What you'll learn:
• Air-fuel ratio vs. equivalence ratio and why you need to consider both when addressing fuel trim related challenges
• How to obtain cylinder air flow measurements for quick and dirty V.E. testing
• Develop a deeper understanding as to how the ECM determines the proper amount of fuel to be delivered to the cylinder at all times.
Fuel trim is a vital data point technicians can leverage when assessing vehicle anomalies related to driveability and emissions challenges. Additive,block learn,integrator,multiplicative.You may have seen any of these terms in the past andessentially,they're all indicative of the samecommon term, fuel trim.Fuel trimisthe correction factor appliedto theair-fuelmassdelivered to the cylinderduring engine operation. Small errors in this cylinder air/fuel massestimationsare expected but larger errors are an indication of trouble.Further,it typically takes asignificant errortoresult in an illuminated malfunction indicator lightaccompanied bydiagnostic trouble codes (DTCs).The source of these errors is not alwayseasily identifiedand therefore the goal of this article is to provide a deeper understanding of how errors can occur and what tools and techniques a technician can applytoformahypothesisto recommend a service/repair operation.
Engine basics
Thefour-cycle internal combustion engine (ICE) has evolved tremendously over the years. The advancements have been primarily driven by the desire to squeeze more energy from the ICE while using less fuel and producing fewertailpipeemissions. What we’ve witnessed over the past decade or so with engine design and strategies is amazing. With the implementation of variable valve timing, engineering teams in some caseswere able toeliminate the use of an external EGR valve andwere abletoexpandthe torque outputof the engineacross a wider engine speed range.However, the engine controller needs to have aclearunderstandingof the actual mass of airalways entering the cylindertodeliver the right amount of fuel to match the desired air to fuel ratio. The way this is measuredand interpreted has changed dramatically over the years and we’ll discuss this further within this article.
Sensors mounted in the exhaustupstream from the catalytic converter are the primary feedbackindicatorsused to report back to the ECM how well the fuel deliveredto the cylinders.matched theestimated cylinderair charge to achieve thetargeted air-fuel ratio.A wide-band oxygen sensor (Universal Exhaust Gas Oxygen – UEGO) is more expensive and complex, but the benefits it provides is crucial in obtaining accurate air-fuel ratio measurements across a wide range of mixtures.However,the narrow band sensor is only accurate to mixtures very close to stoichiometric.
In years past, vehicles used narrow band sensors to achieve a tight average of stoichiometric air to fuel ratio to the combustion chamber by constantly switching the air to fuel mixture rich-lean-rich-lean repeatedly. The wide-band sensor being used today is far superior and allows calibration engineering teams to squeeze more efficiency out of today’s internal combustion engines and provide the optimal catalytic convertor feedgas to achieve the lowest possible tailpipe emission output. Additionally, these sensors are typically ready for work shortly after engine startup.Narrow-band sensors can still be found downstream of the catalytic converter and are primarily used formonitoring catalyst efficiency. However, some vehicles use this sensor to “fine tune” thecatalyst’s feed gas by trimmingfuel in a way so that the catalyst can do everything it can to further reduce tailpipe emissions.
Air-fuel ratio
Today’s pump gastypically contains up to 10 percent alcohol which means that the proper air-fuel ratio is now less than thecustomary 14.7:1ratio weexperienced in the past.Some vehicles are designed to operate onvaryingpercentages ofalcohol therefore the ECM needs to havea clear understandingof this content so that the properstoichiometric air-fuel ratio can be targeted(see Figure1).Some vehicles use direct measurements to detectalcohol contentandothersuse inferencingto estimatealcoholcontent.Today, with most pump gas containing approximately 10 percent alcohol, the stoichiometric air fuel ratiotargetisin the lower 14’s.In most cases the scan tool offers air-fuel ratio PIDs in different formatssuch asequivalenceratio, lambda, phi,and air-fuelratio. Later in this article you will see whyone would want tochooseone overanother orobserveboth.
Figure 1– Ethanol percentage stoichiometric air-fuel ratio table
This method is still in use todayhowever some of the behind-the-scenes routines may have changes. Essentially, a speed density system uses a3-D table to express the volumetric efficiency profile of the engine acrossa widerange.The system typically usesengine speed on one axis andmanifold pressure on the other.This table is generated during engine development and then written to the engine calibrationresiding within the engine control module. This “map” is used as a baseline referencetounderstand what the percentage of cylinder fill is when operating within specific coordinates. For example,using Figure2, say the engine was operating at1600 rpm and the manifold pressure was50 kPa. The reference table states that the cylinder fill capabilities are75.09which means that the engine can fill the cylinder75.09 percent of the actualvolume of the cylinder.So,on anengine with a4,278cc engine, the cylinder volume is 713cc. Essentially, thismeans thatair massenteringthecylinderiswouldonly fill it to75.08 percent of the 100 percent rated size of the cylinder.Since the engine controller knows the cylinder size, the injector flow rate, thecompositionof the fuel,density of theair,and the desired air-fuelratio, it will then know how long to turn on the fuel injector.It will then look to the result of the air-fuel ratio indicated in the exhaust to see how that math worked out. Any errors in hitting that target will result in acorrection issued via fuel trim.For the driveability technician, the manifold pressure sensor key on, engine off (KOEO) reading should always be referenced when performing analysis on any driveability or emissions related complaint. If this value isn’t properly reporting your station pressure, then the powertrain control module (PCM) will not be referencing the proper coordinates within that table during engine operation.
Figure2 – Speed density table
Mass air flow systems
Many enginesuse mass air flow sensors to estimate/measure cylinder air flow.Most engines utilize a single sensor while some usemultiple. The ECM uses the input from the massair flow sensorto calculate the cylinder air flow.The output from the mass air flow sensor can be used in conjunction with other data parametersby technicians to assess various conditions. For example, if the technician suspected that the engine wasn’t flowing the amount of air it should, either because they suspected a restricted exhaust or an under reporting sensor, they could use this as an input. However, what I have found is thatwhere this value is sampled is not always the best. Recently I was hosting a training session and polled the class for where one would typically want to samplethe mass air flow reading when performing a volumetric efficiency test.The majority ofthe attendeesstated that while at wide open throttle (WOT), therpm just before the shift pointshould be where the mass air flow value shouldbesampled. Well,this answer is correct only if that rpm is where the engineunder test is rated to produce itsmaximum torque rating. But in mostcases,this is notwhere the engine produces itsmaximum rated torque.So as a best practice, one should try and sample the mass air flow reading at the rpm where the engine is rated to produce maximum torque.From there,onecan take that value into a volumetric efficiency (VE) calculatorfor calculation. Oronecould use some quick napkin math toroughly figure theefficiencyof the engine.To do so, one needs to calculate using the following equations.
To findcylinder air we need the following:
Engine speed in rpm
Mass air flow (MAF) in grams/sec
Constant (based upon the number of cylinders)
The equation is as follows:(MAF x constant)/rpm
To find the constant weuse the following formula(120/cylinder count)or use the following table:
| Cyl | Constant |
| 4 | 30 |
| 5 | 24 |
| 6 | 20 |
| 8 | 15 |
| 10 | 12 |
| 12 | 10 |
Then one would need to know what thetheoretical100 percent-cylinderair mass would be.Say that we had a 2.7L5-cylinderdiesel,you would take 2700ccand divide that by 5 to get540cc or .540L.Next,we need to know the mass of air that would fill that area. To find this we’ll reference a knownvalue and that is 1.184 grams of air at standard temperature and pressure will fill 1L.So,if wetake 1.184 X 0.540 = 0.639. Thismeans thatif we perform our measurement from mass air flow and arrive at.639 grams/Cylwe would be at 100 percent volumetric efficiency.
Mass air flow andspeed density methods have been the traditionalpathway for predicting cylinder air during engine operationfor most manufacturers. However, there are some today using other methods such as neural networks tobuild artificial intelligencewith great success.
Fuel trim diagnostics
Fuel trim is akeyindicator of how well theoverall engine performance and fuel delivery systemsare performing. For service technicians, it can providean invaluable path towards a resolution. As mentioned earlier, it usually takes awildswingin fuel trim to flag a fault and therefore fuel trim can be used toidentify trouble before it flips on the malfunction indicator lamp (MIL). It’s always a good practice to monitor fuel trimalong with a few other parameters when assessing engine performance issues. Knowing whatcan drive fuel trim numbers can help technicians arrive at the righthypothesis and therefore issue the proper repairs that generate success.Lookingat Figure3you can see in the lower graph that>20 percent fuel is being removedvia long-term fuel trim (LTFT) on both banks.Based on thedata, what do you see?
Figure3 –Fuel trimanomaly due to incorrect fuel composition
This vehicle is a flex fuel vehicle and the revealing factor here is thatwhile the engine control module (ECM) was targeting EQ or Lambda 1,thecommandedair-to-fuel ratioat11.63:1. This means that theECM had inferred the composition ofthefuel to haveroughly52 percent ethanol.The otheranomalyhere is that themass air flow signal is over reporting air flow. Figure4shows theengine data following the replacement of the mass air flow sensor (MAF) and a reset of the fuel composition using the scan tool.
Figure4 – Fuel trimcorrection after service
Monitoring and recordingengine performance parameters canproducegreat reference material fordiagnosticians. Having a deeper understanding of the inner workings of the ECM anditsvarious strategies willno-doubt bear success when dealing withengine performance and emissions related challenges.
