Germany plus Austria-Hungary vs France and Russia were fairly evenly matched on paper, with France and Russia having bigger manpower reserves, but the Germany central position, the high quality of its army, and its railroad network means that the Central Powers eventually wins that one, even with substantial British support for France and Russia short of war. The British army was small, and if the POD is no German invasion of Belgium there is really nowhere to deploy it on the Western Front anyway. But after two years of war and recruitment, the British army was much bigger and a real factor, and needed after the French losses in Verdun. Also after two years the blockade started to really bite. This all implies that without Britain, and no German invasion of Belgium, the French and Russians are fine trading blows with Germany and Austria-Hungary, since they get the British munitions anyway, but things start to fall apart in 1916, and they have to make peace in 1917. If the British government, which will be Liberal only up to January 1916 at the latest, can't get the public reasonably unified behind openly joining the war against Germany, they will focus on arranging a peace settlement that doesn't weaken France and Russia too much.
Without the American army, maybe the British and French can defeat the German 1918 western offensives on their own (remember they had to ask the Italians for assistance!), but I don't see how they can throw the Germans out of France, British propaganda claims about that year to the contrary. By the Armistice, the Americans actually had more troops in France than France or Britain. Once the Americans were in France in force and conducting their own offensives, OHL concluded quite reaonably there was no way for Germany to win.
So more Central Powers operational success could do it, but they had a good deal of operational success IOTL. The best approaches seem to keep Britain out and keep the USA out or find a way that the war ends in a CP victory before the Americans can get deeply involved, which was the German calculation in early 1917.