The science of greener driving

The dramatic improvements in vehicle efficiencies seen in recent years are very welcome, but there is also evidence of a widening gap between manufacturers’ official fuel consumption figures and what’s achieved in real life. A report from the International Council on Clean Transport published last year showed that on average this gap had grown from eight per cent in 2001 to 21 per cent today. This surely means there’s a stronger case than ever for training to help drivers achieve – or at least get closer to – the official figures.
 
The benefits
Reduced fuel consumption and CO2 emissions are the biggest benefits with an average improvement of just under 15 per cent on the day of training. As with any training – driving or otherwise – this will decrease over time, but we believe five per cent is a realistic long term saving. This is supported by evidence from German research of between 3.7 & 6.2 per cent fuel savings.
    
Because of the focus on greater anticipation, efficient driving also reduces accident rates.  Arriva North West is a good example: the
bus company ran eco-driving training for their drivers in 2007 and the following year it recorded a 29.6 per cent reduction in ‘at fault’ collisions involving other vehicles and an 18.3 per cent reduction in non-fault collisions. Eco-driving also reduces vehicle wear and tear which, in turn, reduces maintenance costs.  
    
Our experience shows that there’s no substitute for one-to-one training. And the training doesn’t need to be lengthy. Just under an hour per driver can go a long way.  Short-duration eco-driving training is the basis of government subsidised eco-driving programmes in England and Scotland.  

In-car technologies, especially those giving drivers instant feedback on their driving can be effective in providing on‑going feedback, encouraging drivers to use and develop their skills.

The techniques
For most drivers, the biggest benefits come from greater anticipation, better use of gears and by slowing down on the motorway.
    
In more detail, drivers should drive smoothly, and anticipate situations and other road users as far ahead as possible to avoid unnecessary braking and acceleration.
    
Maintain a greater distance from the vehicle in front so that you can regulate your speed when necessary without using the brakes.
    
When slowing down or driving downhill, remain in gear but take your foot off the accelerator as early as possible. In most situations and for most vehicles this will reduce fuel flow to virtually zero.

When accelerating, shift to higher gear early, usually by around 2,000-2,500rpm.
     
High speeds greatly increase fuel consumption so avoid excessive speed.
    
Other tips for better fuel efficiency include keeping tyres correctly inflated. Under-inflated tyres are not only dangerous but also increase fuel consumption.
    
All ancillary loads, but particularly air conditioning, add to fuel consumption so use it sparingly.

Turn off your engine if you expect to be stationary for more than a minute or so.

Remove racks, roof boxes and bike carriers when not in use as they significantly increase air resistance and fuel consumption at higher speeds. Keep windows shut at high speed and avoid carrying unnecessary weight as this will increase fuel consumption.

The science    
If you consider the basic physics behind the motion of a vehicle, it’s no surprise that greater anticipation is the single most important technique for urban drivers. Energy is required for two main reasons: to accelerate the vehicle and to overcome air resistance. Air resistance is fairly negligible at low speeds which means in typical urban driving the vast majority of fuel is used for acceleration. So even at low speeds a vehicle that’s constantly accelerating and braking will use a lot of fuel whereas the vehicle with the driver that’s anticipating well, has perhaps dropped back slightly from the vehicle in front and is managing to smooth out much of the stop-go will use a lot less fuel.
    
When driving downhill or slowing down, a modern vehicle will usually use less fuel if you remain in gear but take your foot off the accelerator, than if you ‘coast’ in neutral.  This is because it’s intelligent enough to recognise that the momentum of the vehicle is driving the engine. This activates the fuel cut-off switch, stopping the flow of fuel to the injectors. In contrast a vehicle coasting in neutral would still be burning fuel to keep the engine ticking over.  

Use of gears
Some of the clearest research in to the effect of the use of gears on fuel consumption comes from TNO in the Netherlands in a paper published in 2006. TNO assessed three different acceleration strategies for both petrol and diesel cars and concluded that shifting up at low rpm and 50 per cent accelerator position resulted in the lowest fuel consumption.

Air resistance or drag increases by the square of a vehicle’s speed, so when speed is doubled air resistance increases by a factor of four. At motorway speeds most of the fuel burned by a vehicle is used to overcome drag and this relationship between drag and speed mean that relatively small increases in speed add significantly to fuel consumption.  
    
Data on the effects of speed on fuel consumption for different vehicle types is available from the  Department for Transport (DfT), based on work carried out by the Transport Research Laboratory in 2009. The data shows that for a typical modern car, fuel consumption increases by around 11 per cent between 60 and 70mph and 18 per cent between 60 and 75mph. For a typical large van (>1760kg) the increases in fuel consumption are even more marked: 23 per cent between 60 and 70mph and 37 per cent between 60 and 75mph.  

Tyre pressure
As any cyclist knows, it’s harder to move a vehicle with under-inflated tyres. Tyres are flexible and flatten at the bottom where they are in contact with the road. This means the shape of a tyre is constantly changing as it rotates and a different section comes into contact with the road. This process, which is exacerbated in an under-inflated tyre, creates friction and heat and increases rolling resistance. The EU TREATISE project states that four tyres under-inflated by 25 per cent (e.g. 24 PSI instead of 32PSI) will add approximately 10 per cent to rolling resistance and about two per cent to fuel consumption. According to Michelin research from 2011, 39 per cent of British motorists were driving with tyres >= 8 PSI under-inflated. Maintaining correct tyre pressures is also an important for safety.

Locking in the benefits
After training, it’s crucial to ensure the benefits last. One key factor is how drivers pay for their private fuel use. If company car drivers with fuel cards repay the cost of their private mileage as the appropriate proportion of their actual fuel costs, then they will have a personal financial incentive to drive efficiently for both business and private mileage. In contrast, the more common system in which private mileage is repaid as a fixed ‘pence per mile’ provides no such incentive.
    
Other measures can be employed such as providing drivers with reminder information, producing fuel consumption league tables to encourage competition between drivers and offering incentives for the most efficient drivers.  
    
Fuel consumption league tables are well‑suited to fleets of identical vehicles and are associated more with van than car fleets.  But league tables can also work for a mixed fleet of company cars as each driver’s actual fuel consumption can be normalised against his or her car’s official fuel consumption.

The results are then ranked not by absolute fuel consumption but by percentage of the vehicle’s official consumption. The likely results within a large fleet are that the best drivers will achieve close to their cars’ official fuel consumption while the worst will achieve little more than 50 or 60 per cent of the official figure.
    
Incentive schemes to identify and reward the most efficient drivers can be an inexpensive and effective way to promote eco-driving. Examples include awarding vouchers to the most efficient drivers and funding monthly social events for the most efficient team.
    
Fleet managers should explore why some drivers are using significantly more fuel than their peers. There may be mechanical problems or different duty cycles such as more stopping and starting or heavier loads. If not, then drivers should be offered additional help, advice or training.

Get the training
The Energy Saving Trust (EST) has trained more than 30,000 drivers in Smarter Driving (eco-driving) since 2009. EST is part of a European project ECOWILL in which partners have cumulatively trained more than 100,000.  
    
The project involves 15 partners from 9 countries and trains instructors to deliver short-duration eco-driving training and works with licensing authorities to increase the emphasis on eco-driving in driving tests.


Further information
www.energysavingtrust.org.uk/smarterdriving