ECU Fuel Trim - use in Diagnostics

Fuel trim is technology used to keep the fuelling control operating correctly over vehicle lifetime, in order to comply with emissions regulations. Trim values are accessible via scan tools and this knowledge can help your diagnostic procedures.

What does it do?
Short-term fuel trim (STFT) and long-term fuel trim (LTFT) are expressed as a percentage, Positive fuel trim percentages indicate that the ECU is attempting to richen the fuel mixture, to compensate for a lean condition. Negative fuel trim percentages indicate the opposite. Fuel trim values are generated and stored with respect to engine operating condition – generally speed or load.

Why do you need it?
As vehicles operate over their lifetime, and accumulate mileage. The engine system components accumulate operating hours, and thus experience wear and tear. With respect to the fuelling control system – air flow meters can become dirty and contaminated, and this has an effect on their initial calibration and response characteristic. Oxygen sensors can also become contaminated and this impacts on their accuracy and reliability of the signal. The fuel injection wet system also suffers from long term fatigue effects – injectors can clog, thus affecting their spray pattern, and ability to provide a homogenous charge for combustion. Fuel pressure regulators lose their calibration and accuracy due to the continuous operation mode. Add to these factors, general engine wear, due to loading and thermal effects and it’s clear that over a long period of time, the accuracy and capability of the engine control system, with respect to correct fuelling, becomes compromised. In the past, this gradual loss of accuracy for fuel metering would have been accepted – just wear and tear. However, due to ever tightening emissions regulations, this is no longer accepted! The engine control system must maintain emissions within prescribed limits over vehicle life, and if it can’t (perhaps due to component failure) then the system must inform the driver that his vehicle has exceeded critical limits. So that’s why we need fuel trim – to monitor and compensate the closed loop emission control system, over a long term period (the life of the car).

Adaptive Learning
Fuel trim values are the result of an adaptive learning strategy within the ECU, this monitors the effectiveness of the control system during engine operation. For example, a gasoline engine ECU system will determine deviations from stoichiometry in the exhaust gas content over time. The information is stored in non-volatile memory and used to adjust or offset the fuelling value from the original stored fuel map value, to the required value, taking into account wear and tear factors. This is known as a long term trim value – it is learned by the ECU and it is not lost when the battery is disconnected, it generally can only be reset by a technician with a scan tool. 

In addition, there are short term trim values, these are offset values based on short term effects, in response to temporary changes. Typically, for a gasoline engine, the short term trim values are a result of the closed loop lambda control system, comprising of the pre-catalyst oxygen sensor signal, that helps maintain fuelling at the stoichiometric point. Short and long term fuel trim correction values work together to maintain the correct, required lambda value. The short term trim will provide immediate correction, and if this correction is needed to maintain the required value on an on-going basis, then the long term fuel trim will learn this and provide a permanent offset to the basic fuelling map. Note though, this process is monitored and maintained within calibrated limits that correlate with tailpipe emissions. If the long term trim value becomes greater than a certain, pre-set limit. Then this is used as a trigger in the diagnostic (OBD) system to inform the driver of an emissions related problem, via the MIL (Malfunction Indicator Lamp) and storage of an appropriate DTC (Diagnostic Trouble Code).




Fig 1 - Fuel trim is effected by modifying the basic injector demand, for the engine operating condition

Fuel trim diagnostics
A useful aspect of fuel trim values is the potential of being able to use them as part diagnostic procedures. Many fault coder readers, including low cost, generic devices, can provide fuel trim values. If you know and understand what these values are, and how they are generated, then you can use them to help you understand the nature and root cause of many fuel systems related faults that you may come across.

Assuming you can access fuel trim information, how does it help you? During normal engine operation, the ECU records long (LTFT) and short (STFT) term fuel trim values as ECU labels (accessible via a scan tool). STFT normally cycles at the same frequency as the oxygen sensor switches, in effect it reflects the operation of the closed loop lambda control. LTFT is normally quite stable, and any adjustment of the value is made over a long period of time, so is less noticeable. Remember that LTFT is held in non-volatile memory (also known as KAM – keep alive memory), but STFT is dynamic, thus constantly changes during run-time, but starting at zero, both values re-set when DTC’s are cleared.


Gasoline engines are inherently unstable! The cycle-to cycle, and cylinder to cylinder variations cause loss of efficiency and roughness. So the ECU is continually adjusting spark timing and fuelling to compensate for this fact. As injectors wear, the delivered quantity and spray pattern are compromised, and this can cause cylinder specific fuelling faults that can be identified very efficiently using fuel trim values! The long and short term fuel trim value will reflect any variations in injector delivery or combustion efficiency for a specific cylinder, and can help diagnose faults like leaking blocked or dirty injectors.



Fig 2 - Typical fuel trim values at idle, as shown on a standard scan tool, connected to the vehicle OBD connector. Note that STFT is negative and follows closed loop lambda operation. LTFT is much more stable over time, and is a positive value. Both are within acceptable limits


Trim values should generally not exceed 10% either way. Positive means the ECU is trying to compensate for a weak mixture, it does this by extending the basic injector pulse width. This could indicate faults caused by air leaks in the flow system, manifold or vacuum pipework. In addition, MAF (mass airflow meter) or oxygen sensor issues. Lean mixture problems are the most common failure mode. Negative trims mean that the engine is running rich, and the base pulse width is reduced to compensate. This could indicate problems with EGR, MAF, fuel pressure regulator, leaking injectors or oxygen sensor problems. Individual cylinder contributions can also be evaluated. Disconnecting the injectors one-by-one and monitoring fuel trim values allows the technician, to compare each cylinder with the others. Thus individual misfire or injector problems can be identified clearly.



Fig 3 - Fuel trim - high idle speed. STFT is now positive, LTFT has switched negative, this shows how both work together to achieve the required target of Lambda 1.


Summary
It is worth to remember though that different ECU’s will respond differently to a given fault or diagnostic stimulus, in particular with respect to how LTFT and STFT work together to address a deviation or system problem. This is part of the ECU calibration strategy, so there are no hard a fast rules, but if you understand what LT and ST fuel trims are, and what they do. Then you can observe what you see and make an informed judgement. There is no doubt that being familiar with the concept will help your diagnostics of fuelling related problems. If you have time, it’s always worth making some observational measurements of a known ‘good’ vehicle, and record/note the data. Ready for the next faulty vehicle of that type or make to appear!



12:26:00 AM

Markets Roundup April 2013 (Updated)

Leaf's speeding up sales
Editor's note - I've added new figures to this post, for easier identification of this new data, it will be written in blue.


Nissan Leaf Superstar - World Edition

With the risk of repeating this title a lot in the coming months, the truth is that the japanese hatchback is improving its performance everywhere, in April the Leaf conquered the monthly leadership in two more countries, Germany and Austria, adding them to an extensive list of countries at number one (Nine, to be more precise).

Besides the two countries mentioned above, it's important to say that since the beginning of the year, Nissan added three countries (Denmark, Australia, Estonia) to their #1's list, without losing any of those it had (Norway, UK, Hong Kong, Ireland) in December 2012...Not bad, eh?

Another measure of Nissan's electric car is that the lowest position on the several EV sales rankings is one #4 in Holland, but even this position isn't all that shabby, because it is the best among pure electrics...

Although with a handful of units, nevertheless it's important to register the landing of the Leaf in two more countries: The first four units were sold in Iceland and the first three in the Czech Republic. Will they the 10th and 11th countries in Leaf's list of #1's?

In a different level, the Nissan Leaf improves even further in Norway, in April it was #2 in the global ranking with 3,3% share, that's even before the british built, improved and cheaper 2013 model arrives. Will we then see Nissan's electric car #1 in Norway?

All these good performances country-wise contribute to the global leadership of the Nissan Leaf, holding that position since January and allowing a comfortable 10% margin (24% share vs 14%) over the number 2 in the ranking, the Model S. 



Trends in the Great 7 

Of the 7 largest markets for EV's, five (USA, Japan, France, Norway and China) have improved their share, while two (Germany and the Netherlands) saw their EV share decrease.

In the US, Tesla's Model S is leader for the second month in a row, an important feat since it has a price tag several thousands above its main competitors, it will be interesting to see for how long can Tesla keep this title.

In France, the Renault Zoe earns a third consecutive #1 and represents now 48% of the EV market, something that should only improve in the months to come.



Fragmented Markets

While it's normal for this early stage of EV markets to be dominated by one or two models, there are a some countries where things are more fragmented and because of that, more interesting.

Here are three markets where the #1 has less than 30% share:

In the USA, the Model S is leader with 28%, followed by the Volt and the Leaf with 22% and it's this last one, presently #3, that analysts predict to be #1 in the end of this year, so a lot should happen here until then.

With four different leaders in four months, the German EV is in "no mans land" territory, with the #1 (Smart Fortwo ED) managing 29% share and 390 sales, followed by three models at 19% with just 21 units separating them. In December things should be a lot different, by that time VW and BMW should be fighting for #1, leaving present contenders way back.

In Austria and despite the small figures, things are also leveled, with just 8% share separating the number one (Renault Twizy - 27%) from the third place (Opel Ampera - 19%). And between them, the #2, the Nissan Leaf is hoping to increase sales and win over the #1 position in the next months.

A white-ish Fluence ZE in Israel, the only country that actually likes it
Other Markets

Giving some love to less visible markets, it's possible to confirm the fall of EV share in another euro market, the Czech Republic  is down to 0,02% from 0,06% in 2012, but things are looking better elsewhere, in Israel there were 422 EV's sold this year, with EV share improving to 0,58%, up from 0,37% in 2012, making this middle eastern country the 11th largest EV market in the world, ahead of countries like Italy, Spain or Switzerland.

A curious thing about this market is the fact that the only electric car selling significantly is the Renault Fluence ZE, which coincidentally is also the only market where the electric sedan sells in decent numbers, thanks to the Better Place deal.

For all you EV Sales blog readers from Russia, this month i have figures from their home market, in 2012 there were sold a hundred of electric cars there, 49 of which were Mitsubishi's I-Miev.

Will the I-Miev hold on to the russian title in the end of the year?

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