What Determines Fuel Efficiency?

The factors which determine fuel efficiency can be divided into two broad categories.

The first category concerns how efficiently the fuel energy is converted into useful work that propels the vehicle forward. The second category is all about the type of workload that the vehicle is being asked to perform by the driver.

Vehicle fuel efficiency

Vehicle fuel efficiency is determined by the vehicle manufacturer and involves a wide range of engineering influences and design criteria.

Firstly, it’s all about power specific fuel consumption. In other words, maximising the amount of driving force at the road surface under a wide range of driving conditions for a given quantity of fuel that is consumed.

This is known as Brake Specific Fuel Consumption (BSFC) and is determined by the rate of fuel consumption divided by the power produced, expressed as grams per kilowatt-hour and is a measure of the engines fuel efficiency. This involves the combustion and mechanical efficiency of the engine in converting the fuel into a rotational force with minimal frictional and heat losses.

Secondly, it’s about the transmission of that force via the gearbox, driveline and tyres to the road surface, again with minimised frictional losses while also maintaining the engine speed in its most efficient operating range with changes in vehicle speed and load.

Next, the weight and aerodynamics of the vehicle play a significant role. Obviously the heavier the vehicle and the greater the resistance to its forward movement, the more force and therefore fuel that are required to travel a set distance.

Finally, the design application plays a very significant part in a vehicle's fuel efficiency because not all automotive vehicle designs are about minimising fuel consumption.

Some vehicles are designed for speed and acceleration or load carrying as their primary attributes. Therefore sacrifices in terms of kilometres travelled per litre of fuel are justifiably made to achieve these objectives. In other words, each litre of fuel has a specific quantity of energy and it depends on the priorities to which that energy is applied.

What makes for a fuel efficient vehicle?

Engineering design technologies that make for a fuel efficient vehicle are so numerous that it would be virtually impossible to list them all and in any case for the majority of people they would be unintelligible and of little interest.

Thankfully for the vast majority of us there is a very useful and independent fuel efficiency tool that we can use to measure the relative fuel consumption and environment credentials of vehicles currently available in Australia: The Australian Government Green Vehicle Guide.

This tool uses a star rating system to categorise vehicles according to fuel consumption and environmental emissions data. It does not however categorise vehicles according to their design purpose i.e. small vs. large, vehicle performance or load carrying capacity.

So the first task is to decide what type of vehicle meets your own specific requirements, because as stated previously, design application plays a very significant part in a vehicle’s net fuel consumption. The fact is that often a vehicle that is a top performer in terms of fuel consumption is often not the best in terms of performance and or load carrying capacity.

The Green Vehicle Guide/ADR fuel consumption data is obtained from actual vehicle testing performed in a laboratory environment on a ‘rolling road dynamometer’ following very specific driving cycle parameters.

It provides a level playing field upon which all vehicles (regardless of make) can be compared for their fuel consumption and their environmental performance. However, it should be noted that whilst the driving cycle parameters are designed to try to replicate a real world driving cycle, the results only reflect the outcome achieved in respect of those specific parameters. Any departure from the parameters in the real world could mean very different results. It therefore is only a “guide” to the actual consumption you will achieve in the real world but more importantly, acts as the basis of a true comparison of vehicle-to-vehicle fuel efficiency.

NB: The top performers on the Green Vehicle Guide consist mainly of electric or hybrid vehicles and it should be understood that these ratings do not take into account things like the cost of electricity required to recharge, the high cost of replacement batteries or the initial purchase cost and resale value. Total cost of ownership is a consideration that should also be taken into account when making decisions on fuel efficiency and vehicle purchases.

Driving for fuel efficiency

As previously stated, a litre of fuel has a specific amount of energy and the faster we drive, accelerate, slow down, accelerate again or plainly ‘aggressively’ drive and use that energy, the more fuel that is required to cover a specific distance.

Similarly, the heavier the load that the vehicle is required to carry, the higher the fuel consumption. The road itself also has a significant effect- things like the number of uphills, downhills and quality of the road surface can all have a negative impact on fuel consumption.

It’s not rocket science to understand that to obtain the lowest possible fuel consumption the driving style should be as smooth and progressive as possible at reasonable speeds and on the flattest and smoothest roads.

Harder to appreciate is that selecting a vehicle that is not suitable for the intended purpose or driving in a manner unsuited to the vehicle will result in an adverse fuel consumption outcome. That means, for example, overloading or aggressively driving a lightweight vehicle designed for low fuel consumption may mean the fuel consumption achieved is actually worse than a vehicle more suited to that purpose. As stated previously, very careful selection of the right vehicle for the intended purpose is paramount.


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