Green Tech: A Well-Bred Engine Carries Best Genes of Its Parents July 16, 2006
Engineers are looking for new ways to combine the pluses of different engine types.
NO combustion process for converting fuel to forward motion is perfect. Though engineers can choose from a selection of combustion processes that have been developed for use in today?s automotive engines, each has some shortcoming ? in efficiency, cost or environmental friendliness ? that must be weighed against its benefits.
Gasoline engines, for instance, can be designed for very low exhaust emissions, but may leave something to be desired in efficiency. Small diesel engines can return terrific fuel economy, but they are costly, and scrubbing their exhaust gases to meet environmental regulations is a big challenge.
Often, the disadvantages that follow the ?but? overwhelm the gains.
No wonder engineers are looking for new ways to combine the pluses of different engine types. One operating cycle for engines under wide study in the industry, known as Homogeneous Charge Compression Ignition, cherry-picks the best traits from gasoline and diesel designs ? low emissions and high efficiency, respectively.
The H.C.C.I. engine runs ideally as a combination of the two. Like a spark-ignition engine, its fuel enters the cylinder premixed with air. The fuel is evenly distributed in the air ? not concentrated around the fuel injection nozzle ? accounting for the homogeneous charge part of its name; the mixture is ignited without a spark plug, as in a diesel engine. In engineering jargon, this is a compression-ignition engine.
H.C.C.I. does not work at idle or near full throttle, so under these conditions the engine reverts to a conventional spark-ignition mode.
Every automaker is working on the H.C.C.I. combustion process, said Dr. Uwe Grebe, executive director for advanced engineering at General Motors Powertrain. Among the many attractions, he said, is that H.C.C.I. deals with mileage and emissions challenges inside the engine, rather than depending on devices like special catalytic converters to treat exhaust gas after it leaves the engine.
The high cylinder pressures of diesel engines, a result of compression ratios of 17 to 1 or more, raise the temperature high enough that ignition takes place spontaneously as fuel is injected into it. This fuel delivery into hot compressed air creates fuel-air mixtures all the way from 100 percent fuel to 100 percent air as fuel droplets evaporate and diffuse into the surrounding air. Upon igniting, combustion is hottest where fuel and air are ideally proportioned ? hot enough to generate nitrogen oxides, the toughest exhaust emission to control.
As some larger fuel droplets fail to evaporate and do not burn completely, a carbon-rich residue is created. This is the source of diesel particulates ? the black smoke once common from diesel exhaust pipes. If not for nitrogen oxides and particulates, the diesel engine?s high fuel efficiency would make it the most desirable automotive power plant.
In the type of gasoline spark-ignition engines that are dominant in the United States market, fuel and air are mixed before entering the engine cylinder. To cut emissions and increase fuel efficiency, it is desirable to burn lean mixtures in which there is less fuel in relation to the volume of air.
A lean mixture burns cooler than a chemically ideal mixture, and this lower temperature reduces formation of nitrogen oxides. The lower combustion temperature also makes the conversion of heat into pressure more efficient.
But lean mixtures are hard to ignite by spark, so engineers resort to stratifying the charge ? injecting fuel in a layered pattern that is rich enough near the spark plug to ignite easily and leaving it leaner elsewhere. But the hot combustion of the fast-burning mixture near the spark plug again generates nitrogen oxides. If engineers try for higher fuel efficiency by raising the compression ratio to diesel-like numbers, the fuel knocks violently during combustion.
The germ of a compromise solution presented itself by accident. Have you ever had your car?s engine continue to run after its ignition was switched off? This ?run-on,? as it is called, was especially common during the late 1970?s and ?80s. The engine runs roughly, bucks and finally stops ? especially if you open the throttle.
- Posted in : Uncategorized
- Author : arnold
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