Engine Tuning - In Cylinder Pressure - Supercharged Engine

[nitro2]

This graph shows indicated torque for each of the cylinders vs. rpm, as determined from the measured pressures. You can see that the individual cylinder torque contribution tracks pretty closely with the energy release in each cylinder (see previous graph) and that Cylinder 7 leads the way despite there being minimal differences in VE between the cylinders (see previous graph). The amount of energy released in the cylinder is not the only contributing factor to torque/power, but it is certainly king.

Torque vs RPM vs Cylinder Number.jpg

More info to follow.

[MadBill]

Interesting that it is #7 that outperforms and #5 that lags. If this engine has a x-x-x-x-x-5-7-x FO, the former's induction cycle occurs 90° after the adjacent latter's so it may be robbing some of its charge.

[nitro2]

Interesting that it is #7 that outperforms and #5 that lags. If this engine has a x-x-x-x-x-5-7-x FO, the former's induction cycle occurs 90° after the adjacent latter's so it may be robbing some of its charge.

Yes to a degree (see VE graph). However the energy release/torque advantage of Cyl 7 over the rest outpaces the VE advantage.

[Nut124]

Very interesting. Thanks for sharing.

Am I reading your charts correctly: VE 200% +?

[MadBill]

Clint, does the TFX system support adjusting the scales on graphs? In the last two examples shown it's not too much of an issue, but fine detail would be more discernable if the Y axis values ran from say 150 to 300.

[jacksoni]

Very interesting. Thanks for sharing.

Am I reading your charts correctly: VE 200% +?

Supercharged engine.

[nitro2]

Clint, does the TFX system support adjusting the scales on graphs? In the last two examples shown it's not too much of an issue, but fine detail would be more discernable if the Y axis values ran from say 150 to 300.

No problem, see zoomed graphs.

Zoom VE vs RPM vs Cylinder Number.jpg

Zoom Torque vs RPM vs Cylinder Number.jpg

[Belgian1979]

Interesting that it is #7 that outperforms and #5 that lags. If this engine has a x-x-x-x-x-5-7-x FO, the former's induction cycle occurs 90° after the adjacent latter's so it may be robbing some of its charge.

Yes to a degree (see VE graph). However the energy release/torque advantage of Cyl 7 over the rest outpaces the VE advantage.

Exhaust playing a role maybe? Not sure about that engine but nr 7 usually has a better exhaust pattern than nr 5. 1 has a better approach to the collector as well but a longer tube (usually)

[nitro2]

Interesting that it is #7 that outperforms and #5 that lags. If this engine has a x-x-x-x-x-5-7-x FO, the former's induction cycle occurs 90° after the adjacent latter's so it may be robbing some of its charge.

Yes to a degree (see VE graph). However the energy release/torque advantage of Cyl 7 over the rest outpaces the VE advantage.

Exhaust playing a role maybe? Not sure about that engine but nr 7 usually has a better exhaust pattern than nr 5. 1 has a better approach to the collector as well but a longer tube (usually)

Supercharged engine. Exhaust can be an issue at very high boost/very high rpm on a supercharged engine, but this engine doesn't reach those levels.

[nitro2]

Time to look at some lower hanging fruit.

So far problems to fix are:

1) excessive cylinder to cylinder variation

2) excessive cycle to cycle variation

3) VE variation

4) Energy release variation between cylinders that is only partly due to VE


Next up is timing of the combustion process. This is low hanging fruit. Fixing the combustion process timing for every pressure instrumented cylinder, is easily done and it only takes one engine test to do it.


First some background info, before looking at fixing the combustion process timing.

Three things make IMEP, torque, power at a specific rpm : 1) the amount of energy released in the cylinder per combustion cycle 2) the rate of the energy release during a combustion cycle, 3) the timing of the energy release relative to piston motion i.e. the timing of the combustion process.

So how much does the timing of the combustion process matter ?

Below are 2 consecutive combustion cycles for Cyl 7 (the better running cylinder) which just happen to have essentially the same energy release per cycle (see circled ENR values of 831 and 834), but the timing of the combustion process is different. You can see the large difference in cylinder pressure, IMEP, torque,and power between the two cycles.

The circled value APP indicates the efficiency of converting the energy release into power (APP is not to be confused with thermal efficiency). The late occurring combustion (cycle 320) has an efficiency of 45.9 while the better timed combustion (cycle 319) has an efficiency of 51.5. The better timed cycle makes 12.6 % more power even though the energy release is essentially the same.

Actually neither cycle has the combustion process timed optimally, as this data is from the engine as tuned by conventional methods, as previously mentioned, but cycle 319 is a lot better than cycle 320 in terms of bang for the buck.

Combustion Process Timing - Cycle 319 - Cyl 7.jpg

Combustion Process Timing - Cycle 320 - Cyl 7.jpg

So next up is how to get the combustion process timed right, for every cylinder, for the entire rev range, in just one engine test. Easy as pie.

[nitro2]

As mentioned before you have to have the pressure traces in order to do cylinder pressure tuning of course, BUT simply looking at the pressure traces is only about 25% of the benefit of having them. The greatest benefit of having the pressure traces in hand is to have software that can analyze the data and generate other types of plots, things too complicated for a person to grasp by just looking at pressure plots.

One of these analysis parameters is what we call CMBL (combustion location). The software AUTOMATICALLY analyzes and generates these important parameters such as the CMBL. No one has to mess around in Excel with formulas and the like, the software does it on its own.

The software analyzes every combustion cycle and determines how many degrees sooner or later that the combustion process should have occurred in order to have made the most power/torque on that cycle, then a plot of the whole test is generated vs. rpm. After just one test you can then see how much the ignition timing should have been advanced/retarded for all engine speeds. The software does this for every test.

CMBL is a target, it is NOT based on peak pressure location nor is it based on MBF 50 (50 % mass fraction burned). CMBL is based on analysis of the entire combustion process.

A CMBL graph is shown below for cylinders 5 and 7 vs RPM. You want the CMBL trace to be located at zero, for the entire rpm range, if using head mounted combustion pressure sensors, this will ensure you are making the most power/torque out of the energy you are getting from the burning A/F mixture. Obviously the CMBL plot for these cylinders is not at 0.

The difference between the CMBL plot and the target (which is 0), tells you how much the ignition timing has to be changed at different engine speeds so that it is perfect for the next test (assuming you changing nothing but ignition timing for the next test).

The amount you change the ignition timing is NOT the amount indicated on the CMBL graph, it is a known percentage of the amount indicated on the CMBL graph. As an example if the CMBL value at some rpm is +9 then you need to advance the timing 3 degrees for it to be perfect.

CMBL vs RPM for Cyl 5.jpg

CMBL vs RPM for Cyl 7.jpg

As a side note, in general engines with a good combustion process will be sensitive to timing and ones with a poor combustion process will be less sensitive to timing.

[nitro2]

Backing up a little bit. Most people assume cycle to cycle variation of cylinder pressure in an engine would look like this :

Cylinder Pressure - Cycle to Cycle Variation - Diesel Engine
https://app.photobucket.com/u/nitro1385 ... 51bcc245c0


It does look like that, for a diesel engine. It does NOT look like that for a spark ignition engine (see examples of cycle to cycle variation from the supercharged engine on PG 4). Cycle to cycle variation on a spark ignition engine is much greater.


Think of standing 100 yards away from a target and shooting a pistol, then think of the same but with a rifle and scope. The diesel engine is like a rifle with a scope, every shot is just about the same. A spark ignition engine is like a pistol, every shot hits a different spot, your goal (one of them) is to get as many shots as possible as close as possible to the bullseye with the pistol i.e. get as many combustion cycles as possible as close to right as possible.

[nitro2]

Just to make it clear, if it isn't already, the supercharged engine being discussed here so far is not a street car with a blower stuck on it by a hobbyist, it is a near 2000 HP professionally built engine, probably costing in the neighborhood of $50 to $70k. The graphs I am putting up here show what was going on inside the engine prior to cylinder pressure tuning.

A person (including us) never knows what's going on inside an engine until they look. The engine could have had better cylinder to cylinder variation, it could have had better cycle to cycle variation, the timing could have been better, and so on, it was anyone's guess, the nice thing about "seeing" inside the cylinders and finding problems is that every single problem that is improved (doesn't have to be eliminated, just improved) adds power/torque so the more problems found, the more power/torque gained, as well as reliability.

[nitro2]

Getting back to timing of the combustion process, looking at Cyl 7 the more powerful cylinder of the 4 that were instrumented (1,3,5,7).

It's easy to use the CMBL data to set the ignition timing right to get the combustion timing right, for the whole rpm band for the very next test, that is very low hanging fruit, BUT there is more to see. The CMBL plot that I posted is a moving average of a small number of cycles, so that one knows where to set the ignition timing, as per graph below.

CMBL vs RPM for Cyl 7.jpg

The question is how much cycle to cycle variability is there in the location of the combustion process. We know there is a lot of cycle to cycle variation in the pressure traces so it stands to reason that there will be a lot of cycle to cycle variation in the location of the combustion process relative to the optimal location (0 on the CMBL graph). In the graph below the red line shows the variation in the location of the combustion process relative to optimal, the pink line is the moving average (the same as the red line in the graph above). The cycle to cycle variation in CMBL is huge and remember the target for every cycle for the CMBL value is 0.

CMBL per Cycle vs RPM for Cyl 7.jpg

You want the CMBL trace to be 0 straight across the rpm range BUT that is not achievable in a spark ignition engine. However 0 (+/-5) degrees is achievable. This conventionally tuned pro engine is roughly 11 (+/-12) degrees on the best cylinder. Every cycle that has a CMBL value that is not 0 leaves something on the table, power is lost for each of these cycles and the further away from 0 the greater the amount of power that is lost. Sure the engine as a whole might seem impressive because it is a big expensive engine with a blower and makes around 2000 HP, and not just everyone has one of these in their backyard, BUT there are many problems happening when you look inside at what is actually going on.

Addressing the engine problems (shown so far) gives you more power/torque without adding boost, and as a BIG side benefit also allows you to go significantly higher in boost before you run into trouble (excessive pressures or detonation or pre-ignition).

It is common for a tuner or engine builder that isn't familiar with this of data and knowledge to be intimidated, but the fact of the matter is that the equipment is nothing more than a tool, it shows you problems you didn't know you had, so you can fix them and make more power/torque and increase reliability.. The data may be unfamiliar, but how one goes about fixing a problem is in familiar territory because EVERY PROBLEM gets fixed by physically changing something on the engine, and every tuner and engine builder is an expert at changing things.

Next up energy release vs cylinder pressure.

[Roundybout]

This all is very eye opening. I feel like I should be paying for this education.