Summary: Fuel economy

On this one subject only I have read the BoK postings but given up trying to edit them into anything sensible. Instead I've drawn on my own thermodynamics training and compiled this treatise which is biased with my own opinion! - editor.

We've chosen Townies and the like for various reasons. High fuel consumption (worse than 10km/litre? see mpg chart designed by Clive Mucklow.), black smoke, poor acceleration and top speed may be things that you are conscious of, or not. Maybe you would have chosen a different type of vehicle if you had known about them beforehand.

Fuel economy is a broader subject than just how often you have to fill your tank. It can affect your top speed. It can affect wear and tear on the engine - since energy not used to push you along will usually end up making something get hot. It can affect how much you pollute the environment. So it's worth thinking about what you want your vehicle to do and about the total cost including repairs, depreciation and your time. Improve consumption by 5% and you'll only save £40 on fuel bills over the next 10,000km - based on 10km/l @ 80p/l.

These vehicles were designed for (urban use in?) the Japanese market and the speed limit there is 100Kph (62Mph). Don't be surprised to have problems hitting over 80 in a townace but a lot depends on the conditions. Would you want to pull an emergency stop from 90 mph in one? The brakes and handling just aren't up to it. These machines are by no means racers and it can be a bit galling when Galaxies pass you by on the motorway. Townies are heavy (which helps with overall coomfort), and have a lot of frontal area (goes with a high driving position and good view). Plus the 2.0TD only puts out around 85HP. Something like a Ford Galaxy TDI can have 115HP, is a lot more slippery, and the FWD system has less loss than the RWD+auto in the townie.But then they don't compare with the amount of space, versatility and spec of the Townace. It depends upon what you want most from an MPV.

Fuel is consumed to produce mechanical energy which is transmitted to the wheels to do two things: to overcome friction at whatever speed the car is travelling and to increase the momentum (speed) of the car on demand. Along the way all these things affect fuel consumption:

• The calorific energy available in each litre of the type of fuel used. (I have not yet got much information on this. Types of diesel are said to vary a bit. Anyone know if diesel and petrol are intrinsically different? - worth bearing in mind when making mpg comparisons and choosing a vehicle, but otherwise not at the heart of the issue discussed here).
• The efficiency with which the engine is converting fuel to mechanical energy rather than, say, to heat. Usually this means having the engine adjusted and operated as far as possible as the designer intended. It depends to some extent on the atmospheric air density - low altitude, temperature and humidity, and high pressure will mean higher density and so oxygen drawn in faster. Plus it depends on the design - the Diesel combustion cycle is inherantly more efficient than the Otto ("petrol") one. Good automatic transmission goes a long way towards ensuring the engine is being operated as intended.
• The amount of energy diverted into auxilliary systems such as fans, pumps and generating electricity for lights and recharging the battery. The aircon compressor can take up to 10HP off the engine when it's running - though it's thermostatically controlled and goes on and of, so the net effect will depend on how you set the temperature lever, the ambient temperature and whether you have the blinds, curtains, fresh air vent, or even windows, open.
• The first type of friction, rolling resistance, mainly of the tyres on the road surface. (Think of the difference between the way the ball rolls in lawn bowls and in ten-pin bowling.) Roughly proportional to speed, i.e. doubles as speed doubles. Less fuel/energy used if tyres are at higher (but safe) pressure and have less chunky tread, and if the road surface is smooth and wet (or greasy, i.e. lubricated).
• The second type of friction, pushing the car through the air. Roughly proportional to the square of speed, i. e. doubles for 40% increase in speed. Less fuel/energy used for a given car shape if its surface is smooth/polished, if the windows are closed, and if the air is less dense - i.e. at higher altitude, temperature or humidity, or lower barometric pressure. (N.B. the opposite of the best conditions for the engine, above).
• The style of driving. It is impossible to get from A to B without increasing the speed/momentum to a reasonable level, and doing so over and over again every time traffic slows you down. Fuel/energy is used every time you increase speed and you don't get any back when you slow down! So save fuel by not increasing speed more than necessary and avoiding braking (either using the brakes or engine-braking). Allowing the two types of unavoidable friction to slow you down IS efficient because the momentum/speed, which you have already consumed fuel for, is now being used to overcome the friction instead of using fuel to do that.