Remaps and ECU tweaking

[obi_waynne]

I'd be curious to know more about engine maps.

What factors does a map consider, I presume rpm is only a small factor now. I guess engine load, throttle position and knock sensor readings play a part.

What is the difference between an economy map and performance map.

Hybrid maps for both economy and power I presume give economy at low rpm and performance at higher rpm?!

 

[RobBentley]

Hi Wayne.

I'll try and help you with a little insight here, without doing myself out of a job

This depends entirely on the engine and the ECU fitted. The maps on say a Siemens ECU and the way they are laid out is different from say the Bosch ECU that'll be fitted to your car. There are of course fundamental differences in maps between Petrol and Diesel given the Diesel is controlled by volume of fuel injected (mg/stroke) as the Diesel has no throttle.

In case of a Diesel... you'll find multiple different maps are considered before for the overall fuel injection quantity, start of injection etc are finally decided upon.

At the simplest level a Bosch EDC16 ECU has a 'drivers wish' map. This has RPM down one side, and engine load down the other side. These will normally step in say 500rpm and 8% load increments and the ECU will interpolate between the pre-set values. It can then determine when you request a load of 38% at 2000rpm that the fuel injected should be X mg/stroke.

This doesn't go into the engine though, its just the initial value suggested. It'll then go through a 'smoke limiter' map which will cap the fuel at different RPM based on quantity of air to stop you bellowing black smoke out. Beyond this there are torque limiters. Some are based on load vs revs, some on rpm alone. Some will be based on temperature etc. Again all of these are considered before arriving at the lowest value which doesn't violate any map. Remember all this is done per stroke so imaging the calculations your ECU is having to do at 4000rpm.

Obviously this is for injected quantity of fuel. There will be similar maps for Start of Injection (SOI) as to how many degrees of timing when the relevant decided IQ (Injected Quantity) will be started and of course from all this is needs to calculate from its duration calibration maps, how long the injector needs to stay open for - to achieve that IQ.

Of course (are you bored yet?)... the fuel rail pressure will also determine the IQ as higher pressures flow more fuel in the same amount of time. At higher RPM there won't be enough time to inject the fuel, before you find the flame is chasing the piston and producing no power - so the fuel pressure will increase with RPM (and there is a map for this too).

With a Turbo Diesel (You've now fell asleep haven't you?) there is also not a fixed quantity of air because the car is turbo charged. So now you've a series of boost maps to go through. There will be requested boost maps in the same way in fuel there was the drivers wish map, but again there will be absolute boost limiters (safety cut off if you like) which could be higher or lower than this. Wastgate Duty Cycle maps to help the car be able to achieve the boost its being asked to achieve. The ECU will of course make minor corrections on these, but it gives it a base to work from. You'll then have overall pressure limiter maps.

Economy vs Performance is about getting a better burn from the fuel. Building the boost earlier gives better cylinder charging which helps, and also releasing torque lower down allowing the driver to use taller gears (less rotational friction) is another factor... amongst many other things we've worked out, so we have a job

I hope that gives you some sort of insight as to whats going on

 

[obi_waynne]

Thanks, a really informative post!

You guys know your stuff. I'll be in touch soon about a remap for my car - finally ! I've heard good things about you & your prices seem reasonable. TorqueCars members & admin staff get a discount do they?

 

[RobBentley]

Thanks, a really informative post!

You guys know your stuff. I'll be in touch soon about a remap for my car - finally ! I've heard good things about you & your prices seem reasonable. TorqueCars members & admin staff get a discount do they?

No problem Wayne. My pet hate in this industry is the 'black magic smoke and mirrors' quite often found. If you're modifying your car in any other way, suspension, brakes etc... The guy doing it tells you whats happening - the same cannot be said for remapping in general.

Of course - I'll always look after TC staff and members with suitable discounts

 

[RobBentley]

All the above was based on a modern Turbo Diesel. For the petrol owners amongst us, I'll try and run through the same sort of brief analysis of whats happening...

Firstly, Petrol is different as you've got spark, a throttle and much tighter air fuel ratio's to consider to stop the car running too hot. In the case of my Audi, a Turbo Direct Injected Petrol, again with Bosch ECU you'll have throttle maps as everything really is going to be based around the amount of air going into the engine, so the correct amount of petrol can be injected to maintain a proper AFR (Air Fuel Ratio). You'll have a map for this which states for X percentage of throttle peddle pressed, the throttle value should open by X percent. It'll also have an axis in this map which is a reference to how much air will flow at any given throttle angle. Obviously compensations will be made for this by way of Lamba, Air Flow meters, boost pressure etc - but its a base and tells the car how much to open the throttle valve and will also have a massive effect on your throttle response too. A map that opens the throttle value with less peddle press, is of course going to have a sharper throttle than one that doesn't. This will be one of the reasons tuners talk about improved throttle response. You'll also have a maximum throttle value angle map which is the maximum throttle angle for any given RPM (usually lower than your other map). This is a great way for manufacturers to starve power on lower models FYI... cap the throttle opening.

You'll then have engine load request maps, which will be engine load as a percentage of air vs RPM vs the percentage of torque you're looking for, from 0% to 100% and various throttle opening vs airflow calibration maps and quite often for any given amount of air vs RPM a map to cap things based on exhaust temperatures so if you're going nuts for a long time, you don't go and melt everything. So for certain volumes of air, at certain RPM you'll be allowed a maximum EGT.

There will be torque maps to determine how much air is required at different torque requests and calibration maps to go with these to compensate for different conditions, tempratures, speeds and gears. There can quite often be different versions of all these maps for 'sport' modes to either produce more power, or better throttle response.

So now we've determined the throttle opening, and the volume of air going through for the requested load we now need some fuel. This is tightly controlled by the lamba sensors, to maintain a correct air fuel ratio. At lower RPM and low engine load you'll have a requested lamba of 1.0 (which equates to 14.7:1 AFR) the ratio when unleaded petrol should in theory completely burn with no excess air or fuel. You're not rich, your not lean, your perfect and this gives the best fuel economy. The problem is at higher RPM/load the engine will get too hot. The Lamba maps will determine that at higher loads, air flows and RPM to pull back the lamba and richen the mixture to keep the engine cooler. This not only helps protect the engine from high cylinder temperatures but it can also help protect again knock/timing as cooler temperatures are less prone to early detonation.

You'll also have turbo pressure maps (and limiters) and wastegate maps as discussed above on the Diesel, so I won't repeat all that.

There will be RPM (rev) limiters. These can quite often be per-gear, and different based on temperature. You'll often see a slightly lower RPM limiter when the engine is cold and of course vehicle speed limiters. Most commonly set on the german machinery at 250kph (155mph).

We've now got fuel and charged air, so its time for spark to make it all go bang. You'll have various spark tables for different conditions but essentially they're rpm vs volume of air (as of course, volume of air is your effective load) and these tables will determine your degrees of timing. Again the ECU can make changes here. You'll have a base map and an ideal map. Think of them as upper and lower limits. The ECU will try and get to the ideal map providing knock detection doesn't come into play. Obviously if we can't even achieve the base map - this is when you're going to get an engine light and fault code.. usually something along the lines of spark advance lower limit exceeded.

There's a tonne of other things going on in a modern petrol, but essentially unlike a diesel its all based on volume of air (which in turn is your load) and the RPM of the engine.