Auto Tech: Direct fuel injection vs port fuel injection green scene1 auto tech
Direct fuel injected engine in the 2006 MazdaSpeed6. Click image to enlarge

Auto Tech: Direct fuel injection vs port fuel injection green scene1 auto tech Share this story on Facebook

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By Jim Kerr; photo by Chris Chase

There are a few car owners who prefer the simplicity of the vehicle fuel systems that existed before fuel injection was introduced: a mechanical fuel pump, a carburetor and a few hoses, and the fuel system was complete. Although these systems may have been simple, they were far from trouble-free. Carburetors often required overhauls or cleaning. Sticking choke mechanisms either meant the vehicle wouldn’t start or it would start but pour black smoke (unburned fuel) out the tailpipe. Many vehicles required the carbs to be adjusted every spring and fall as temperatures changed. The fact is, fuel injection eliminated most of these problems.

Early fuel injection systems could be grouped into two types: port and throttle body. Throttle body injection places the fuel injectors in a central housing similar to a carburetor that sprays fuel into the intake manifold. These systems were economical to produce but had some of the disadvantages of carbs. Fuel had to travel through the intake manifold with the incoming air, so manifold design had to be simple. Place a few curves in the manifold and fuel would drop out of the airflow. Then the air fuel ratio wouldn’t be the same for all cylinders.

Port injection however, sprays the fuel into the intake ports of the cylinder heads at the back side of the intake valves. This allowed the engineers to design manifolds that directed only air, so they could be any length and shape. Better volumetric efficiency was achieved (better cylinder filling) with these new manifold designs, so fuel economy improved along with better power.

Regardless of the type of fuel delivery system, the goal is to have a 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 (below 14.7 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, does the catalytic converter reduce emissions properly: that is why vehicles operate at this air fuel ratio.

If we had converters that worked effectively at leaner air fuel ratios, then engines could be programmed to operate at a leaner ratio and we would have better fuel economy, but research hasn’t produced a cost-effective one yet. You can be assured they are working on it, though.

Direct fuel injection is the newest type of injection system on the North American market. Companies such as Nissan and Mitsubishi have had systems overseas for several years, but it was only in the past few years that materials and manufacturing technology improved so that direct fuel injection would meet our emission standards.

Today, many companies produce vehicles with direct fuel injection and others will soon. You don’t have to look under the hood of a luxury car either. Mazda and Chevrolet have direct fuel injection on lower priced vehicles already.

So why choose direct fuel injection? Better fuel economy and more power are the benefits. Direct fuel injection can increase fuel economy by 15 to 20 per cent and when combined with turbocharging, may see 20 to 30 per cent economy improvements. A 15 per cent increase in power output is possible at the same time.

Direct fuel injection sprays the fuel directly into the cylinder instead of the intake port like a port injection system. The difference in position is small, but it decreases the surface area that fuel can come in contact with before it ignites. If fuel comes in contact with the sides of the cylinder, the back of the intake valve or the intake ports, some of it condenses into fine droplets. Fuel only burns after it becomes a vapour, so these droplets pass through the combustion chamber without being burned. Direct fuel injection reduces this possibility.

Direct fuel injection sprays the fuel into the combustion chamber at a much higher pressure. Some systems operate in the 2,000 to 3,000 psi range and if you listen closely under the hood, you can hear the mechanical clicking of the injector solenoids and pressure regulator as they control fuel spray. This is normal. The higher pressure causes the fuel to come out of the injector in a very fine mist that will quickly turn into vapour.

High swirl combustion chamber designs and/or turbocharging are often used in conjunction with direct fuel injection to mix the fuel with the air faster. More power, better fuel economy and better engine control make direct fuel injection the way of the future for gasoline engines.

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