Managing engine thermal state to reduce friction losses during warm-up

The thermal behaviour of a 2.4 l direct injection diesel engine has been investigated to identify how the fuel consumption penalty associated with operation during warm–up can be minimised. A version of PROMETS (Programme for Modelling Engine Thermal Systems) was developed to support the investigations. The developments improved the representation of thermal-friction conditions in the oil circuit, extended the piston heat transfer sub-model to account for the effects of piston cooling jets and introduced a main bearing thermal-friction model to predict friction and oil film temperatures. Computational studies were complemented by an experimental investigation of the effectiveness of pre-heating the oil feed to the bearings. Results show that heat transfer from the oil film to the bearings shells and crankshaft journal reduces the benefit in friction savings. Other measures considered were exhaust gas heat recovery, repositioning of the oil main gallery within the block, thermal energy storage, reductions in engine thermal capacity and a novel split-EGR cooler able to cool the EGR gases and heat either the coolant or oil streams. All of the above measures were investigated in isolation, but where appropriate different measures were adopted in conjunction to achieve even greater fuel savings. During warm-up the energy available to raise fluid temperatures is small. As a result, over the New European Drive Cycle, thermal energy storage showed the greatest benefits. Given an available source of thermal energy which can be transferred to the oil over a chosen time, simulations indicate that a higher power input over a shorter period is most beneficial. This reflects the increased sensitivity of oil viscosity to temperature changes at colder temperatures which in turn means that the potential to reduce friction is highest in the first minutes after engine start up but drops rapidly hereafter. Results also show how the balance of energy transfers out of the oil changes as the engine warms up and point to the importance of oil interaction with components in the lower parts of the engine which have a large thermal capacity, such as elements supporting the main bearings, the crankshaft and the lower liner which limit the rate of temperature rise of the oil. A combination of supplementary heat introduction into the oil circuit from a thermal store and an elimination of heat losses from the oil to the lower parts of the engine resulted in a fuel consumption saving close to that achieved by starting the engine fully warm, which equates to around 6% improvement