Vehicle Engines - Decreasing Fuel Consumption and Exhaust Gas Emissions in Transportation - page 136

Table 9.4 Impact of construction on fuel consumption and exhaust gas emissions
Construction parameters Physical properties

Combustion chamber profile A compact combustion chamber can save engine volume. However, low cracks and piston strokes increase NO x

emissions, reduces flame quenching, and the cold wall could lead to the expiration of the flame [ 8 ]

Combustion chamber design In spark ignition engines, the combustion chamber is predominantly in the cylinder head in contrast to self ignition

engines with direct injection, by which the piston serves for the admission into the combustion chamber [ 9 ]. The
pistons of spark ignition engines possess either a light hollow or they are flat. In the multi valve engine an inlet
channel can be constructed as a swirl channel which can increase the mixture preparation in the lower partial-load
area especially with a low mass flow. The second channel, which operates at a higher load and higher number of
revolutions, serves as a filling channel [ 10 ]

Compression ratio The compression ratio can be extremely high in the full load range. In a spark ignition engine, the limit is given by the

knocking characteristics. The NO x emissions increase with extremely high compression ratios. Leaner burning with a
higher Lambda number can decrease NO x emissions. Thermal efficiency in spark ignition engines is between 0.35 and
0.45 on average and in self ignition engines between 0.50 and 0.60, depending on construction [ 11 ]

Spark plug position The situation of the ignition plug, the number of valves, and the conception of the valve impulse, e.g., a variable valve

impulse influences the fuel consumption and the exhaust gas emissions. Four or more valves allow the spark plug
to be centrally positioned [ 12 ]

Ignition point Fuel consumption and exhaust gas emissions can be influenced by the ignition energy, and by the shape and position

of the spark plug [ 13 ]. Late ignition points result in increasing exhaust gas temperatures, which produce favorable
conditions for post reactions of HCs and CO. Additionally, late ignition increases fuel consumption. However, it
shortens the warmup phase of the engine and optimizes the starting characteristics of the catalyst

Idle stroke bore relationship to
cylinder volume

The longer the stroke of an engine, the smaller the HC and CO emissions and the partial-load fuel consumption.
However, the size of the idle stroke bore relationship to the cylinder volume is not freely dimensionable, because
criteria such as the mass forces, combustion chamber design, task of the construction, existing manufacturing
plants, etc., usually permit only a closed range for the application [ 14 ]
The long stroke engine offers a great potential for the improvement of the efficiency degree, in particular in four-
valve combustion chambers by higher compression ratios [ 15 ]