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Fixed valve lift characteristics and timings of internal combustion engines usually represent a compromise with regard to the requirements placed on power output and torque. In order to minimize the compromise, numerous Variable Valve Actuation (VVA) systems have been proposed that give adjustable valve operation depending on the engine speed. This paper presents a review of VVA systems which have been investigated mainly for the optimization of engine torque. Of most interest are systems which have already been tested in complete engines or even installed in production line cars.

An electro-hydraulic variable valve actuation (henceforth, referred to as WA) system has been developed for spark ignition engine applications. This is a report of its successful initial testing. Motoring tests were first conducted to determine any cylinder to cylinder variations in charging efficiencies. These were then corrected by individual intake valve timings. Then testing was conducted on a firing engine at both part and full load over a full range of engine speeds. The brake specific fuel consumption (bsfc) was considerably improved at low speeds by the WA system. However, the bsfc results of the WA system were worse than those of the throttled engine system at higher speeds and low loads. The cause of this problem was determined to be poor mixture preparation and various corrective measures are now being studied.

1 A spark ignition engine's pumping loss increases over-proportionally when the engine load is reduced. Without throttling, control of the charge of the S. I. engine can be realized by variation of the intake valve opening period. Therefore, the variable valve timing has great potential for reducing pumping losses. A mechanically operating variable valve timing system has been implemented on a Porsche 924 TOP. Under both full load and part load, improvement is obtained with optimization of intake valve closing at any load and speed condition. Under low speed wide open throttle conditions, intake valve closing at the correct time causes an increase in volumetric efficiency and therefore a torque increase of up to 8%. Under lower part load conditions, fuel consumption can be improved by as much as 4%, and at idle by 10%. These improvements are the result of a 20%-50% reduction of pumping losses and of an improved combustion process due to less residual gas fraction.

The purpose of the study reported here was to investigate the applicability of monolithic honycomb catalysts for exhaust gas purification on small two-stroke engines. Investigations were carried out to assess the relations between operating conditions and the catalysts' CO- and HC converstion efficiency and to optimize conversion efficiency under stationary and transient engine operating modes. Furthermore, the influence of a catalyst on smoke emission was investigated as were the aging characteristics of various catalysts under European operating conditions with the engine running on leaded fuel

Exhaust and fuel characteristics during the warm-up phase of engine operation is in great need of improvement, especially for carburetted engines. Due to the number of short trips made by vehicles, a large proportion of urban driving occurs during warm-up operation. The paper presents possibilities for achieving these improvements through modification of engine components (e.g., exhaust recirculation, oil and intake manifold heating). Tests were conducted on an Audi 80 LS. Through use of the described changes, fuel consumption beyond normal, hot-operation figures was reduced from 25% to only a few percent of the latter. Reductions of CO and HC emissions were 80% and 50%, respectively, although the erratic engine performance which is often characteristic of warm-up operation was somewhat aggravated. However, the results indicated that the problems associated with engine warm-up can be solved at acceptable costs. Further possibilities for improvements remain to be studied.