This is, obviously, going to be a speculative answer. I would say that the Allies, France and the UK, with their Empires, would, eventually have won. The Central Powers were struggling economically: Britain's blockade was really biting by as early as 1917, and by 1918 Germany and the Hapsburg Empire were close to starvation. With the Treaty of Brest-Litovsk, Germany had shown the world war a German dictated peace would be like, which encouraged Britain and France to keep fighting. The Ottoman Empire was struggling in Mesopotamia and Palestine, despite German help, as was Bulgaria. The Hapsburgs were just about holding in. Indeed, they weren't doing quite as badly as the Germans themselves thought, and were able to make a massive offensive at Caporetto, which pretty much demoralised the Italians, who had to be propped up by the Allies. However, by 1918 Britain had, at long last, adopted the convoy system, and supplies were getting through on a more regular basis, so the U Boat threat was, at least, under control. The German occupation of Russia, although freeing lots of troops to be transferred to the West, was not the economic salvation that the Germans thought it would be. The Russian railway system was broken down, and the anarchy that obtained in Russia post-Revolution, meant that the hoped for raw materials from German occupied Russia was far less than what was needed. So, the German Spring offensives, assuming that they were still launched, which were stopped without significant American help, would still have ultimately failed. The Allied, mainly British, Autumn offensive which broke the German Army in the West would possibly not have taken place until the Spring of 1919. By which time the Hapsburgs would have already collapsed after Vittorio Veneto, the Greeks would have kept the remnants of the Ottomans occupied, so Germany would have been both isolated and near collapse. The Bolsheviks had already been able to influence the German troops in the East during 1918, so there would have only been further problems in that area, so perhaps Revolution would have broken out before the Allied Spring offensive of 1919 had even begun. So, I would suggest that Germany would still have lost, but it would have taken until early to mid 1919. Probably.

The US had a huge impact on the war. By May 29th of 1918 a grand total of 650,000 American troops were stationed in France, with as many as 10,000 men being added every day. Germany just couldn't keep up -- they were unable to replace their losses. What is interesting to note however is that American officers made the same strategic mistakes which other countries made earlier in the war. German Lt. Col. Herman von Giehrl said of the Americans:

The military significance of the Americans lay not only in their numbers but, more than anything else, in their unused nervous force. The troops were conspicuous for their absolute freshness, their physical and hence their moral superiority, and a certain ignorance of, and, therefore, and indifference to danger. Training technique, attacking power, and real fighting value, on the other hand, were less in evidence.

Nevertheless, within the first two weeks of August, Germany advanced with her last reserves. By the end of September, General Ludendorff realized that war must stopped and advised the Kaiser to immobilize his men.

Furthermore, it isn't necessarily important to ask the question of "what if?" because American intervention was inevitable. Wilson thought, "The world must be made safe for democracy. Its peace must be planted upon the tested foundations of political liberty." You can read Wilson's "War Message to Congress" here.

EDIT: Words.

Not particularly big. By the time US forces arrived the Germans were on the brink of defeat, suffering shortages of pretty much everything, especially food. Arriving American troops were completely unprepared for the war that awaited them and were lacking in necessary equipment and vehicles, particularly machine guns. Without American involvement I can see the war being extended by a few months with the same outcome