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During recent years there has been a continuing increase in the demands for higher braking performance of commercial vehicle engines. Mercedes-Benz had introduced the engine brake with continuously open decompression valve (‘Konstantdrossel’) into series production in 1989 as an option (1). A further increase of braking power was to be achieved while retaining the additional decompression valve in the cylinder head. For this, the decompression valve was no longer kept open during the whole working cycle (continuously open decompression valve), but only for a short period from just before compression TDC to about 90...120° crank angle after compression TDC (pulsed decompression valve). The hydraulic actuating system which opens and closes the decompression valves was developed in cooperation with Mannesmann-Rexroth GmbH, Lohr, Germany. The engine braking performance attainable with this system is shown in comparison to other known engine braking systems.

The demand for fuel-efficient, low-displacement engines for future passenger car applications led to investigations with small DI diesel engines in the advanced engineering department at Mercedes-Benz. Single-cylinder tests were carried out to compare a 2-valve concept with 241 cm3 displacement with a 422 cm3 4-valve design, both operated with a common rail injection system. Mean effective pressures at full load were about 10 % lower with the smaller displacement. With such engines a specific power of 40 kW/I and a specific torque of about 140 Nm/I should be possible. In the current stage of optimization, penalties in fuel economy could be reduced down to values below 3 %. The “4-cylinder DI diesel engine with 1 liter displacement” is an interesting alternative to small 3 cylinder concepts with higher displacement per cylinder. An introduction into series production will not only depend on the potential for further improvement in fuel economy of such small cylinder units.

As the late combustion phase in SI engines is of high importance for a further reduction of fuel consumption and especially emissions, the impacts of unburnt mass, located in a small volume with a relatively large surface near the wall and in the top land volume, is of high relevance throughout the range of operation. To investigate and quantify the respective interactions, a state of the art Mercedes-Benz single cylinder research SI-engine was equipped with extensive measurement technology. To detect the axial and radial temperature distribution, several surface thermocouples were applied in two layers around the top land volume. As an additional reference, multiple surface thermocouples in the cylinder head complement the highly dynamic temperature measurements in the boundary zones of the combustion chamber.