, not acceleration. Acceleration is dictated solely by the rod ratio.

For the above example of the three 370 engines, we would have to ensure the mass of all three rotating assemblies was kept the same, so the inertia would be the same.

C5_Pete

iTrader: (8)

But also remember different bore size will have different surface area of the rings to the cylinder walls. So, even if you have the same mass for the piston/rods/crank assembly, you'll likely have different friction values for each engine.

Really cool discussion, but my head hurts.

joecar

I don't think there's an easy correlation for cylinder pressure...

there are various factors that change the burn rate (e.g. AKI/octane-rating, temperature, pressure, mixture ratio/content, spark intensity/indexing, chamber material/geometry/quench...)...

any change in burn rate changes the peak pressure (it seems to be proportional to the integral of burn rate (and some other things));

and there's especially no correlation when knock occurs (an instant explosion producing a huge pressure spike vs a progressive burn producing a controlled pressure envelope).

DavidBoren

All the variables that effect cylinder pressure, or at least most of them (afr, compression, octane, temperature, head casting materials) are still present in every other aspect that we measure, and tune, and try to control... Why has the actual cylinder pressure reading just been neglected for so long?

With a couple different sets of pistons, you could run one high tech sensor plug in a junkyard ls1, and run 10 individual tests each for 5 different octane rating fuels and 5 different compression ratios. You control intake air temperature, coolant temperature, same heads for all, same cam, same intake, same spark plug intensity and indexing, blah blah blah.

And nobody, nowhere, has done even one test at one compression ratio with one octane fuel, AND PUBLISHED THEIR RESULTS?

joecar

I believe the larger and older OEM's would have already done this (possibly long ago), and of course they would not have published externally.

Does anyone here have access to SAE papers...? If ever this stuff was published it would possibly be there in the SAE archives.

DavidBoren

See, growing up in Montana, I reloaded my own ammunition... Not that reloading is in any way isolated to only Montana... And everything is based off of cylinder pressure, everything. And the actual sensors to get empirical data are expensive for that, too. Very few people own the equipment, but every reloading manual has the maximum pressure for each cartridge listed. And it has the velocity predictions for that pressure. It has a medium and low pressure load listed, with their respective velocities.

Nobody has the pressure equipment, nobody. But as a reloading enthusiast, you are expected to have some tools at your disposal. The reloading manuals list velocity, which can be measured with a cheap chronograph, and it lists predicted bullet drop impact points at known ranges. So if you have a chronograph, you can get a rough approximation of your cylinder pressure.

If you know your target's range/distance, and measure the bullet drop, you can get a rough approximation of your cylinder pressure.

Ranges are usually listed from muzzle velocity out to the point of going subsonic, usually a distance of some 1500 meters, in 50 meter increments, with bullet drop measurements accurate to the 1/4".

And these books are damn accurate in their calculations, the velocity and bullet drop measurements are generally spot on when empirically measured.

So, umm, why don't we have "reloading manuals" for motors? Same things are present in reloading... Powder burn rate is like octane rating, there are different intensities of primers not unlike spark plugs, the volume of the cylinder expands as the combustion is occurring, you can load the case to different fill levels and ratios, as well as different compression of the powder, different crimp tightness of the case on the bullet like piston rings, bullets have different friction levels with different types of rifling in barrels made of different materials and of differing lengths... Lots and lots of variables that effect cylinder pressure and velocity and bullet drop, but I have bookcases of cylinder pressure manuals dating back to world war two.

Guns and cars are redneck fun. Literally cavemen do this for entertainment. Okay? Both are extremely dangerous and potentially lethal to the caveman or anyone around them. And yet multiple vendors all post comparable data for their products in the gun world. Like Berger and Barnes and Speer and nosler and hornady all are competitors in the bullet production business. They all list their own reloading data in their own reloading manuals. You can cross reference the different manufacturers manuals and they all have the same maximum cylinder pressure. The bullet manufacturers have taken it upon themselves to buy the equipment and do the tests to provide the customer with a solid knowledge of their potentially dangerous products.

Why don't car manufacturers or aftermarket piston/rod/crank/whatever do the tests and give us the information to provide us with a better more solid understanding of their potentially dangerous products?

Darth_V8r

iTrader: (4)

I'm sorry there is too much to try to quote. I don't think you could do the tests with a constant mass, because the crank shaft with the longer stroke will have more mass. It's unavoidable. Even if you could do identical mass you can't get identical MOI

On the acceleration, I have a very practical example. In steel making, we have to oscillate the molds so the steel doesn't stick to the molds. When I calculated out all the parameters, I had to make sure the acceleration stayed below 4 m/s/s. The molds oscillate only in the vertical plane. Now, if the stroke of the mold is increased and frequency held constant, the acceleration increases automatically. The reason is that the mold has to start, cover more distance in the same amount of time, stop, reverse itself, cover that distance again, and stop. Velocity is distance per time. So velocity increases with stroke at a constant frequency. Since velocity is higher, the acceleration needed to slow it down and speed it up is higher.

In an engine, the Pistons move up and down the same way. It's almost perfectly sinusoidal. If you increase the engine stroke, the acceleration forces are higher at the same rpm vs a shorter stroke. The Pistons have to travel faster to cover more distance in the same time.

DavidBoren

I get that people on forums like this are very open to discuss ideas because there's not potentially billions of dollars of technology supremacy in a competitive market at stake for us mere mortals.

So I can somewhat, kind of, understand some industry secrets to maintain a competitive edge in the market.

But the Barnes reloading manual lists multiple and separate vendors of powder. If imr powder is what gives them their medium pressure load, they specify "X" number of grains of imr-4350 produces "Y" feet per second out of blah blah length barrel. And an entirely different company's powder may be recognized for the high or low pressure load.

And the bullet manufacturers ARE motivated to keep industry secrets in an attempt to maintain a competitive edge in the market. But they openly recognized different company's products, all in the name of providing the customer with better data about their products.

Darth_V8r

iTrader: (4)

Here's where I'm struggling on the cylinder pressure. If it were an adiabatic system, you could take PV=nRT, calculate the heat from the fuel to get the new temperature, using the new mass get the pressure, and go from there.

Problem is, you lose heat through the cylinder walls, and out the tailpipe. I think this is why higher thermostats get better gas mileage. If less heat is transferred from the flame front to the cooling system, less fuel is consumed to perform the same work.

Then, like David is saying the data is so sparse. However the two actual measurements I have been able to find combined with a 0,0 data point make a .99 r squared linear correlation.

For the purpose of estimating power losses above peak power, it's probably adequate. And it's probably minor compared to the friction and centrifugal forces working against the motor already.

For designing an engine? I wouldn't even consider it. I'm betting GM and Ford etc have cylinder heads fitted with these sensors running all kinds of configurations to determine the combinations that give the best performance and efficiency on the worst fuels.

Darth_V8r

iTrader: (4)

I'm honestly with you on the pressures. Really not any harm in publishing them. I bet they don't think anyone would actually want to know them.

joecar

Yes, not quite sinusoidal...

piston motion is simple harmonic motion (due to rotating crank) plus complex harmonic motion (due to angularity swept by conrod)...

see the graph attached, from here:
https://en.m.wikipedia.org/wiki/Piston_motion_equations

DavidBoren

Do we have any representatives on here from piston manufacturers? Piston manufacturers deal with cylinder pressure ratings every day, and maybe we could coerce someone on the inside to get us some information.

The funniest part about all of this talk about cylinder pressure, is we don't even know if it's directly correlated to what we are looking for.

All of this discussion hinges around finding what causes, or at least correlates to, the point of peak horsepower. We simply brought up that peak horsepower and peak cylinder pressure MAY correlate, which led us to the startling realization that cylinder pressure data is nonexistent. But we could very well be incorrect altogether, and we could be chasing the wrong thing entirely.

In every physics class I have had the pleasure of being in, they always have the disclaimer that whatever model or equation is based on the absence of friction, air resistance, etc. They give you models that should be accurate enough to give you a good idea of what is actually happening. When they say the force of gravity will accelerate a body of mass at 10m/s/s, negating air resistance, the math is 10m/s/s, but you know it will be slightly less because of the air resistance not in the equation. Sure, it's a huge factor, but for the sake of simple math just to give you the idea, it not only works but is commonly accepted.

So why, after all these years of car enthusiasts modifying cars, do we not even have the slightest semi-accurate, close-F'ing-enough equation for cylinder pressure? I honestly don't care if it's an equation built from 100 tests on the same engine, using the same octane fuel, and the same compression/timing/whatever. Mankind has formulae to quantify and explain everything else under the sun, and every one of those equations is negating some stupid factor that is hard to measure or account for.

We, car enthusiasts in general, don't even have a perfect world mathematical model for cylinder pressure. Oh the variables... **** on the variables, who cares about the variables when we don't even have a theoretical model accounting for perfect world circumstances.

Could you imagine the equation required to explain the acceleration of gravity on an object falling through the atmosphere? Air resistance would increase as elevation decreases due to the increase in density... It would be a colossal pain in the ***, so we settled on just saying 10m/s/s... Screw air resistance.

We don't we have an equation for cylinder pressure? Screw air temperature, use 72° or 100°, I don't care so long as you state what you used. Screw compression ratio, use 10 or 9 or 11, who cares so long as you state what you used. We don't have any equation using any variables, and it's discouraging because I know that there has to be more than the four people in this discussion who want to know.

Darth_V8r

iTrader: (4)

**** you're right. That means I need to recalculate peak and average acceleration. Back to the diff-eq books.

This will be fun

kingtal0n

iTrader: (1)

I found this:
Patm X CR = Max Stat. Cyl. Pressure

14.7psi X 8.5=125psi. I don't see any problem.


here:
http://www.sr20-forum.com/forced-ind...test-woes.html

Can anyone verify that its a real quick easy dirty way to calculate what it claims to?
How are you all doing DCR maths? I saw a thread about that and I was pretty sure darth was in it.

And as to this...
If you want more power you get a larger turbo. Largers turbochargers flow more air and thus more horsepower. More air = more power. Peak air mass flow rate= peak power.

Cylinder pressure is completely separate entity all together. Pressure gives us torque, it gives us force on the lever system that the engine pretends to own. Some of the energy goes up in heat; we dont know how much so math involving energy flow is fairly useless. We can correlate power with airflow, but not cylinder pressure with airflow, or cylinder pressure with power. In order to get power (work) from cylinder pressure, a basic equation would at least need to include rpm, and an advanced equation would contain variables involving the moment of leverage and friction variables, and other sorts of numbers of which I can vaguely conceive enough of to know we are not going to get anywhere useful with that sort of equation.

DavidBoren

We have seen evidence, in this discussion, gathered from people on this forum, that directly shows measured air volume/flow continue to rise after peak horsepower. So it's not that simple.

And, yes, the compression pressure is easy to calculate with the equation you presented. But it's only part of finding what we want.

If we had access to the data for various octane rating fuels' thermal expansion, we could find out how much the mass in the cylinder expands in the time between intake valve closing and exhaust valve opening events. We can then calculate how much the volume of the cylinder expands in the same time. We can calculate the pressure of the expanded mass of the burning fuel in the expanded volume of the cylinder. We can then calculate perfect world mathematical cylinder pressure by multiplying whatever expanded mass to expanded volume pressure ratio against the original compression pressure... Or something along those lines. But we don't have access to the data for 85 octane @ 14.7psi @ 10:1 = ANY USEFUL INFORMATION, PLEASE! If we had access to any conversion factor, any information relating octane and compression to some useable unit of pressure we could do the rest mathematically.

DavidBoren

For example, why doesn't Exxon have published data for their products? You could use it as a marketing tool to show us stupid farmers that buying the expensive gas results in more power (or can). You make visual models of the standard dimension sbc350 (or whatever is popular and easily relatable) cylinder @ 9:1 compression (or whatever is easily relatable) and you show us apes the secret of man's red fire.

But the data doesn't exist. That's probably inaccurate to say. The data for sure exists, we just don't have access to it.

We have clearly seen the benefits of higher octane rating fuels, even without exact pressure measurements, so we all know that it's not just a marketing gimmick to sell the expensive gas. So if publishing the data as to why the expensive gas is better isn't going to debunk your marketing gimmick, then why not publish it? The data would actually support the price of the expensive gas. The actual data would be a marketing tool in itself.

gtfoxy

A lot of interesting discussion in this thread. Started off shakey but it is coming around.

I will touch on one thing, inner-related to rod/stroke ratio: Of course it impacts chamber volume pressure Delta. It also impacts energy conversion. (that is ofcourse the primary point in having an engine run, to convert chamber pressure into torque) The easiest way I can explain the phenominon rod/stroke plays is matching acceleration rates of the piston to that of the gaseous mixtures combustion.

Compression ratio, both static & dynamic play role here that is paramount in energy conversion.

The big picture I know most do not know or see is that a very large majority of energy is wasted in pumping losses due to the necessity for increased advance in ignition timing.

Ideally the optimal scenario is to have an uber high compression ratio, vaporization, homogenization & distribution in the chamber so the required ignition timing is only a few degrees BTDC. Then matching the piston acceleration rate to the rise in chamber pressure ATDC.

Until those things are accomplished much theory in how certain areas are impacted is mostly negated by the elusive predict of pumping losses.

Aside: Boren, I am a reloader as well & I see where you are coming from. Using the cartridge scenario you can see what I'm saying. Imagin if a bullet had to go into the cartridge after combustion has commenced. The bullet would have to work against the pressure building. As it turns out this may actually increase velocity of there is a large portion of powder not burned before the projo leaves the barrel. But if the powder charge & burn rate are correct, & the barrel long enough, you can achieve 100% powder burn, resulting in near max velocity. In that case the pressure generated as the bullet would be moving into the chamber is wasted. Now we also have to consider how the barrel itself alters the pressure curve via twist rate. That is liken to rod/stroke ratio in my mind anyway, as it alters the pressure curve starting & ending pressures as the bullet accelerates.

Also look up "pressure trace" I know a few guys with this hardware & it isn't all that expensive, relative to cars, & can provide a world of info to your testing.