AUTE3450UCombustion and Engines

Air and Fuel Induction

Anand Joshi

Academic Associate, UOIT

Fuel Injectors

• Fuel injectors are nozzles that inject a spray of fuel into the intake air.

• They are normally controlled electronically, but mechanically controlled injectorswhich are cam actuated also exist.

• A metered amount of fuel is trapped in the nozzle end of the injector, and a highpressure is applied to it, usually by a mechanical compression process of somekind.

• At the proper time, the nozzle is opened and the fuel is sprayed into thesurrounding air.

• Most modern automobile SI engines have multipoint port fuel injectors.

• In this type of system, one or more injectors are mounted by the intake valve(s)of each cylinder.

Fuel Injectors

Throttle body injection Multipoint fuel injectionSource: https://www.google.ca/

Fuel Injectors

• They spray fuel into the region directly behind the intake valve, sometimesdirectly onto the back of the valve face.

• Contact with the relatively hot valve surface enhances evaporation of the fueland helps cool the valve.

• The injectors are usually timed to spray the fuel into the quasi-stationary air justbefore the intake valve opens.

• High liquid spray velocity is necessary to assure evaporation and mixing with theair.

• Because injection starts before the intake valve is open, there is a momentarypause in the air flow, and the air velocity does not promote the needed mixingand evaporation enhancement.

Fuel Injectors

Ref: R. Stone, J.K. Ball, Automotive Engineering Fundamentals, SAE International, 2004. ISBN 0-7680-0987-1

Fuel Injectors

• When the valve then opens, the fuel vapor and liquid droplets are carried into thecylinder by the onrush of air, often with the injector continuing to spray.

• Any backflow of hot residual exhaust gas that occurs when the intake valve opensalso enhances the evaporation of fuel droplets.

• Multipoint port injector systems are better than carburetors or throttle bodyinjector systems at giving consistent AF delivery.

• Some multipoint systems have an additional auxiliary injector or injectorsmounted upstream in the intake manifold to give added fuel when rich mixturesare needed for startup, idling, WOT acceleration, or high RPM operation.

• Because there is such a short duration (time and length) after fuel injection forevaporation and mixing to occur, it is essential that port injectors spray very tinydroplets of fuel.

Fuel Injectors

• Ideally, droplet size could be varied with engine speed, smaller at higher speeds,when real time is shorter.

• Because little or no air-fuel mixing occurs in most of the intake manifold, highvelocity is not as important, and larger diameter runners with less pressure losscan be used. There is also no displacement of incoming air with fuel vapor in themanifold.

• Some systems have a single fuel pump (common rail) supplying all cylinders or abank of cylinders.

• Other systems have a fuel pump for each cylinder, with the pump sometimesbuilt as a single unit with the injector.

Fuel Injectors

• Time average of fuel flow into an engine at different operating conditions canvary by as much as a factor of 50.

• The amount of fuel injected for each cycle can be adjusted by injection time,which is of the order of 1.5 to 10 ms.

• Various kinds of fuel injectors are available. Most operate by trapping a smallamount of fuel behind the nozzle orifice.

• The nozzle is closed by a needle valve held against its seat by a spring or magneticforce. On lower pressure nozzles, injection is initiated by increasing pressure andpushing open the valve, allowing flow to occur.

• On high-pressure nozzles, flow is initiated by lifting the valve needle off its seat byaction of an electric solenoid. Spray duration, and sometimes pressure, isgenerally controlled electronically.

Fuel Injectors

Ref: R. Stone, J.K. Ball, Automotive Engineering Fundamentals, SAE International, 2004. ISBN 0-7680-0987-1

Fuel Injectors

• Some fuel injection systems, including most very early ones, consist of throttlebody injection.

• This consists of one or more injectors mounted near the inlet of the intakemanifold, usually just downstream of the throttle plate.

• This injector or set of injectors supplies fuel for all cylinders, allowing thedistribution to be controlled by the intake manifold.

• This is simpler technology than multipoint injection and a fair amount cheaper tomanufacture.

• Fewer injectors are needed and coarser nozzles can be used, as there is a longerflow duration to evaporate and mix the larger fuel droplets.

Carburetors• For several decades,

carburetors were usedon most SI engines asthe means of addingfuel to the intake air.

• Over the period,carburetors are beingreplaced with fuelinjectors as pollutionlaws become morestringent.

• Some Small engines -lawn mowers, fewautomobiles and modelairplanes still usecarburetors.

• The basic carburetor is aventuri tube (A) mountedwith a throttle plate (B)(butterfly valve) and acapillary tube to input fuel(C).

• It is usually mounted on theupstream end of the intakemanifold, with all airentering the engine passingfirst through this venturitube.

• Most of the time, there willbe an air filter mounteddirectly on the upstreamside of the carburetor.

• Other main parts of the carburetor are the fuelreservoir (D), main metering needle valve (E), idlespeed adjustment (F), idle valve (G), and choke(H).

• As air enters the engine due to the pressuredifferential between the surroundingatmospheric air and the partial vacuum in thecylinders during intake strokes, it is accelerated tohigh velocity in the throat of the venturi.

• By Bernoulli's principle, this causes the pressurein the throat P2 to be reduced to a value less thanthe surrounding pressure P1, which is about oneatmosphere.

Carburetors

• The pressure above the fuel in the fuel reservoir isequal to atmospheric pressure as the reservoir isvented to the surroundings (P3 = P1 > P2).

• There is, therefore, a pressure differential through thefuel supply capillary tube, and this forces fuel flow intothe venture throat.

• As the fuel flows out of the end of the capillary tube, itbreaks into very small droplets which are carried awayby the high-velocity air.

• These droplets then evaporate and mix with the air inthe following intake manifold.

• As engine speed is increased, the higher flow rate of airwill create an even lower pressure in the venturethroat.

Carburetors

• This creates a greater pressure differential throughthe fuel capillary tube, which increases the fuel flowrate to keep up with the greater air flow rate andengine demand.

• A properly designed carburetor can supply the correctAF at all engine speeds, from idle to WOT. There is amain metering valve (E) in the fuel capillary tube forflow rate adjustment.

• The level in the fuel reservoir is controlled by a floatshutoff. Fuel comes from a fuel tank supplied by anelectric fuel pump on most modern automobiles, by amechanical-driven fuel pump on older automobiles,or even by gravity on some small engines (lawnmowers).

Carburetors

• The throttle controls the air flow rate and thus the engine speed. There is an idlespeed adjustment (throttle stop) which sets the closed throttle position such thatsome air can flow even at fully closed throttle.

• This adjustment, which is usually about 50-150 of throttle plate rotation, controlshow fast the engine will run at idle conditions.

• Because the air flow rate through the venturi throat will be minimal at idleconditions when the throttle is closed, the pressure in the throat will only beslightly less than atmospheric pressure.

• The pressure differential through the fuel capillary tube will be very small,resulting in a low fuel flow rate and very poor flow control.

• An idle valve is added (G) which gives better fuel flow control at idle and almostclosed throttle position.

Carburetors

• When the throttle is closed or almost closed, there is a large pressure differential acrossthe throttle plate, and the pressure in the intake system downstream of the throttle (B) isvery low.

• There is, therefore, a substantial pressure drop through the idle valve, allowing forproper flow control and a greater flow rate of fuel.

• Engines are usually run with a richer air-fuel mixture at low and idle speeds to avoidmisfires caused by a large exhaust residual resulting from valve overlap.

• Another butterfly valve called the choke (H) is positioned upstream of the venturi throat.This is needed to start cold engines.

• It is not really the air-fuel ratio that is important for considering combustion, but the air-vapor ratio; only fuel that is vaporized reacts in a combustion process.

• When an engine is cold, a very small percent of fuel will vaporize in the intake andcompression processes.

Carburetors

• When starting a cold engine, the first step is to close the choke. This restricts airflow and creates a vacuum in the entire intake system downstream of the choke,even at the very low air flow rates encountered in starting.

• There is, therefore, a large pressure differential across both the fuel capillary tubeand the idle valve, causing a large fuel flow to mix with the low air flow.

• This gives a very rich air-fuel mixture entering the cylinders, up to AF = 1:1 forvery cold starts. With only a small percent of fuel evaporating, a combustible air-vapor mixture is created, combustion occurs, and the engine starts.

• Only a few engine cycles are required before everything starts to heat up andmore normal operation occurs. As the engine heats up, the choke is opened andhas no effect on final steady-state operation.

Air and Fuel Flow in Carburetors

Example Problem 5.3

Supercharging and Turbocharging:Superchargers

• Superchargers and turbochargers are compressors mounted in the intake systemand used to raise the pressure of the incoming air.

• This results in more air and fuel entering each cylinder during each cycle. Thisadded air and fuel creates more power during combustion, and the net poweroutput of the engine is increased.

• Pressure increase can be anywhere from 20 to 250 kPa, with most engines on thelower end of this scale.

• Superchargers are mechanically driven directly off the engine crankshaft. Theyare generally positive displacement compressors running at speeds about thesame as engine speed (Fig. 1-8).

• The power to drive the compressor is a parasitic load on the engine output, andthis is one of the major disadvantages compared to a turbocharger.

• Other disadvantages include higher cost, greater weight, and noise.

Superchargers

• A major advantage of a supercharger isvery quick response to throttle changes.

• Being mechanically linked to thecrankshaft, any engine speed change isimmediately transferred to thecompressor.

• Some high-performance automobileengines and just about all large CIengines are supercharged.

• All two-stroke cycle engines which arenot crankcase compressed (a form ofsupercharging) must be eithersupercharged or turbocharged.

Superchargers

𝑾𝒔𝒄 = 𝒎𝒂(𝒉𝒐𝒖𝒕-𝒉𝒊𝒏) = 𝒎𝒂𝒄𝒑(𝑻𝒐𝒖𝒕-𝑻𝒊𝒏) (5-13)

Superchargers

Superchargers

(𝜼𝒔)𝒔𝒄=𝑾𝒊𝒔𝒆𝒏

𝑾𝒂𝒄𝒕=[ 𝒎𝒂(𝒉𝟐𝒔−𝒉𝟏)]

[ 𝒎𝒂(𝒉𝟐𝑨−𝒉𝟏)]=

[ 𝒎𝒂𝒄𝒑(𝑻𝟐𝒔−𝑻𝟏)]

[ 𝒎𝒂𝒄𝒑(𝑻𝟐𝑨−𝑻𝟏)]=

(𝑻𝟐𝒔−𝑻𝟏)

(𝑻𝟐𝑨−𝑻𝟏)(𝟓 − 𝟏𝟒)

η𝒎 =(𝑾𝒂𝒄𝒕)𝒔𝒄

𝑾𝒇𝒓𝒐𝒎 𝒆𝒏𝒈𝒊𝒏𝒆(5-16)

Turbochargers

• The compressor of a turbocharger is powered by a turbine mounted in theexhaust flow of the engine (Figs. 1-9 and 5-8). The advantage of this is that noneof the engine shaft output is used to drive the compressor, and only waste energyin the exhaust is used. However, the turbine in the exhaust flow causes a morerestricted flow, resulting in a slightly higher pressure at the cylinder exhaust port.This reduces the engine power output very slightly. Turbocharged enginesgenerally have lower specific fuel consumption rates. They produce more power,while the friction power lost remains about the same.

• Maximum pressure in an engine exhaust system is only very little aboveatmospheric, so there can only be a very small pressure drop through the turbine.Because of this, it is necessary to run the turbine at very high speeds so thatenough power is produced to run the compressor. Speeds of 100,000 to 130,000RPM are common. These high speeds, and the fact that exhaust gas is a hot,corrosive environment, demand special materials and concern for long-termreliability.

Turbochargers

Turbochargers

Turbochargers

• A disadvantage of turbochargers is turbo lag, which occurs with a sudden throttlechange. When the throttle is quickly opened to accelerate an automobile, theturbocharger will not respond quite as quickly as a supercharger. It takes severalengine revolutions to change the exhaust flow rate and to speed up the rotor ofthe turbine. Turbo lag has been greatly reduced by using lightweight ceramicrotors that can withstand the high temperatures and that have very little massinertia. Turbo lag can also be reduced by using a smaller intake manifold.

• Most turbochargers, like superchargers, are equipped with an aftercooler to againlower the compressed air temperature. Many also have a bypass that allows theexhaust gases to be routed around the turbocharger when an inlet air pressureboost is not needed. Some modern turbines are being developed which have avariable blade angle. As the engine speed or load is changed, the blade angle canbe adjusted to give maximum efficiency at each flow rate.

Turbochargers

• Radial flow centrifugal compressors, turning at high speed, are generally used onautomobile-size engines. On very large engines, axial flow compressors are usedbecause of their greater efficiency at the higher air flow rates. The isentropicefficiency of a compressor is defined as:

Turbochargers

Turbochargers

Example Problem 5.3

Reference:

• Engineering Fundamentals of the Internal Combustion Engines, by W. W.Pulkrabek, 2nd edition, Prentice Hall, Upper Saddle River (2004).