Thursday, 18 June 2015

Fuel economy, and the factors affecting it

7.1l/100km is a pretty good fuel economy figure for a five-cylinder 3.2l diesel that can produce 200bhp and 470Nm, like this Ford Ranger XLT.


Yes, it is a matter of growing concern. Fuel is expensive, and the Ringgit is deteriorating.
So, here’s the low down on what affects fuel consumption and how you, as the driver, can adjust your driving style to maximise efficiency.

Modern car engines are mostly run by computers which interpret what the driver wants from his/her inputs at the throttle. These inputs are translated into precisely measured quantities of fuel injected into the engine’s combustion at the right time. Many cars also have computers controlling the gearboxes, in sync with the engines, to further optimise fuel efficiency.

If you have a newish vehicle of a particular make, model and specifications, and achieves fuel economy that is drastically from another person who has the same type of vehicle and it is of similar age, that difference can be attributed to different driving conditions, such as ratio of urban driving to highway driving.

You may, for example, be getting poor economy because you drive a lot in congested city streets while he travels mostly between cities and towns, using mainly the highways. Stop-start motoring wastes more fuel than constant cruising in top gear.

But, before getting into the nitty-gritty of good or bad fuel economy, it's worthwhile to consider how consumption is measured. A common unit used today is litres per hundred kilometres, expressed as the amount of fuel used to travel 100km. Other popular units include km per litre and the old Imperial miles per gallon. I advocate going metric, for reasons of simplicity.

A car counts the kilometres it has travelled by a sensor that measures the number of rotations of a part of the car's drivetrain, that co-relates to the number of times its wheels have turned. This is where it becomes tricky to determine if you have really travelled 100km even when the odometer says you have.

Most new cars have meters that over-read a little, i.e., it may indicate you are going 100km/h when the reality is that you are doing 97km/h. There is a good reason for this - you cannot blame the car maker if you get penalised for speeding. By the time you get zapped at 119km/h, there is no way you can argue that the car's speedometer was showing 110km/h. It was more likely to be showing 124km/h, or something like that.

This optimistic reading does not matter if you keep your car standard and measure fuel consumption under one set of circumstances against another. But it becomes a problem once you change tyres to a different size than the original. If your new tyres are of a bigger diameter, the odometer will show a lower reading for the same distance travelled.

For example, if your odometer showed 200km for a journey from a fixed point near KL to another fixed point in Ipoh, then the odometer may show 194km after the same journey with tyres that are 3% bigger in diameter. This has to be taken into account when calculating fuel consumption. And if you rely on the car's computer, remember that the car does not know that you now have bigger tyres. (It is possible to get the meters re-calibrated but that's another story.)

Heavy Vs Light foot
If you and your housemate work the same hours in the same building downtown and hang out together after hours, and still record widely differing fuel economy with the same type of car - in short, all things being equal - then it is highly likely that driving style is causing the difference.

It takes a lot of energy to accelerate a stationary vehicle from standstill, and a lot more to keep accelerating to a higher speed. It takes a lot less to maintain the vehicle at a moderate constant speed, and then a lot more is need again to maintain it at high speeds, when wind resistance increases.

You burn a lot of fuel to build up speed, converting chemical energy into heat and then into kinetic energy. When you brake to slow down you, turn that kinetic energy into heat again, with the brakes being the parts that gets hot. That energy then gets dissipated into the atmosphere.

So, in city traffic, if you stomp on the accelerator to get up to 50km/h and then stomp on the brakes to come to a screeching halt again, you will have burnt a lot of fuel to heat the air and move just a short distance. Like from one set of traffic lights to the next.

Or you could press just enough on the accelerator to move about half the distance, and lift off the pedal. Let the car coast (other traffic permitting, of course) to the next red light. This way, you burn just enough fuel to get you going, and let momentum or knietic energy carry you along. Friction and air resistance will act to slow you down. If your judgement and timing is spot on, you will need just a gentle press on the brakes just as your car is almost at a standstill anyway, just to avoid hitting the car in front.

In such a scenario, a heavy-footed driver and a calculative light-footed driver will travel the same distance in more or less the same time, but one will use a lot less fuel, all other factors being equal. Plus, one will wear out his brake pads a lot sooner.

There are many variables in city traffic, such as intersections, right of way, congestion, etc, all of which conspire against a smooth, constant drive, which is the sure way to achieve the best fuel economy. But an alert driver who thinks things through can still use fuel more efficiently than one who does not.

Mind the brakes
Even when traffic is flowing smoothly, the ability to read the traffic ahead can aid in fuel efficiency. If the brake lights come on six or seven cars ahead, for example, a careful driver who maintains a safe distance between himself and the car in front may not need to brake but only lifts off his right foot and coasts for a while as he assesses the situation. If it develops into an emergency, he brakes. If it does not, and that driver up ahead braked for a reason that no longer exists or no reason at all, he resumes driving normally.

Each time you brake, you are wasting energy gained from burning fuel, i.e., wasting fuel. So, by keeping a safe distance between yourself and traffic in front and constantly scanning conditions far ahead, you can leave the brakes alone until you really need them. You save on fuel, and over time, also on brake replacement parts.

Having said that, do use the brakes when necessary. No point taking fuel economy to extremes and crashing the car. Common sense and all that, ya?  

Unnecessary overtaking is another way to waste fuel. If you are in a long line of cars on a single-lane trunk road, downshifting gears and stomping on the gas pedal may be necessary to achieve an overtaking manoeuvre, burning copious amounts of fuel to build up speed, then slamming on the brakes to slow down and merge back into the long line as an oncoming lorry whizzes by.

You will have wasted a lot of fuel and heated up your brakes, frighten a few other motorists and your passengers and yourself, and be exactly one car farther ahead in the same long line of cars.

Hit the open road, and you will get better mileage, but ... Yes, there is always a but.

Speed
There is a speed at which fuel efficiency is at its optimum for a particular vehicle. It is the slowest speed at which you can drive comfortably (for the engine, not the driver) in the tallest gear.

This optimum speed is determined by the torque characteristics of the car’s engines and its gear ratios (itself a combination of the main gearbox, the final drive and, in the case of 4X4s, the transfer case ratio).

Learn to read power and torque charts like this, also for the Ford Ranger 3,2l. Choosing the right gear to keep the engine revs between 1,500 and around 2,800 rpm yields the best combination of performance and fuel economy.
For many vehicles, this happens to be between 70 and 90km/h. Go any slower, and the torque may not be enough to push the car along in top gear. When you go faster, wind resistance or drag increases in proportion to the square of the increase in speed. Meaning, if you double the speed, drag rises four times. Pick any speed as a baseline, go 10% faster, drag increases 21%, 20% faster, 44% more drag, etc.

At the risk of sounding repetitive, remember a lot of the fuel that is burnt is the energy needed to overcome the greatly increased drag at increasing speeds. Driving at 140km/h requires a lot more revs and fuel to overcome four times more wind resistance compared to cruising at 70km/h.

Driving style, as in fast & furious (FF) versus cool, calm, collected (CCC), also has a bog impact. For maximum efficiency, read the traffic just like you would in congested areas, ease off when you see vehicles braking ahead, when you see a road hog on the overtaking lane who does not give way to others, etc. If you charge right up behind a recalcitrant road hog, then have to brake hard and wait until he moves over, then accelerate hard again, fuel is going to be wasted.

The alternative is to study the situation and plan ahead, lift off the gas to slow down gently, find a way past, then resume cruising at the speed you want to travel. Smoothness is the key.

A good friend and I once travelled from KL to Penang, each of us driving identical new VW Golf GTIs of the same age, and we both refuelled to the brim at the R&R (rest area) at Tapah. Upon reaching Batu Ferringhi, I had more than two-thirds of a tankful left, he had low fuel warning light blinking.

There is no question about YS Khong’s driving ability or credentials, having been the Malaysian rally champion five times. He does enjoy speed, though. Seeing as I also neared the car’s speed limit and cruised at ... ahem, above legal limits, for most of the journey, the difference between his consumption and mine was mainly in aggressiveness.

Weight 
It takes more energy to accelerate a heavy car than a light one. It takes more fuel to accelerate a double cab pick-up with a full load of passengers and maximum payload than it does to accelerate that same vehicle with just a driver aboard. And each time the vehicles slow down, the built-up energy is wasted, and more fuel needs to be burnt to build up speed again.

Theoretically, an unladen vehicle and a heavily loaded version of the same make and model would return similar consumption if both could start rolling together and then travel, say 500 kilometres at a constant speed of 90km/h. The heavy vehicle would incur a fuel penalty only when accelerating from standstill to 90km/h, after which both will need the same energy to overcome the same level of wind resistance. Overall consumption figures will see only a tiny difference favouring the lighter car.

This is all well and good in theory, but in real life, the heavier car will be thirstier than the lighter car because cycles of acceleration and deceleration are very much a part of getting around.

A slippery slope
From walking and running, we know that climbing hills take more effort than cruising on flat ground. In theory, though, the extra energy you put into gaining altitude becomes potential energy which should be recoverable when you descend the other side of the hill. After all, you don't climb up forever, and what goes up must come down.

In reality, that does not always happen. If the slopes are gentle, it is possible to offset the extra fuel need to climb up by coasting down the other side. But, if the slopes are steep, you may need to burn a lot of fuel while climbing, and descend slowly with some braking to ensure a safe descent so some energy will be lost.

DO NOT ever put your gearbox in neutral when coasting, whether you are driving an auto or manual. It is silly and dangerous. Silly because it does not save any fuel, and dangerous because it can result in loss of control and a crash..

All modern cars rely on electronic fuel injection, which cuts off fuel when you don't press on the accelerator pedal. When coasting down a gentle in slope in top gear, it is the car's momentum that keeps the engine turning so whether it is running at 2,000rpm or 3,000rpm, no fuel is burnt. Many modern cars have multi-information displays that show, among other things, instantaneous fuel consumption. Select this and you will see that fuel burn is 0 l/100km when you coast in gear, with right foot off the pedal.

However, if you put the gear in neutral, the ECU (engine control unit) actually needs to inject fuel to keep the engine turning over at idling speed. Idling does not use up much fuel but even a small amount is still more wasteful than none at all.

On top of being wasteful, coasting downhill in neutral is dangerous because you no longer have the benefit of engine braking, and no means of propulsion if you suddenly encounter a situation that requires power to get around or away from. Engine braking is the practice of selecting a lower gear than usual when descending a slope so that the car's momentum or kinetic energy is used to turn the engine over, working against the compression in the cylinders, resulting in a retardation of speed.

Tyres are another factor that has a bearing on fuel economy. Some tyres are especially engineered to give low rolling resistance, possibly with compromises in other aspects of performance. Chances are that changing to other tyres on the basis of aggressive looks, or bigger sizes for more macho style, will hurt fuel economy. There’s a price for looking good, beyond the sticker price of the tyres, and other sacrifices in noise, grip, comfort, etc.

Tyre pressures also influence economy, with higher tyres generally giving better mileage at the expense of comfort. Refer to the vehicle manufacturer’s guidelines on pressures. If a range of pressures is given, know that the lower figures usually give a softer, more comfortable ride while the higher figures give better fuel economy. Choose wisely.

There are no rigid guidelines on when an engine is at its most efficient, with some schools of thought insisting a brand new engine is still tight and rough before it is truly run-in, when the working, moving components have polished themselves by rubbing against each other. Some say engines are nicely bedded in at 10,000km, others say 30,000 or 40,000, while some engines begin to get tired at 100,000km or even sooner.

What is not disputed is that a well-serviced and maintained engine will run more efficiently than one that is neglected. Service regularly, use the best quality lubricants and filters recommended by the engine maker, and fuel consumption should remain at optimum levels.