With gas prices hovering at their highest level since 1981, there's a lot of loose jabber about the comeback of the diesel. Diesel sales did skyrocket back then, with oil-burning Rabbits and Jettas thick on the highways and diesels accounting for as much as 79 percent of Mercedes' sales in this country in 1982.
But as the ever-skeptical Patrick Bedard points out in his column, modern turbocharged diesel engines are substantially more expensive than gasoline powerplants, and they face daunting technical challenges before they can meet the EPA Tier 2 exhaust-emissions standards that began phasing in with 2004 models.
Notwithstanding wild claims that Dubya has "destroyed the environment," these standards are the strictest in the world, offering no breaks for diesel engines. In contrast, European governments—often credited with showing more concern for the planet than Washington displays—have not implemented emissions regulations nearly as strict as those currently taking effect here.
So before fretting about whether Americans will buy diesel-powered vehicles, the industry must first figure out whether it's even possible to build them to meet the required emissions standards at a reasonable price.
But what if someone could combine the best aspects of a diesel engine and a gasoline engine? In simple terms, that would be mating a gasoline engine's power and low emissions with a diesel's fuel efficiency.
Several automakers are working on it, trying to create what they've dubbed the "homogeneous charge compression ignition" engine. Let's just call it HCCI. I recently spent an afternoon with Graham Hoare, Ford's director of powertrain research and advanced engineering, to find out how such a half-breed powerplant might work.
The very name of the HCCI engine suggests how difficult the task is. That's because in a conventional gas engine you inject the fuel, let it mix during the intake and compression strokes, and light it off with a spark plug when the compression is just about complete.
A diesel engine sucks only air during the intake stroke and squeezes this air to a high temperature and pressure, owing to a compression ratio of about 18:1—at least one- and-a-half times as high as that of most gasoline engines. Then when diesel fuel is injected into this superheated air near the top of the compression stroke, the fuel ignites spontaneously and burns, pretty much as quickly as it is injected.
A diesel makes less power than a gasoline engine of similar size because even with injection pressures that reach 20,000 psi in modern diesels, the atomized fuel cannot mix with all the air in the cylinder in the instant between injection and the beginning of the power stroke.
This inability to burn all the air is not a problem in a gasoline engine, in which air and fuel have plenty of time to mix during the intake and power strokes. That's one reason why a naturally aspirated diesel makes less power and torque than a naturally aspirated gasoline engine.
The other reason is that when diesel fuel is injected into the combustion chamber, the fuel requires a short but essentially constant amount of time before it starts to burn. This so-called delay time limits a diesel engine to a maximum power peak in the low-4000-rpm range. A gasoline engine can rev much higher, using its cylinders more often, which is the second major reason for the gas engine's power advantage. Modern diesels are turbocharged to overcome these inherent power deficiencies.
In the HCCI engine, the spark-ignition mode would be used when high power is needed and the compression-ignition mode would provide efficient cruising power. The spark-ignition part is pretty easy. The HCCI engine uses direct fuel injection into the combustion chamber during the intake stroke with plenty of time for mixing with the air. A spark plug would light off the charge in normal fashion with some accommodation for the engine's higher-than-normal compression ratio, perhaps in the area of 14:1.
In the compression-ignition mode, however, things get challenging because, unlike a diesel, HCCI doesn't use the injection of fuel to initiate combustion. Because fuel and air are already mixed, the combustion process is essentially detonation, which must be controlled well enough to get the charge to explode at precisely the correct moment for optimal efficiency.
That brings us to the second problem. In a conventional gasoline engine, detonation, also called knock, is highly destructive and to be avoided at all costs. In an HCCI engine it is a part of the combustion process in its compression-ignition mode. What will prevent this detonation from blowing the HCCI engine apart?
For one thing, the engine will only operate in HCCI mode during cruising conditions at less than full power. Furthermore, in HCCI mode, the engine will be running lean, with comparatively little gasoline injected into each cylinder. It will also be running with lots of exhaust-gas recirculation, which not only further dilutes the mixture but also warms it up sufficiently to make compression ignition possible.
To make this all work, the engine must be able to switch from the HCCI mode to conventional spark-ignition mode from one cylinder firing to the next, using radical alterations in valve timing, fuel metering, and spark-plug operation. And the HCCI combustion must be consistent enough to maintain low emissions performance and flexible enough to be used in the majority of the engine's operation.
Hoare says Ford has achieved fuel efficiency with HCCI that is 10 percent better than a diesel's under some conditions. When combined with the promise of gasoline-engine power, this sounds like a winning combination.
However, these results were achieved with a single-cylinder laboratory test engine fitted with infinitely variable, electronically controlled valves. Hoare also points out that peak cylinder pressures in the HCCI engine are about 10 percent higher than in a turbo-diesel and twice as high as in a turbo gas engine. Durability development will not be easy.
In other words, don't expect to see an HCCI powerplant under the hood of an Explorer much before 2015. But that still might be sooner than when fuel cells hit the market.
Csaba Csere joined Car and Driver in 1980 and never really left. After serving as Technical Editor and Director, he was Editor-in-Chief from 1993 until his retirement from active duty in 2008. He continues to dabble in automotive journalism and LeMons racing, as well as ministering to his 1965 Jaguar E-type, 2017 Porsche 911, and trio of motorcycles—when not skiing or hiking near his home in Colorado.