should peak cylinder pressure always occur at 14 degrees ATDC?

[dorset]

hey folks

i run old motorcycles in LSR, but i have a general question.

when an engine is running with a certain amount of spark advance, the cylinder pressure after ignition before TDC will be rising due to both expanding gases and the compression due to the piston. then after TDC, the prior compression of the piston going up will be cancelled by the de-compression of the piston going down, but the pressure due to combustion will still be rising, until the exhaust valve opens.

i think its correct that maximum power-- maximum BMEP-- will occur when combustion (not pressure rise!) is exactly half-over at exactly TDC. is this right?

the crankshaft position at average pressure has to be after TDC or the engine will be trying to overcome forces that want it to run backwards. but best power will be with peak pressure some rotational distance after TDC. i've heard that best power will occur by having peak cylinder pressure at around 12 to 14 degrees after TDC. so here's my question.

is this 12-14 degrees ATDC a fixed geometric thing, or is it variable and derived from some esoteric feature of gas dynamics?

if it's geometric, i'm wondering whether it's affected by rod length, in that the longer rods cause the piston to dwell longer near TDC than short rod motors. in a short rod motor, th episton is raised, jerked to a stop, and then yanked down, faster than in a long rod motor. so would this affect the degrees after TDC that peak cylinder pressure occurs? long rod triumph 650s have an ignition advance of 38 degrees BTDC. short rod triumph 750s use the same advance, as far as i know. should they, or should they be tweaked one way or another? both use similar two valve hemi heads.

if its gas dynamics, what would cause the optimum degrees ATDC for peak cylinder pressure to be different from 12-14? would changing camshaft timing change the position of optimum cylinder pressure? say i took an engine with a pressure peak at 14 ATDC, and retarded the valve timing four degrees. would the position of th eoptimum pressure peak still be at 14 ATDC?

i don't understand this stuff very well, and i'm interested in hearing what other people think.

[PackardV8]

Interesting question.

Aren't there many variables which determine ideal ignition advance? Given ignition advance can vary over such a range, typically 20 - 40 degrees BTDC, wouldn't that influence when peak cylinder pressure occurs?

And how does combustion chamber design and cylinder diameter figure into the equation? "Better" chamber design LS heads make more power with less initial advance than does the bad old days 23-degree heads.

And then, there's RPM making it a moving target. The time for the peak pressure to occur half as long at 10,000 RPM than at 5,000.

[modok]

If you adjust the timing for maximum power then the peak pressure will probably be at 12-14, it's true.
Why??? is an interesting question, but.... I don't think knowing why is very important.

The amount of ignition advance the engine needs TELLS you how fast the burn is, I suppose in order to accept that, one needs to accept the idea that peak pressure should be centered at a narrow range of crank angle regardless of rpm. So, maybe it is important to know, kinda, in that respect

[Nut124]

How would anyone know where the peak pressure is, or what it is, in real life, other than software simulation?

[CMIdiesel47448]

Cylinder pressure transducer output referenced to crank angle and measured with high speed data acquisition. Easy peasey...

How would anyone know where the peak pressure is, or what it is, in real life, other than software simulation?

[Nut124]

Cylinder pressure transducer output referenced to crank angle and measured with high speed data acquisition. Easy peasey...

How would anyone know where the peak pressure is, or what it is, in real life, other than software simulation?

Really? I'm impressed. I did not think such things existed. I work in precision industrial controls, although more like in the 1 second real time vs microsecond or nanosecond.

Is this factory instrumentation or aftermarket?

[pastry_chef]

Is this factory instrumentation or aftermarket?

One example.
tfxengine.com

[CMIdiesel47448]

Yep, that’ll do...

Is this factory instrumentation or aftermarket?

One example.
tfxengine.com

[rebelrouser]

I was under the impression that engine RPM, Fuel burn time, rod stoke ratio, stroke length, and the amount of dwell time of the piston at TDC was the determining factor for ignition timing.

You want the mixture to burn just as the piston reaches TDC so the full force drives the piston down the bore. Late timing and the piston is running away from the flame front, early timing and the piston is pushing against the flame front. At least with the engines I have raced, the big Mopar's with a long stroke like more advance and the small block Chevy's for example like less. On a good cool day, my Mopar runs best with 38 to 40 degrees timing, never ran a small block Chevy that liked much more than 34 to 36 degrees.

[David Redszus]

hey folks

i run old motorcycles in LSR, but i have a general question.

when an engine is running with a certain amount of spark advance, the cylinder pressure after ignition before TDC will be rising due to both expanding gases and the compression due to the piston. then after TDC, the prior compression of the piston going up will be cancelled by the de-compression of the piston going down, but the pressure due to combustion will still be rising, until the exhaust valve opens.

i think its correct that maximum power-- maximum BMEP-- will occur when combustion (not pressure rise!) is exactly half-over at exactly TDC. is this right?

the crankshaft position at average pressure has to be after TDC or the engine will be trying to overcome forces that want it to run backwards. but best power will be with peak pressure some rotational distance after TDC. i've heard that best power will occur by having peak cylinder pressure at around 12 to 14 degrees after TDC. so here's my question.

is this 12-14 degrees ATDC a fixed geometric thing, or is it variable and derived from some esoteric feature of gas dynamics?

if it's geometric, i'm wondering whether it's affected by rod length, in that the longer rods cause the piston to dwell longer near TDC than short rod motors. in a short rod motor, th episton is raised, jerked to a stop, and then yanked down, faster than in a long rod motor. so would this affect the degrees after TDC that peak cylinder pressure occurs? long rod triumph 650s have an ignition advance of 38 degrees BTDC. short rod triumph 750s use the same advance, as far as i know. should they, or should they be tweaked one way or another? both use similar two valve hemi heads.

if its gas dynamics, what would cause the optimum degrees ATDC for peak cylinder pressure to be different from 12-14? would changing camshaft timing change the position of optimum cylinder pressure? say i took an engine with a pressure peak at 14 ATDC, and retarded the valve timing four degrees. would the position of th eoptimum pressure peak still be at 14 ATDC?

i don't understand this stuff very well, and i'm interested in hearing what other people think.

OK, you asked for it.

The instant torque curve is the product of several curves that interact with each other with each crank degree.
Each curve has a different amplitude and shape. The are: compression pressure curve. combustion pressure curve, crank pin lever angle curve and piston inertial curve.

The Dynamic Compression Ratio will produce a bell shaped pressure curve centered about TDC. It will require energy to raise the pressure before TDC, which will be returned after TDC. Its shape and position will not change with rpm.

The Combustion Pressure curve, in a pressure bomb is also a bell shaped curve, with a peak pressure that will occur at about 50% mass fraction burned. But, in a running engine, the piston is moving downward after TDC which increases chamber volume and reduces combustion pressure even while the mixture is still burning. Consequently, peak combustion pressure will occur (50% MFB) about 15 to 20 deg ATC, depending on ignition timing and flame speed.

The crank pin lever curve is a function of stroke radius and con rod length. The stroke radius (one half of total stroke) is the dominant factor, while rod length is a weaker factor. Together they determine the crank angle that produces the greatest leverage on the crank.The lever ratio is zero at TDC and will increase with crank angle. The maximum leverage is typically about 72 to 76 deg ATC, depending on the specific engine.

The piston inertial curve is also determined by the stroke radius and rod length. And by piston speed, but
not by rod ratio.

When the gas pressure curves are converted to gas forces (while acting on the piston crown area), and combined with crank pin leverage and piston inertia forces, we have a resulting instant torque curve. If we calculate the average torque over the entire cycle, we have a torque curve. And if we multiply by engine rpms, we have horse power.

The torque value will decrease with increasing rpm. But what is lost in torque is made up by more power strokes
per unit time.

Realizing that each of these four curves can be shifted around, due to mechanical features and combustion functions, it is easy to understand that several different combinations can produce the same power output. Of course, ultimately it all depends on how much fuel we can completely burn.

Once again, the factors that make an engine work are invisible but can be measured. They can be seen with a good engine simulation program that does much more than predicting horsepower.

[Roundybout]

I’ve been reading a lot of papers on detonation. One take away I’ve noticed is the closer the peak cylinder pressure is to just after TDC, say 3-4 degrees the more you’re on the verge of detonation or in detonation. If it happens before that we call that an oops and pick up parts of course. The happy spot seems to be what’s been mentioned as 14-20 degrees ATDC as optimum? But that’s not the best mechanical leverage angle. What if we could have peak combustion pressure then?

[Geoff2]

My understanding is that the 14* number is for best mechanical leverage, ie the crank angle where the fully burned/expanding gas can exert maximum force to rotate the crank. Obviously at TDC, any combustion pressure does not rotate the crank, but tries to push it through the bottom of the engine.
Hence, max force applied a few degrees ATDC.

[ptuomov]

My understanding is that the 14* number is for best mechanical leverage, ie the crank angle where the fully burned/expanding gas can exert maximum force to rotate the crank. Obviously at TDC, any combustion pressure does not rotate the crank, but tries to push it through the bottom of the engine.
Hence, max force applied a few degrees ATDC.

I think it’s not, instead I agree with David that best mechanical leverage is at about 75 degrees ATDC. Think of it as the cousin of the maximum piston speed crank angle.

The way I prefer to think about it is the following. Suppose that the cylinder pressure is about constant regardless of where the piston is (untrue, but taken into account elsewhere). Suppose that the pressure moves the piston down by 0.001 inch. The pressure times piston area equals the force and force times distance equals work done. You get the most torque at the crank angle at which the crank rotates the least number of crank degrees when piston goes down by 0.001”, or where this work done moves the crankshaft the least. That also happens to be the maximum piston speed crank angle when the crankshaft rotates at constant speed. Or did I make a logical error?

The reason why the peak cylinder pressure is usually optimally located at 14 degrees ATDC in an engine that is not knock limited is that combustion takes time. If you could burn the charge immediately in an instant without any mechanical strength concerns, you would want to burn it at the top dead center. This way, you would get the greatest expansion and cooling of the gas during the power stroke and this means that you would also be extracting the most work from the combustion event and wasting the least amount of energy thru the exhaust pipe. Again, subject to the caveat that high school physics is no longer fresh in my mind.

[ptuomov]

I’ve been reading a lot of papers on detonation. One take away I’ve noticed is the closer the peak cylinder pressure is to just after TDC, say 3-4 degrees the more you’re on the verge of detonation or in detonation. If it happens before that we call that an oops and pick up parts of course. The happy spot seems to be what’s been mentioned as 14-20 degrees ATDC as optimum? But that’s not the best mechanical leverage angle. What if we could have peak combustion pressure then?

It is my understanding that the best predictor of knock is the maximum temperature that he unburned end has during the cycle. If the unburned mixture in the cylinder gets too hot when the flame front burns the mixture still some distance away, the engine detonates. This temperature can be high for four reasons: intake air is hot, there is hot exhaust gas left in the combustion chamber (although there is an offsetting purity effect), compression and combustion processes increase pressure and hence temperature, and the combustion itself directly heats the unvented end gas. You’ll get more knock (holding other factors constant) if the intake air is hotter, hot exhaust gas isn’t evacuated, compression ratio is higher, ignition is more advanced, burn is faster, and/or the burning charge has a way to radiate heat to the end gas. That’s my understanding anyway.

For a knock limited engine the peak power happens when combustion pressure is tuned to peak at 12-14 degrees ATDC. With knock limited pump gas turbo engines, the optimum can be much later peak cylinder pressure because the retarded ignition will allow increasing the manifold pressure.

[Camaromeo]

The otto (gasoline) cycle is a constant-volume cylce.

To achieve max. efficiency (max output), the process (cumbustion) would have to happen in "constant" volume, meaning infinitely fast, no piston movement at all.

Thats not possible, since flame needs to travel.

Therefore the tighter the crankdegree window of the whole combustion is, the more it resembles the ideal "constant volume".

All otto cylce engines have the thermodynamically ideal mass fraction burned MFB-50% point at ~8° ATDC, resulting in ideal peak pressure between 10-20° ATDC.

Why about 8°? Its because part of the mass needs to be burned BTDC(!), so the most ideal "ratio between fraction burned before and after TDC" is -> 8° ATDC, resulting in peak pressure betw. 10-20°, with peak pressure being "a function of" the 8° MFB-50% point, depending on "burn rate".

Some engines burn faster, some burn slower, but all share the MFB-50% point of 8° being ideal. Therefore sometimes peak pressure is sooner at 10°, sometimes a littler later at 20°, depending on burn-rate.

So coming back to you question: 14° ATDC "peak pressure" ? -> Depends on your burn rate! If this means, that 50%-MFB is at 8°, then yeah, perfect. If 50%-MFB sooner or later that that -> less than max output