[paulgato]

Hi fearone,

To be honest, no I haven't done the maths on the coolant heater re cost of mains electricity used vs cost of diesel saved. My guess is that there's not much money saving available there. Also, I don't yet know how much fuel I'm saving by starting off with a pre-heated engine. (10% over the first ten miles?) I suspect that there is a 'sweet spot' whereby if I heat the engine quickly to a medium temperature like 50 degrees, I will avoid the really bad fuel economy of a stone cold engine without using too much electricity.

I also haven't costed out the battery charger vs alternator thing, but I am totally convinced that the mains electricity used to charge the battery is a lot cheaper than charging it using diesel. I'm saving about 10% overall on diesel through the overnight battery charger mod, and over a year that's at least £150 for me. Recharging the battery uses only about half a kwHr overnight, or less than 200 KwHr per year. At a generous 20p/kwHr, that would cost just £40 a year, or 10p a day. (And that's an absolute maximum cost, assuming I take the battery down to 50% charge every single day, which I don't by any means.)

[It's also possible to put the charger circuit on a timer so it uses only cheap, night-time electricity. (I'm on an Economy 7 tariff.) In my case that would just mean a timer that doesn't allow power through to the external socket before 11pm, as by morning the charger will have cut its output right down to a trickle anyway.]

Once the battery is fully charged, leaving it plugged in on trickle charge uses almost no power, whereas a warm engine is constantly cooling down, and the warmer the engine, the faster it will cool down, so a short, sharp burst of heat just before drive-off time is the thing to aim for as far as economy goes. I found yesterday that 20 minutes at 1.8kw heated my engine by 40 degrees from 10 degrees (Centigrade) to 50 degrees, but it took a further 50 minutes at 1.8kw to raise the temperature by the next 38 degrees to 88 degrees. It's really difficult to know, as engine temperatures keep rising as I drive, but my feeling is that the really bad mpg happens below 50 degrees C. 20 minutes at 1.8kw costs about 6p, but heating the engine fully would cost an additional 15p.) 6p buys about 30g of diesel or 0.04 litres. Hmmm... I reckon it's roughly 'cost neutral' in terms of fuel as long as I don't keep the heater on for more than 30 minutes or so. But in a vehicle which is less fuel efficient than mine, the saving would be that much greater.

The coolant heater mod was a brave experiment for me. It does improve mpg, but it was expensive and complicated to fit. I could not have justified it to myself on fuel-saving grounds alone, but it will also contribute to safety, save wear on the engine, save me time and trouble on icy mornings, reduce the polution the car produces and will mean I don't have to idle the engine on my driveway, annoying the neighbours and poisoning their children.

I would definitely, definitely recommend the alternator delete mod though. Real cash money to be saved there.