My most recent vehicle has an interesting feedback display on the dash.
What it shows is the fuel burned in real time, expressed in the number of litres of fuel the engine is using to travel a distance of 100 km, or xxL/100 Km.
I have watched this read-out while traveling at various speeds and have concluded that optimum fuel efficiency is obtained when I drive around 80 km per hour.
For example, at 80 km/h my fuel burn is about eight litres per 100 km, and that rises to nine litres at 90, and close to 10 litres per 100 km at a speed of 100 km/h. And if I want a bit of a downer, I can simply multiply the litres/100 km by the price of a litre of fuel to give me my fuel costs for a 100-km trip.
The mileage of any vehicle deteriorates with increased speed, mainly because of the increased drag, or wind resistance.
So, on a windless day, my mileage is the same going either east or west on the highway. On a windy day, it is another story.
If the west winds are blowing, say at 30 km/h on my trip home, against the wind I will consume about 10- to 15-per-cent more fuel, than going with the wind. This is a result of the increased aerodynamic drag.
Going against the wind at a groundspeed of 80 km/h the car senses a headwind of 110 km/h . . . (ie 80+30=110). Going downwind at a groundspeed of 80 km/h, with a tailwind, the vehicles senses a wind speed of only 50 km/h . . . (ie. 80-30=50) and very little aerodynamic drag.
My daily travels over the same portion of Hwy 54 near Spruce View have shown me some other interesting things.
The portion of the highway travelled is relatively flat, with not more than a few feet of elevation change over 20 km. Such a flat stretch of highway gives a good indication of true fuel burn.
Most geography is a function of geology, and this area of Central Alberta is flat on account of being at one time the lake bed of a glacial dammed lake.
Sediment coming into the lake settled to the bottom, mantling over any previous ground relief and making it smooth and flat.
The glacial lake drained away almost 10,000 years ago leaving a flat plain. So flat that that many of the farmers around here took to building drainage canals almost 100 years ago.
The land was too flat to drain properly for agricultural use and just needed a little man-made help. It would not be proper to drain your own land at the expense of flooding your neighbour, so the farmers all co-operated for the greater good and built a vast network of drainage channels across most of the farms. The soil was good, but often too wet to get a crop in, or off, since the rainwater just pooled and killed off any growth. These drainage canals are still active and operated under the banner of the Dickson drainage system. Nowadays the flat farmland drains in a matter of days into the Red Deer River. Now that you know what to look for, you can see these drainage channels in the Markerville, Dickson, Spruce View area as you travel along Hwy 54 in the summer. It is a flat land devoid of many hills or valleys.
But I digressed. I was talking about fuel burn under various conditions. There are lots of good reasons for poor mileage.
This winter I watched my fuel burn display and noted that, in total, my fuel burn is almost double on a cold day, depending on temperature, wind, ground cover and frame loading. That’s right, a decrease of 30 to 50 per cent in how far a tank of fuel will take you. When you consider the various factors, it is easy to understand why your vehicle don’t go far on a cold day. On a really cold day, I like to warm the car up. It sits there, going nowhere, and idling away until I am ready to go. So the five-15 minutes of idling time is consuming fuel while my mileage is zero. Idle time is really idle time.
And the greases and fluids in the wheel bearings, axles, differentials, transmission are all thick and heavy, or viscous. The increased friction requires extra horsepower to overcome that resistance to movement, at any speed. Consequently, the rolling resistance is greatly increased and is a drag on fuel economy.
The air is cold, and because of that the air density is much higher and the resistance to movement through the air requires more horsepower to overcome this aerodynamic drag. Ask any pilot about air density, and he will tell you how the aileron, elevator and rudder surfaces ‘bite’ the air so nicely on a cold crisp morning and he can perform turns and such with the flick of a wrist. Further, his take-offs and landings are done in an incredibly short distance because of the additional lifting power of cold dense air. Conversely, many a runway has been overshot on a hot day with less dense air, resulting in a costly prang job. On a really hot day, your speed over the ground (on take-off, or landing) may be almost twice the actual speed on a cold day. Air density affects equipment in many ways.
The cold winter air makes an engine run ‘richer’ or with a higher fuel to air ratio. Years ago, most vehicles had manual choke controls, and pulling the choke lever essentially closed off the mouth of the carburetor to inhibit the inflow of air and make for a richer mixture. Nowadays, almost all vehicles have automatic chokes, and with all that cold air coming in, these automatic chokes will begin to restrict the air intake. And most vehicles today have sensors that measure these things and adjust the fuel flow accordingly. And I believe the fuel itself is less well atomized in cold air, so the sensors may also inject more fuel. Then there is the extra weight you carry in your vehicle in the winter. Some emergency supplies, a shovel, maybe some sand bags, winter clothes, etc. And add to that the amount of snowload you are carrying on the frame in the manner of slush, snow, ice and crud. Adding extra weight further decreases your mileage.
And much of your trip may be made with higher electrical loads. Lights, heaters, fans, all going full bore. So the alternator has to put out more and is drag on horsepower. But to be fair, so is Air Conditioning, which can cut your mileage by 10 per cent alone. Your tires flex when they roll along, and different tires have different degrees of rolling resistance.
Yet cold rubber does not flex easily and some additional power is required to overcome this resistance to rolling.
Lastly, the ground cover has a big effect on mileage. Road drag increases if anything is on the road, either rain, snow, slush, or simply even softer ground. If you are pushing snow you are using extra power. If you are pushing slush, you are using extra power. If it is icy and you are spinning your wheels, you are wasting horsepower away.
At any rate, the cold weather is energy intensive, and wallet draining, whether we are talking about the fuel burn in our vehicle or to heat your house. A tank of gas won’t take you as far in the winter as it does in the summer.
Paul Hemingson is a freelance writer who lives near Spruce View. His column appears every other week in LIFE. Contact him at paulhemi@telusplanet.net or www.paulhemingson.ca