C180 petrol courtesy car just 27mpg?

[Sp!ke]

I think this comes back to the thread I started the other day.

27mpg on a 1.8l Petrol... in 2012?

Ask an owner of a 2.0L 190E or even the owner of a W124 (220 model) what they average and then tell me how much petrol engine technology has moved on in the last 35 years.

It appears to be going backwards imho.

 

[Paul/]

At the risk of taking an overly simplistic abstract view on all this (and recognising that many of these points have been made on in previous posts):

The only thing providing energy/power is the fuel, the only way to get more power is to burn more fuel.

To cruise, you need just enough force to overcome friction, wind resistance and energy lost in the power chain.

To accelerate, you also need enough force to increase the potential energy of the vehicle sufficiently to achieve the new target speed, vehicle mass being the key factor.

Assuming 2 vehicles identical in all respects except engine and final drive ratio (to match petrol or diesel rev ranges):

If both engines produce the same power but one is smaller (e.g. has a turbo), the smaller one should require slightly less fuel for identical performance because it's lugging a slightly lower total weight;

If just comparing fuel consumption at identical cruising speeds, max power is irrelevant provided there’s enough to maintain the cruise. A substantially smaller engine could maintain the same legal cruising speed and have economy improved in proportion to the resultant lower total vehicle weight;

If both engines produce the same power but one is diesel and the other petrol, then the diesel benefits from having more energy per litre of fuel, and less energy loss in the engine, but requires a larger and therefore heavier engine to match the petrol power, because power is proportional to the operating rpm and diesels operate at lower rpm. The diesel will also on average need more gearchanges to accelerate by the same number of mph as the petrol, so actually needs to be more powerful to compensate for power lost during additional shifts.


...I’m not sure where I’m going with this now so I’ll stop there. Seemed like a good idea when I started.

 

[Pittsey]

I have a 2009 C180K Blue efficiency. I get about 25 MPG, non motorway driving.

I borrowed a loan car from Mercedes which was a C180 CGI with Eco mode. I found this to be awesome on fuel compared to mine. I was getting 40 MPG.

 

[wemorgan]

Maybe it’s fair to say that the economy potential of cars has improved over the years, but the way the cars are driven hasn’t.

 

[V12]

Strange, as most if not all the BM's I've owned are nearly spot on.

Brim to brim figures?

Would be good to know, I have very little BM experience, but do know that VAG and MB do fall within the 10%-20% rule.

@Paul/ - Love the theory behind it.

I have real world differences between a 2001 e220cdi and a 2001 e320cdi over 40k miles (each car) of the same daily 35 mile commute.
E220cdi - 43mpg
E320cdi - 38mpg.

The 320cdi was immaculate and driven from 90k on the clock up to 140k.
The 220cdi was to get me by after an incident with the 320, but ended up lasting over a year in the end. Mileage was over 300k when it parted.

It was a few years back when I had a second car and only used this for business mileage.

The 320cdi also seemed to really start to drink a bit if I ever had to do some town mileage. But i guess we were discussing motorway speeds.

 

[Creepy coupe]

Brim to brim figures?

Always. I found the OBC figures are much more optimistic.

My M3 showed 23 MPG when driving carefully. The actual was 21 mpg

My 335i showed 31.5 mpg actual was 29 MPG

I'd like to think the SLK's engine has been hand crafted by one of MB's finest, to the tightest tolerances. And can only get better with age

 

[V12]

Always. I found the OBC figures are much more optimistic.

My M3 showed 23 MPG when driving carefully. The actual was 21 mpg

My 335i showed 31.5 mpg actual was 29 MPG

I'd like to think the SLK's engine has been hand crafted by one of MB's finest, to the tightest tolerances. And can get better with age

Thanks.

Keep being optimistic!

 

[Creepy coupe]

I’m convinced modern forced induction engine, whether petrol or diesel have their max boost point so low in the rev band, that fuel consumption has suffered greatly. Our 520d touring backs up my theory.

I can drive at a reasonable constant throttle angle along an A road at 55 MPH and see 55+ mpg
Drive alone a country lane at 45 MPH. on and off the pedal and it drops the 42 MPG.

Do the same trip in my old runabout ( 2.0ltr Petrol normally aspirated) 34 MPG with either scenario

 

[smillion]

i think this comes back to the thread i started the other day.

27mpg on a 1.8l petrol... In 2012?

Ask an owner of a 2.0l 190e or even the owner of a w124 (220 model) what they average and then tell me how much petrol engine technology has moved on in the last 35 years.

It appears to be going backwards imho.

exactly

 

[Paul/]

Keep being optimistic!

That's us too- my wife's had her new SLK200(7g+) a month now and the computer is declaring 26.6mpg average. When I pointed this out to her and set up the eco driving display (the 3 bars and overall % rating) she managed 98% on one trip and was rewarded with declared 30.2 mpg (for the trip). All with Eco start/stop enabled, aircon off, and from day 2 in 7g E mode. 20m mile commute, mainly flowing Mway, allegedly well under 80. Not remembered to do a BtB calculation yet.

 

[Deleted member 65149]

That's us too- my wife's had her new SLK200(7g+) a month now and the computer is declaring 26.6mpg average. When I pointed this out to her and set up the eco driving display (the 3 bars and overall % rating) she managed 98% on one trip and was rewarded with declared 30.2 mpg (for the trip). All with Eco start/stop enabled, aircon off, and from day 2 in 7g E mode. 20m mile commute, mainly flowing Mway, allegedly well under 80. Not remembered to do a BtB calculation yet.

Which all goes to prove that the part of every car that has most influence on its fuel consumption is the nut immediately behind the steering wheel. All the torque/power & diesel/petrol rubbish spouted by the likes of DM whilst trying to sound clever is of absolutely no consequence in the real world.

 

[Creepy coupe]

For me all this stop/start is codswallop. It's sold as a ECO thing, which is in fact counter initiative. Because if you using the car in traffic so much as to benefit from such devices, you're never going to see good MPG.

 

[Paul/]

Which all goes to prove that the part of every car that has most influence on its fuel consumption is the nut immediately behind the steering wheel.

I didn't realise you knew her. <Only joking dear>

I give her part of the credit for that result, but I think another factor was that she travelled earlier than usual so less start-stop...

 

[Stratman]

I too was confuzzled about the relative consumption of cars of the same model with different engines driving alongside each other on a motorway, for example an E200 and an E55.

I posed the question here and did some interweb research and it turns out it's to do with engine efficiency at the given power output. The two cars in the situation mentioned above will both be putting out the same power (within a smigeon), but the smaller engine will be doing it more efficiently. It will be filling its cylinders quite nicely and the mixture will be be generating high combustion pressures.

The E55 engine will be severely throttled and turning over quite slowly. It will be taking in hardly any air and when compressed the pressures will be nothing like those in the 200. There is also the matter of valve overlap. At the lower revs these factors ensure the V8 will not come close to filling the cylinders. If the throttle restricts the incoming charge by say 75%, it's pretty much the same as turning a 10:1 compression ratio into 2.5:1 with the attendant loss of efficiency. Combine that with shoving a portion of the incoming charge straight out of the exhaust valves because of overlap and the bigger engine needs to ingest more fuel/air mixture to make the same power as a smaller engine.

At wider throttle openings, when the inlet and exhaust systems are moving meaningful amounts of gas things change

 

[Paul/]

For me all this stop/start is codswallop. It's sold as a ECO thing, which is in fact counter initiative. Because if you using the car in traffic so much as to benefit from such devices, you're never going to see good MPG.

...I have my suspicions that the ECO label is deliberately foreshortened to muddy the waters between Economy- and Ecology (i.e. emissions). Must ask her to do a comparative test.

 

[Creepy coupe]

. There is also the matter of valve overlap.

Does the E55 not have VVT then?

 

[Paul/]

The two cars in the situation mentioned above will both be putting out the same power (within a smigeon), but the smaller engine will be doing it more efficiently.

Thanks for the info. I suspected that efficiency would differ with engine size but ignored that element as didn't know why...to be honest I'd have guessed that it would have been the larger engine that would be more efficient. Oops.

 

[Creepy coupe]

That's us too- my wife's had her new SLK200(7g+) a month now and the computer is declaring 26.6mpg average. When I pointed this out to her and set up the eco driving display (the 3 bars and overall % rating) she managed 98% on one trip and was rewarded with declared 30.2 mpg (for the trip). All with Eco start/stop enabled, aircon off, and from day 2 in 7g E mode. 20m mile commute, mainly flowing Mway, allegedly well under 80. Not remembered to do a BtB calculation yet.

Sounds like our Missus's have similar cars, with similar mileage. It will be interesting to compare over the next few months. Well for me and you anyway. Everyone else will be bored ridged

 

[Paul/]

Sounds like our Missus's have similar cars, with similar mileage. It will be interesting to compare over the next few months. Well for me and you anyway. Everyone else will be bored ridged

Indeed. Except everyone else would be advised to keep a watching brief, 'cos I've a feeling that our ladies are riding on the leading edge of engine evolution, and we're all destined (forcibly) to join them there at some not too distant point As I mentioned on another thread, if I'm ever going to tick the V8 ownership box, I'd better do it PDQ.

 

[Colin_b]

That's us too- my wife's had her new SLK200(7g+) a month now and the computer is declaring 26.6mpg average. When I pointed this out to her and set up the eco driving display (the 3 bars and overall % rating) she managed 98% on one trip and was rewarded with declared 30.2 mpg (for the trip). All with Eco start/stop enabled, aircon off, and from day 2 in 7g E mode. 20m mile commute, mainly flowing Mway, allegedly well under 80. Not remembered to do a BtB calculation yet.

My SLK350 has averaged 25.5mpg over the last 5 years. Mainly urban type use, S mode and aircon usually engaged. It doesn't get thrashed (much), not driven with economy in mind.

Makes the 200 look rather thiryst!