Eye on electronics

Motor, Mar 2001 by Dale, Mike

Once nothing more than a pipe dream, direct injection is fast becoming reality, as stricter fuel economy and emissions regs come to the forefront.

Recently, Ford and other manufacturers made pledges that they'll improve the fuel mileage on their gas-guzzling SUVs by as much as 25% over the next four years. At the same time, there are new emissions regulations coming in Europe, California and the rest of the United States. Major new technology will be needed to fulfill those promises and meet those requirements.

Gasoline Direct Injection (GDI) has the potential to make the internal combustion engine up to 20% more fuel-efficient. While the basic technology goes back to the 1930s, it's only now that the combination of electronic technologies and other system improvements has made it possible to bring this engine concept to fruition.

Currently, GDI is in production by a number of manufacturers, and the list is growing. Mercury Marine, for example, has an Optimax system that uses a dual air/fuel direct injection system on its large outboard engine applications. Mitsubishi has had GDI engines under development since 1996. Siemens and Renault have teamed up to put GDI into Renault's new Megane coupe and cabriolet models. Delphi, meanwhile, has recently announced a joint venture with the Orbital Engine Co. to make a combined spark plug/fuel injector that requires only one access hole into the combustion chamber.

The original idea behind direct injection is that under most circumstances, an air/fuel ratio can be made to run very much leaner than stoichiometric (14.7:1), with no negative effects on engine performance. The limitations are that such lean mixtures can be hard to ignite. There are also consequences regarding emissions, particularly NOx.

Still, the advantages of GDI far outweigh the negatives. With direct injection, fuel enters the cylinder as a mist of tiny droplets, rather than a vapor. This means that the cylinder is actually cooled when the fuel absorbs the heat needed to turn it into a vapor. This cooling effect reduces an engine's octane requirement, so its compression ratio can be increased. And just like diesels, higher compression ratios translate into improved fuel efficiency.

Another advantage of GDI is that it produces a very fast bum of the air/fuel mix, which makes the engine very tolerant of exhaust gas reirculation in comparison with conventional port-injected engines. In the Mitsubishi engine, for example, during idling, when combustion is most unstable, the engine can run comfortably at a 40:1 air/fuel ratio (55:1 if you include EGR).

The key to making a very lean mixture practical is figuring out a way to reliably ignite it. What's needed is a mixture that's rich enough near the spark gap to start the flame kernel. Because the size of the flame kernel is so much larger than a spark gap, this beginning of combustion can then spread to leaner areas of the combustion chamber. Much of the early development work in GDI focused on coming up with mega-zap ignition systems that bum very hot and long in the hope of hitting something combustible. But while the big, hot sparks from such systems are great for igniting lean mixtures, the heat produced from "lightning bolt" strikes makes short work of the plug electrodes.

The breakthrough technology is the use of computers to map and model the flow of fuel and air into and out of the combustion chamber, The shapes of the piston and combustion chamber, the energy and direction of the injected pulses, the movement of the piston and the heat of the engine all influence the position of each droplet of fuel and air. It takes cutting-edge computer technology to determine what the flows should be and how best to position the fuel and air injectors, and the spark plug itself.

Two Basic Systems

When it comes to GDI technology, there are two basic systems out there-- HPDI and LPDI. HPDI relies on high pressure (100 bar, or 100 times atmospheric pressure) to force fuel into a combustion chamber that has already been filled with air. In the case of the Renault IDE engine, Siemens is supplying a three-piston pump needed to generate the high pressure in the fuel rail. Electromagnetically controlled valves, meanwhile, allow the engine controller to determine intake and exhaust valve timing based on the operating needs of the engine.

Orbital's low-pressure direct injection (LPDI) system is a further development and refinement of the one it had hoped would bring two-stroke engines to production cars some years ago. With LPDI, a measured amount of fuel is injected into an air chamber located at the top of a combined air/fuel injector. A belt- or cam-driven air compressor provides pressure to an air rail at about 6.5 bar. When the air rail solenoid is fired, air pressure drives both the fuel and air into the combustion chamber. The key to making this system work is that the fuel blown into the chamber be in a ready-to-burn condition. One nice thing about this setup is that since the fuel is not under extraordinary pressure, no special pump is required, so presumably there's less danger from a split or leaking fuel rail.