Fuel Economy of the Gasoline Engine

On the subject of motor car fuel consumption, apparently, ‘everybody is an expert’. In the aftermath of the oil supply crisis in the autumn of 1973, public consciousness of this topic is again high, and the technical world is now engaged in this subject area at a new level of priority. Yet the information available on the factors that govern fuel consumption is, in general, surprisingly poor and more so in Europe than in the US. For instance, the measurement of vehicle fuel economy in relation to the way the motorist drives is a surprisingly difficult thing to do repeatably, because of the variety of ways and weather conditions in which motorists actually drive, combined with the rather poorly researched experimental techniques currently available. Methods for improving the fuel economy of a vehicle without materially affecting its performance in other respects are at an early stage of development, and much work needs to be done before the most cost-effective solutions can emerge.

The power unit in the majority of passenger cars on the road today is still the gasoline engine. As the name implies, its primary function is to produce the appropriate power as and when the driver commands. However, the demands of the current political and economic climate have brought back into focus yet again the increasing need for this power unit to be an efficient one.

The purpose of this chapter is to provide the general reader with an overall review of the principal characteristics of a motor gasoline, to study in some detail the effect of compression ratio on octane requirement and fuel economy and then to look at the overall economy when the car and the refinery are considered as a single economic unit.

The gasoline engine in its most widely used configuration invokes the use of a carburetter to meter liquid fuel into a moving airstream in response to the degree of throttling applied through the accelerator pedal. The rate at which this fuel is metered is intended to provide an appropriate compromise between requirements (a) to provide maximum power, (b) to provide maximum economy and (c) to comply with exhaust emission levels.

Crude petroleum consists essentially of hydrocarbons of various kinds boiling over a wide temperature range. In the early days of the spark ignition engine it was possible to use some of the light fractions as fuel without modification. Similarly it was possible to use selected heavy fractions as the crankcase lubricant. However, advances in engine and petroleum technology over the last several decades have led to the development of more efficient and much more powerful engines which need more complex fuels and lubricants for their operation. The performance of oil products can be improved to a substantial extent by selection and modification of the hydrocarbon structures, but there are practical and economic limitations to the performance levels that can be reached by hydrocarbon processing alone.

The quality of the fuel—air mixture supplied to a gasoline engine affects its fuel economy in many ways, some of which are subtle and are not fully understood.

In discussions about fuel economy much emphasis is usually placed on the influence of driving habits, and there is little doubt that attention to such factors can lead to economies. However, the mechanical hardware that constitutes a modern car is and always has been subject to degradation with usage, and so attention needs to be focussed on the fairly substantial savings that can be made by attention to vehicle maintenance.

The last seven years have seen a progressive lowering of permitted vehicle emission levels, particularly in the US (table 8.1) where signs of a levelling-off are perhaps beginning to be evident. While this was going on, scientists within the industry were noting what sacrifices in performance were needed to achieve these levels, and one of these was fuel economy. When the fuel crisis of 1973 developed, the question of the deleterious effect of emission control on fuel economy was raised more sharply than before, and a public debate ensued in the US in particular.

It is a truism to state that the study of the effect of fuel and lubricant properties on gasoline consumption must be based on adequate techniques for the measurement of consumption (or its reciprocal expression, fuel economy). However, although this is self-evident, it is unfortunately also true that outside the US there are no widely accepted methods for the realistic assessment of fuel consumption by vehicles, and it is only since 1973 that the US has been in advance of the rest of the world.

The internal combustion engine, whilst more efficient than its predecessor the steam engine, is still only capable at present of converting about a quarter of the energy available from its fuel into useful work. The remaining three-quarters are lost as heat in the exhaust gases, in the cooling water and in removal from the external surfaces of the engine (see also appendix E).

Very little work has been carried out on the effect of transmission lubricants on fuel economy. In fact most of the work reported has been done quite recently by the Lubrizol Corporation in the US. This means that the investigations into lubricant-related fuel savings have involved large American-built vehicles and not the generally smaller European models.