Europeans Advance on Gasoline DI

Mitsubishi Motor Corp. led the way with its patented GDI (gasoline direct injection) system, and now three major German auto makers are heading the European advance into this promising powertrain development.

Big gains in fuel economy and emissions reduction are direct gasoline injection's (DGI) main advantages — along with the possibility of downsizing engine displacement, a practice that can chop engine weight and production costs.

Although company names for the DGI systems differ — Mitsubishi has patented the “GDI” acronym — the basic technology is common to all.

Volkswagen AG calls it FSI (fuel stratified injection) and had an early start, benefiting from its TDI (turbo direct injection) turbodiesel development dating back to 1989.

Ford-Werke AG of Cologne originally named its system DISI (direct injection spark ignition) to underline the evolution of direct injection from advanced-diesel development programs. Its initial work is with a 1.1L turbocharged 3-cyl. delivering 110 hp — and probably slated for initial production in the Fiesta.

DaimlerChrysler AG liked the moniker CGI ([stratified] charged gasoline injection), to single out one member of a new range of four 1.8L 4-cyl. engines currently earmarked for the Mercedes C-Class cars. Designated C200 CGI, it develops 170 hp with the aid of supercharging.

An outline of DGI characteristics and benefits starts with improved performance. An important factor is operation at part-load with a wide-open throttle valve in the inlet manifold, avoiding the considerable pumping losses of today's conventional port- injection systems.

With all DGI systems, gasoline is injected directly into the combustion chamber, with throttle action initially controlling fuel delivery rather than intake airflow. Mixture turbulence and swirl rate can be enhanced by precise positioning of the injector nozzle, with fuel droplets impacting on a specially-contoured piston crown.

Injection pressures of up to 1,740 psi (120 bar) — compared to the usual 55 psi (3.8 bar) of a typical indirect-injection gasoline engine — provide much-finer fuel atomization. And higher compression ratios are possible since DGI reduces the knock tendency, thus enhancing thermal efficiency.

Offered the combination of such pluses, auto makers see engine downsizing for a given performance as an economy-angled possibility. One company reports up to 30% fuel saving, particularly at idle and the lower speed ranges. Because of DGI's increased power potential, smaller engine size, as well as fewer cylinders, becomes a viable option.

The flip side of these gains: the “lean” air/fuel mixture inherent to DGI operation, with its surplus of oxygen, upsets the stoichiometric 14.7:1 air-fuel mixture ratio needed for complete combustion. That presents a new challenge in exhaust emissions — mainly how to reduce excess oxides of nitrogen (NOx) production.

The common solution is a new type of NOx “reservoir” catalytic converter that temporarily absorbs excess NOx during times when the engine operates in lean-burn mode, then releases it periodically to react with other exhaust components, forming harmless nitrogen.

Unfortunately, sulfur in gasoline can poison these catalysts and drastically reduce their effectiveness. To guarantee emissions-compliance, DGI engines require sulfur-free (10 parts per million or less) gasoline, not yet widely available in Europe and even less so in the U.S.

GDI is a 2-phase combustion process, starting with a form of stratified charge from idle to mid-rpm engine speed or during part-load operation. In this mode, there is a large surplus of inducted air, with the direct-injected fuel precisely guided to the spark plug. The localized concentration of air-fuel mixture is easily ignited, and combustion spreads to the entire chamber for an exceptionally lean-burn function that offers limited power.

When increased performance is needed, the engine controller automatically switches to the homogenous-combustion mode, where the entire stoichiometric charge is easily combusted.

In this mode, the cylinder is uniformly filled with a normal stoichiometric mixture and the throttle valve in the intake manifold is activated to regulate air delivery.

With homogenous-charge operation, though, the ultra-lean economy gains of GDI are lost — though partly recovered by the higher permissible compression ratio. High EGR (exhaust gas recirculation) rates also help decrease fuel consumption. Reduction in NOx emissions is another important benefit.

Volkswagen cites its Lupo FSI as a pacesetter for this new generation of gasoline engines, and claims an average fuel consumption of 48 mpg (4.9L/100 km) for the small-but-peppy hatchback.

Today's standard, indirect-injection gasoline engines soon will be completely replaced by FSI engines throughout the Volkswagen Group, says a company spokesman. “Direct injection will be the dominant technology for gas engines, as it is already today for diesels,” he says.

The 1.6L DOHC FSI unit in the Golf and Bora has a 12.1:1 compression ratio, against the standard 11.5:1, and output is 5% greater. Maximum torque of 114 lb.-ft (155 Nm) is enhanced by the same 5%. Performance thus is marginally increased, yet there's a claimed 10% improvement in average fuel consumption.

Since sulfur-free gasoline is available in Germany — though not in many other European countries — the sale of VW FSI-engined cars is largely confined to the home market.

The look-ahead solution to the fuel problem is, of course, the widespread availability of sulfur-free gasoline. VW has been cooperating with Shell Petroleum to merge the experience of both worldwide concerns in their respective fields. One result: FSI-friendly Shell Optimax gasoline, now on sale in Germany.

For DaimlerChrysler, a 4-cyl. gasoline engine in a prestigious Mercedes demands unusual refinement. The sophisticated C200 CGI variant of the company's all-new DOHC 1.8L family fills the bill with two counter-rotating balance shafts, supercharging with a Roots-type blower, an intercooler and variable valve timing for both camshafts.

With this layout, the engine is said to have the tractive power, smoothness and low-noise characteristics of a 6-cyl. engine, coupled with the economy of a 4-cyl. powerplant.

For the C200 CGI engine, consumption in the Mercedes CLK coupe is reported to be 19% less than the previous equivalent C-Class model, or about 30 mpg (7.8L/100 km) on the NEDC combined cycle with sulfur-free Super-Plus gas.

The CGI engine has two separate inlet ports with flow characteristics designed to optimize mixture swirl. Fuel is injected at a 42-degree angle, when the turbulent mixture reaches the spark plug via sculpted recesses in the piston crown.

Ford of Europe's direct-injection DISI project is a key part of its futuristic Fusion concept vehicle. The 1.1L (110-hp) turbocharged engine is notable for having three cylinders, which trims the total weight to 220 lbs. (100 kg) as well as reducing friction and the parts count. Compression ratio raised to 11.7:1 contributes to the fuel economy, as tested in the small Fiesta, which gave a 14% advantage over the car powered by a comparable multi-point injection gasoline engine.

Ford sees further economies in mating the Duratec SCI engine with an automated layshaft gearbox called Durashift EST.

Both the DISI engine and gearbox are integral parts of the Fusion concept. Its full potential depends on sulfur-free gas. Until that is common, Ford says it is holding back on any product launch.