How It Works

Elec­tronic ig­ni­tion

Classic Car Weekly (UK) - - This Week -

‘Fuel in­jec­tion it­self has been around for more than a cen­tury’

fuzz town­shend CCW’s master me­ChaniC

En­thu­si­asts of­ten be­moan the loss of tin­ker-abil­ity on mod­ern clas­sic cars, but the re­al­ity is their en­gines still work on the 150-year-old suck-squeeze-bang-blow prin­ci­ple.

Fuel in­jec­tion has been around for more than a cen­tury. Diesels have been us­ing it since the year dot, dol­ing out ac­cu­rately mea­sured squirts of atom­ised fuel at pre­cisely the cor­rect mo­ment.

Ear­lier petrol in­jec­tion sys­tems, such as the Bosch in-line pumps in early- to-mid-1960s Mercedes, used tech­nol­ogy al­lied to that of diesel in­jec­tion, but as the 1970s pro­gressed, me­chan­i­cal Bosch K-Jetronic be­came king of the mod­ern en­gine bay. Solid-state elec­tron­ics even­tu­ally re­placed me­chan­i­cal op­er­a­tion, bring­ing things pretty much up to date into the mod­ern clas­sic era.

Elec­tronic fuel in­jec­tion re­quires a num­ber of sen­sors to gather in­for­ma­tion for an elec­tronic con­trol unit (ECU) so that it, in turn, can tell the in­jec­tors to sup­ply the cor­rect amount of fuel at pre­cisely the right time, while si­mul­ta­ne­ously sup­ply­ing ig­ni­tion tim­ing in­for­ma­tion to the coil pack.

First and fore­most is the crankshaft po­si­tion sen­sor, which feeds in­for­ma­tion to the ECU so that it can de­ter­mine ba­sic en­gine tim­ing and ori­en­ta­tion. Ba­sic ig­ni­tion tim­ing is de­ter­mined us­ing this in­for­ma­tion too, al­though the throt­tle po­si­tion sen­sor feeds ad­di­tional data that can al­ter this as­pect. En­gines with vari­able valve tim­ing also have a camshaft po­si­tion sen­sor, pro­vid­ing in­for­ma­tion re­gard­ing valve ori­en­ta­tion.

Throt­tle bod­ies fea­ture a throt­tle but­ter­fly valve, sim­i­lar to that found in a car­bu­ret­tor and like­wise op­er­ated by the throt­tle pedal, which con­trols air flow into the in­let man­i­fold. At­tached to the pivot spin­dle of this is a throt­tle po­si­tion sen­sor, which feeds data re­gard­ing the but­ter­fly’s po­si­tion to the ECU. A tem­per­a­ture sen­sor re­lays in­for­ma­tion re­gard­ing coolant and en­gine tem­per­a­ture to the ECU, where it is used to de­ter­mine the amount of fuel supplied by the in­jec­tors. This re­places the choke, sup­ply­ing ex­tra fuel to a cold en­gine.

In the fuel sup­ply cir­cuit, a low­pres­sure pump may be used to draw fuel into a swirl pot, which in rudi­men­tary terms is a reser­voir of read­ily avail­able fuel from which an in­jec­tion-pres­sure pump can meet the needs of the in­jec­tors.

This pump is of­ten sit­u­ated in the petrol tank, negat­ing the need for a sep­a­rate swirl pot.

The high-pres­sure pump sup­plies fuel di­rect to the in­jec­tors in readi­ness for when they’re opened by the ECU. Steady pres­sure is main­tained by a reg­u­lat­ing valve that opens and re­turns fuel to the tank when the pres­sure rises above op­er­at­ing lev­els. The ECU-con­trolled in­jec­tors spray a given amount of fuel over a given amount of time, de­pend­ing on tem­per­a­ture, throt­tle po­si­tion and crank po­si­tion.

Other sen­sors of­ten used in­clude a mass air­flow sen­sor/me­ter, which de­ter­mines the vol­ume of air en­ter­ing the in­let man­i­fold and a Lambda sen­sor, which analy­ses ex­haust gas oxy­gen con­tent to en­able the ECU to achieve more ac­cu­rate con­trol of the fuel/air ra­tio and the waste prod­ucts of com­bus­tion.

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