The purpose of this study is to investigate the effects of the mean cylinder temperature and pressure from the start of injection to ignition on ignition delay period in a D.I. diesel engine, and to clarify the difference in the ignition delay period between the engine and a constant volume bomb. The ignition delay period in the engine was measured by changing the following factors such as injection timing, engine speed, swirl ratio, injection quantity, nozzle opening pressure, nozzle hole diameter and piston cavity shape. The cylinder temperature was measured by a fine thermocou-ple with a compensation circuit under the motored condition. A single shot injection was adopted to reduce the residual gas from a previous cycle. Results are as follows:
Ignition delay period in the engine can be arranged with the cylinder temperature and pressure. The engine shows shorter ignition delay than the constant volume bomb. The difference in the ignition delay between them is mainly due to the existance of the piston cavity wall on which the fuel spray impinges.

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This Paper deals with the micro-explosion of a single emulsion droplet in high pressure and high temperature. From the analysis of simplified models and the observations of the vapor bubbles in the droplet of diesel fuel and of the micro-explosion of a single emulsion droplet, the main results are as follows;
(1) In high pressure (3 MPa), the phenomena of micro-explosion are observed. From the temperature characteristics of a droplet, it is reasonable to expect that micro-explosions can occure at higher pressure.
(2) By increasing the pressure, forming of vapor bubbles is advanced while the vapor growth rate decreases, but formation of vapor bubbles is not inhibited up to 3MPa.

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An experimental research is performed to clarify the combustion characteristics of the present bunker fuels by the methods mentioned above subtitle.
The sample fuels are several kinds of base oils taken out from the conventional and some recent refiner process, these mixture oils on trial and several bunker fuels in practical use.
The main result are showed as follows:
(1) The thermal analysis makes possible to predict the original components of bunker fuels by comparing the burning profile with it of the base oil.
(2) Two correlation formulas on droplet burning were obtained by the method of multiple regression analysis. The ignition lag and the burning rate on droplet burning are able to be predicted by these formulas.
(3) The sample fuels are able to be classified by the characteristic component on the diagram between the ignition lag and the burning rate.
(4) On the same diagram, bunker fuels which were reported troubles happened in use, are distributed the characteristic range.
The recent low grade bunker fuels have caused to increase troubles on combustion in the marine diesel engines. Troubles on combustion are able to be predicted by adapting the analysis result above mentioned to these low grade bunker fuels.

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