Huge amount of by-products is still considered as waste and is simply disposed, for example by-product gas is usually flared. Political and social pressure to reduce air pollution and national needs for energy security make these waste fuels interesting for near-future power generation. Unfortunately most of these waste fuels, even when liquefied or gasified, have very low quality and can hardly be used in high-efficiency power systems. Among main challenges are low calorific value and composition fluctuation. Additionally very often there is a high content of sulphur, siloxanes, tars, etc., which have to be removed from the fuel. Modern 4-stroke gas engines designed for power generation applications provide very high efficiency, high reliability and availability. Unfortunately, these gas engines require high quality fuel with stable composition. Horus-Energia together with Cracow University of Technology developed a novel gas supply system HE-MUZG that can adapt to current gas quality and change engine settings accordingly.This article will present results from the HE-MUZG system tests on modern 4-stroke spark-ignition gas engine. Tests focus on low quality gas, such as gas with low calorific value, gas with very low methane number and gas with very big variations of calorific value. Test results compared with performance of that engine in the original configuration show huge improvements. Moreover the HE- MUZG system is easy to implement in commercial gensets.