But in what specific ways do higher ambient temperatures benefit fuel economy, directly and indirectly, and should it get better and better, as the ambient temperatures increase? And what other ambient conditions also affect fuel economy? Well, obviously the major beneficial effects come from quicker engine warm-up, and lower overall heat losses along with reduced engine friction, as lubricant viscosity falls. Over the 15 miles that’s an average winter warm-up distance, there’s probably an increase of around 15 to 25 per cent in fuel consumption over a fully warmed up car, or the same car after just five miles in high summer. So, if your motoring trips are typically less than 15 miles, then you’re going to notice these seasonal fuel economy differences more than longer distance drivers.
There’s another significant negative cold weather effect originating at the tyres; cold tyres have a higher rolling resistance than hot tyres, resulting from the temperature of the rubber itself, whilst the warmer air in hot tyres also increases the tyre pressure, which again helps reduce rolling resistance. In warm weather, the tyres, like the engine, heat up quicker, because both the air and the road surface temperatures are higher, and optimum rolling resistance is reached faster.
The fuel economy effects of air temperature itself on engine efficiency are rather more complex; with turbocharged engines, the air compression in turbocharging heats up the intake air, whilst the intercooler lowers it again by cooling the compressed intake air with ambient air. At moderate engine power outputs in relatively cool weather, there’s no great effect of intake air temperature on fuel economy, and older non-intercooled Ford/PSA/Volvo 90bhp engines were capable of delivering impressive economy. When ambient temperatures rise though, and an engine is working harder, the intercooler really earns its keep by facilitating higher engine power outputs without the risk of engine overheating, and possible damage, rather than by improving engine efficiency. (Although with turbocharged petrol engines the negative effect of hot air’s lower density is more critical.) Air temperature does have an opposite effect on fuel economy though, because cold air is more dense and therefore your car’s aerodynamic drag is necessarily lower when the air is hotter, and it’s worth as much as a three to five per cent increase in mpg with a 20 degrees Celsius rise in ambient air temperature, with higher figures applying at higher cruising speeds.
In the real world though, theoretical aerodynamics are often destroyed by the turbulence created by static roadside objects, other vehicles, and crosswinds, and the energy losses from aerodynamic drag are probably somewhat greater than is assumed. That’s yet another explanation why real life economy figures don’t match the “official” EC figures that are all derived in still air conditions! This raises another valid point regarding following other vehicles closely, because you generally need to be dangerously close to the vehicle in front to get any positive slipstreaming effect, whilst there is a distance where you’re actually picking up turbulence and increasing your aerodynamic drag, and the turbulence is even worse if you’re following a number of similarly spaced vehicles. So you’re better off keeping well behind, particularly from larger vehicles with irregular shapes. But electronically controlled slipstreaming, maybe employing adaptive cruise control, where the collective drag of a controlled group of closely-spaced vehicles is reduced for all of them is highly beneficial and could just be something for the future. It’s apparently why some truck drivers follow each other so closely, although that’s probably a generous interpretation of this habit. But geese, as we know, fly in closely-spaced V-shaped formations, and seemingly it’s down to the intelligence of the geese, and not some presumed order of animal hierarchy. In fact, the goose in front is the one working hardest. A final thought: have geese ever watched cycling team pursuit races, and do they similarly regularly change their flying positions to take turns at the front, just as the waiting Emperor penguins take turns standing in the very coldest and windiest spot, on the outside of the group?