Stoichiometric Control

[Dynamotive]

I have an application where an engine (which I can chose) will be installed permanenty on a test bed. The purpose is to control Stoichiometric at all times. i.e. no open loop control.

I am new to this and although I understand control system in general I am not an expert in ECU technolgy.

Is there an ECU that controls to Stoichiometric conditions at all times?

If not can one be made to do this? I assume that the it goes from closed loop to open for 2 reasons - power and protection of over temperature of exhaust gases. The latter I can live with this as the exhaust temperature will be monitored seperately.

What kind of accuracy of Stoichiometric Control can I expect. I understand I would need a wide band ECU? Is Stoichiometric Control implemented using PID alogorithmns?

The ECU apart from providing Stoichiometric Control

Any help or comments greatly appreciated.

[Matt Cramer]

There's a couple more reasons why Megasquirt does not work in a 100% closed loop mode:

Oxygen sensors are a bit more failure-prone than some of the other sensors because they are exposed to higher temperatures. A purely closed loop ECU can cause a lot of trouble if the sensor fails - or if misfires throw its reading off.

Also, open loop has a somewhat slower response time. There are a couple logs on this forum that demonstrate this very well - an engine that has its VE table off and is relying on EGO correction to bring things under control may show the AFR bouncing up and down as it corrects, overshoots, and returns, while the same engine with better tuned VE tables will have a nice, steady AFR reading.

Is there some particular reason you must run a strictly open loop system?

[Dynamotive]

Matt

Thanks for your response.

The purpose of this rig is not related to the efficiency, performance or even long life of the engine.

The critical requirement is to be able to produce a measurable flow of gas (grams / second - using a sophiticated mass flow sensor) under Stoichiometric conditions. We simply adjust the engine speed and to increase the exhaust flow and use a simple water brake dyno to absorb the power the engine is producing.

The rig is to actually test exhaust systems.

I assumed I need to always be in 'closed' loop control for Stoichiometric using a wide band oxygen sensor. Having looked at the assembly program for the mega squirt it might be difficult to achive what I want unless I can heavily modify it.

Is it true to say that for stoichiometric control is completly a function of the duration that the injectors are open for and nothing to do with the point in the cycle they are first opened. In other words if I used a mega squirt injector firing circuits to trigger another board I design that controls the actual width of the pulses. This special board would take a wide band oxygen sensor as its feedback and create the actual pulse width required, (within limits, ie a min and mac pulse width)

Still learning so please feel free to correct me!

Cheers

Andy

[Matt Cramer]

In that case, I would not try a pure closed loop, but rather set Megasquirt to work in its usual way, speed density with EGO correction. You do not necessarily need a wideband sensor if you want to hold the mixture at stoichiometric; that's more important if you want to hold a steady mixture elsewhere, such as around 12:1.

A combination of good speed density control with EGO correction will often do a better job of holding the AFR at stoich than a purely closed loop system, for the reasons I have outlined. Pure closed loop is sometimes like driving by looking in the rear view mirror - it can't tell something has changed until after the change has had an effect on the air/fuel ratio.

[Dynamotive]

Hmmm

Interesting, I think I understand where you are coming from. You are saying that a lookup table is better than a feedback system?

What sort of control accuracy can be achieved? for example 14.7 +/- 0.1, 0.25, 0.5 etc?

Is it only good for steady state RPM / Loads? Have you any idea how long it would take to settle, say ramping from 1000 RPM to 3000 RPM at a reasonable (compatible for engine) loadings? are we talking milliseconds or seconds?

Perhaps you would be kind to PM me your email / telephone. If this is the way we go (and at the moment I dont have any other options) we will need 4 or 5 plus sensors, plus spares, plus possible (probably ) help tuning the first engine. We can choose the engine as well as long as it is easy to maintain and reasonably cheap. The rigs will be installed in Michigan. The intention will be to strip the engine of all other non essenial stuff.

It would be nice to have an engine with a 0-10 volt input to control its RPM ie electronic throttle. Do you know of any such animal? Obviously needs to be seperate to the ECU as we would throw away the standard ECU and use the MS.

Thanks

Andy

Cheers

Andy