Working out fuel consumption per hour...

[Stalk]

Does anyone know how to do this?
I know that this is not strictly bike related and I am sure that it will be moved if I have got it wrong, but it is a sort of mechanical / theory question.

Theoretrical starting point.
1500 cc diesel engine that would do 45MPG at 60 MPH reading 4000 RPM.

Same engine running at 1800 rpm would use how much fuel per hour?

Reason behind this is that heating oil and diesel are very similar.
1800 RPM would give 50 Hz on an ac generator.
Said engine would also be able to heat water due to hot exaust manifold

Current energy costs are oil / electric at £200 a month, the generator could go to standby during the day when not needed (I will ignore the fridge freezzer issue at this point). Would the overall cost go down? The idea has been kicking around in my head for a while so I thought I would do a paper exersise on this and see what I come up with.

Thanks in advance
Stalk.

[Teflon-Mike]

Very difficult to back-calculate specific fuel consumption from MPG.
Query you have however is not so novel as you may think, its called Combined Heat & Power or CPH, and YES it is normally more efficient than a boiler.
Roughly, the mechanical output of the engine is 'free'. The heat produced is the same as if you burned it in a conventional boiler, so you size for heat, and get whatever electricity the genny makes as a bi-product.
Not ALWAYS or EXACTLY true, there are a few added variables in the equation, but its a reasonable aproximation.
Main thing is that diesel engines are rather noisy, compared to a conventional boiler..... you wouldn't want one clattering away in your airing cupboard, so for domestic instalations, normally means a 'remote' siting, say in a semi-submerged shed at the bottom of the garden, which means piping the heating water from the CPH unit to the building being heated, with insulation and pumping losses, that can deminish the savings, considerably.
Then there's the servicing, miantenence, & repair costs. Compared to a conventional boiler, a CPH unit is expensive to buy, and install, and then maintain and repair... all them moving parts!
That also eats into the gains from 'free' electric, and theres alsorts of 'optimums' in the ecconomics to find where it becomes more ecconomical to run CPH or conventional heating boiler and mains electricity, and typically, the ecconomics dont stack up until you are at a commercial scale and looking at buildings the size of a small hospital or average sized school, where a re-commissioned gas-turbine is most ecconomical.
Had to do an assignment on alternative energy as part of a diploma I studied a few years back, it was all interesting stuff.
Domestically, the only case study for 'domestic' situations where the costs over-all added up was in a shared CHP scheme, a single CHP unit supplying an 'assisted-living' complex.
It didn't look closely at the 'ameteur' situation with some-one running an old Transit engine in thier shed, but DID mention it!
Was a completely DIY CHP system, using a 'scrap' diesel engine and bits of reclaimed heating system, and a few 'new' components, all DIY maintained. On the ecconomics, it was described as an 'anomoly', but did qote the chap commenting on having Norweb come out to check why his meter was aparently running backwards, AND the precident that they were actually made to pay him for his surplus electricity!
Best of luck with it, sure that theres now a few web-sites devoted to the topic, if you search.
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[herulach]

Doesn't use a gen, but british gas have started installing boilers with a Stirling engine in them to generate electricity. Think its something like 1kw with the heating on, which according to my nifty monitor gadget would power the lamp &tV fine.

Only problem is they're silly money to buy.
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