Horn explained that heavy-duty engine oils have responded to evolution in engine design, much of it resulting from efforts to improve fuel economy. In 1993, when Volvo introduced its current baseline on-road truck engines, the D12/D13 platform, they used a 15W-40 engine oil with high temperature, high shear rating (HTHS) of 4.2 centiPoise. The engines had a fixed oil pump without a relief valve, no oil thermostat and relief valves that helped cool pistons. Oil operating temperatures were 95 degrees C.

Since then, engine sizes were reduced to improve fuel economy, reducing oil sump volumes. This caused oil operating temperatures to rise enough that OEMs had to monitor and manage the temperature. When oil temperatures reached 105 degrees, Volvo installed wax thermostats, and when they reached 118 degrees, the company switched to electrical thermostats.

Engine manufacturers also began to install components such as exhaust gas recirculation valves and crankcase ventilation oil separator valves and oil filter bypass valves, which require fluid pressure provided by the engine oil. Increasing the number of oil-consuming components reduced oil pressure and oil flow.

To reduce engine friction (in the interest of improving fuel economy), engine manufacturers began recommending thinner oils, and Volvo now recommends a 10W-30 with HTHS of 3.5 cP for its baseline engines. It still uses a fixed oil pump, but that pump now has a relief valve. The piston relief valves have been replaced by electrically powered relief valves. Todays oils allow a 2 percent improvement in fuel economy compared to those used in 1993.

Horn said the trends of the past quarter century will continue in the future. In the short term, oil viscosity will need to decrease further, as will oil pressure and oil flow. For now, oil temperature should remain steady. State-of-the-art heavy-duty engines will soon employ variable displacement oil pumps and use 0W-20 engine oils with HTHS of 2.9 cP. Compared to the oils used today, these lubricants will enable fuel economy to improve an additional 1.5 to 2 percent.

In the longer term, Horn said, oil viscosity will probably have to fall even further, while temperature may have to increase again. The number of components requiring engine oil will continue to increase, causing a further decrease in oil flow and pressure. To ensure sufficient flow, some of these components will need to be electrified.

Our current lubricant is using a Group II base oil, Horn said. Short-term development is targeting a Group III base oil, and medium-term development is targeting a Group III, IV, V or a mix thereof, depending on future requirements and needs. Group I, II and III base stocks are refined from crude oil. Group IV base stocks are polyalphaolefins, which are synthetic petrochemicals. Group V is a catchall category for other types of base stocks, including other synthetics such as esters.

It will become increasingly difficult to formulate oils that meet the needs of future engines, Horn said, so that success will require oils to be a factor of engine design, along with engine component materials, coatings and textures.

The design of the oil needs to be balanced with engine hardware, he said, a proper oil temperature, oil pressure, oil flow and filtration level. One key to success will be a parallel development of lubricant and engine hardware.