Design and Simulation of Two Stroke Engines

Chapter 7 • Reduction of Fuel Consumption and Exhaust Emissions
ally low-cost applications such as mopeds, scooters and outboards are also found in the literature
[7.44, 7.45, 7.46, 7.48, 7.52].
Liquid gasoline injection
Among the first reports of experimental work in this field are the papers presented by Fuji
Heavy Industries from Japan. The first of these was published as long ago as 1972 [7.17] and
more recently by Sato and Nakayama [7.2] in 1987. Nuti [7.12] from Piaggio, and Plohberger
et al. [7.19] from AVL, have also published experimental data on this subject and they too
should be studied. The papers just mentioned all use high-pressure liquid-injection systems,
not unlike that employed for diesel engines, but modified to attain smaller droplet sizes when
using gasoline fuel. The droplet sizes required are of 10-15 um mean diameter, usually measured
by laser-based experimental techniques to acquire the Sauter Mean Diameter (SMD) of
the fuel droplets [7.16].
The results measured by Sato [7.2] are reproduced here as Fig. 7.48. The peak power
performance characteristics of the engine are unaffected by the use of fuel injection, as is
their reported levels of NOx emission. However, considerable reductions in the bsfc values
are seen in the center of the "oyster" map, with the best contour being lowered to 300 g/kWh
with fuel injection, from 380 g/kWh when the engine was carburetted. The equivalent picture
is repeated for the hydrocarbon emission levels with the carburetted engine showing some
3000 ppm HC/NDIR values, whereas the fuel injection engine is reduced to 400 ppm. This
effect is also reported by Nuti [7.12] and the fuel consumption levels in his engine would be
even lower at the best possible condition, these being bsfc values of 270 g/kWh. These are
significantly low levels of fuel consumption and hydrocarbon exhaust emissions.
However, Sato [7.2] also reports that the direct in-cylinder fuel injection did not improve
the misfiring behavior (four-stroking) at light loads and speeds, and this can be seen in Fig.
7.48. In the lower left-hand corner of the "oyster" maps, at the light load and speed positions,
the bsfc and hydrocarbon emission values are the same for the carburetted and fuel-injected
engines. The values of bsfc are an unimpressive 500 to 600 g/kWh and the unburned hydrocarbons
are at 3000-5000 ppm. In short, stratified burning at light load and low speed was not
achieved.
Plohberger [7.19] shows very complete test data at light load over the speed range, and
below 3000 rpm his results confirm that reported by Sato [7.2]. The direct in-cylinder injection
of fuel has not solved the vital problem of light load and idle running, in these two
instances.
Although Nuti [7.12] makes no comment on this situation, his HC emission is reportedly
only slightly better than that given by Sato [7.2] and his CO levels are rising rapidly in that
zone.
One might conclude from the experimental data presented that the direct fuel injection of
liquid gasoline does not provide stratified combustion at light loads and speeds, and thereby
does not improve the emissions and fuel economy of the two-stroke engine at the crucially
important urban driving condition.
It would appear that there is sufficient time at higher speeds and loads to vaporize the
gasoline and mix it with the air in time for a homogeneous combustion process to occur. The
onset of injection required is remarkably early, and well before the trapping point at exhaust
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