should peak cylinder pressure always occur at 14 degrees ATDC?

[David Redszus]

Dude, it's "area under the curve" thing, a "force across distance" thing, not really a leverage thing.

Dude, to which curve(s) do you refer?
Remember we are discussing "instant torque" resulting from a number of factors so area does not apply.
If we choose to examine the mean torque over one complete cycle, we must then include all negative forces as well as positive gas torque force.

Crank pin leverage is quite important if you can quantify it properly.

An increase in stroke length will increase instant torque and move the crank pin maximum leverage angle closer to TDC.

Stroke .....Torque......angle
1.5"...........1621........76 deg
1.65..........1794.........75.5
1.8............1969.........74

Engine performance is not ALL about combustion gas pressure.

[Tom Walker]

Piston pin offset is predominantly to soften piston slap when cold, the tiny amounts in a production piston would have negligible affect on any power difference, unless you are looking for the tiniest gains.
Now, offsetting the crank to bore center lines, like Toyota does, opens up some areas worth discussion..

[David Redszus]

Piston pin offset is predominantly to soften piston slap when cold, the tiny amounts in a production piston would have negligible affect on any power difference, unless you are looking for the tiniest gains.
Now, offsetting the crank to bore center lines, like Toyota does, opens up some areas worth discussion..

Well said, Tom.

Piston pin offset will affect the piston inertial side forces.
pin offset
thrust side........Thrust........Anti T
+.100".............1732..........-1650
0.000..............1691..........-1691
-.100"..............1650..........-1732

But pin offset will have virtually no effect on piston inertial axial forces.
pin offset
thrust side........TDC...........BDC
+.100".............6504..........-11769
0.000..............6405..........-11766
-.100"..............6505..........-11769

Piston pin offset will affect the angle of peak piston inertial axial forces.
pin offset
thrust side........TDC.........BDC
+.100"............-1.0 deg....177 deg
0.000..............0.0.........180
-.100".............+1.0........180

Whether the piston pin is offset or the crank is offset, makes no performance difference.
However, we are limited to the amount of pin offset possible; crank offset allows much more offset.

[modok]

I mean you don't need to have a crankshaft.
I think all you need is a PV loop and also plotted the volume of the combustion space.

What if we have a very light flywheel, what if we have two crankshafts? what if it's a rotary?
I think your point of most leverage is more like the END of the power stroke, as past that point the rate of change slows down. The majority of the expansion stroke has already passed

[David Redszus]

I mean you don't need to have a crankshaft.
I think all you need is a PV loop and also plotted the volume of the combustion space.

What if we have a very light flywheel, what if we have two crankshafts? what if it's a rotary?
I think your point of most leverage is more like the END of the power stroke, as past that point the rate of change slows down. The majority of the expansion stroke has already passed

Yes, I agree with you. The point of maximum crank pin leverage occurs at maximum piston speed, which is well past peak gas pressure or peak piston force. But, the point to be made, is that several factors go into the production of torque, not merely gas pressure.
And, that if we could produce significant gas pressure at a later crank angle, we will increase gas torque.

But we have not considered: compression pressure, piston inertial forces and friction forces.

If a fuel could be injected into the chamber after TDC (like a diesel or perhaps DI) and the fuel burned completely so that we have higher pressure at a later crank angle, we would increase gas torque significantly.

A large part of the problem is related to the properties of fuel.

Just curious. What is the bore and stroke of a rotary engine?

[dorset]

lol

i'm still here, people, trying to digest this stuff.

please do not think this discussion is unappreciated.

[RCJ]

Can you explain piston inertial forces

[David Redszus]

Can you explain piston inertial forces

Piston axial inertial forces are the result of piston acceleration x piston mass; they are maximized at TDC
and minimized at BDC. Axial forces are a function of stroke length, rod length and rpm. They will change with rpm.

Gas pressure exerts a downward and lateral force on the piston which increases friction. Maximum friction will
occur at low piston speeds and high gas pressure.

When the inertial forces, along with gas force friction is plotted against crank angle, a complex curve results that
will either contribute or detract from instant torque at each crank angle. It can be computed if you remember where
you put your trig book.

[Relic]

Interesting aside on the Honda L13 engine in rgard to this 14 degree figure.

Crank offset is quoted as appr. 13-14mm (cant find exact details)
Crank throw = 40mm (44.7mm L15)
Rod length = 134.5mm (149mm L15)
Bore = 73mm
1340cc 4 pot engine.

Now the thing to bear in mind with this offset crank, is that the effective TDC (where piston pin, big end and main bearing align) also gets rotated after TDC.
What we are interesting in then, is this shifted TDC alignment and the condition where the conrod is paralell with the bore of the cylinder.
Ideally then....
Asin (12.361mm / 40mm) = 18 degrees ATDC
Asin (12.361mm / (40mm + 134.5mm)) = 4.06 degrees ATDC
18 ATDC - 4.06 ATDC = 13.96 degrees effective

L13...
Asin (13.5mm / 40mm) = 19.72 degrees ATDC
Asin (13.5mm / (40mm + 134.5mm)) = 4.43 degrees ATDC
19.72 ATDC - 4.43 ATDC = 15.28 degrees effective

L15...
Asin (13.5mm / 44.7mm) = 17.58 degrees ATDC
Asin (13.5mm / (44.7mm + 149mm)) = 3.99 degrees ATDC
17.58 ATDC - 3.99 ATDC = 13.59 degrees effective

[The SAE quote is actually 12-14 ATDC]
https://www.sae.org/publications/techni ... nt/890759/

[nitro2]

Peak cylinder pressure will always be around 14-18° ATDC.

Nonsense. That is 1990's information from back when people didn't know what they do now.

[Tom68]

Peak cylinder pressure will always be around 14-18° ATDC.

Nonsense. That is 1990's information from back when people didn't know what they do now.

Tell us more, mentioned recently 15 degrees seemed late.

Considering there's a couple of degrees between pinging and not pinging I always thought peak cylinder pressure should occur only a few degrees after TDC where high pressure pushes within a small volume, the later you get there the bigger a volume that needs to be expanded into. Of course with that I'm ignoring geometry.

[Geoff2]

Nope, not 1990s information.

Allen W Cline wrote an extensive article on ign, detonation etc, printed in Contact Mag, Jan 2000. I printed it out, 11 pages.
I believe he is a GM engineer. 14* ATDC was the magic number. He also talks about 'pinging'. It is NOT the pistons rattling in the bores...

[Tom68]

Chatgpt.

User
With an internal combustion engine, how many degrees after TDC should peak cylinder pressure occur to make maximum power ?
ChatGPT
In a typical internal combustion engine, peak cylinder pressure should occur a few degrees after top dead center (ATDC) to maximize power. The exact timing of peak cylinder pressure can vary depending on the engine design, fuel type, and other factors, but it's generally in the range of 10 to 20 degrees ATDC for many gasoline-powered engines.

This specific timing is optimized to take advantage of the expanding combustion gases to drive the piston down the cylinder and maximize the engine's power output. It's important to note that this timing is determined through careful engineering and testing, and it can be different for different engines and operating conditions.

For more accurate and specific information about a particular engine, you would need to refer to the engine's specifications and potentially consult the manufacturer's documentation or an expert in engine tuning and performance.




User
Is it possible that peak cylinder pressure 5 degrees after TDC may in fact make more power than 10 to 20 degrees after ?
ChatGPT
Yes, it is possible that peak cylinder pressure occurring 5 degrees after top dead center (ATDC) can make more power than 10 to 20 degrees ATDC in certain engine designs and operating conditions. The optimal timing for peak cylinder pressure depends on various factors, including the engine's design, compression ratio, fuel type, ignition timing, and other variables.

In high-performance or modified engines, it's not uncommon to experiment with different ignition timing and other parameters to determine the ideal timing for peak cylinder pressure that results in maximum power output. This process is typically done through dyno testing and fine-tuning to find the sweet spot for the specific engine and its intended use.

The best timing for peak cylinder pressure may vary based on factors such as the engine's displacement, turbocharging, supercharging, and other modifications. The goal is to achieve efficient combustion and take full advantage of the expanding gases to produce the maximum amount of power while avoiding issues like knocking or excessive heat buildup.

So, while 10 to 20 degrees ATDC is a common range for peak cylinder pressure in many engines, it's not a fixed rule, and engine tuning and optimization can lead to variations in the ideal timing for different applications.

[Tom68]

Chatgpt again.

User
Are there people that think engine pinging is the pistons rattling ?

ChatGPT
Yes, some people, especially those who are not well-versed in automotive mechanics, may mistakenly believe that engine pinging is caused by the pistons rattling. The sounds emitted by an engine can be confusing to those who are not familiar with the intricacies of engine operation.

Engine pinging, also known as engine knocking or detonation, is typically associated with improper combustion in the engine's cylinders, resulting in a sharp, metallic knocking or pinging noise. It is not related to the pistons physically rattling within the engine.

Piston rattling, on the other hand, refers to a different issue, such as piston slap. Piston slap occurs when the pistons make a slapping noise against the cylinder walls, usually when the engine is cold or when there's excessive wear in the piston and cylinder wall surfaces.

While engine noises can be confusing and sometimes misattributed, it's important for vehicle owners to seek professional advice and diagnosis from a qualified mechanic when they encounter unusual sounds coming from their engine. Proper diagnosis is crucial to identify and address the specific issue, ensuring the engine's continued performance and longevity.

[ptuomov]

Can you ask the chatgpt version you’re using which gsx-r750 l1 aftermarket exhaust makes the most peak power on a dyno?

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