Also known as the heating value. The energy content of diesel fuel is its heat of combustion; the heat released when a known quantity of fuel is burned under specific conditions. In the U.S., the heating value is usually expressed as British thermal units (Btu) per pound or per gallon at 60°F. For gross (high) heating value, the water produced by the combustion is assumed to be recondensed to a liquid. For the net (lower) heating value, the water remains as a gas. Since engines exhaust water as a gas, the net heating value is the appropriate value for comparing fuels.