Ichang Lemon
It is also called Shangyuan and is widely grown and used in some regions of China to perfume rooms and cabinets. The Chinese name Hsiang Yuan means "fragrant ball". It exists in several varieties and is able to withstand colder temperatures than the mandarin or kumquat. In the US it has been used for lemon pie and some connoisseurs think the juice makes a better flavored pie than lemons. Each fruit can yield up to a half cup of juice, but is very seedy. The rind oil has a strong flavor. It forms a small tree and is very thorny.
From the research I've been reading about, it seems it may have almost the same level of cold hardiness as Yuzu, which is really saying a lot.
here is from another source:
Hsiang Yüan, Sangyuan, Shangyuan, Xiang yuan
'Ichang lemon' originates in Hsing shan, Hupeh province, central China. It was discovered by E.H. Wilson and it probably is a natural cross of ichandarin C. junos Sieb. ex Tan. and ichang papeda. According to other sources it is a cross of a shaddock and ichang papeda. That's why it was referred to as shaddock in the past.
The tree is average, very thorny. Its leaves are elliptic, heart-shaped and have wings on their petioles. These small wings usually overgrow to the leaf part and cover about 1/4 of the leaf. Leaves are softer than those of orange or lemon, dark green and have sturdier petiole. It could be classified as frost hardy variety, it can take about -10°C without any damage and can be grown in cooler areas with proper protection. The fruit is usually elongated, slightly pointed and about 7-11cm long. Its rind is yellow, pebbled, thin (2mm), well attached to the pulp. The pulp is yellow, sour, aromatic, sometimes hot and tastes like a lemon, better than citranges. It has typical, intense aroma and is divided into 10-12 segments with lots of seeds (often polyembrionic, usually about 40 seeds). It is also very juicy and can be used for preparing refreshing drinks. It is an excellent rootstock that has (in case of oranges) increased the production rate by 15% (Botanical garden in Batumi). This variety is also used in China to perfume the room and that's what it's name came from, 'Hsiang Yüan' means aromatic balls. The fruit is in USA used in baking, because the hot air eliminates the bitter flavor.