The U-Groove Design Denso received a patent for the popular U-Groove ground electrode design back in 1972. This electrode was developed to meet the demand for improved ignitability, which is necessary for an increase of efficiency in emission gas purification and fuel consumption. The U-Groove gives the flame a groove to grow in, thereby creating a larger flame front for a more complete combustion even in the case of lean air-fuel mixture. A conventional, flat ground electrode design of regular plugs crushes the flame preventing full spark potential.