Direct Petrol Injection

8/9/2019 Direct Petrol Injection

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Direct Petrol Injection

All the detail on how direct petrol injection systems work

byJulian Edgar

At a glance...

Direct fuel injection components

Direct fuel injection operating modes

Electronic control

Increased efficiency

Direct petrol injection is the latest technology in injection systems youll find it in models

from Mazda, Audi/V, !M and Alfa, amongst others" !ut ho# does it #or$%

As the name suggests, Direct &etrol Injection uses injectors that add fuel directly to the

com'ustion cham'er" (i$e diesel engines, the air/fuel mi)ing occurs inside the com'ustion

cham'er, rather than in the inlet ports" *a$ing this approach gi+es far greater control o+er

the com'ustion process, allo#ing for a +ariety of com'ustion operating modes, including

those ha+ing ultralean air/fuel ratios"
http://www.amazon.com/Julian-Edgar/e/B00C3MRYN4/?_encoding=UTF8&camp=1789&creative=9325&linkCode=ur2&tag=auto0e22-20http://www.amazon.com/Julian-Edgar/e/B00C3MRYN4/?_encoding=UTF8&camp=1789&creative=9325&linkCode=ur2&tag=auto0e22-20


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System Mechanicals

*his diagram sho#s the layout of the !osch direct injection system" Direct injection

systems differ from con+entional port injection in se+eral #ays"

*he fuel supply system uses t#o fuel pumps a con+entional electrical fuel pressure pump

-in the past du''ed a high pressure pump 'ut no# referred to in this system as a lo#

pressure pump. and a mechanicallydri+en high pressure pump" *he lo# pressure pump

#or$s at pressures of "0 "1 M&a #hile the high pressure pumps 'oost this +ery

su'stantially to 1 23 M&a"

*he high pressure fuel is stored in the fuel rail that feeds the injectors" *he fuel rail is

made sufficiently large that pressure fluctuations #ithin it are minimised as each injector

opens" *he pressure of the fuel in the injector supply rail is controlled 'y an electronically

controlled 'ypass +al+e that can di+ert fuel from the high pressure pump outlet 'ac$ to its

inlet" *he fuel 'ypass +al+e is +aried in flo# 'y 'eing pulse#idth modulated 'y the

Electronic 4ontrol 5nit -E45." A fuel pressure sensor is used to monitor fuel rail pressure"


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*his diagram sho#s a crosssectional +ie# of an injector" 4ompared #ith a con+entional

port fuel injection system, the fuel injectors must 'e capa'le of #or$ing #ith huge fuel

pressures and also injecting large amounts of fuel in +ery short periods" *he reason for the

much reduced time in #hich the injection can 'e completed is due to the fact that all the

injection must sometimes occur #ithin just a portion of the induction stro$e" 4on+entional

port fuel injectors ha+e t#o complete rotations of the cran$shaft in #hich to inject the fuel

charge at an engine speed of 6 rpm, this corresponds to 3 milliseconds" 7o#e+er, insome modes, direct fuel injectors ha+e only5milliseconds in #hich to inject the fullload

fuel" *he fuel re8uirements at idle can drop the opening time to just "9 milliseconds"


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Direct injection fuel droplets are on a+erage only onefifth the droplet size of traditional

injectors and onethird the diameter of a human hair"

*he +ery lean air/fuel ratios at #hich direct injection systems can operate results in the

production of large 8uantities of o)ides of nitrogen -:;)." As a result, direct injected cars

re8uire 'oth a primary catalytic con+erter fitted close to the engine, and also a main

catalytic con+erter incorporating a :;) accumulator that is fitted further do#nstream"


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. Homogenous Stratified Charge Mode

Initially, this mode doesnt seem to ma$e sense ho# can the com'ustion process 'e 'oth

homogenous and stratified% 7o#e+er, #hat occurs is not one 'ut t#o injection cycles"

*he initial injectionoccurs during the inta$e stro$e, gi+ing plenty of time for the fuel to

mi) #ith the air throughout the com'ustion cham'er" *hen, during the compression stro$e,

a second amountof fuel is injected" *his leads to the creation of a rich zone around the

spar$plug" *he rich zone easily ignites, #hich in turn ignites the leaner air/fuel ratio #ithin

the remainder of the com'ustion cham'er" ;f the total fuel addition, appro)imately >1 per

cent occurs during the first injection and 31 per cent in the second" *he 7omogenous


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*his diagram sho#s the t#o primary com'ustion modes stratified charge and

homogenous modes"

lectronic Control Systems


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As #as indicated earlier, the injectors must 'e opened against +ery high fuel pressures"


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*his diagram sho#s ho# this occurs" During


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*he system incorporates an operatingmode coordinator #hich maps operating mode

against engine speed and tor8ue re8uest" *his diagram sho#s a schematic diagram of the

functioning of this controller" As can 'e seen, a 2stage prioritisation is used #hen

deciding on the re8uired operating mode" !efore the selected com'ustion mode starts to

occur, control functions for e)haustgas recirculation, fuel tan$ +entilation, charge

mo+ement flap -ie port tum'le +al+es or +aria'le length inta$e manifold., and electronic

throttle settings are initiated as re8uired" *he system #aits for ac$no#ledgement that

these actions ha+e 'een carried out 'efore altering fuel injection and ignition timing"

*he ad+antage of ha+ing the electronic throttle +al+e fully open at lo# loads is a huge

reduction in pumping losses the engine is no longer trying to 'reathe through the

restriction of the nearlyclosed throttle" -A similar scheme is used in the latest 7onda 4i+ic

'ut it is achie+ed through +al+e timing changes". 7o#e+er, the do#nside of this approach

is that the partial +acuum that is normally a+aila'le for the 'ra$e 'ooster is lac$ing" *o

o+ercome this pro'lem, a +acuum s#itch or pressure sensor monitors 'ra$e 'ooster

+acuum and if it is necessary, the com'ustion mode is altered so that +acuum again

'ecomes a+aila'le"


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Increased !!iciencies

In addition to the reduction in pumping losses occurring as a result of the throttle 'eing

#ide open at lo# loads, during


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Conclusion

Mitsu'ishi has 'een 'uilding direct injection petrol engines since 2BB6, 'ut they ha+e not

'een a'le to achie+e #orld#ide success #ith their designs due, it is said, to the engines

reliance on high 8uality fuel" :o# !osch has de+eloped technology #hich is allo#ing direct

injected engines to 'e sold around the #orld" 4om'ine direct injection technology #ith the

hy'rid cars 'eing #idely de+eloped plus perhaps do#nsized and tur'ocharged engines

and #ere certainly in for some interesting times ahead"

"o Starter Motor Starts#

Another ad+antage of Direct Injection is that it is possi'le to start a hot engine #ithout

using the starter motor" *his approach also reduces startup hydrocar'on emissions,

ma$ing more attracti+e the use of engines that automatically s#itch off once the car has

'een stationary for a period"

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Direct Petrol Injection