It has taken a long time, but many South African motorists are – at last – becoming concerned about reducing their carbon dioxide output. It’s obvious that there should be a link between fuel consumption and carbon dioxide emissions, because the latter is formed from the carbon in the fuel and the oxygen in the air. This means that saving fuel is equivalent to reducing carbon dioxide emissions.
A calculation will show how close the connection is. South African petrol contains approximately 86 per cent carbon, and has an average density of 740 g/litre. One gram of carbon combines with 2,667 grams of oxygen to form 3,667 grams of carbon dioxide. This means one litre of petrol forms (0,86)(740)(3 667) = 2 334 grams of carbon dioxide. If a particular car has an average fuel consumption of 7,0 litres/100 km it will emit 7(2 334) = 16 338 grams of carbon dioxide per 100 km, which amounts to 163,38 g/km. Hence the conversion factor from consumption in litres/100 km to carbon dioxide emissions in g/km is 23,34 for petrol engines.
South African diesel has an average density of 830 g/litre but the other numbers stay the same so that the conversion factor for diesel engines is simply the ratio of masses multiplied by the above conversion factor, which gives the answer 23,34(830/740) = 26,18.
Ninety-eight per cent of the energy in fuel is unlocked by the combustion process, but is lost again in the following ways: 1. Engine inefficiency. New petrol engines are only about 25 per cent efficient at the maximum torque speed at full throttle, but this rapidly drops to less than 10 per cent at very small throttle openings. Automotive diesels are about 34 per cent efficient under the same conditions, but remain above 15 per cent even at small throttle openings. One of the reasons is that they do not have a throttle butterfly so that the intake manifold is usually at atmospheric pressure instead of being under a partial vacuum at low engine speeds as is the case for petrol engines.
Another important reason is that diesel engines always run very lean compared with petrol engines. 2. The aerodynamic drag, ie the pressure on the car exerted by the air that it moves through, increases as the square of the speed. As vehicle speed rises, the drag will eventually be equal to the maximum power developed by the engine so that the car will achieve its maximum speed, ie it will stop accelerating. This means the faster you go the more energy you waste.
3. The transmission, final drive and tyre/road interface absorbs between 15 and 20 per cent of the power measured at the flywheel while travelling on smooth roads in a straight line. Traversing rough roads and cornering at speed will increase these losses. 4. Braking destroys the kinetic energy that is inherent in any moving vehicle. The above breakdown helps us to visualise ways to save fuel and hence reduce carbon dioxide emissions.
HOW TO SAVE FUEL
Here is a list of ways to save fuel, in approximate order of the savings potential: 1. Buy a diesel-engined vehicle. It is an expensive option, but you can expect to save up to 25 per cent in fuel consumption, depending on driving conditions. Keep in mind that diesels only have a major consumption advantage over petrol engines at part-throttle, but the advantage dwindles as the driver demands more power from the engine.
2. Buy a hybrid. It’s also an expensive option, but the savings potential is close to what you can expect from a diesel, except at speed on the open road, where a diesel is still unbeatable because the electric component cannot be used for any length of time.
3. Scale down to a smaller car. The fuel saving you can expect obviously depends on how much smaller you’re prepared to go.
4. Cruise at speeds below the legal limit on the open road. The difference in consumption between cruising at 100 instead of 120 km/h is quite significant, because the energy required to move a car goes up as the square of the speed, so that at 120 km/h a car uses 44 per cent more energy to maintain its speed than at 100 km/h. This means that lower cruising speeds usually result in lower fuel consumption. Some people counter this advice with the argument that engines are most efficient when they’re delivering maximum torque, and this often equates to a cruising speed of 130 km/h. This argument ignores the fact that the engine speed at which maximum torque occurs is measured at full throttle, but at any part-throttle cruising speed the maximum torque occurs at a lower engine speed.
5. Accelerate slower in traffic. Using large throttle openings not only increases fuel consumption but also increases stress levels in the engine, drivetrain and tyres.
6. Have your engine modified for fuel economy by a good tuning shop. Any change that improves the torque will also reduce fuel consumption provided you don’t use the extra power.
7. Drive smoothly and stay in the higher gears as much as possible. Driving wildly or staying in a lower gear for no reason will waste fuel. Conversely, staying in top gear when the engine would be happier in a lower gear is also wasteful.
8. Remove exterior add-ons. Roof racks (ie the cross rails) and bull bars increase the aerodynamic drag on a vehicle by a larger percentage than most people realise. An empty roof rack may increase fuel consumption by as much as 10 per cent, but once it is loaded the consumption will take an even more serious dive. If you have to carry a load on the rack, keep the speed down.
9. Use your car less often. This may not always be practical, but planning trips so as to reduce frequent travelling is always worthwhile.
10. Buy a car fitted with a CVT (continuously variable transmission). These are usually economical because the quick and smooth response of such a transmission allows the manufacturer to employ very high gear ratios (low numbers) for cruising.
11. Have the car serviced regularly. Unfortunately, this may mean that you have to pay a small fortune for what in many cases is only 90 minutes worth of work.
12. Check the pressures and condition of the tyres regularly. A large percentage of cars are running around with tyres that are too soft. This soaks up energy that should rather go towards maintaining your selected speed. Conversely, don’t try to save fuel by pumping-up the tyres harder than they should be. This often reduces roadholding and may even lead to increased tyre temperature and consequent tyre damage. Have the wheel alignment checked if you notice any unusual wear patterns.
13. Reduce idling times. A few modern cars feature a stop-start system using a beefed-up starter motor and computerised controls. These controls switch off the engine when the car is stationary for more than a few seconds, and start the engine automatically when the brakes are released or the clutch pedal is depressed. This is claimed to reduce fuel consumption by as much as 10 per cent in heavy traffic. We’re not suggesting that you try to do this manually because your starter motor won’t survive for very long, but it does give you an idea of the potential savings.
14. Avoid heavy traffic. Constant stopping and starting involves a lot of throttle movement that feeds excess fuel into the engine. Engines also run rich at very small throttle openings, resulting in heavy consumption and oil contamination.
15. Anticipate vehicle movement ahead to reduce the number of times you have to brake. Braking destroys energy, which has to be recuperated by burning more fuel.
16. An open window results in a slight increase in the aerodynamic drag that opposes a car’s movement, and this effect increases with the square of the vehicle speed. An air-conditioning compressor takes between three and five kilowatts away from the engine, but this effect is more pronounced at slow speeds where the loss is a larger percentage of what the engine is delivering. This implies that when trying to save fuel one should cool down by opening some windows at slow speed but close them and use the air-conditioner at higher speeds.
17. Heavy loads carried inside a vehicle do not increase the drag significantly, but their effect is felt when accelerating. This means that you will end up using more fuel in heavy traffic or while motoring uphill, but hardly notice a difference at constant speeds.
18. Many of the latest models incorporate designs that help to save fuel. Devices such as electric power steering, computer-controlled oil and water pumps, “smart” alternators, cylinder cut-outs, direct petrol injection, lean burn combustion and reduced internal friction, while not significant on their own, add up to well over a 15 per cent saving in the amount of fuel being used.
HOW NOT TO SAVE FUEL
Buy a fuel saving gadget. Item 18 above is a summary of what the industry has been doing in the last 10 years to reduce fuel consumption. These changes have been made at an enormous cost, yet if the gadget merchants are to be believed it is all a waste of money. The savings achieved could have been anything from 20 to 50 per cent if engines were fitted with fuel-line magnets, air bleeds or little fans in the intake manifold, chemical pellets in the fuel, hydrogen generators in the boot or any of the other devices that litter the internet like bad smells on a sewage farm.
CAR has often been criticised for not testing these devices. Our stance is that there’s no point in testing something unless it makes scientific sense. We are in regular contact with engineers at three universities and if they cannot make sense of the explanations given to us by gadget vendors then they are usually not worth testing.
