Power to Gas has attracted much attention as a solution to handle the electricity overproduced from renewable energy. This overproduced electricity can be converted into H-2 and injected into the existing natural gas grid. Therefore, efficient and safe usage of H-2 enriched natural (HNG) gas is a key issue to spread Power to Gas further. In this paper, the influences of H-2 addition on the engine combustion process is discussed, focusing on large-bore lean burn gas engines operated at high specific loads. The experimental setup consists of a single cylinder research gas engine with a displacement of 4.77 L. The results show a big influence of the addition of H-2 on the operating range and emissions even with small amounts of H-2. The misfire limit extends to the leaner side, while knocking must be prevented by a later ignition timing (IGT). Pre-ignition phenomena are limiting the operating range at richer conditions with rising H-2 amounts. While NOx emissions increase, unburned hydrocarbon (THC) and formaldehyde (HCHO) emissions decrease due to an enhanced combustion. This and the leaner operating conditions, which can overcompensate the actual NOx increase, facilitate a higher efficiency, which is discussed based on a detailed loss analysis. (C) 2015 Elsevier Ltd. All rights reserved.