A question of economy

'At defined intervals we bring the test vehicles into the laboratory and measure them on a chassis dynamometer. Essentially this is like a treadmill designed for a vehicle. It simulates a car running on a perfectly smooth, flat road, with all of its vital functions being monitored by computers.'

Once the car is on the dynamometer and hooked up to the measuring devices, a computer guides the driver through a very precise drive cycle in which he is told the speed to drive at, when to change gear and so on. One of the key test cycles used for this is the New European Driving Cycle (NEDC), which simulates both city and non-urban driving. The NEDC is an industry accepted test and is primarily used to determine whether new vehicles meet the emissions regulations set down by European Union directives - in the European Union, vehicle advertising is required to show carbon dioxide emissions and fuel consumption data.

Measuring emissions also enables engineers to determine with pinpoint accuracy the car's fuel economy, using a carbon mass balance.

'From the emissions measurements of carbon monoxide, carbon dioxide and hydrocarbons we know how much carbon is in the exhaust,' Rogerson explains. 'Through careful analytical measurement of the fuel we know the amount of carbon in it. By applying a mass balance based on the carbon that goes into the engine coming out as carbon in the exhaust, we can very accurately calculate the fuel consumed.'

To make the dynamometer tests even more rigorous, BP is continuously improving the procedures - sometimes, for example, replacing the driver with a robot: the robot can be used to drive a car or a motorcycle digitally, and will drive absolutely perfectly, the same today, tomorrow and the day after that.

One example of BP's fuel economy testing is focused on the company's premium Ultimate fuels, first launched in 2003. The BP Ultimate range of high-performance unleaded gasolines and diesels was extensively evaluated at test tracks and on dynamometers. The fleet trials, which compared Ultimate to reference products such as ordinary unleaded gasoline, involved more than 60 vehicles ranging from city 'runabouts' to top-of-the-range vehicles like BMWs and Jaguar sports saloons (Frontiers, December 2003). The testing protocols were designed to cover a range of different situations. The protocol for unleaded gasoline, for instance, was carefully developed to be representative both of motorists switching from ordinary fuels to BP Ultimate, and of brand new cars using BP Ultimate from the start.

Overall, the tests, which were independently carried out by automotive technology consultants Ricardo and Millbrook to BP's specification, showed that Ultimate in the UK offered significant benefits over ordinary fuels. Ultimate unleaded gasoline reduced fuel consumption by an average 3.1 per cent. In a dramatic post-test demonstration, two identical cars drove repeatedly round one of the Millbrook circuits. After the ordinary unleaded vehicle came to a halt the Ultimate vehicle lapped its competitor nearly nine times - a 4.5 per cent improvement in fuel economy.

Small but significant

Having refined their tests to a high degree, BP's Pangbourne technologists are now taking the process one step further by employing the dynamometer not only to test the performance of the final product, but as an aid to developing it.

John Moffa, lubricants technology manager for Central Europe, is interested in the fuel economy benefits that arise from engine lubricants rather than the fuels themselves.

'Lubricants improve the efficiency of the engine in two ways,' says Moffa. 'The first route is the chemistry of additives. Lubricant formulators add surface active chemicals to the base oil, and these coat engine component surfaces to reduce the friction between the moving parts of the engine. The second way is to formulate the lubricant in such a way that you improve the oil's viscosity - lower viscosity means the engine can rotate with less effort because there is less drag from the oil.'

To test these effects, a chassis dynamometer is used to establish the fuel economy changes that result from variations to the chemistry or viscosity of the lubricants. The anticipated benefits from both can individually be very small. It is when these and other improvements, such as further formulation optimisations, are added together that the beneficial effects become quite significant. But when a dynamometer is used as a tool to examine small fuel economy benefits, there can be issues of repeatability and reproducibility to take into account.

Lubricants researchers describe repeatability as the ability to run a test over and over again and always get the same result. Reproducibility is the ability to run identical tests in separate laboratories and again achieve exactly the same outcome.

'Poor repeatability and reproducibility made evaluation of fuel economy changes of less than 0.5 per cent unreliable when using a dynamometer,' says Moffa.

But meeting the repeatability and reproducibility parameters is essential to back up marketing claims about fuel economy, leading BP, in collaboration with the University of Bath, to try to determine why the dynamometer sometimes got it wrong. The researchers examined a wide range of variables, from the engine loading to how tightly one tied the straps that hold the vehicle on the rolling road. Moffa says the analysis indicated that amongst the main 'culprits' are variables such as temperature, the state of the charge in the vehicle's battery, and tyre pressures.

'The project clearly demonstrated that by controlling certain variables to defined limits, the repeatability and reproducibility of this type of test work would improve significantly, meaning this could enhance the development of new fuel efficient lubricants. Now we understand just how closely we have to control these factors to remove them as contributors to the difference in test results.'

Customer satisfaction

Gordon Lamb, advisor in the diesel engine oil product development group at Pangbourne, takes a special interest in lubricants designed for heavy duty vehicles such as trucks. He agrees with Moffa about the importance of being able to reliably measure small differences in the efficiency of lubricants. Highly efficient test bed equipment and rigorous methodology mean that lubricants formulators can now detect these small benefits.

'When we add all the small benefits together we hope to arrive at a big number,' says Lamb.

Heavy duty vehicle engines are usually put through their paces on special test beds. The engine is removed from the vehicle and coupled directly to a dynamometer. As with passenger vehicles, these tests can provide large amounts of detailed information about lubricant performance, but ultimately, says Lamb, most fleet owners demand something extra. They want to see customised 'real world' measurements. They argue that test bed results are generic and cannot really provide them with the sort of reassurance they want about how the lubricants will perform in their own particular vehicles.

'While engine test beds accurately measure fuel consumed during a test cycle,' says Lamb, 'they don't always replicate the exact operating conditions experienced by our customers' vehicles. For example, some trucks might be going over the Alps, some might be travelling on relatively flat terrain surrounding a city. The trucks are all very different, all carrying different loads, operating in different environments.'

While some fleet owners may be content to see the results of field trials from operators similar to themselves, translating that experience to their own fleet, many want something that is more customised. Lamb expects to run between five and ten major trials a year of this kind to measure fuel economy.

'Our customers are a fairly sceptical audience. They want to see evidence and quite rightly so.'

One trial, completed in March 2008, looked at the impact of BP's diesel engine lubricant, Elixion Low SAPs 5W-30, on a fleet of 28 MAN trucks run by the Austrian haulage company Koenig. The trial monitored the trucks over a nine month period, during which they collectively covered 1.5 million kilometres, equivalent to travelling almost 40 times round the earth's equator.

All of the trucks were first run using a reference lubricant, Castrol Enduron, after which half the fleet changed over to Elixion. An independent analysis showed that Elixion delivered a 2.2 per cent average benefit in fuel consumption compared with the reference lubricant.

Improvements such as this, though seemingly small, can translate into big cost savings.

'For illustrative purposes,' says Lamb, 'imagine just one truck covering 100,000 kilometres a year. At last year's diesel prices you would be spending about 60,000 Euros a year on fuel. So if you make a two per cent fuel economy benefit you are saving 1200 Euros, three per cent 1800 Euros. And that's just one truck. If you have 100 trucks the savings start to become significant. It can obviously affect the bottom line of some companies.

'For reasons such as these it is very important that we can accurately predict the fuel efficiency of our products in a way that is easily understood by our customers. Fortunately in BP we have developed the methods of testing fuel economy that allow us to do just that.'

So what about the ordinary driver, out on the public highway? Next time you look at your fuel consumption on the trip computer, just think - was it a warm day, were the tyre pressures correct, were the lights on, how loaded was the vehicle, was it all uphill or against the wind, and were you gentle on the accelerator pedal? See if you can spot the difference for yourself.

Source: BP