Page 17 - GMC Vintage RVing - Winter 2018

For additional technical presentations, papers, manuals, and other information go to gmcmi.comthe position of the throttle (an indication of how much power the driver wants), the manifold vacuum (an indication of the load the engine is under), the water temperature and a few other factors to estimate the amount of fuel that should be injected into the incoming air stream for the next combustion event. The oxygen sensor then feeds back information about how much oxygen the resulting exhaust still contains (a measure of how complete the burn), so the next time the electronic control unit sees this same set of conditions it will know whether to add more or less fuel in an attempt to get a completeburn of the fuel.Whew! Sounds like a lot of stuff to do every time a spark plug fires, and it is. Thanks to the newer, faster electronics and the newer and better sensors available today, it is possible to do this time and again for the life of the engine, often without any intervention on the part of the owner.But, as we will see in the following two charts, reaching actual stoichiometric A/F ratio for the fuel used is not necessarily the objective. Note that the greatest power output from the enginecomes from air/fuel ratios richer than stoichiometric – in the range of 12.4 to 12.8 to 1, while the greatest economy (and lower emissions) comes from A/F ratios leaner than stoichiometric – in the range of 15.6 to 16 to 1. And, remember these numbers are for unadulterated gasoline and will be different for gasoline with 10 percent ethanol added but the shape of the curves will be similar. So, the designer of the EFI self-learning software has to take these competing objectives into account and urge the electronics to achieve air/fuel ratios the designer considers the best compromise – not simply trying to achieve the stoichiometric A/F ratio of the fuel itself.Hence, not all manufacturer’s self-learning EFI systems will achieve the same performance, or the same fuel economy, or the same exhaustemissions for a given driving pattern. But, all of them will achieve a far better balance than even the best tuned carburetor under the same set of conditions given the changed nature of the fuel we now purchase at the pump. And, that is the bottom line as to why any brand modern self-learning EFI system will out perform a carburetor in our GMCs.No more worries about the “quality” of the fuel since the self-learning EFI will automatically compensate for the burn characteristics of the fuel used while you drive. No more worries about black smoke or stumble during cold start as the self-learning EFI will supply just the amount of fuel you need whether the engine is cold or hot. No more worries about the effects of altitude on the performance of your GMC as the self-learning EFI will compensate for altitude automaticallyas you drive. No more worries about cranking endlessly to refill the carburetor bowl with fuel before starting as the self-learning EFIwill start nearly instantly. (Note that you can program these systems to take a few more cranks before starting if you are worried about starting with little or no oil pressure.) No more worries about engine run on (dieseling) after you shut it off as the EFI will stop the engine immediately when you turn the key off. And, best of all, no more worries about whether you are damaging your engine by consistently running too rich or too lean as the self-learning EFI will do all it can to keep the air/fuel ratio within proper bounds automatically mile after mile.Are these new self-learning EFI systems “perfect”? No more so than the EFI in a new car. A high pressure pump can fail, an injector can clog, a sensor can fail, a wire can pull loose or any number of other failures can theoretically occur. But, as the driver of any new car can attest, compared to the reliability of a 40-year-old carbureted engine there is no comparison, especially since now the fuel is formulated for EFI instead of carburetors.Typical Fuel Efficiency (Gasoline)RICHLEAN100755011:113:1 15:1 17:1 AFR – gasoline100500Typical Power Output (WOT and Gasoline)RICH11:1 13:1 15:1 AFR – gasolineLEAN17:1WINTER 2018 GMC MOTORHOMES INTERNATIONAL 17% of optimal efficiency% of optimal powerstoich.stoich.