The major advantage of the diesel engine over the gasoline engine is its high fuel efficiency.
Diesels are more fuel efficient than gasoline engines for two reasons. First, the diesel cycle uses
high compression ratios (16:1 to 24:1) to ignite the fuel spontaneously upon contact with hot
compressed air, which leads to high engine efficiency. Gasoline engines cannot employ such high-
compression ratios because the gasoline/air mixture would ignite prematurely under such
conditions; the octane number of the fuel limits the compression ratio to about 10:1 for an engine
using regular gasoline. Second, diesels do not experience the pumping losses characteristic of SI
engines because they do not throttle their intake air; instead, the power output of the diesel engine
is controlled by regulating the amount of fuel for each combustion event while the air inducted is
unthrottled. The SI engine’s throttling of intake air leads to power losses (refereed to as pumping
loss) that increase at light loads (typical in city driving) which are absent in the diesel, and its fuel
efficiency benefit under light load conditions over a gasoline engine is impressive.
On the negative side, diesel engines have much higher internal mechanical fiction because of
their high cylinder pressures, and they must expend additional energy to drive their high-pressure
fuel injection pumps. The high compression ratio and combustion process also lead to higher
engine weight relative to a similar displacement gasoline engine, as well as reduced specific output
and increased noise and vibration. These last three factors of reduced power, increased noise, and
higher vibration are often blamed for the lack of widespread acceptance of the diesel in the U.S.
marketplace, where the value of the diesels’ enhanced fuel efficiency is low.
A potentially more serious factor affecting diesel engines in the United States is potential
difficulty in meeting current and future emission standards. Diesel engines have very low gaseous
HC and carbon monoxide (CO) emissions but relatively high nitrogen oxides (Nox) and
particulate emissions. The very lean air-fuel ratios employed by the diesel under most driving
conditions and the resulting low exhaust temperature has made catalytic treatment of NOX and
particulates difficult, but recent developments with higher pressure, electronically controlled fuel
injection systems, and improved oxidation catalysts have reduced the particulate emission
problem. Diesels have a waiver from current NOX standards for cars, but, if the waiver were
revoked, their ability to meet Tier I, Tier II, and California LEV standards is still uncertain.
The status of diesel technology relative to its fuel efficiency, power output, acceptability,
ability to meet emissions standards will be discussed.
Performance of New Diesel Engines
andThe latest designs of diesel engines recently unveiled in Europe provide significant
improvements in virtually all of the characteristics of interest. Most of the development in diesel
technology is centered in Europe. Diesel penetration in the Japanese market is low, and Japanese
automakers are focusing primarily on lean-bum gasoline engine concepts. Diesel penetration is
occurring, however, in the Japanese sports utility vehicle market.
Until 1991, diesel powered passenger cars and light trucks sold in the United States were all of
the IDI type, where fuel is sprayed into a prechamber, partially mixed and combusted with air
before further mixing and combustion occurs in the main combustion chamber. The prechamber