The prevention of thermal losses from automotive lubricants to improve cold start efficiency

Roberts, Andrew P. (2015) The prevention of thermal losses from automotive lubricants to improve cold start efficiency. PhD thesis, University of Nottingham.

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The internal combustion (I.C.) engine remains unrivalled as the primary means of road vehicle propulsion. The frequency of re-fuelling stations, combined with the high energy density of both gasoline and diesel fuel provide users with unrivalled flexibility and vehicle range. However a range of environmental and health concerns exist surrounding I.C. engine emissions; in particular carbon dioxide (CO2), nitrous oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO). There is therefore increasing pressure on vehicle OEMs to reduce vehicle emissions through tightened emission standards and regulations. A significant challenge in meeting these tightened regulations is the reduced efficiency of the I.C. engine during cold-start which reduces from typical values of 40% when fully warm to values as low as 10% when cold. Increased friction in the engine caused by overly viscous lubricants providing sub-optimal lubrication during cold starts is a primary cause of this reduction in efficiency during cold-start. This is despite the advancements in lubricant technology made that has reduced the sensitivity of lubricant viscosity to temperature variation.

It is therefore desirable to increase the rate of lubricant heating during engine warm-up so that optimal lubrication conditions are reached sooner and frictional losses are reduced. The reduction in frictional losses therefore reduces fuel consumption and hence emissions. In this thesis, the merits of insulating engine oilways are investigated as a means to reduce thermal losses from the lubricant and thus accelerate warm-up rates using a bespoke oil flow rig and simulation model. Through this work, it has been found that, using insulating inserts, it is possible to reduce the thermal losses from the lubricant to the surrounding wall structure by up to 58%. Such reductions have been achieved by installing an insulating insert into the oilway (also commonly referred to as a gallery in I.C. engines) that combines a low thermal conductivity material but also introduces a contact resistance between the insert and the surrounding metal. It has been found that the contact resistance is a highly significant and beneficial feature and, using special inserts designed to enhance the contact resistance, reductions in thermal losses of up to 40% can be achieved using the contact resistance alone without using low thermal conductivity materials.

A computational finite difference model has been developed to simulate heat transfer between flowing engine lubricant and the gallery walls. The model has been correlated with experimental data from the oil flow rig and is capable of simulating the effects of changing the materials properties (density, specific heat capacity and thermal conductivity) of both insulating inserts and the surrounding metal structure. The model is also capable of investigating the effect of changing contact heat transfer coefficients and changing flow geometry and velocity. Through computational experiments with this model, it has been found that the optimum strategy to achieving reduced thermal losses from the lubricant through the gallery walls is to ensure that the thermal conductivity of the insulating insert and that the thermal mass of the surrounding structure are minimised. Computational experiments have also highlighted the need to consider the flow geometry of different regions of the engine with the variation in bore diameter affecting both the heat transfer surface area and the convective heat transfer coefficient through the Reynolds’ effect. It has been found that increasing the lubricant flow velocity for a given bore diameter increases thermal losses to the gallery walls as a result of the Reynolds effect. If the bore diameter is increased, the thermal losses from the lubricant reduce in uninsulated galleries owing to a reduction in the Reynolds number but the reverse happens in insulated galleries owing to the increase in heat transfer area. The change in trend is a result of the interactions between the changing convective heat transfer area, heat transfer coefficient and the temperature differential between the lubricant and gallery wall. In addition, implementation of the insulation into a running engine needs careful consideration to ensure that the insulation does not isolate the lubricant from a potential heat source (such as the cylinder head). The optimum locations will vary between engines but investigations suggest that the return galleries from the head to the sump represent a positive opportunity to reduce thermal losses from the lubricant with a clear reduction in lubricant temperature observed as the lubricant moves down the gallery.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Brooks, R.
Shipway, P.H.
Keywords: Internal combustion engines; Thermal insulation; Heat transfer, Lubricant heating; Gallery insulation; Cold-start efficiency
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 29357
Depositing User: Roberts, Andrew
Date Deposited: 25 Jan 2016 14:13
Last Modified: 15 Sep 2016 09:08

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