MPG & Webasto

[jml]

Those vehicles fitted with the Webasto supplementary heater clearly use diesel from the main fuel tank.

Does anyone know whether the car’s mpg readout take into account this fuel usage, or is it only the fuel coming through the main fuel supply to the engine ?

Any thoughts gratefully received.

Cheers

Julian

[admiral51]

I think it all depends on where the various ECU's take readings from but having used them in HGV as "Night Heaters" for years i think they use something silly like 0.1-0.5 litres per hour depending on kW so over a tank full of diesel ( 65 -70 ltrs ) the impact on mpg would be minimal and probably less than having the A/C on.

And they work like the central heating, it has a thermostat in built so when the temperature is reached it cuts off and then restarts when the temperature is below what it has been set at, so not like having it running all the time.

Not really on topic but a HGV with a HIAB etc will use approx 3-4 litres per hour via the PTO and that does make a difference to MPG if only fuel added/miles driven are taken into account. Most trucks will show the amount of fuel used in PTO operation and hours operated. If i remember tomorrow i will take a pic to show it and post tomorrow evening.

Colin

[jml]

Thanks for your thoughts.

The consumption rate of the thermo top C is 0.6 l/hr at full load and 0.3 l/hr at half load. IIRC, It goes from full load to part load once engine temperature reaches 72 degrees. At full load it pumps out 5kw.

So, as a very rough and ready calculation, if I’m doing a journey of 15 minutes at an average of 30mph and, say, 30 mpg (although at the cold temperatures we’ve had recently I suspect this is rather overgenerous, especially from a cold start) then the engine will use about 1.1 litres of fuel.

I don’t think the Webasto will reach 72 degrees in that time (again, especially in the recent cold conditions) so let’s assume it’s running at full load for those 15 minutes. This would mean it uses 0.15 litres in the same time.

So, 1.1 litres for the engine and 0.15 litres for the Webasto means more than 15% extra fuel, which is not an inconsiderable increase, and certainly not what I would describe as minimal !! Unless I’ve messed up on my maths somewhere which is quite likely.

Obviously, once on a longer journey the consumption of the Webasto will decrease and, before too long, become zero, but I am looking at a typical morning journey for me and, I suspect, many others.

But is this reflected in the mpg gauge shown on the car ? Any other thoughts would be most gratefully received.

Cheers

Julian

[jml]

I did some further thinking on this subject overnight and I reckon the following:

15 minute trip @ 30mph & 30 mpg will use 0.25 galls = 1.135 litres. With the Webasto running, it will use an extra 0.15 litres. So my 15 minute journey without Webasto uses 1.135 litres but with Webasto uses 1.285.

My maths gets shaky here so please correct me if I’ve c***ed up but I reckon, translating this to mpg means that my 30 mpg drops to around 26.5 mpg - quite a sizeable increase in consumption.

I’d still be interested to know though whether the car’s mpg readout takes account of this - I suspect so because I’ve logged my cars fuel consumption diligently for nearly 150,000 miles and it always increases in the winter months. Previously I’d just put it down to colder weather but perhaps it’s because of the Webasto working in that colder weather!

Any thoughts gratefully received

Cheers

Julian

[GiveMeABreak]

Hi Julian

I have little experience with these Webasto units, but the trip / fuel economy would be determined from the fuel measured / used against engine speed using the various sensors. I could see that there would be engine load through increased electrical use with operation of the additional heating equipment depending on system (electric coolant pump, sensors etc.) which may be a contributing factor to the increased fuel consumption though.

The one fitted on the X7 was a bit more involved, although yours seems more simplified, it still has some electrical connections that I can see.

[jml]

Many thanks for your thoughts Marc.

I thought that quite a few Citroen models featured the Webasto unit so this thread might have been interesting to others.

In the early days of fitment (Xantia Exclusive ?) there was a problem if the car had only been run for a couple of minutes and then restarted, whereupon a massive cloud of smoke could encircle the car as unburnt fuel was burnt off. Later models featured a better combustion chamber venting arrangement to avoid this problem, however I remember well a senior manager at Citroen uk coming back to his car to find the fire brigade in attendance after a passer by thought it was on fire !

The Webasto unit can take up to about 6amps on start up but, if I remember correctly, it is only about 2 when running. On some other makes ( later Rover 75 and some Land Rover models) they use the same Webasto ThermoTop C, but it is plumbed into the CAN bus system however on Citroen C8 and Peugeot 807 it is a much simpler system and whilst it ‘feeds into’ the multiplex system, the actual unit has just 3 wires - one a permanent 12v positive, one an earth and one a trigger wire.

Here’s the unit removed from the car :

How come I know a bit about the Webasto ? Well I built a unit comprising 3 units (putting out a total of 15kw) to heat my swimming pool !

However, despite all this I’m still not sure whether my C8’s mpg readout takes account of the Webasto consumption - I rather suspect it does.

Cheers

Julian