As engine fuels, the most popular alternative fuels are bioetanol, biodiesel, and hydrogen. Recently, in addition to these, there are intensive researches on methyl-, and ethylalcohols, natural gas, liquefied petroleum gas, P-series, electricity, and solar fuels. Alternative fuels for diesel engines are becoming increasingly important due to diminishing petroleum reserves and the environmental consequences of exhaust gases from petroleum-fueled engines. One of the advantages of P-series is that they are very easy to use. There is no need for any special fuel management because gasoline and P-series can be freely intermixed in any proportion with fuel that is already in the vehicle's fuel tank. So, even if P-series is not available at a particular location, simply fill up with gasoline. These fuels are inexpensive fuels generated by municipal and agricultural wastes. The National Renewal Energy Laboratory (NREL) showed that P-series would be 96% derived from domestic resources and reduce petroleum use by 80% as compared to gasoline. Use of P-series fuels also greatly reduces toxic emissions. P-fuels are economically competitive with gasoline. As of May 2003, the projected retail price for P-series, including all taxes, is $1.49 per gallon, about the same as mid-grade gasoline in a $/mile calculation. There are 3 million cars on the road today that could run on P-series fuels.