It's a topic that pops up frequently. "What sort of fuel consumption should I be getting from my car?" Or, "I'm getting a fuel consumption of 17-18 litres per hundred kays - is that OK?" Those readers quoting their fuel consumption always seem to have radically thirsty cars - probably the reason that they're asking the question in the first place! So how do you get your fuel consumption down?
1. Mechanical Condition
First up, to get the best possible fuel consumption, your car needs to be in good condition. Starting with the engine, the compressions should be high and even, the airfilter clean, and the ignition system in good nick. The brakes should not be dragging and the wheel bearings should allow easy wheel rotation. Tyre pressures should be high - usually a few psi over the carmaker's recommendation will improve both fuel consumption and handling. If your car won't roll on a very slight slope, maybe you should take a look at these aspects.
One real killer of fuel consumption is a partially blocked exhaust. As cars fitted with cat converters get on a bit in age, it's becoming more and more common to find cars with internally collapsed cats. The best way to find if the exhaust is imposing a greater than normal restriction is to perform a back-pressure test. This can be easily done by unscrewing the oxygen sensor, and then screwing in an adaptor connected to a normal pressure gauge. While it varies from car to car, a standard exhaust shouldn't have more than about 0.5 Bar (~7 psi) back-pressure at peak power.
Thinner engine, gearbox and final drive oils will reduce drag and so improve fuel consumption. If the carmaker gives you a choice of oil grades, select the thinnest multigrade oil (lowest numbers) that still satisfy other criteria like the operating ambient temperature range.
While this probably isn't of much use if you already have a car from which you're trying to extract better fuel consumption, the very best power/consumption compromise comes from turbocharged, relatively small (eg three and four cylinder) engines. These avoid the major internal frictional drag of larger engines, while the turbo aspiration means that substantial power and mid-range torque figures can still be realised. And incidentally, around town, auto transmissions these days are often more fuel-efficient than manual 'boxes, and have very close to the same fuel efficiency on the open road.
2. Engine Management
It's not unknown for uninformed people to disconnect the oxygen sensor when chasing better fuel economy. Quite how this is supposed to improve things we don't know - but it certainly doesn't! In fact, having an oxy sensor that's in good health is very important to gaining the most kilometres per litre. Oxy sensors lose their responsiveness if they have been contaminated by lead or by the use of non-appropriate silicone sealer, or have been clogged-up with carbon. If the management doesn't have an on-dash indication of problems (eg by a Check Engine light), a self-diagnosis should be manually performed to make sure that there are no problems with the oxy sensor or other engine management input sensors. How to do this is normally outlined in the workshop manual.
Two engine management problems that can cause high fuel consumption - but won't show up on a self diagnosis - are an overly high fuel pressure, and partially blocked injectors. High fuel pressure can occur if the hose connecting the fuel pressure regulator to the plenum chamber develops a leak, or the fuel pressure regulator itself has problems. While fuel pressure can be measured with a gauge, if you suspect that this is the problem it's probably worthwhile going to the next step below - dyno testing. Partially blocked injectors will not atomise the fuel correctly, leading to poor running. Workshop-level dedicated on-car fuel system cleaners and checkers can be used to clean all but the worst of blocked injectors.
3. Dyno Testing
A chassis dyno test undertaken with an air/fuel ratio meter up the exhaust will quickly show whether the mixtures are appropriate for acceleration, deceleration, and cruise conditions. Testing the mixtures in this way can save hours of self diagnosis, fuel pressure measurement and the like - so the cost is usually worth it. If possible, go to a dyno shop that frequently sees your type of car - that way, they'll be familiar with the mixtures that are used by standard, good condition examples of the model. And if the mixtures do show up as rich, further diagnosis is much more easily carried out on the dyno than with the car idling or being road tested.
4. Modifications
Good modifications will usually improve light load fuel consumption. For example, a bigger exhaust, free-flow intake and ported head will improve fuel consumption on almost all cars. However, if a high overlap cam has been used, the air/fuel ratio will have to be richer than standard to allow the car to idle and run smoothly at light loads, so offsetting the gains made through higher engine efficiency. Increasing the compression ratio will also improve fuel consumption, as more energy will be gained from each burn.
But improved fuel consumption in a modified car will only be achieved if the air/fuel ratio and ignition timing remain appropriate for road use. It's not uncommon in modified cars running programmable management for little attention to be paid to the mapping of light load cruise - exactly where most road cars spend the majority of their lives! When you are driving down a flat road at 60 km/h, only a tiny throttle opening - perhaps 5 per cent - is being used. The engine is developing 5 or 10 kilowatts of power - and if the dyno tuning of the management wasn't conducted at these loads, who knows what air/fuel and ignition timing is being used! Many engine dynos, for example, cannot operate effectively at these types of very light loads - the eddy current design shown here is an exception to that rule.
Of course, if your engine has been modified to develop 30 per cent more power than standard - and you like using every one of those thirty per cent - don't expect better than standard economy!
5. Aerodynamics and Mass
At about 80 km/h and above, overcoming the aerodynamic drag of the car takes most of the power that is being developed by the engine. Making a substantial decrease to the drag co-efficient of a car by adding spoilers and the like is almost impossible, but it's easy to go the other way - to degrade aero performance. Leaving a roof rack or roof bars on a car all of the time is one way of throwing petrol money away. Also, leaving a heavy toolbox in the boot, or carting around a heavy subwoofer means that you need to pay their fares at the petrol pump every time that you fill up.
6. Driving Style
This is the biggy - the single aspect that has the greatest affect on the overall fuel consumption of a car. A good driver will generate excellent fuel consumption figures by reading the traffic flow - not being constantly on and off the power. If you are in a stream of traffic approaching a red traffic light, you should never have to move your foot straight from the accelerator to the brake - if you do, it shows that you're not reading the traffic very well. Instead, there should be enough time for you to roll in gear before starting to brake. If you leave an adequate gap to the car in front, this will help you to be smooth and consistent in throttle applications. Unless you particularly want the front row of the grid, when moving from red light to red light there's probably not a lot of point in redlining it - not when you're gonna be stopped again in a few moments anyway.
Making sure that the car doesn't lug - or scream its head off - means that you should change gears at the right time. In normal driving in most cars, that means changing up a gear somewhere near peak torque (ok, ok, that advice doesn't apply to an Integra Type R). Pulling high revs while engine braking doesn't use any fuel, so there's no drama in doing that.
When driving along at a constant speed - whether that's 60 km/h or 150 km/h - on a flat, straight road, your accelerator foot should be almost still. Just as there are people who constantly saw at the wheel when driving in a straight line, there are others who are always on and off the power. This is just lousy driving! If you have a car with cruise control, you should be able to compare your driving with the cruise system - in all but those cars equipped with the very best systems, your driving should be smoother and less jerky than the cruise control can achieve.
If you fang it all of the time, expect to pay heavily at the petrol pump. But if you drive conservatively and well most of the time - and then drive hard only when you want to take on someone at the lights or put away that section of twisty road - you will be able to achieve very good consumption. Oh yes - one last point. When the engine is cold, don't warm it up by waiting around idling. Instead drive off as soon as the engine has been started, but drive gently until it reaches operating temp.
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