Elsbett's six main rules for cooling a d.i. diesel engine are: 1. Heating engine components so that they have to be cooled is a waste of energy. Thus develop a combustion method and engine components that reduce the heat-flow from the combustion air to the components. 2. Thermal stresses are due to differences in temperature rather than to the absolute level of temperatures so that cooling should be concentrated only on the very hottest spots such as for instance the valve bridge. 3. Cooling the inside of the cylinder is more effective than cooling the outside. Thus, whenever possible, eliminate the outside cooling totally or at least cool only the very top of the cylinder block - the part that is usually the uncooled thick plate for the cylinder head bolts. 4. Water or air cooling is less well suited than oil cooling. The best solution is to use the available lubricant to do all the required cooling and design the engine and components accordingly. 5. Any solution that reduces cooling requirements but tends to make the engine more expensive or less reliable is unacceptable. Thus for instance ceramics still cannot be used in the present stage. 6. The reduced cooling requirements of the engine should be so directed that a larger proportion of the energy is used for work, that is to say for expansion of the combustion gases. Up to the point where this is not possible, the resulting increased heat in the exhaust gases should be used, for instance for turbocharging. The paper describes the cooling system of different prototype engines on which the above rules were applied. These engines' total displacement is between 1.4 liters and 16 liters. These d.i. diesel engines are all characterized by the total absence of any water or air cooling and all or almost all cooling of the block is done from inside the cylinder and the cooling of the head is reduced to a bore, through which lubricant flows, in the valve bridge area.