The ultimate green diesel engine—the one with zero harmful emissions, with theoretically minimal fuel consumption, and running on environment-friendly renewable fuels, is something that will always remain out of reach. However, reaching the exact theoretical limits is not as important as is the question: how close to these limits can we push a modern diesel engine. By looking back into the last decade, one can only be astonished what a tremendous progress was achieved by diesel engine developers and producers. Fuel consumption decreased substantially, exhausts became much cleaner, and the engines became lighter and smaller due to downsizing in piston displacement volume and number of cylinders. From that point of view one can be very optimistic for the future. In spite of that, one should be aware of several hard nuts still waiting to be cracked in the coming years. Two of the perhaps mostly exposed are briefly addressed in the following.