 The simplest glow-engines operate on the two-stroke cycle.These engines are inexpensive, yet offer the highest power-to-weight ratio of all glow-engines, but can often generate a great deal of noise, requiring substantially-sized expansion chamber mufflers to reduce their noise output, of both tuned exhaust and non-tuned varieties.Glow engines which operate on the four-stroke cycle, whether using ordinary poppet valves or occasionally rotary valves offer superior fuel-efficiency (power-output per fuel-consumption), but deliver less power than two-stroke engines of the same displacement – yet, often because the power they deliver is more suited to turning somewhat larger diameter propellers for lighter weight, more drag-producing airframe designs such as biplanes and scale aircraft models of pre-World War II full-scale subjects, four-stroke model engines, fueled either with methanol or gasoline fuels are slowly increasing in popularity from their generally lower noise output when compared to similar displacement two-stroke engines, and are available (for larger displacement, multi-cylinder four-stroke engines) in opposed twin and radial engine layouts.