mixture but it is ignited by the injection of a small volume of Diesel fuel. Energy balance Thermal efficiency Gas engines that run on natural gas typically have a thermal efficiency between 35-45% (LHV basis)., As of year 2018, the best engines can achieve a thermal efficiency up to 50% (LHV basis). These gas engines are usually medium-speed engines Bergen Engines Fuel energy arises at the output shaft, the remainder appears as waste heat. Large engines are more efficient than small engines. Gas engines running on biogas typically have a slightly lower efficiency (~1-2%) and syngas reduces the efficiency further still. GE Jenbacher's recent J624 engine is the world's first high efficiency methane-fueled 24-cylinder gas engine. When considering engine efficiency one should consider whether this is based on the lower heating value (LHV) or higher heating value (HHV) of the gas. Engine manufacturers will typically quote efficiencies based on the lower heating value of the gas, i.e. the efficiency after energy has been taken to evaporate the intrinsic moisture within the gas itself. Gas distribution networks will typically charge based upon the higher heating value of the gas. i.e., total energy content. A quoted engine efficiency based on LHV might be say 44% whereas the same engine might have an efficiency of 39.6% based on HHV on natural gas. It is also important to ensure that efficiency comparisons are on a like-for-like basis. For example, some manufactures have mechanically driven pumps whereas other use electrically driven pumps to drive engine cooling water, and the electrical usage can sometimes be ignored giving a falsely high apparent efficiency compared to the direct drive engines. Combined heat and power Engine reject heat can be used for building heating or heating a process. In an engine, roughly half the waste heat arises (from the engine jacket, oil cooler and after-cooler circuits) as hot water, which can be at up to 110 °C. The remainder arises as high-temperature heat which can generate pressurised hot water or steam by the use of an exhaust gas heat exchanger. Engine cooling Two most common engine types are an air-cooled engine or water cooled engine. Water cooled nowadays use antifreeze in the internal combustion engine Some engines (air or water) have an added oil cooler. Cooling is required to remove excessive heat, as overheating can cause engine failure, usually from wear, cracking or warping. Gas consumption formula The formula