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Megasquirt/aftermarket fuel management overview

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Megasquirt/aftermarket fuel management overview

Post by dirtytorque on Mon Aug 10, 2009 1:54 pm

This isn't a Howto more of a genereal overview of how MS or indeed any other ECU goes about the business of making an engine work.
Writing this in small chunks as I have a short attention span.

MegaSquirt Fuel Equation

PW = REQ_FUEL * VE * MAP * E + accel + Injector_open_time

*PW

Pulse width.
Simply the amount of time in milli seconds that the injector stays open to achive our required amount of fuel.

*required fuel constant.
Just think of this as a predefined constant that takes into account your engine size,number of cylinders/injectors flow rate of said injectors and how many times you wish to fire these injectors for any given ignition event.

* VE.
"
The volumetric efficiency (VE) is a percentage that tells us the pressure inside the cylinder versus the pressure in the manifold."
This is what the megasquirt manual says.I prefer this description.

"volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%."
Much better.Cool
Simples.Basically describes an engines ability to suck in charge.(air and fuel) Cool
At 100 % VE the air pressure in the cylinders is the same as the air presure in the inlet manifold.

You can't know your engines ability to suck in air (it's VE) under different conditions unless someone else has previously modelled it for you.So this has to be tuned.And so megasquirt and emerald use a VE table that can be configured by you so that you can educate the ecu to advise it how much air and indeed fuel your engine tends to breath in at a particular engine speed and throttle position.
The reason why Forced induction is so good is that we can create more pressure inside the cylinders than than would normally be present if the engine was just operating at normal atmospheric pressures,effectively increasing the cubic capacity of an engine.I love force induction.Some people think it is crude but to me it is making an engine more efficient and simply increasing displacement is crude and wasteful.
If you don't like FI your gay,but I digress.

* MAP


manifold pressure,as measured in your inlet manifold plenum.
This is obvioulsy closely related to throttle position.Pressure can be either positive or negative,i.e boost or vacuum.

* E Gamma enrichment.
A fractional multiplier.You can ignore this one.Most of the time it will= 1 and so doesn't effect the fueling calculation.The manual doesn't go into much detail about it but it is not important for what I am trying to get across.

*accel + Injector_open_time
All injectors take a finite amount of time to open and so this has to be taken into account.
Also when the throttle suddenly opens or closes fueling has to be adjusted on the fly to compensate for these transient conditions.The accel constant takes this into acount.

ok enuff 4 now.

Just trying to get people used to the jargon.
Any corrections comments welcome.
Don't bother with pointing out spelling mistakes as I'll have to shoot you.
ta. Cool
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Re: Megasquirt/aftermarket fuel management overview

Post by mrbeige on Wed Aug 12, 2009 3:54 am

Mmmmmm transient fuelling....that's one of the hardest things to calibrate!

Anyway top bloke for stating this thread. Should come in handy I reckon Smile

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Re: Megasquirt/aftermarket fuel management overview

Post by dirtytorque on Sun Aug 23, 2009 2:00 pm

Ok
so carrying on with the fueling theme.
It is pretty obvious that under different engine load conditions the engine needs different amounts of fuel.
Stoichiometric at idle because we love are planet and petrol is expensive.
Maybe a little leaner on light cruise.
leaner still on overrun
And then richer conditions for harder driving when the engine is under load.
The below is a VE table.The vertical scale is in kpa.However kpa translates easily into the more familiar Bar pressure scale.10 kpa is 0.1 bar.50 kpa is 0.5 bar.So 100 kpa is 1 bar,i.e atmospheric pressure.Thus anything above 100 kpa is boost.
The horizontal scale is obviously engine speed(rpm) as seen on your instrument cluster.
[You must be registered and logged in to see this image.]

It is used by the ecu to hit predefined air fuel targets.Its numbers are increased or decreased by the tuner until the fueling requirements of the engine at the particular operating condition is met.Below is a fuel target table.MS has a self learning feature as do most engine ecu's so that the fueling can be adjuested under closed loop conditions so that the measured air fuel ratio as measured by the exhausts lambda sensor is the same or close to the specified target air fuel ratio.
[You must be registered and logged in to see this image.]
The below is an example of a fuel target table.

There are other constants that you must set so that the ecu fuel the car properly but once these are done then it is the above VE table and the target afr table that are the major factors of how the car fuels.
It is important to get your VE table to closely follow the behavior of the engine so that the afr's seen at the tailpipe,or in most cases b4 the CAT are the same as in your target afr table.
That way when you decide that you want to run richer/leaner at a certain point in your map table you can just chang your target afr table and then you should see the desired change with your lambda sensor.
Once the VE table is setup it can be forgotten about,unless you change boost levels ,different cam,change engine displacement blah blah blah...
The point is I'm trying to show how these changes relate to parameters in software.
So what about these target afr's?

As hinted at above , under certain engine conditions i.e idle and light cruse;economy and emissions come into play.So at idle an afr of about 14.7:1 will be the most efficient.Nearly all the fuel entering the engine will be burnt to keep the engine turning over and so it will give the best compromise for emissions aswell.
On overrun the engine is under maximum vacuum as the throttle is closed and the engine is turning relatively quickly with alot of momentum so very little fuel is needed to keep the engine turning and so you will see 15:1 16:1 and maybe even 17:1 afrs briefly under these types of conditions briefly.
Under WOT throttle conditions when low vac reading and on FI cars,boost readingas are seen;the engine needs richer afr mixtures to ensure that all the air that is entering the engine has anough fuel to combine with to combust.Extra fuel is also used to help aid cooling for the engine components in the combustion chamber to prevent hot spots forming that may bring on detonation.
You will hear the expression Lean best torque used by tuners and this is basically the leanest air fuel mixture that creates the most torque for the engine at a particular engine operating point.
Conventional wisdom says engine make best torque at an afr of 13:10 and best bhp at 12.5:1.These are just guide lines and on high performance engines especially those that are force fed on higher targets are sometimes used.

So basically these are the main things to have in mind when attempting to set up fuel tables for an engine.Hopefully you can have an understanding of what your paying your tuner for when you take your car to their rolling road for a remap.

clicking on the images should enable you to see the whole image I hope..
Too lazy to resize.
Sorry.
Next i'll try to summarize ignition timing.
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Re: Megasquirt/aftermarket fuel management overview

Post by dirtytorque on Mon Aug 24, 2009 3:40 pm

emerald fuel maps.
This map is using throttle positions on for the vertical scale as opposed to engine MAP.
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Re: Megasquirt/aftermarket fuel management overview

Post by mrbeige on Wed Aug 26, 2009 4:24 am

So how do you go about defining your VE table? Is there a set method for getting it 'sort of right'?

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Re: Megasquirt/aftermarket fuel management overview

Post by dirtytorque on Fri Aug 28, 2009 12:57 am

mrbeige wrote:So how do you go about defining your VE table? Is there a set method for getting it 'sort of right'?

with Megatune you can get a startup table with this.
[You must be registered and logged in to see this image.]

You put some ball park peak torque/horsepower figures in along with the rpm & map pressures that they occur in.
Not sure how you go about getting a base fuel MAP for emerald though.
Would be interested to find out.Emerald also doesn't seem to use a target afr table either?!!!

Would like to learn more about how other managements systems do it too.


Last edited by dirtytorque on Sat Aug 29, 2009 6:43 am; edited 1 time in total
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Re: Megasquirt/aftermarket fuel management overview

Post by tshirt2k on Sat Aug 29, 2009 2:13 am

Goodinfo here mate Good work

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Re: Megasquirt/aftermarket fuel management overview

Post by kevhaywire on Tue Sep 01, 2009 8:32 am

dirtytorque wrote:Not sure how you go about getting a base fuel MAP for emerald though

The old fashioned way Very Happy

That VE guesstimation table seems pretty clever. If it gets the fuelling to within +/- 5% of the target AFRs first time round I'd be amazed!

dirtytorque wrote:Emerald also doesn't seem to use a target afr table either?!!!

According to Emerald's site it has lamdba targetting, but I've not seen it with my own eyes in the software though.

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Re: Megasquirt/aftermarket fuel management overview

Post by dirtytorque on Wed Sep 02, 2009 12:55 am

kevhaywire wrote:That VE guesstimation table seems pretty clever. If it gets the fuelling to within +/- 5% of the target AFRs first time round I'd be amazed!

Nah it will just get you to a state where you can drive the car and then refine it manually or use he autotune feature which I don't like.
Danger with the auto tune is that a rich misfire can be interpreted by the lambda as running too lean and so the situation gets exaccerbated by the ecu then trying to add more fuel.
It is easier to keep track of such things when your doing it manually.
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Re: Megasquirt/aftermarket fuel management overview

Post by kevhaywire on Wed Sep 02, 2009 4:21 am

Yeah Autotune can be a bit hit and miss. Fully adjustable correction table settings to allow for short term spiking and other situations as you describe. Voltage filtering on the lambda input can help too. My DTA's correction table is a rolling update type with adjustable refresh rates and correction bands etc, but it needs a good few journeys to build up a half decent fuelling picture.

Some of the suggested part throttle corrections are wild and ridiculous and can be ignored, but the full throttle corrections are usually pretty good.

It will never be as good as a proper map, but it certainly gets you close enough to drive to the dyno!

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Re: Megasquirt/aftermarket fuel management overview

Post by dirtytorque on Wed Sep 02, 2009 2:41 pm

So you can actually see this correction table in software?
Is it user adjustable?
I guess the whole idea is to let the ecu control it though as it will learn typical deviation values?!??

Just for kicks,I mentioned the required fuel number above.
Here is the software in MS used to calculate it.
Just to make the idea less abstract.
[You must be registered and logged in to see this image.]


Last edited by dirtytorque on Fri Sep 04, 2009 4:25 am; edited 2 times in total
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Re: Megasquirt/aftermarket fuel management overview

Post by kevhaywire on Thu Sep 03, 2009 5:19 am

Yep, it's a 20x14 table mirrored from the base fuel map and is indeed user adjustable.

As each fuel cell is passed through, the correction table compares the actual wideband AFR with the target AFR and then builds up an average correction figure. I check the correction map every 100 miles or so, but the full throttle areas are never any more than 5% adrift. The part throttle areas still need a lot of work though Very Happy

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