DETROIT — Volkswagen AG's 3 liter/100 km (78 mpg combined) Lupo, introduced last year, has been such a huge success that production of its unique 1.2L turbodiesel engine is well short of demand and has delayed introduction of the remarkable aluminum 4-door Audi A2, which also delivers 78 mpg with the same engine (see WEVTU — Mar. 1, '01, p. 7).

Not satisfied to leave well enough alone, VW releases information at the recent Detroit Society of Automotive Engineers World Congress here (paper #2001-01-0258) that states, “We intend to exchange the diesel engine of the Lupo car for a stratified charge engine with a ring nozzle.”

In this system, fuel is introduced into a “ring” surrounding the combustion chamber at the top of the cylinders; the ring then injects fuel from the sides at various points to the center of a nearly spherical combustion chamber cavity — part in the cylinder head and part in the piston top. In the design, “fuel jets enter the open chamber, impinge each other in the center of the combustion chamber, where — after self ignition — they form a fireball by not touching the (chamber) walls.”

The paper authored by VW, Institute fur Fahrzeugbau and Technical University engineers then explains that “rapid combustion takes place without any wall quenching in order to prevent unburned fuel. The formation of NOx and soot can be minimized by surrounding the fireball with already burnt gasses.”

The authors then indicate the consumption of the Lupo will be cut to 2L/100 km (117 mpg) but does not define the specific conditions at which this extremely low energy use is achieved. Also discussed is another combustion system invented in 1993 called the MESC-stratified-charge-engine, as well as the Latch engine, which evidently led the developers to the preferred system identified (and illustrated) as a “Uniflow 2-stroke diesel-like HTSC-engine with a ring nozzle.”

The rather rough drawing of the engine indicates there is only one valve (intake) for each cylinder, mounted centrally overhead relative to the piston. If VW selects a 2-cyl. version, which would have the same torque-pulse smoothness as a 4-cyl. 4-stroke engine, there will be a total of only two valves, which in combination with 2-stroke operation will have low friction losses. With the near-spherical combustion chamber and clever combustion system, low emissions with high efficiency are evident.

The intake air scavenging system is neither explained or illustrated but could well be an electrically assisted turbocharger. Exhaust exits through ports in the cylinder wall.

In effect, the developers have turned to the established advantages of uniflow diesel technology, proven over a period of decades, to develop very high efficiency. But they have applied another proven feature: an ideal combustion chamber shape, as well as the ability, of course, to use advanced common-rail, multiple-injection fuel systems.

The paper also describes a spark-ignition gasoline version of the engine, which may increase interest for use in the U.S., where diesel is not as widely adopted.