Contribution to the Problem of Starting and Operating Diesel Vehicles at Low Temperatures

Abstract

Using systematic cold starting trials with diesel (and also gasoline) vehicles in a large cold room, means of extending the serviceability of Swiss Army vehicles down to temperatures of about -13°F (-25°C) were studied.

The first requirement for a successful cold start is a sufficient cranking speed. After study of the relation between motor oil viscosity at low temperatures and cranking speed, the minimum speed limits for a successful cold start were determined. It was found that the design and working temperature of the battery also had a very great influence. In addition, special ‘cold-starting’ batteries, or conventional batteries protected from excessive cooling gave much higher cranking speeds than unprotected types. It is therefore possible to have successful starts either at lower temperatures or at the same temperature using an oil of higher viscosity, the latter giving better lubrication in the warmed-up engine.

When the minimum cranking speed has been ascertained, the properties of the fuel necessary for trouble-free operation must be considered. For starting diesel engines at temperatures down to -13°F (-25°C) starting aids such as the injection of a volatile auxiliary fuel, or the use of heating plugs becomes indispensable. In this case, the ignition quality (cetane number) of the fuel, so long as it stays within commercial limits, is of less importance. The greatest difficulty arises with the separation of paraffin wax which clogs lines and filters. With the aid of a rig test, consisting of all the main parts of a Saurer fuel system, the filterability of different fuels was studied in some detail. It has been found that different fuels behave quite differently. If the wax separates in large crystals, for instance, the coarse pre-filters may sometimes clog at temperatures very little below the cloud point of the fuel. More frequently, finely divided wax separates out and the fine filter proved to be the more critical in which case clogging only occurred at temperatures well below the cloud point of the fuel.

It therefore proves rather difficult to correlate the behaviour of a fuel in practice with any of the conventional laboratory yardsticks used to assess the cold properties of gas oils. It is also unlikely that a ‘single-point’ laboratory test can be devised that covers all practical cases. A certain relationship between the results of the rig-filterability-test on the one hand and cloud point, pour point, and ‘filterability’ (DIN 51770) on the other is undeniable.

The use of additives to improve filter-clogging tendencies of diesel-gas oil was not very promising. The best method appeared to be dilution with kerosine.