Propane is one alternative fuel which is already widely available. Its use as a transporation fuel has largely been confined to spark ignition engines, however. This paper reports on an investigation into liquid propane injection as a means for fueling diesel engines.
A single-cylinder CFR cetane rating engine was used to carry out the experimental work. The fuel system was revised to ensure that propane remained liquified in the fuel injection pump and injector. Since propane has a very low cetane number, some means of ignition must be provided. Two means of ignition were evaluated in this investigation, diesel pilot injection and a continuously operating glow plug.
Tests were carried out at compression ratios of 19:1, 22:1 and 25:1 and at engine speeds of 1200, 1500 and 1800 rpm for each of the two ignition methods. The results are similar for the same test conditions. The main advantage of liquid propane injection is reduced smoke emission at a given power output compared to the same engine operating on diesel fuel. Alternatively, for a given smoke limit, the liquid propane injection engine produces significantly more power, about 20% more at full load. The reduction in smoke emission is accompanied by a modest improvement in energy efficiency. Gaseous emissions are comparable to those of the original diesel engine.
The use of glow plug ignition allows complete substitution of propane for diesel fuel, but difficulty was experienced in obtaining reliable ignition throughout the range of engine operating conditions. Proper interaction between the fuel spray and the glow plug is crucial to achieving reliable ignition and reasonable ignition delays. Additional development work is needed to arrive at a combustion chamber geometry which will provide satisfactory operation at all loads and speeds.
Diesel pilot injection provided smooth, reliable ignition at all loads and speeds but requires a second complete fuel system. The amount of diesel fuel used for the pilot injection was typically between 5 and 10% of the total fuel mass.