Feature request - Boost control X axis

Testing and development of Megasquirt 3

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DisgruntledMotorist
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Feature request - Boost control X axis

Post by DisgruntledMotorist »

Request to change the X axis from RPM to Mass Airflow (g/s) on the following tables:
Boost Control Duty Table
Boost Control Bias Duty Table
DisgruntledMotorist
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Re: Feature request - Boost control X axis

Post by DisgruntledMotorist »

How do feature requests normally work?
Should I expect a response with reasoning of why/how the feature can not/will not be implemented?
Or should I expect to have my request completely ignored?
muythaibxr
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Re: Feature request - Boost control X axis

Post by muythaibxr »

At the moment both James and I have been busy (me with a new job).

This wouldn’t be that hard and can be put on the list.
Megasquirt is not for use on pollution controlled vehicles. Any advice I give is for off road use only.
DisgruntledMotorist
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Re: Feature request - Boost control X axis

Post by DisgruntledMotorist »

Amazing.
Thanks for the reply. I honestly do think that directly modelling the airflow (as opposed to indirectly via rpm) will give much more robust boost control, as it will help smooth out issues relating to changes in air temperature and atmospheric pressure.
TurboBob
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Re: Feature request - Boost control X axis

Post by TurboBob »

This is an interesting idea, Certainly worth trying.

Perhaps we should put together a "boost control top 5" list. so we can discuss what makes sense to develop.

I have done some experiments in years past with the spool-to-run transitions, including convergence prediction, Smith Compensator, and other tricks.
All work really well in the specific instances where they were developed. The trick is to make it a good general solution.

TB
JoseMiguel
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Re: Feature request - Boost control X axis

Post by JoseMiguel »

A few months (or years) ago, I requested the Y axis on boost control table to have the option to select a simple raw analog input so that anyone could have a boost control, controlled by a simple potentiometer like most other aftermarket ECU's do.

It was completely ignored... so you are lucky that you were not...

I tried coding it myself on MS2, unfortunately, never worked as it should :(
elaw
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Re: Feature request - Boost control X axis

Post by elaw »

Have you guys considered that airflow is dependent on boost?

So effectively you'd be making the boost control target a function of itself... that could have some nasty unintended consequences.
Eric Law
1990 Audi 80 quattro with AAN turbo engine: happily running on MS3+MS3X
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DisgruntledMotorist
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Re: Feature request - Boost control X axis

Post by DisgruntledMotorist »

TurboBob wrote:This is an interesting idea, Certainly worth trying.
I have done some experiments in years past with the spool-to-run transitions
I think the important thing here is to build a control strategy that is dynamic in how it responds to boost demand.
The model inputs in Megasquirt's case are throttle position, demanded boost pressure, actual boost pressure and engine RPM. The output is wastegate position (i.e. duty cycle).

The issues this method of boost control has is that it doesn't compensate for the inertia in the turbine. For a set throttle position, wastegate position and engine speed you will have a variety of boost pressures due to thermal and rotational inertia.
If you set the correct duty cycle to achieve a set level of boost pressure under steady state conditions that same duty cycle will not achieve that same boost pressure under transient conditions. I believe many people will have witnessed this when they see different boost in different gears during a WOT acceleration pull.

Replacing the RPM input with mass airflow (or exhaust mass flow which is simply the addition of the fuel mass to the air mass) will help to reduce the effects of rotational inertia.
Monitoring the exhaust gas temperature pre-turbine (with a thermocouple for example) and calculating the change of turbine temperature over time, a "Turbine Temp dot" or "Turbine Temp delta" variable could be used as an additive (or subtractive if reducing boost demand) value to increase the duty cycle.
For example a large increase in turbine temp in a short period of time (indicative of a throttle stomp) would result in a large addition of duty cycle from the "turbine temp delta" variable. This would help to reduce effects caused by thermal inertia. Conversely if the temp at the turbine is stable (i.e. steady state running) there will be no addition to duty cycle from the "Turbine Temp delta" adder.
DisgruntledMotorist
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Re: Feature request - Boost control X axis

Post by DisgruntledMotorist »

elaw wrote:Have you guys considered that airflow is dependent on boost?

So effectively you'd be making the boost control target a function of itself... that could have some nasty unintended consequences.
The Y axis is boost!

The only issue I can think of is if there isn't a unique solution for the look up table to use i.e. the mass air flow is not increasing with engine speed.
Last edited by DisgruntledMotorist on Mon Mar 12, 2018 11:55 pm, edited 1 time in total.
TurboBob
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Re: Feature request - Boost control X axis

Post by TurboBob »

I can see the "circularity" of using airflow on an axis looking up something that controls boost pressure.

But from a response / tuning perspective, it might make sense as an axis to look up PID gains. Probably a huge pain to tune though.

I do like the idea of having a factor in the target pressure calc that can include a 'boost knob', and other things like that.

TB
vw_chuck
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Re: Feature request - Boost control X axis

Post by vw_chuck »

I developed some boost control a few years back that used mass flow on the x axis. I would say that is NOT what you want on the x-axis. Based on what we found you need to have the pressure ratio on the x-axis not massflow. It is the pressure ratio that matter not the massflow. You can actually have different wastegate duty cycles at the same massflow. I know that sounds strange but it was the issue we ran into. This in turn made it quite hard to tune the overshoot from target. Most all new factory turbo cars use pressure ratio as an axis. FYI.
DisgruntledMotorist
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Re: Feature request - Boost control X axis

Post by DisgruntledMotorist »

vw_chuck wrote:I developed some boost control a few years back that used mass flow on the x axis. I would say that is NOT what you want on the x-axis. Based on what we found you need to have the pressure ratio on the x-axis not massflow. It is the pressure ratio that matter not the massflow. You can actually have different wastegate duty cycles at the same massflow. I know that sounds strange but it was the issue we ran into. This in turn made it quite hard to tune the overshoot from target. Most all new factory turbo cars use pressure ratio as an axis. FYI.
Oh, never mind then. No point in developing something that's been proven not to work.
What was on the Y axis if P ratio was on the x axis?
vw_chuck
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Re: Feature request - Boost control X axis

Post by vw_chuck »

I forget there was a ton of maps that went together to give you wastegate effort. It has been so long. I think the pressure ratio was on the x-axis and I think we had TPS on the y axis but not super sure.
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