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fuel economy

Old 02-08-2012, 04:13 PM
Thread Starter
Join Date: Jan 2011
Posts: 32
Default fuel economy

My C220 estate has poor mpg compared to cars of similar spec from rival manufacturers (VW Passatt / Ford Mondeo). I have tried to think through why my C220 auto estate does 15mpg less than my mates passatt and cant believe the auto box (with torque converter lock up clutch at >30mph), bevel gears in the rear axel and 100kg more mass make that much difference.
The energy loss of 15mpg is "kilowatts" and if this were caused by friction somewhere (rubbing brake for example) it would soon get hot enough to notice. I have no heat issues like rubbing brakes.
I think the energy from the ignited fuel is not being converted into foward motion by the pistons but is going up the exhaust pipe as heat. If the point at which the diesel is added is late in the cycle (i.e. injection timing is retarded) the diesel will burn when the piston is on its way down the bore and heat the cylinder wall and the wasted energy will go up the exhaust pipe as heat.
A friend once bought a peugeot 405 whose owner was selling it because it was "gutless". He promptly loosened the rotary pump and advanced it a tiny bit and transformed the performance completely.
So how would we be able to check the timing is correct with an ECU controlled engine and could the average man in the street adjust it? A retarded engine will run smoother and is this one way that MB make their cars "luxury". I am interested in peoples opinions and feed back.

I really fail to see how a 2.0 litre VW can really achieve 55mpg at 70 mph when my 2.2 litre MB struggles to get 40mpg.
Gary Smith.
Old 02-09-2012, 03:21 PM
Thread Starter
Join Date: Jan 2011
Posts: 32

Okay, lets debate the point about weight. The driver of a 38 ton articulated Volvo lorry (diesel) was telling me recently if he drove carefully he could get a reliable 9 mpg (imperial galls). So things in his favour are :- all motorway work with minimal stop start, only goes at 55mph so less aerodynamic losses, manual gear box. Things agains are :- 7 litre turbo charged engine, 12 massive road wheels (with 12" tread width) with 12 sets of brakes to potentially rub, un-aerodynamic flat fronted cab and slab sided trailer, bevel gears in the back axel, air conditioning, 200 amp alternator, air pump for brake system....etc.
So I have to agree that making the car lighter would probably improve mpg but making my MB 25 times heavier (i.e. same waight as the lorry) does not appear to make a 25 fold reduction in mpg (just a mere 4 fold reduction). Something just does not add up and I cant figure it out.
I have tried to evaluate the losses by comparing my MB with the Passat and it comes down to :- MB has back axel bevel gear losses, oil pump in the auto gear box runs all of the time (even with torque converter clutch engaged), it weighs more by an estimated 200lb, is driving 2 extra bearings in the prop shaft (the VW is front wheel drive). Everything else is the same, both have heated seets, air con, electric everything, catalytic converter, EGR system, kept in serviced condition.
I do not believe the formentioned losses account for my 15mpg shortfall and being unable to identify losses elsewhere I have to conclude that the engine is less efficient.
So why is a 4 cylinder turbo MB engine less efficient by 25% than a 4 cylinder turbo VW engine. When you break things down its a piston, two valves and an injector. If the engine its self were producing huge ammounts of heat due to friction then I dont think it would last long, hence my thoughts that the wasted energy is diasappearing down the exhaust pipe as heat.
If you think i am barking up the wrong tree then bring some ideas to the table and reason with me. Incidently the VW has the same 150Hp output so dont think the MB engine is tuned for performance at the sacrifice of economy...because the VW is able to do both. Hmmmmm.
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