ELEC­TRONIC FUEL IN­JEC­TION (EFI)

NZ Performance Car - - Promotional Feature Efi Vs Carbs -

For many of our read­ers, elec­tronic fuel in­jec­tion is the sta­ple of fuel de­liv­ery sys­tems; it can be found un­der the bon­net of prac­ti­cally all ‘mod­ern’ ve­hi­cles from the ՚’80s on­wards. How­ever, when it comes to what ac­tu­ally goes on, it can be a bit of a mine­field of physics, ther­mo­dy­nam­ics, and maths — not our strong­est sub­jects at school, we’ll ad­mit. There’s a hell of a lot of po­ten­tial for power hid­den in there, as an en­gine re­ally is based on how much air and fuel you can cram into it, safely. We wanted to find out ex­actly how EFI func­tions and what you need to know when up­grad­ing things. Nat­u­rally, we turned to the team at Link ECU to get the low-down.

WHAT COM­PO­NENTS MAKE UP AN EFI SYS­TEM, AND HOW DO THEY WORK?

“Elec­tronic fuel in­jec­tion tra­di­tion­ally con­sists of some sort of high­pres­sure fuel sup­ply, be it a me­chan­i­cal or an electric pump, which is reg­u­lated to sup­ply fuel to a rail that is fit­ted with the in­jec­tors. Sen­sor mea­sure­ments — things like an RPM trig­ger, en­gine speed and throt­tle­po­si­tion sen­sors, or man­i­fold ab­so­lute pres­sure (MAP) sen­sor — al­low the ECU to pre­dict air­flow into the en­gine; when it does so, it can in­ject the right amount of fuel. This gives the ECU a broad idea of the air­flow go­ing in. An O2 sen­sor in­stalled in the ex­haust reads ex­haust gasses to also pro­vide valu­able pa­ram­e­ters in or­der for the ECU to de­ter­mine whether the car is run­ning lean or rich.

“In older-style sys­tems, it was es­sen­tially bro­ken down into a ta­ble where, if X amount of air was flow­ing in, the ECU would look for the cell on the ta­ble that re­lated to the amount of air and re­ceive an an­swer of how much fuel was to be fed per mil­lisec­ond. As ECUs have de­vel­oped, it’s more of a live cal­cu­la­tion based on a num­ber of fac­tors.”

WHEN CHAS­ING POWER, HOW DO I KNOW MY IN­JEC­TORS ARE MAXED OUT?

“This is best checked through some form of en­gine-man­age­ment di­ag­nos­tics, be it an after­mar­ket ECU or a fac­tory sys­tem that al­lows you to look at pa­ram­e­ters such as the in­jec­tors’ duty cy­cle. This will give you a rough win­dow of where your in­jec­tors can op­er­ate.

“The whole idea of fuel in­jec­tion is to try to mix the right amount of fuel with the air that is flow­ing into the en­gine, so, if your in­jec­tors aren’t big enough to flow an ad­e­quate air–fuel ra­tio, that can cause is­sues. We recommend that you don’t ex­ceed an 85-per-cent duty cy­cle, which ba­si­cally means that the in­jec­tor is open for 85 per cent of the time and is reach­ing the limit of what it can phys­i­cally flow. The higher the duty cy­cle, the less pre­dictable it is on how much fuel is added.

“For ex­am­ple, you could bump your duty cy­cle per­cent­age to 90 per cent, think, Oh there is more here, I’ll go higher, and up it to 95 per cent. How­ever, there may be no dif­fer­ence, be­cause the in­jec­tor is open for nearly the en­tire time, mean­ing that there isn’t any more flow to use. More power re­quires more fuel; if your in­jec­tors can’t sup­ply the cor­rect amount of fuel for your air–fuel ra­tio, then the en­gine will lean out. Run­ning lean usu­ally means more heat, and that can cause pis­ton wear, seiz­ing, etc.”

HOW DO YOU CAL­CU­LATE WHAT SIZE IN­JEC­TORS ARE RE­QUIRED?

“A few fac­tors need to be con­sid­ered: num­ber of cylin­ders/in­jec­tors, ap­prox­i­mate tar­get power, as­pi­ra­tion type, and type of fuel. Most in­jec­tor man­u­fac­tur­ers have cal­cu­la­tors to sug­gest the cor­rect size in­jec­tor for your ap­pli­ca­tion, but it’s re­ally down to ba­sic physics and ther­mo­dy­nam­ics. There is such a thing as go­ing too big, too, when is­sues arise at idle be­cause the en­gine needs very lit­tle fuel but the in­jec­tor can­not open for a short enough time to de­liver the small amount re­quired, caus­ing the en­gine to over­fuel. Many high-pow­ered cars run mul­ti­ple stage in­jec­tors, hav­ing only one at idle feed­ing the en­gine and turn­ing on more as RPM and power in­crease.”

WHAT KIND OF SUP­PORT­ING UP­GRADES ARE GEN­ER­ALLY RE­QUIRED?

“The main one is hav­ing a fuel pump that can sup­ply the cor­rect amount of fuel. Oth­er­wise, most of the ba­sic com­po­nents usu­ally have a lot of head­room in them to al­low for power in­creases.”

HOW IM­POR­TANT IS AN ECU IN THE SYS­TEM?

“Vi­tal! The bet­ter the ECU, and how it is set up, the bet­ter the sys­tem will be con­trolled and run. Not only is the qual­ity of the ac­tual ECU it­self im­por­tant, but so is the sup­port sys­tem be­hind it and whether you’ve got a good, use­ful dealer to com­mu­ni­cate with for tech­ni­cal sup­port and proper doc­u­men­ta­tion for set­ting it up.”

WHAT ARE THE BIG­GEST MIS­TAKES PEO­PLE COM­MONLY MAKE?

“Am­a­teur wiring — where some­one doesn’t un­der­stand the im­por­tance of elec­tri­cal fun­da­men­tals and tries to tune it them­selves with­out hav­ing the knowl­edge or train­ing to work through it. A lot of the time, the most im­por­tant thing is who is go­ing to tune it — when tak­ing it to a tuner, it’s im­por­tant to con­sult them regarding what com­po­nents they recommend and what ECU they are com­fort­able with tun­ing. If you take a car to a tuner and he’s not fa­mil­iar with, or doesn’t like, what you’ve used, the fi­nal prod­uct won’t be as good.”

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