標題: [教學] Direct fuel injection vs port fuel injection [打印本頁] 作者: carwing 時間: 2009-2-9 01:28 PM 標題: Direct fuel injection vs port fuel injection
By Jim Kerr
There are a few car owners who prefer the simplicity of thevehicle fuel systems that existed before fuel injection was introduced:a mechanical fuel pump, a carburetor and a few hoses, and the fuelsystem was complete. Although these systems may have been simple, theywere far from trouble-free. Carburetors often required overhauls orcleaning. Sticking choke mechanisms either meant the vehicle wouldn’tstart or it would start but pour black smoke (unburned fuel) out thetailpipe. Many vehicles required the carbs to be adjusted every springand fall as temperatures changed. The fact is, fuel injectioneliminated most of these problems.
Early fuel injection systems could be grouped into two types: portand throttle body. Throttle body injection places the fuel injectors ina central housing similar to a carburetor that sprays fuel into theintake manifold. These systems were economical to produce but had someof the disadvantages of carbs. Fuel had to travel through the intakemanifold with the incoming air, so manifold design had to be simple.Place a few curves in the manifold and fuel would drop out of theairflow. Then the air fuel ratio wouldn’t be the same for allcylinders.
Port injection however, sprays the fuel into the intake ports of thecylinder heads at the back side of the intake valves. This allowed theengineers to design manifolds that directed only air, so they could beany length and shape. Better volumetric efficiency was achieved (bettercylinder filling) with these new manifold designs, so fuel economyimproved along with better power.
Regardless of the type of fuel delivery system, the goal is to havea 14.7 to 1 air to fuel ratio in the combustion chamber as it burns.The reasoning for this is simple: current catalytic converters are two-stage devices. One stage works best with rich air fuel ratios (below14.7 to 1) and the other stage works best with lean air fuel ratios(above 14.7 to 1). Only at 14.7 to 1, the stoichiometric ratio, doesthe catalytic converter reduce emissions properly: that is why vehiclesoperate at this air fuel ratio.
If we had converters that worked effectively at leaner air fuelratios, then engines could be programmed to operate at a leaner ratioand we would have better fuel economy, but research hasn’t produced acost-effective one yet. You can be assured they are working on it,though.
Direct fuel injection is the newest type of injection system on theNorth American market. Companies such as Nissan and Mitsubishi have hadsystems overseas for several years, but it was only in the past fewyears that materials and manufacturing technology improved so thatdirect fuel injection would meet our emission standards.
Today, many companies produce vehicles with direct fuel injectionand others will soon. You don’t have to look under the hood of a luxurycar either. Mazda and Chevrolet have direct fuel injection on lowerpriced vehicles already.
So why choose direct fuel injection? Better fuel economy and morepower are the benefits. Direct fuel injection can increase fuel economyby 15 to 20 per cent and when combined with turbocharging, may see 20to 30 per cent economy improvements. A 15 per cent increase in poweroutput is possible at the same time.
Direct fuel injection sprays the fuel directly into the cylinderinstead of the intake port like a port injection system. The differencein position is small, but it decreases the surface area that fuel cancome in contact with before it ignites. If fuel comes in contact withthe sides of the cylinder, the back of the intake valve or the intakeports, some of it condenses into fine droplets. Fuel only burns afterit becomes a vapour, so these droplets pass through the combustionchamber without being burned. Direct fuel injection reduces thispossibility.
Direct fuel injection sprays the fuel into the combustion chamber ata much higher pressure. Some systems operate in the 2,000 to 3,000 psirange and if you listen closely under the hood, you can hear themechanical clicking of the injector solenoids and pressure regulator asthey control fuel spray. This is normal. The higher pressure causes thefuel to come out of the injector in a very fine mist that will quicklyturn into vapour.
High swirl combustion chamber designs and/or turbocharging are oftenused in conjunction with direct fuel injection to mix the fuel with theair faster. More power, better fuel economy and better engine controlmake direct fuel injection the way of the future for gasoline engines.
