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The commercial grade of propane for automotive use is known as HD-5 in North America. “HD-5” stands for Heavy Duty (propane) containing a maximum of 5% propylene (also called propene) and a maximum of 2.5% butanes and heavier hydrocarbons (also shown as C4+). This grade was developed back in the 1970's to establish a grade of propane, based on composition, that would be suitable for automotive use. That is, if the composition of propane met the HD-5 requirement, then the fuel would be suitable for automotive engines both for stability and for octane quality, without having to actually test every batch for octane or stability.

Propane fuel also goes by the name of LPG (Liquefied Petroleum Gas) in many parts of the world. LPG does not refer to a specific type of gaseous hydrocarbon compound but instead refers to a mixture of liquefiable gaseous hydrocarbons that includes propane, butane, isobutane, propylene, and butylene. Commercially, LPG consists mainly of propane or butane or a mixture of the two.

While it might seem that there would be other grades of propane in addition to HD-5, there never has been an HD-1, -2, -3, or -4. However, some states (such as possibly California) have allowed an HD-10 (meaning 10% propylene) as a means of extending the fuel supply and/or reducing production costs, but there has not been any move within the industry to follow this lead. As I am not familiar with the market outside of Ontario, I don’t know if any product is actually being sold to the HD-10 specification.

Generally, there are no blends of propane and butane sold at retail in Canada or the US. However, such “PB” blends are apparently sold in many tropical and sub-tropical countries around the world – mostly for heating purposes. Butane has a higher heating value than propane, so a tank of PB fuel would have more energy than the same size tank of propane. Propane fuel composition varies in other parts of the world and may well not be called HD-5 but some local name or grade.

There can be a wide range of composition of commercial propane-propylene mixtures and PB mixtures, and the composition of these mixtures are normally agreed between seller and buyer (for large accounts). However, there are no distinct ‘grades’ or ‘types’ as such. Such mixtures can be used as petrochemical feedstocks or as fuel for heating purposes. The only widely available (in Canada and the US) specific product is HD-5 propane. Where ‘commercial propane’ is sold, it is usually a ‘waste product’ or byproduct that a producer wants to get rid of.

While HD-5 is the common grade of propane (LPG) available in Canada and the US, some marketers sell ‘commercial propane’, which can have a wide range of propylene (generally 10% – 50+ %). This product is satisfactory for most heating applications but might potentially cause a problem as a motor fuel.

The issue around propylene is that propylene is much less stable than propane and can polymerize (under hot conditions, as occur in an engine compartment) to form gums and varnishes – which interfere with engine performance. Just remember that propylene is the base for polypropylene plastic. The 30+ years of experience with HD-5 has shown that limiting the propylene concentration to 5% results in an acceptable product.