by Jim Kerr
Fuel injection has long replaced carburetors because of the advantages it provides in fuel economy, lower emissions and drivability. Most vehicles now use port injection systems, where the fuel is sprayed into the engine’s intake manifold near the backside of the intake valve. There, the fuel mixes with the air and is drawn into the cylinder when the valve opens.
Some engines use multi-port injection where more than one injector is fired at a time, but most use sequential injection where each injector fires individually. This optimizes fuel economy and emissions at low engine speeds, but there is still another way to inject fuel that can provide a huge improvement in economy and emissions. It is called Direct Injection.
Diesel owners may be familiar with direct injection because most current light duty diesel engines use direct injection. With this design, the fuel is injected directly into the engine cylinder instead of into a pre-cup chamber in the diesel cylinder head. Direct injection diesels used to have a lot of combustion knock but the power of electronics has enabled the engineers to precisely deliver the fuel in pulses that reduce most of the noise. Gasoline direct injection, like its diesel counterpart, also delivers the fuel directly into cylinder instead of the intake manifold.
Gasoline direct injection (GDI) development can be traced back to the 1930’s, but only recent developments in electronics and metallurgy are allowing the manufacturers to design systems that will withstand our current driving environment. Mitsubishi, Toyota and Nissan have had GDI engines on the road in Japan for several years, but the technology couldn’t meet tough North American emission standards. That is about to change, as both Audi and Toyota are ready to introduce GDI powered vehicles into Canada.
The key to GDI technology is a high-pressure injector that can withstand the heat and harsh environment of the engine’s cylinder. Diesel fuel acts as a lubricant, to help a diesel injector survive, but gasoline washes any lubricant away, so the injector material and design is critical. The fuel is delivered to the injectors under high pressure (several thousand PSI) and is sprayed into the cylinder in a fine mist. Typical fuel economy gains of 15% to 20% are possible by using GDI as compared to conventional port injection, but stratified charge GDI can make these savings even higher.
Stratified charge systems inject a small amount of fuel directly into the cylinder where it mixes with the air. This mixture is too lean to self ignite, but when it is time for the cylinder to fire, a small amount of fuel is injected as a rich mixture to start the combustion process. This rich mixture then ignites the lean mixture in the rest of the cylinder to produce power. Stratified charge engines can also reduce emissions by up to 10% because of the lean fuel mixtures used.
Like its diesel counterpart, a GDI engine doesn’t need an ignition system, but some designs will still use a spark plug ignition to control combustion timing. Using an ignition system allows the fuel to be sprayed into the cylinder before it is compressed, placing less load on the injectors, unlike a diesel engine where combustion takes place as soon as the fuel is injected at the top of the compression stroke.
Another similarity to diesels is engine speed is controlled by the amount of fuel injected, not by the amount of air entering the engine. Throttle plates are not needed to control engine speed so volumetric efficiency improves.
I predict GDI technology will see rapid development in the near future as gasoline prices continue to rise. In future articles, we will look at specific designs of the new Audi and Toyota GDI engines that I am sure will make them popular in North America.